Judging and Actualizing Intrapersonal and Interpersonal ...pacherie.free.fr/COURS/MSC/Richardson_Marsh_Baron_JEP-HPP200… · der Kamp, Savelsbergh, & Davis, 1998; Warren, 1984)

Judging and Actualizing Intrapersonal and Interpersonal Affordances

Michael J. Richardson, Kerry L. Marsh, and Reuben M. BaronUniversity of Connecticut

The current study investigated the perception of intrapersonal, interpersonal, and tool-based graspingpossibilities. In Experiment 1, participants judged whether they would grasp planks of wood—presentedin ascending, descending, and random orders of length—using one hand (1H), two hands (2H), or witha tool that extended their reach (TH). In Experiment 2, participants physically grasped the planks using1H, 2H, or TH. In Experiments 3 and 4, the choice of TH was replaced with a choice of grasping theplanks with another person (2P). The results showed that presentation order influenced the participants’behavior differently in the judgment and action experiments. The same behavioral patterns, however,were observed when participants switched between 1H and 2H, 2H and TH, and 2H and 2P grasping. Thepoint at which participants judged they would switch between the different modes of grasping, as wellas the point at which participants physically switched between the grasping modes, occurred at similaraction-scaled ratios. The equivalence of perceiving intrapersonal and interpersonal affordances is dis-cussed.

Keywords: affordances, dynamics, grasping, interpersonal interaction, tool use

A fundamental issue in understanding human behavior is deter-mining how individuals spontaneously come together to engage injoint actions. Many everyday tasks require that individuals cometogether to perform cooperative actions such as moving largedesks or tables. What is perhaps most intriguing about basic jointactions is that they often occur without prior planning. That is,cooperating individuals come together and act in real time byperceiving the constraints of the environment and task in relationto the limitations of their own individual capabilities. Interestingly,the ability of individuals to spontaneously act together often par-allels the way in which individuals spontaneously employ envi-ronmental objects or tools to complete simple tasks. For instance,a single individual can use a dolly (a pushed mobile platform) tomove large desks or tables. So when do individuals spontaneouslyjoin forces with other individuals or employ environmental tools inthe performance of everyday activity? Moreover, how can re-searchers begin to understand the emergence of such interpersonalor tool-based activity? To answer these questions, it is important to

first understand how individuals perceive whether interpersonal ortool-based activity is necessary or even possible. One way tounderstand this perception is to examine whether individuals per-ceive interpersonal and tool-based action possibilities in the sameway that they perceive intrapersonal action possibilities.

Affordances and Action-Scaled Ratios

Over the last several decades, researchers investigating theperception of basic intrapersonal action possibilities have focusedon the concept of affordance and how individuals perceive envi-ronmental surfaces and objects in relation to their own bodilydimensions or capabilities (e.g., Carello, Grosofsky, Reichel, &Solomon, 1989; Klevberg & Anderson, 2002; van Leeuwen,Smitsman, & van Leeuwen, 1994; Warren & Whang, 1987).Affordances are opportunities for action, where the frames ofreference for these actions are the relations that exist between ananimal and its environment. The term affordance was first intro-duced by J. J. Gibson (1979) to capture the complementarity of ananimal and its environment and to highlight the interdependence ofperception and action (Michaels & Carello, 1981; Sanders, 1997;Stoffregen, 2003; Turvey, Shaw, Reed, & Mace, 1981). Thus, toperceive an affordance is to perceive what an environmental sur-face, object, or event offers a perceiver relative to the actioncapabilities or effectivities1 of that perceiver (Michaels, 2003;Reed, 1996; Shaw & Turvey, 1981; Turvey & Shaw, 1979). Forinstance, individuals perceive the climbability of stairs not by theheight of the stair risers but by the height of the risers relative totheir own leg length (Warren, 1984).

1 The term effectivity was coined by Shaw and Turvey (1981; Turvey &Shaw, 1979) and refers to the functionally defined action systems used inthe actualization of an affordance. Affordances and effectivities are mutu-ally defined concepts, and the realization of an action possibility reflectsthe fit between animal and environment (Shaw, Flasher, & Kadar, 1995;Warren, 1984).

Michael J. Richardson and Kerry L. Marsh, Department of Psychologyand Center for the Ecological Study of Perception and Action, Universityof Connecticut; Reuben M. Baron, Department of Psychology, Universityof Connecticut.

This research was supported by a University of Connecticut ResearchFoundation Large Faculty Research Grant, and preparation of this articlewas supported by National Science Foundation Grant BSC-0342802awarded to Kerry L. Marsh, Claudia Carello, Reuben M. Baron, andMichael J. Richardson. We thank Oscar Whelan, John Flynn, and DivyaPatel for assisting with Experiments 1, 3, and 4 and Robert Isenhower forhelping with the design of Figure 4. Finally, we also thank Claudia Carello,Harry Heft, Lucy Johnston, Lynden Miles, Rachel Kallen, and R. C.Schmidt for their helpful comments and suggestions.

Correspondence concerning this article should be addressed to MichaelJ. Richardson, who is now at the Department of Psychology, ColbyCollege, 5550 Mayflower Hill, Waterville, ME 04901. E-mail:[email protected]

Journal of Experimental Psychology: Copyright 2007 by the American Psychological AssociationHuman Perception and Performance2007, Vol. 33, No. 4, 845–859

0096-1523/07/$12.00 DOI: 10.1037/0096-1523.33.4.845

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This latter example highlights the foundational work of Warren(1984). Measuring leg length (L) and riser height (R) using thesame units, Warren found that the resulting dimensionless ratio(R/L) predicted critical shifts in perception and action. This rationot only predicts whether an individual will perceive a stair riser asclimbable, it also captures the affordance of stair-climbabilityacross different individuals. Dimensionless ratios such as this oneare commonly referred to as body- or action-scaled ratios or pinumbers. With a more general formalism (E/A, where E is themeasured environmental property and A is the measured action-relevant property of an intentional agent), these ratios have beenused to investigate such affordances as the cross-ability of gaps(Burton, 1992; Jiang & Mark, 1994), the reach- and grasp-abilityof objects (e.g., Carello et al., 1989; Cesari & Newell, 2000b;Newell, Scully, McDonald, & Baillargeon, 1989), the walk-through-ability of apertures (Warren & Whang, 1987), the walk-up-ability of slopes (Kinsella Shaw, Shaw, & Turvey, 1992), andthe sit-ability of surfaces at different heights (Mark, 1987; Mark &Vogele, 1987; Mark, Balliett, Craver, & Douglas, 1990).

To a large extent, this previous research has centered on twoimportant issues. The first concerns the degree to which actionscaling, given its intrinsic nature, is invariant across differences inthe absolute metrics of environmental properties or the propertiesof an individual’s action system (e.g., Mark & Vogele, 1987; vander Kamp, Savelsbergh, & Davis, 1998; Warren, 1984). For in-stance, research has demonstrated how the grasping patterns of 3-to 5-year old children are determined by the same hand-size toobject-size ratio rather than the size of the object grasped or thesize of the hand doing the grasping (Newell, Scully, Tenenbaum,& Hardiman, 1989). The second issue concerns the degree towhich the perception of an affordance boundary constrains themode of activity an individual engages in (e.g., Gardner, Mark,Ward, & Edkins, 2001; Mishima, 1994; Warren & Whang 1987).For instance, research on reaching behavior has demonstrated notonly that individuals perceive what is reachable but that an indi-vidual’s perception of whether an object can be reached by ex-tending the arm, or by bending the hip and extending the arm, orby bending from an upright posture while extending the arm isdetermined by a scaling of the distance and height of the object tobe reached (Carello et al., 1989; Mark et al., 1997).

Affordances and Dynamical Systems Theory

Collectively, this previous research has demonstrated that whenthe ratio between the relevant properties of the environment and anindividual’s action system reach a value that corresponds to anaffordance boundary, other, more stable affordances emerge (orbecome attuned to), and a goal-directed agent spontaneously re-configures its action system or behavioral mode accordingly. Thisspontaneous reorganization of an individual’s behavioral mode hasled some researchers to argue that the perception and actualizationof affordances should be understood from the perspective of dy-namical systems theory (e.g., Marsh, Richardson, Baron, &Schmidt, 2006; van der Kamp et al., 1998; Warren, 1984).

Dynamical systems theory assumes that stable patterns of mac-roscopic behavior emerge and change over time due to the lawfulnonlinear interaction of animal and environmental properties andcomponents (Kelso, 1995; Kugler & Turvey, 1987; Turvey, 1990).In other words, rather than assuming that stable patterns of behav-

ior are solely the result of mental or representational processes,researchers employing dynamical systems theory argue that thephysical and informational constraints that exist between an indi-vidual and the environment operate to dynamically self-organizethe observed patterns of human behavior (Kelso, 1995; Kugler,Kelso, & Turvey, 1980; van der Kamp & Davis, 1998).

From a dynamical systems perspective, the behavioral mode thatactualizes an affordance is understood to be a stable state (i.e., astate toward which the system is pulled when it is otherwiseperturbed from stability) of the animal–environment system, andthe action-scaled ratio that captures an affordance is understood tobe a control parameter (e.g., Mark et al., 1997; Warren, 1984). Acontrol parameter is a variable that influences the existence orstrength of a behavioral state whereby a system’s current statebecomes unstable at a critical control parameter value, and thesystem suddenly transitions to a different, more stable state (Stro-gatz, 1994). In addition, the spontaneous and abrupt transitionfrom one stable mode of behavior to another that occurs at affor-dance boundaries (i.e., critical action-scaled ratios) is referred to asa nonlinear phase transition (Kelso, 1995; Turvey, 1990). Al-though such transitions can occur at a single control parametervalue, reflecting a critical point transition, nonlinear phase tran-sitions often occur at different control parameter values dependingon whether the parameter is scaled from small to large values orfrom large to small values. This is true for the transition betweenaffordances, with individuals transitioning between different ac-tion possibilities at different action-scaled ratios depending onwhether the action-scaled ratio that captured the relevant affor-dance boundary was increasing or decreasing over time (Fitz-patrick, Carello, Schmidt, & Corey, 1994; Hirose & Nishio, 2001;van der Kamp et al., 1998). In an investigation of one-hand totwo-hand grasping by van der Kamp et al. (1998), individuals werefound to transition from one-hand to two-hand grasping at a higheraction-scaled ratio than when they transitioned from two-hand toone-hand grasping. That is, objects presented to individuals inascending size (from smallest to largest) continued to be graspedwith one hand beyond the action-scaled ratio at which the indi-viduals grasped the same objects with two hands when the objectswere presented in a descending order (from largest to smallest).

This latter type of transition behavior is called hysteresis andoccurs when the history of a system’s past behavior affects thesystem’s present behavior. Interestingly, the opposite kind of non-linear behavior can also occur, whereby the transition from onebehavioral mode to another occurs at a smaller value of the controlparameter when that parameter is decreased over time comparedwith when it is increased over time. This is known as enhancedcontrast (Tuller, Case, Ding, & Kelso, 1994) and occurs when asystem’s future behavior influences its present behavior. This typeof behavior has also been observed in affordance research involv-ing perceptual judgments. Hirose and Nishio (2001) found thatperceptual judgments of what could be sat on and what could bestepped over were larger for descending orders of presentationthan for ascending orders of presentation. Similarly, Fitzpatrick etal. (1994) found that participants who visually or haptically eval-uated a slanted surface tended to perceive the boundary betweenslants that could support upright posture and those that could notto be at a lower angle of inclination when presented in an ascend-ing order than when presented in a descending order.

846 RICHARDSON, MARSH, AND BARON

Affordances and Extended Action Systems

The action systems or effectivities that actualize an affordanceneed not be limited to systems that entail bodily components of anindividual. Rather, an action system can also be comprised of anindividual with action capabilities that have been extended withthe use of external objects or tools (Bongers, Michaels, & Smits-man, 2004; Bongers, Smitsman, & Michaels, 2003; Shaw et al.,1995; Smitsman, 1997). Just as changes to an individual’s biolog-ical degrees of freedom due to growth (or improved dexterity)enable the actualization of different affordances, so does embod-ying previously external objects or tools enable an individual toactualize different affordances, some of which would have other-wise been unactualizable (Gibson, 1979; Hirose, 2002; Wagman &Carello, 2001, 2003). For example, 3 ft of fresh snow does notafford effective locomotion due to the small surface area of one’sfeet. Once snow shoes are fitted, however, one’s action systembecomes more functionally refined, resulting in 3 ft of fresh snowaffording effective locomotion.

Tools, then, have a dual function. First they serve as externalobjects of the environment, separate from a potential user’s body,which afford certain actions, such as grasping and throwing (Gib-son, 1979; Wagman & Carello, 2003). Once in use, however, theyare no longer external objects but serve another function. Theybecome functional parts of the user’s action system, just as integralto the realization of affordances as bodily components (e.g.,Bongers et al., 2003, 2004; Peck, Jeffers, Carello, & Turvey, 1996;Smitsman & Bongers, 2003). Mark and colleagues (Mark et al.,1990; Mark & Vogele, 1987) demonstrated this by showing thatwhat an individual perceives as too high to sit on increases whenblocks are attached to the feet and that the action-scaled ratio thatcaptures the boundary between what was perceived as sit-on-ableor not was the same for the body and body-block systems. Thesignificance of these findings is that they not only highlight howindividuals are able to accurately perceive action possibilities thatentail the use of tools or external objects, they also reveal the roleof action-scaled information in constraining such perceptions.

This should also be true for interpersonal activity, in that coop-erating individuals come together to actualize interpersonal affor-dances in much the same way as two limbs come together toactualize intrapersonal affordances. The similitude between affor-dances at multiple levels of the animal–environment system—thebody, the body–tool, and the body–body—is being suggested here,where the emergence of cooperation and coordination at each level(both intrapersonal and interpersonal) is a result of the sameintrinsic informational constraints. That is, despite the intuitionthat cooperative action is substantially different from solo action,an understanding of cooperative activity in terms of affordancessuggests that there is a similarity in how joint and solo activity isconstrained and organized (Richardson, Marsh, & Schmidt, 2005).It is important to note that a similitude between the perception andactualization of intrapersonal, interpersonal, and tool-based affor-dances is consistent with adopting a dynamical systems perspec-tive of affordances. This is because an underlying premise ofdynamical systems theory asserts that the same lawful propertiesof the animal–environment system regulate and constrain thebehavior of a broad range of systems, independent of the compo-nents that comprise them (Kugler & Turvey, 1987; Schmidt &Turvey, 1994).

The current experiments were therefore aimed at demonstratingthe similarity of perceiving intrapersonal, interpersonal, and tool-based action possibilities and testing whether the theory of affor-dances and action scaling can be used to understand the sponta-neous emergence of interpersonal and tool-based action. Weconducted four experiments in which individuals made judgmentsabout the graspability of a series of wooden planks of differentlengths or actually grasped and moved planks using either onehand (1H); two hands (2H); two hands with a grasp-extending toolin one hand (TH); or another individual to assist them, with eachperson holding one end (2P). Of interest was the action-scaled ratioat which individuals or pairs of individuals transitioned from 1H to2H, 2H to TH, and 2H to 2P grasping. The transitions betweenperceiving and actualizing 1H, 2H, TH, and 2P affordances werecalculated to be functions of the relevant action-scaled ratio (e.g.,plank length/hand span for the 1H grasping, plank length/arm spanfor 2H grasping). By presenting the planks in ascending anddescending orders, we also examined the patterning of the transi-tions between 1H and 2H, 2H and TH, and 2H and 2P. Drawingfrom the notion that the same dynamic regularities constrain be-havior across a broad range of systems, we expected that similardynamic patterns of behavior would be exhibited for solo and jointaction systems and for action systems with embodied tools.

The perception–action task of object grasping was used becauseit has been used in a number of previous studies examining theperception and actualization of affordances and action scaling(e.g., Cesari & Newell, 1999, 2000a; Newell, Scully, McDonald,& Baillargeon, 1989). For instance, researchers have demonstratedhow the same hand-size to object-size ratio can be used to predict1H and 2H grasping for adults, children (Newel, McDonald, &Baillargeon, 1993; Newell, Scully, Tenenbaum, & Hardiman,1989; van der Kamp et al., 1998), and individuals with mental orphysical disabilities (Savelsbergh, Steenbergen, & van der Kamp,1996; Savelsbergh, van der Kamp, & Davis, 2001). Thus, a grasp-ing task provides a well-studied paradigm for examining shiftsfrom solo action to action involving a tool or cooperative actioninvolving another individual.

Experiment 1

Experiment 1 examined the similarities of perceiving body- andbody–tool-based grasping affordances by having individuals judgewhether wooden planks of different sizes could be picked up using1H, 2H, or TH. We expected that participants would perceive theboundary between 1H and 2H grasping and between 2H and THgrasping and, thus, judge that they would transition between 1Hand 2H and between 2H and TH at an action-scaled ratio less than1. Moreover, we expected that a similar patterning of transitionswould be observed between 1H–2H and 2H–TH transitions, re-flecting the dynamic similitude between body and body–tool ac-tion systems.

Method

Participants. Six female and 6 male undergraduate studentsfrom the University of Connecticut participated in the experimentas partial fulfillment of a course requirement. The participants hadan average hand span of 20.10 cm (SD � 0.89 cm) and an averagearm span of 168.8 cm (SD � 7.57 cm).

847INTRAPERSONAL AND INTERPERSONAL AFFORDANCES

Materials. Three sets of narrow wooden planks, 2 cm high �6.5 cm wide, were used as objects. Each set contained 69 planksthat varied in length from 2 cm to 35 cm (in 1-cm intervals) andfrom 80 cm to 2.5 m (in 5-cm intervals). The planks were keptbehind a curtain, out of sight of participants, and were presented toparticipants on a 10-cm-high shelf that stood on a 3.2-m-long �80-cm-high table.

A 1.25-m-long lifting tool, which consisted of a metal clawattached to the end of a wooden handle, was placed on the tablenext to participants. This tool, when grasped at the base of thehandle, extended a participant’s reach by approximately 1.2 m.When used by someone with an arm span greater than 1.3 m, thetool could be used to lift planks up to or greater than 2.5 m long.By holding the handle in one hand and extending the claw so thatit clamped (grasped) onto one end of a wooden plank, participantscould then grasp the other end of the plank with their free hand andlift the plank successfully.

Procedure. Individual participants were required to make per-ceptual judgments about how they would2 grasp and lift thewooden planks, with 1H, 2H, or TH from the shelf to the tabletop,with the instruction that if they were to grasp and move the planks,they could only do so by touching their ends. It was made clear toa participant that this was purely a perceptual judgment experimentand at no stage would they be required to physically grasp andmove the wooden planks. To ensure that the judgments were basedon a correct understanding of how the planks could be grasped,both with the hands and with the tool, the experimenter performedthree demonstrations, one with a plank that could be grasped with1H, one with a plank that required 2H, and one with a plank thatrequired use of TH.

Each participant completed three trials in which the sequence ofplanks was presented one by one in an ascending, descending, andrandom order. To eliminate any possible confounds that couldoccur due to participants becoming aware of the number of planksused during the ascending and descending trials, only planks up toand including 5 plank lengths after the point of transition werepresented to a participant. Specifically, for the ascending trials,planks were presented to participants one by one, starting with thesmallest plank (2 cm) and then in an ascending order (3 cm, 4 cm,5 cm, etc). During the presentation of the smaller set of planks (2cm to 35 cm), a transition from 1H to 2H was deemed to haveoccurred once a participant perceived 5 planks in a row as requir-ing 2H to grasp. Following the presentation of the 5th plankperceived as requiring 2H, the experimenter began presenting thelarger set of planks (80 cm to 250 cm) in an ascending order. Aswith the 1H–2H transition, the transition from 2H to TH wasdeemed to have occurred when a participant perceived that 5planks in a row required the tool to be grasped and lifted. The trialended after the 5th plank perceived as graspable with the tool waspresented. A similar procedure was used for the descending trial,with the only difference being that a participant was presented withthe 250-cm plank first and then presented with planks in a de-scending order until 5 planks in a row were perceived as onlyrequiring 2H to grasp. Planks were then presented in a descendingorder from the 35-cm plank until 5 planks in a row were perceivedas requiring 1H. All 69 planks were presented to participantsduring the random order of presentation. Presentation order wascounterbalanced across participants (but with the random condi-

tion always occurring second), and a different set of planks wasused for each trial.

A participant stood at the center of the table with his or her eyesclosed and arms held comfortably at the sides. This ensured that aparticipant was not able to see how the experimenter placed theplanks out on the table and, moreover, was not able to extend hisor her hands or arms in order to gain any physical informationabout the graspability of the plank. After a plank was placed on theshelf, the experimenter signaled the participant to open his or hereyes and make a response. The participant responded verbally byindicating whether he or she would grasp the wooden plank with1H, 2H, or TH by saying “one,” “two,” or “three,” respectively.After making a response, the participant was instructed to close hisor her eyes again and wait for the next plank to be placed on thetable. Following the three trials, anthropometric measures of handspan (length between the end of thumb and index finger of dom-inant hand) and arm span (length between the end of right-handindex finger and left-hand index finger with arms outstretched atshoulder height) were taken with an accuracy of 1 mm.

Design and analysis. The experiment had a 2 (transition: 1H–2H, 2H–TH) � 3 (presentation order: ascending, random, descend-ing) repeated measures design. To determine the action-scaledratio at which the participants judged that they would transitionbetween 1H and 2H and 2H and TH grasping, we calculated themean plank length in centimeters at which each participant tran-sitioned from 1H to 2H or from 2H to TH for each presentationorder and then divided by hand span or arm span, respectively (forthe descending trials, the transition reflects a change from TH to2H or from 2H to 1H grasping). For example, in the case of 1H–2Htransitions, this was done, first, by determining the length of theshortest plank lifted with 2H and for which all planks longer thanthis plank were also lifted with 2H and, second, by determining thelongest plank lifted by 1H and for which all planks shorter thanthat were also lifted by 1H (van der Kamp et al., 1998). The sumof these two plank lengths divided by 2 (PLt) was then divided bythe participant’s hand span to obtain the perceived point of tran-sition as an action-scaled ratio (ASt).

Results and Discussion

It was expected that participants would correctly perceive theboundaries between 1H, 2H, and TH grasping. Accordingly, thepoint at which the participants judged that they would transitionbetween these different modes of grasping was expected to resultin ASts less than 1. Consistent with this expectation, participantsjudged that they would transition between 1H and 2H grasping andbetween 2H and TH grasping at average ASts of 0.74 and 0.97,respectively. Interestingly, an inspection of Table 1 reveals that theparticipants judged that they would transition between 1H and 2Hgrasping at a lower ASt for the ascending order of presentationcompared with the descending order of presentation. This was alsothe case for the 2H–TH transitions, with participants judging theywould transition between 2H and TH grasping at a lower ASt for

2 The instructions (how they “would” move the plank rather thanwhether they “could”) and the pacing of the experimenter’s instructionswere designed with no emphasis on accuracy or urgency of response so asto minimize pressures to engage in an effortful, analytic style of responding(Heft, 1993).


the ascending order of presentation compared with the descendingorder of presentation.

To examine whether this effect of presentation order was sig-nificant, we conducted a 2 (transition) � 3 (presentation order)repeated measures analysis of variance (ANOVA) on ASt for eachparticipant. The analysis revealed a significant effect of order, F(2,22) � 21.92, p � .05, indicating that participants judged the pointof transition to be lower for the ascending compared with thedescending order of presentation. The effect of presentation orderwas verified by a post hoc analysis that found the differencebetween the ascending and descending orders to be significant( p � .05) for both 1H–2H and 2H–TH transitions.

The effect of presentation order indicates that participantstended to exhibit enhanced contrast (see Figures 1a and 1b). A

close examination of the individual data confirmed this, with 10 ofthe 12 participants exhibiting enhanced contrast for both the1H–2H and 2H–TH transitions. Of the 2 remaining participants, 1exhibited hysteresis for both transitions, whereas the other exhib-ited a critical point transition for both transitions. As with theprevious findings of Fitzpatrick et al. (1994) and Hirose and Nishio(2001), this result suggests that for the ascending and descendingorders of presentation, the participants perceptually anticipatedhaving to transition between the different affordances. Such aresult is consistent with the perception of affordances being pro-spective, whereby the perception of affordances can be understoodas being the process of detecting what action possibilities lie ahead(Turvey, 1992; Turvey & Carello, 1986; Turvey et al., 1981).

Recall that we expected there to be a dynamic similitude be-tween the perception of the body and body–tool affordances. Thefact that the effect of presentation order was the same for the1H–2H transitions and the 2H–TH transitions appears to provideinitial evidence for this similitude. Indeed, the similarity in thepatterning of the judgments for the 1H–2H and 2H–TH transitionswould seem to indicate that tool-based action possibilities areperceived in the same way as intrapersonal action possibilities—that is, by means of intrinsic action-scaled information. Not sur-prisingly, however, the 2 (transition) � 3 (presentation order)ANOVA conducted on ASt yielded a significant effect of transi-tion, F(2, 22) � 39.36, p � .05, with participants judging that theywould transition between 2H and TH grasping at a higher ASt thanbetween 1H and 2H grasping. This discrepancy was partially dueto the fact that when grasping the planks with 1H or 2H, partici-pants needed to extend the tips of their fingers over the ends of theplanks. At a minimum, this required participants to have theirgrasping fingers (e.g., the middle finger and thumb for a 1H grasp,the middle finger of each hand in a 2H grasp) over the plank ends(which were 2 cm high/deep) in order to grasp and lift the plankssuccessfully. Thus, planks less than 4 cm of a participant’s handspan could not be grasped with 1H, and planks less than 4 cm ofa participant’s arm span could not be grasped with 2H. Note thatthis 4-cm grasping region results in an asymmetry in 1H and 2Hgrasping, in that 4 cm is a much greater proportion of the 1H actionsystem than of the 2H action system. Consequently, the actionratios at which the participants judged that they would transitionbetween 1H and 2H grasping and between 2H and TH graspingwere normalized by subtracting 4 cm from each participant’s handspan and arm span. Specifically, the normalized action-scaled ratio(nASt) was calculated as PLt/(hand span � 4 cm) for the 1H–2Htransition and as PLt/(arm span � 4 cm) for the 2H–TH transition.

Table 1Average Transitions Between Perceived Grasping Modes in the Intrapersonal, Perception-Only Experiment (Experiment 1)

Variable

1H to 2H 2H to TH

Ascending Random Descending Ascending Random Descending

PLt 13.33 (2.40) 15.83 (2.93) 15.50 (3.05) 152.50 (21.95) 161.25 (19.05) 173.75 (22.17)ASt 0.66 (0.11) 0.78 (0.14) 0.78 (0.14) 0.92 (0.12) 0.96 (0.10) 1.03 (0.13)nASt 0.82 (0.13) 0.98 (0.17) 0.97 (0.18) 0.92 (0.12) 0.98 (0.11) 1.05 (0.13)

Note. Standard deviations are in parentheses. PL1 � the average plank length at which participants’ perceptual judgments shifted between one-hand (1H)and two-hands (2H) and between 2H and tool (TH) grasps for each ordering of planks; ASt � the average action-scaled ratio (PLt divided by hand or armspan) at each transition point; nASt � the average normalized ASt values.

Figure 1. Mean percentages of perceived 2H (two-hand) grasp judgments(a) and TH (tool) grasp judgments (b) as a function of presentation orderin Experiment 1.


Consistent with the analysis of ASt presented above, a 2 (tran-sition) � 3 (presentation order) repeated measures ANOVA con-ducted on nASt also resulted in a main effect of presentation order,F(2, 22) � 22.61, p � .05, with participants judging that theywould transition between 2H and TH grasping at a lower ASt forthe ascending order of presentation compared with the descendingorder of presentation. Of more interest was that there was nolonger an effect of transition, F(1, 11) � 2.32, p � .15, with nASt

for the 1H–2H and 2H–TH transitions being comparable (seeTable 1). The analysis of nASt did yield a significant Transition �Presentation Order interaction, F(2, 22) � 3.56, p � .05, with thenASts for the ascending trials being slightly lower for the 1H–2Htransition than for 2H–TH transition. However, this appeared to bea result of participants perceiving the tool as being potentiallyawkward to use—several participants indicated this to the exper-imenter during debriefing.

Although our theorizing does not require that the point oftransition between 1H and 2H and between 2H and TH graspingoccur at the same ASt, the correspondence of nASt for the 1H–2Hand 2H–TH transitions further highlights the similarity with whichparticipants in the current experiment perceived body- and body–tool-based action possibilities. This is perhaps best demonstratedby the fact that participants judged they would transition between1H and 2H grasping and between 2H and TH grasping at exactlythe same nASt for the random order of presentation (0.98). Takenin conjunction with the similarity in the patterning of the judg-ments across the three presentation orders, the results of the currentexperiment provide clear evidence that the theory of affordancesand action scaling can be used to understand the spontaneousemergence of tool-based activity as well as individual activity.

Experiment 2

Experiment 2 was aimed at extending the findings of Experi-ment 1 by having individuals actively grasp and move the woodenplanks used in Experiment 1 using 1H, 2H, or TH. Given that theresults of Experiment 1 indicated that normalizing the ASt, pro-vided a more appropriate quantity for examining the similaritiesand differences between the 1H–2H and 2H–TH transitions, anal-ysis in the current experiment (and in Experiments 3 and 4) wasconducted using nASt as the dependent measure.

Method

Participants. Three female and 9 male undergraduate studentsfrom the University of Connecticut participated in the experiment

as partial fulfillment of a course requirement. The participants hadan average hand span of 21.72 cm (SD � 1.08 cm) and an averagearm span of 183.77 cm (SD � 5.49 cm).

Materials and procedure. The same materials and procedureas in Experiment 1 were used, with the exception that instead ofmaking perceptual judgments about how they would grasp and liftthe wooden planks, participants in the current experiment wereinstructed to physically grasp and move the planks from the shelfto the tabletop. As in Experiment 1, the experimenter demonstratedhow this could be done with 1H, 2H, and TH, using a differentplank length for each demonstration.


The current experiment was aimed at replicating and extendingthe findings of Experiment 1 by demonstrating that participantsphysically transition between 2H and TH grasping in much thesame way as they physically transition between 1H and 2H grasp-ing. An inspection of Table 2 reveals that, as in Experiment 1,participants spontaneously shifted between 1H and 2H graspingand 2H and TH grasping at comparable nASt values. In contrast tothe perceptual judgments made by participants in Experiment 1,however, participants in the current experiment actually transi-tioned between 1H and 2H grasping and between 2H and THgrasping at a higher ASt for the ascending order of presentationcompared with the descending order of presentation.

To examine whether this effect of presentation order was in factreliable, we conducted a 2 (transition: 1H–2H, 2H–TH) � 3(presentation order: ascending, random, descending) repeatedmeasures ANOVA on the nASt values for each participant. Theanalysis revealed a main effect for presentation order, F(1, 11) �11.07, p � .05, indicating that the difference in the point oftransition for the ascending order of presentation was significantlyhigher than that found for the descending order of presentation.This was true for both the 1H–2H and 2H–TH transitions (therewas no effect for transition, F[1, 11] � 2.76, p � .12), with posthoc analysis finding the difference between ascending and de-scending orders to be significant for both the 1H–2H and 2H–THtransitions ( ps � .05). The ANOVA also yielded a significantTransition � Presentation Order interaction, F(2, 22) � 5.77, p �.05, with participants making the shift between 1H and 2H grasp-ing and between 2H and TH grasping at similar nASts for therandom and ascending orders of presentation, whereas for thedescending order, the nASt for the 2H–TH transition was greaterthan that for the 1H–2H transition. This was confirmed in post hoc

Table 2Average Transitions Between Grasping Modes in the Intrapersonal, Action Experiment (Experiment 2)

Variable

1H to 2H 2H to TH


PLt 16.92 (2.11) 16.42 (1.56) 13.58 (3.70) 171.67 (9.49) 165.00 (12.34) 162.08 (11.57)ASt 0.78 (0.08) 0.76 (0.06) 0.62 (0.16) 0.93 (0.03) 0.90 (0.05) 0.88 (0.05)nASt 0.95 (0.09) 0.93 (0.07) 0.77 (0.20) 0.96 (0.04) 0.92 (0.05) 0.90 (0.05)

Note. Standard deviations are in parentheses. PLt � the average plank length at which participants’ perceptual judgments shifted between one-hand (1H)and two-hands (2H) and between 2H and tool (TH) grasps for each ordering of planks; ASt � the average action-scaled ratio (PLt divided by hand or armspan) at each transition point; nASt � the average normalized ASt values.


analysis, with the difference in nASt between the 1H–2H and2H–TH transitions being significant for the descending order ( p �.05). As in Experiment 1, this difference was likely due to the factthat participants found the tool unfamiliar or awkward to use and,thus, were more comfortable grasping wooden planks with 2H.

The significant effect of presentation order found in the currentexperiment was opposite to the effect found in Experiment 1, withparticipants making the transition between action modes at ahigher nASt value for the ascending orders than for the descendingorders of presentation. Whereas participants in Experiment 1tended to exhibit enhanced contrast, participants in the currentexperiment tended to exhibit hysteresis. This can be seen in Figure2, which shows the percentages of 2H (see Figure 2a) and TH (seeFigure 2b) grasps as a function of presentation order. A closerexamination of the individual data revealed that for 1H–2H tran-sitions, 9 of the 12 participants exhibited hysteresis, 2 participantsexhibited enhanced contrast, and the remaining participant exhib-ited a critical point transition. Similarly, for 2H–TH transitions, 9participants exhibited hysteresis, 2 exhibited critical point transi-tions, and 1 exhibited enhanced contrast.

The tendency for participants to exhibit hysteresis is consistentwith previous work examining the dynamic transition betweengrasping affordances (van der Kamp et al., 1998) and, like thetendency of participants to exhibit enhanced contrast in Experi-ment 1, provides strong evidence that the actualization of affor-dances should be understood and explained dynamically. Thequestion of why participants exhibit enhanced contrast when mak-ing perceptual judgments about the 1H, 2H, and TH affordances

and hysteresis when actualizing these same grasping affordances isan intriguing one and is discussed in more detail in the GeneralDiscussion. At this point, it is worth noting that although this resultwas not anticipated, the few studies that have previously examinedthe manner in which individuals transition between affordanceshave produced results consistent with the transition differencesfound between this experiment and Experiment 1. That is, studiesthat have required individuals to make judgments about affor-dances (Fitzpatrick et al., 1994; Hirose & Nishio, 2001) havetended to observe enhanced contrast, whereas the experiment byvan der Kamp et al. (1998), in which individuals physically actu-alized an affordance, found that participants tended to exhibithysteresis.

It also worth noting that from a dynamics perspective of affor-dances, the critical prediction is of some form of nonlinear shift inbehavior (i.e., a spontaneous or sudden shift between behavioralmodes) as opposed to a steady, gradual increase in the frequencyof a new mode of behavior and a linear decrease in occurrence ofthe old mode of behavior. Although dynamical systems perspec-tives have not determined a priori when hysteresis versus enhancedcontrast should occur, either phenomenon is compatible with thetheory and can be viewed as evidence of nonlinear phase transi-tions (Tuller et al., 1994). Rather critically, however, results ofstudies finding hysteresis and enhanced contrast using comparablesequencing of stimuli (increasing or decreasing gradually in size)are incompatible with traditional psychophysical, social judgment,or priming perspectives (e.g., Helson, 1964; Herr, 1986; Herr,Sherman, & Fazio, 1983; Manis & Paskewitz, 1984). From suchperspectives, persisting in the use of a previous judgmental cate-gory versus shifting to use a new judgmental category is deter-mined by the degree of difference between the immediate stimulibeing evaluated and the preceding set of stimuli. The studies thatdiffer by finding hysteresis or enhanced contrast do not, however,differ methodologically in the degree of gradualness of stimuluschanges (i.e., do not involve small increases in the former andlarge discrepancies in the latter studies). Therefore, such discrep-ancies in results are challenging to reconcile with priming- oradaptation-level explanations for judgmental phenomena.

Despite the contrasting effects of presentation order to thosefound in Experiment 1, the current findings further illustrate thedynamic similitude by which action scaling constrains the percep-tion and actualization of body and body–tool affordances. This wasdemonstrated by finding that participants exhibited the same hys-teresis effect when transitioning between 1H and 2H grasping andbetween 2H and TH grasping. Finding that the participants tran-sitioned at similar nASts for the random and ascending orders ofpresentation also highlights the similarity of perceiving the body-and body–tool-based grasping affordances. Experiments 1 and 2also reveal the embodied nature of tool-based activity by showingthat individuals were able to accurately perceive their abilities withthe tool, despite never having seen it before (or having practiced itsuse). This finding is consistent with the findings of Mark et al.(1990), in which participants with blocks attached to their feetwere able to perceive the sit-ability of surfaces despite not beingpermitted to sit while wearing the blocks.

A natural extension of Experiments 1 and 2 was to ask whetheraction scaling could also explain the spontaneous emergence ofjoint activity. More specifically, we wanted to examine whetherthe perception and actualization of cooperative interpersonal ac-

Figure 2. Mean percentages of 2H (two-hand) grasps (a) and TH (tool)grasps (b) as a function of presentation order in Experiment 2.


tivity is similar to the perception of solo activity. Experiments 3and 4 investigated this possibility by asking participants to judgeand grasp the wooden planks with either 1H, 2H, or together withanother person.

Experiment 3

Although there has been no research examining the dynamicsimilitude between intrapersonal and interpersonal affordances, thedynamic similitude of intrapersonal and interpersonal action sys-tems has been demonstrated by researchers interested in movementcontrol and coordination. In particular, Schmidt and colleagues(e.g., Richardson et al., 2005; Schmidt & O’Brien, 1997; Schmidt& Turvey, 1994) have shown how the rhythmic movements of twovisually coupled individuals unintentionally and intentionally ex-hibit patterns of coordinated action that are the same as thosefound when a single individual coordinates the rhythmic move-ments of two limbs. Mottet, Guiard, Ferrand, and Bootsma (2001)have also demonstrated a similitude in the dynamics of a one-handed, two-handed, or two-person Fitts law task. Specifically, theauthors found that the movement organization of participantsperforming an aiming task, with either one hand (moving a pointerback and forth between two stationary targets), two hands (movingboth the pointer and the targets), or another individual (one indi-vidual moving the pointer and the other moving the targets)exhibited the same dynamics (i.e., had the same changes in non-linearity as the index of difficulty was increased).

Drawing on these findings, Experiment 3 asked participants tomake perceptual judgments about how they would grasp and movewooden planks of differing lengths with either 1H, 2H, or the help ofanother individual. As was the case in Experiment 1, we expected thatthe point (as a normalized ASt, nASt) at which participants judged thatthey would transition between 1H and 2H grasping and between 2Hand two-person (2P) grasping would be less than 1. Moreover, thepatterning of transitions (e.g., hysteresis, enhanced contrast) between1H and 2H grasping and 2H and 2P grasping was expected to besimilar, highlighting the dynamic similitude of perceiving intraper-sonal and interpersonal action possibilities.

Method

Participants. Six female and 6 male undergraduate studentsfrom the University of Connecticut participated in the experimentas partial fulfillment of a course requirement. Participants had anaverage hand span of 21.62 cm (SD � 1.79 cm) and an averagearm span of 177.89 cm (SD � 7.99 cm).

Materials and procedure. The same materials and procedureas in Experiment 1 were used, with the exception that participantswere instructed to judge whether they would grasp the woodenplanks with 1H, 2H, or 2P—with an imagined other.3 Thus, as wellas demonstrating how the planks could be grasped with 1H and2H, the experimenter also described how 2 people (the participantplus another individual) could lift the planks by having 1 person ateach end of a plank.


As expected, participants judged that they would transitionbetween 1H and 2H grasping and between 2H and 2P grasping atASts of less than 1. This can be seen from an inspection of Table3. Table 3 also reveals that, like participants in Experiment 1,participants in the current experiment judged that they wouldtransition between 1H and 2H grasping at a lower ASt for theascending order of presentation compared with the descendingorder of presentation. This was also the case for the 2H–THtransitions, with participants judging that they would transitionbetween 2H and TH grasping at a lower ASt for the ascendingorder of presentation compared with the descending order ofpresentation.

A 2 (transition: 1H–2H, 2H–2P) � 3 (presentation order:ascending, random, descending) within-subject ANOVA con-ducted on nASt verified that this effect of presentation orderwas reliable, F(2, 22) � 25.21, p � .05. Figure 3 shows thepercentages of 2H (see Figure 3a) and 2P (see Figure 3b) graspsas a function of presentation order. It is clear from these graphsthat for both the 1H–2H and 2H–2P transitions, participantstended to exhibit enhanced contrast. A close inspection of thedata confirmed this, with 11 of the 12 participants exhibitingenhanced contrast for the transitions between 1H and 2H grasp-ing and 10 of the 12 participants exhibiting enhanced contrastfor the transitions between 2H and 2P grasping. Post hocanalysis also confirmed that the difference between the ascend-ing and descending orders of presentation was significant forboth 1H–2H and 2H–2P transitions ( ps � .05).

3 In a separate experiment, six pairs of participants completed this sametask in the presence of another person. That is, rather than perceptualjudgments about whether a plank would require 2P being based on animagined other, another individual was present and both made perceptualjudgments (without interacting or having information about the other’sjudgments). The results of that experiment were identical to those of thecurrent experiment.

Table 3Average Transitions Between Perceived Grasping Modes in the Interpersonal, Perception-Only Experiment (Experiment 3)

Variable

1H to 2H 2H to 2P


PLt 14.67 (2.29) 16.67 (1.89) 17.50 (2.52) 153.75 (18.96) 162.71 (15.76) 171.25 (19.79)ASt 0.68 (0.12) 0.77 (0.10) 0.81 (0.13) 0.87 (0.10) 0.92 (0.09) 0.96 (0.11)nASt 0.84 (0.15) 0.95 (0.13) 1.00 (0.17) 0.88 (0.10) 0.94 (0.09) 0.99 (0.12)

Note. Standard deviations are in parentheses. PLt � the average plank length at which participants’ perceptual judgments shifted between one-hand (1H)and two-hands (2H) and between 2H and joint (2P) grasps for each ordering of planks; ASt � the average action-scaled ratio (PLt divided by hand or armspan) at each transition point; nASt � the average normalized ASt values.


Finding that participants tended to exhibit enhanced contrast rep-licated the perceptual judgment findings of Experiment 1 and those ofFitzpatrick et al. (1994) and Hirose and Nishio (2001). Of moreimportance, however, was finding that the effect of presentation orderwas the same for the 1H–2H and 2H–2P transitions. That is, theANOVA revealed neither an effect of transition, F(2, 22) � 1, nor aTransition � Presentation Order interaction, F(2, 22) � 1.69, p � .21,with participants judging that they would transition between 1H and2H grasping and between 2H and 2P grasping at similar nASt values.Thus, not only do these findings highlight the dynamic similitudebetween the perceptual judgment of intrapersonal and interpersonalaffordances, they also provide the first empirical evidence that theperception of cooperative action possibilities is action scaled and canbe studied using the theory of affordances. Moreover, taken with thefindings of Experiment 1, the current results indicate that the sponta-neous emergence of joint activity also parallels the way in whichindividuals spontaneously use environmental objects or tools to com-plete many simple tasks.

To further investigate the dynamic similitude between theperception of body, body–tool, and cooperative affordances, weconducted a final experiment in which participants were re-quired to physically grasp wooden planks either alone or to-gether. Although participants in the current experiment tendedto exhibit enhanced contrast, the findings of Experiment 2suggest that participants might exhibit hysteresis when physi-cally grasping and moving the planks together with anotherindividual. Thus, this final experiment was also needed tofurther investigate the contrasting effects of presentation orderon judging and actualizing affordances.

Experiment 4

Experiment 4 was aimed at extending the findings of the pre-vious experiments by uncovering whether the actualization of2H–2P grasping also reflects the same dynamic and action-scaledregularities as 1H–2H and 2H–TH grasping.

Method

Participants. Twenty-eight undergraduates from the University ofConnecticut participated in partial fulfillment of course requirements. Theparticipants were randomly paired to form 14 same-sex dyads. Thisresulted in 7 male pairs and 7 female pairs. The participants had anaverage arm span of 176.64 cm (SD � 7.56 cm). The difference in armspan between participants in a pair ranged from 0 cm to 17.5 cm, with anaverage difference of 7.28 cm (SD � 5.08 cm).

Materials. A set of 102 wooden planks, 2 cm high � 10 cm wide,was used in each condition. The planks varied in length from 80 cm to2.5 m, in 5-cm intervals, and there were three planks of each length.

The experimenter presented the wooden planks by placing themon a 2.6-m-long conveyer belt that moved at a speed of 0.9 m/s(see Figure 4). The experimenter placed the planks on the end ofa conveyer belt behind a curtain. The planks then moved along theconveyer belt until they emerged from behind the curtain and sliddown a ramp onto a 54-cm-wide � 3-m-long table (at no timeduring the experiment were participants able to see all of theplanks or the experimenter placing the planks on the conveyerbelt). After the planks had landed on the table, participants wererequired to lift planks, turn around, and move the planks 4.3 m toa 3-m-wide ramp that stood opposite the table. The ramp wasconstructed so that after participants had placed a plank on it, itwould slide down behind a curtain to be collected by an experi-menter for the next trial.

A standard VHS video camera was used to record the partici-pants as they completed each trial. The video recording was alsostreamed in real time to a television monitor positioned behind thecurtain so the experimenter could view the participants as theycompleted the experiment.

Procedure. After both participants in a pair arrived andsigned an informed consent form, they were told that theexperiment was investigating object lifting and that they wouldbe required to grasp and move a set of narrow wooden planksfrom the table (once a plank had slid off the conveyer belt) tothe ramp on the opposite side. As in the previous experiments,the participants were instructed that they could only grasp andmove a plank by touching its ends and that this would requirethem to lift planks either alone using 2H or together with eachparticipant grasping one end of a plank. The experimenter andhis assistant then demonstrated how the planks could be lifted,using an 80-cm plank for the 2H demonstration and a 250-cmplank for the 2P demonstration.

Each pair completed three trials in which the sequence of plankswas presented in a steady stream on a conveyer belt in an ascend-ing, descending and random order. Participants began each trial bystanding next to each other in front of the conveyer belt. Theexperimenter positioned the wooden planks on the center of theconveyer belt so that when the planks slid off the end of the beltand down the ramp they came to rest with the center of the plankaligned with the center of the table. By watching the television

Figure 3. Mean percentages of perceived 2H (two-hand) grasp judgments(a) and 2P (two-person) grasp judgments (b) as a function of presentationorder in Experiment 3.


monitor positioned behind the curtain, the experimenter was ableto present the planks in a continuous stream, with each plank beingplaced on the belt while the previous plank was being lifted fromthe table. This ensured that the participants completed the exper-iment at a comfortable, self-selected but steady tempo. The threeplanks of each length were presented in sequence for each order ofpresentation (i.e., ascending order: 80 cm, 80 cm, 80 cm, 85 cm, 85cm, 85 cm, 90 cm, 90 cm, 90 cm, etc.; random order: 155 cm, 155cm, 155 cm, 230 cm, 230 cm, 230 cm, 95 cm, 95 cm, 95 cm, etc.;descending order: 250 cm, 250 cm, 250 cm, 245 cm, 245 cm, 245cm, 240 cm, 240 cm, 240 cm, etc.). This was done to counteractany possible turn-taking effects—that is, by presenting participantswith three objects of the same length in a continuous stream, weensured that participants who took turns would alternately receivethe object of new size.

As in the previous experiments, all 102 planks were presented tothe pair for the random order, whereas only those planks whoselengths were less than or equal to the 5th plank length after thetransition from 2H to 2P in the ascending order and from 2P to 2Hin the descending order were presented to the participant pair.Given that 3 planks of each length were presented to participantssequentially, this resulted in 15 planks being presented to partic-ipants after the transition hand occurred. For the ascending order,the point of transition from 2H to 2P was deemed to have occurredonce a pair stopped lifting the planks alone and had, with eachother, grasped and moved 5 different plank lengths (15 planks intotal) in a row. For the descending order, the point of transitionfrom 2P to 2H was deemed to have occurred once the participants

in a pair stopped lifting the planks together and had, on their ownusing 2H, grasped and moved 5 different plank lengths (15 planksin total) in a row. Orders were counterbalanced across participantpairs, with the exception that the random sequence was alwayspresented second. Once participants had completed the three or-ders of presentation, participants completed a variety of question-naires about the tasks, after which their arm spans (length betweenthe end of right-hand index finger and left-hand index finger witharms outstretched at shoulder height) were measured with anaccuracy of 1 mm. Participants were then debriefed about the truepurpose of the experiment and thanked for their participation.

Design and analysis. Experiment 4 had a 2 (participant armspan: smaller, larger) � 3 (presentation order: ascending, random,descending) � 2 (gender: male, female) mixed design, with par-ticipants’ arm span and gender as the within-subject factors. Notethat for each pair, the nASt values for the participant with thelonger arm span and the nASt values for the participant withshorter arm span were computed and analyzed (hence, the within-subject factor of participant arm span), rather than the averagenASt value being taken for each pair.


The current experiment was aimed at extending the findings ofthe previous experiments by demonstrating that participants phys-ically transition between 2H and 2P grasping in much the sameway as they physically transition between 1H and 2H grasping. Inparticular, we expected that the patterning of transitions would be

Figure 4. Experimental setup for Experiment 4. (a) Example of 2H (two-hand) grasping. (b) Example of 2P(two-person) grasping.


consistent with the findings for the 1H–2H grasping behaviorinvestigated in Experiment 2, reflecting the dynamic similitudebetween the perception and actualization of intrapersonal andinterpersonal grasping affordances. Consistent with this expecta-tion, participants in the current experiment transitioned between2H and 2P grasping at a higher ASt for the ascending order ofpresentation compared with the descending order of presentation(see Table 4). This not only reflects a patterning of transitionssimilar to that observed for the 1H–2H transitions in Experiment 2but, taken in comparison with the findings of Experiment 3,parallels the contrasting effects of presentation order observedbetween Experiment 1 and 2. Specifically, participants in Exper-iment 3 exhibited enhanced contrast, whereas participants in thecurrent experiment exhibited a patterning of transitions consistentwith hysteresis. This hysteretic effect of presentation order isclearly displayed in Figure 5a, where percentages of 2P grasps areplotted as a function of presentation order.

A 2 (participant arm span) � 3 (presentation order) � 2 (gen-der) mixed-design ANOVA conducted on the nASt values for eachparticipant in a pair revealed that the effect of presentation orderwas significant, F(2, 24) � 20.30, p � .05, with 10 of the 12 pairsexhibiting hysteresis. The remaining 2 pairs exhibited enhancedcontrast. It is important to appreciate that in addition to the currentfindings revealing a similar patterning of transitions between 1Hand 2H grasping and between 2H and 2P grasping, a comparisonof Tables 4 and 2 also reveals that the point at which participantstransitioned between 2H and 2P grasping was similar to that foundfor the 1H–2H transitions in Experiment 2. To test whether thecurrent nASt values were similar to those observed in Experiment2, we conducted independent-samples t tests for each presentationorder comparing the average nASt values for each pair in thecurrent experiment with the nASt values found for the 1H–2Htransitions in Experiment 2. Although this analysis revealed asignificant difference between the 1H–2H (0.93) and 2H–2P (0.83)transitions for the random order of presentation ( p � .05), thedifference between the 1H–2H (0.95) and 2H–2P (0.94) transitionsfor the ascending order of presentation was not found to besignificant ( p � .6), nor was the difference between the 1H–2H(0.76) and 2H–2P (0.72) transitions for the descending order ofpresentation ( p � .6). The similarity between the 2H–2P and1H–2H grasping transitions is most easily seen in Figure 5b, wherethe percentages of 2-effector grasps (2H grasps for the 1H–2Htransition and 2P grasps for the 2H–2P transition) for the transi-tions in Experiment 2 and Experiment 4 are plotted as a functionof the ascending and descending presentation orders.

Interestingly, the 2 (participant arm span) � 3 (presentationorder) � 2 (gender) mixed-design ANOVA yielded a significant

main effect of participant arm span, F(1, 12) � 21.97, p � .05,with the participant who had the longer arm span in a pair makingthe transition between 2H and 2P at a lower nASt than the partic-ipant with the shorter arm span. There was also a significantParticipant Arm Span � Presentation Order interaction, F(2, 24) �13.40, p � .05, with the difference in nASt between the longerarmed and shorter armed participants in a pair being greatest forthe ascending order and smallest for the descending order. Thetransition between 2H and 2P grasping during the ascending andrandom orders tended to occur around the time the smaller partic-ipant in the pair could no longer lift the planks comfortably using2H—the larger participant transitioned at a smaller nASt value inorder to accommodate the participant with the shorter arm span.The greater the difference between the shorter and longer armed

Table 4Average Transitions Between Solo and Joint Action in the Interpersonal Action Experiment (Experiment 4)

Variable

Smaller arm span Larger arm span


PLt 161.79 (15.70) 143.57 (23.18) 126.96 (31.87) 161.79 (15.70) 143.57 (23.18) 126.96 (31.87)ASt 0.93 (0.08) 0.83 (0.13) 0.73 (0.17) 0.90 (0.08) 0.79 (0.11) 0.70 (0.16)nASt 0.96 (0.08) 0.85 (0.13) 0.75 (0.17) 0.92 (0.07) 0.81 (0.11) 0.72 (0.16)

Note. Standard deviations are in parentheses. PLt � the average plank length at which participants’ actions shifted between solo and joint action for eachordering of planks; ASt � the average action-scaled ratio (PLt divided by arm span) at each transition point; nASt � the average normalized ASt values.

Figure 5. (a) Mean percentages of 2P (two-person) grasps as a functionof presentation order in Experiment 4. (b) Mean percentages of two-effector grasps for the 1H–2H (one-hand–two-hand) transition in Experi-ment 2 and the 2H–2P (two-hand–two-person) transition in Experiment 4for ascending and descending orders of presentation.


participants in a pair, the lower the point of transition was for thelonger armed participant. For the descending order, this effectoccurred less (thus the significant Participant Arm Span � Pre-sentation Order interaction), given that the participants continuedto lift the planks together well beyond the point that both partic-ipants could lift the objects with 2H.

In summary, the results of Experiment 4 were consistent with thefindings of the previous experiments and capture the dynamic simil-itude of perceiving and actualizing intrapersonal and interpersonalaffordances. They also support the notions that the spontaneous emer-gence of joint activity results from the perception of interpersonalaffordances and that the realization of such affordances is actionscaled. This latter point is important, as no previous research hasdemonstrated that the perception of simple interpersonal or jointactivity is action scaled. Indeed, previous studies investigating affor-dances in a social context have only involved determining whether anindividual can perceive the affordances of another person (Ramen-zoni, Riley, Davis, & Snyder, 2005; Stoffregen, Gorday, Sheng, &Flynn, 1999) or if an observer can perceive what another personaffords them (Gunns, Johnston, & Hudson, 2002). Such studies ad-dress affordances at the level of the solitary individual. Experiments 3and 4, however, examined affordances at the level of the interpersonalunit. Thus, the current findings provide the first clear indication thatthe affordance concept can be used to understand the spontaneousemergence of social action.

General Discussion

The current study was aimed at demonstrating the similarity ofperceiving intrapersonal, interpersonal, and tool-based action pos-sibilities. More generally, we attempted to highlight how thetheory of affordances and action scaling can be used to understandthe spontaneous emergence of interpersonal and tool-based action.Four experiments were conducted in which individuals made per-ceptual judgments (Experiments 1 and 3) about whether theywould grasp wooden planks (Experiments 1 and 3) or they actuallygrasped and moved these planks (Experiments 2 and 4) alone—using one hand, two hands, or a tool that extended their graspingcapabilities—or cooperatively, using another to assist them. Ofinterest was the ASt at which individuals or pairs of individualstransitioned from 1H to 2H, 2H to TH, and 2H to 2P grasping andthe patterning of the transitions between 1H and 2H, 2H and TH,and 2H and 2P grasping.

Collectively, the results of the four experiments were consistentwith the expectation that participants would transition from 1H to2H, 2H to TH, and 2H to 2P in a similar manner, thereby support-ing the hypothesis of a dynamic similitude between the perceptionand realization of body, body–tool, and interpersonal affordances.Specifically, the results revealed a striking similarity in the de-scending and ascending nASt values across 1H, 2H, TH, and 2Pgrasping. The point and patterning of transitions for the ascendingand descending orders was almost exactly the same for the 1H–2Hand 2H–2P transitions, and despite the awkwardness of using thenovel tool, the effects of presentation order for the 2H–TH tran-sitions were also very similar to those found for the 1H–2H and2H–2P transitions. Thus, the current results suggest that the spon-taneous emergence of joint or tool-based activity results from theperception of action-scaled animal–environment relations. Thecurrent results also highlight the functional equivalence of body,

body–tool, and body–body action systems. That is, body–tool andsocial action systems can be understood and studied as a singlesynergy or effectivity (Marsh et al., 2006).

It is worth noting that the average nonnormalized ASt for the1H–2H transitions (0.73) found in Experiments 1, 2, and 3 iscomparable to that found previously (e.g., Cesari & Newell 1999;van der Kamp et al., 1998). Recall that normalizing the ASt

involved subtracting a constant magnitude from the respectiveeffector length (i.e., 4 cm for the participants’ hand span and armspan), and although this resulted in relatively consistent ratiosacross the four experiments, it is clear that future research shouldobtain the magnitude of the maximum possible object that can begrasped as the measure of the action-relevant property.

Enhanced Contrast for Perception and Hysteresis forAction

The similar occurrence of hysteresis and enhanced contrast forthe 1H–2H, 2H–TH, and 2H–2P transitions reflects the dynamicsimilitude of perceiving and actualizing body-, body–tool-, andbody–body-based affordances. However, why participants exhib-ited enhanced contrast when making judgments and hysteresiswhen actualizing the grasping affordances is unclear. Indeed, al-though we predicted that individuals would exhibit nonlinearphase transitions, we did not expect there would be a difference inthe patterning of these transitions for the judgment and actionexperiments.

Differences between judgments of an affordance and the actu-alization of an affordance have been found previously and have ledsome researchers to question whether judgments should be con-sidered a valid method of investigating the perception of affor-dances (Heft, 1993; Jiang & Mark, 1994; Pagano & Bingham,1998; Pepping & Li, 2005; Rochat & Wraga, 1997). These argu-ments draw from finding that judgments about whether an affor-dance can be actualized in a certain manner do not always corre-spond with whether an individual will actualize an affordance inthat way (Bootsma, 1989; Mark et al., 1997; Pagano & Bingham,1998; Warren & Whang, 1987). A number of studies have alsofound that judgments reflect systematic underestimations (Mark,1987; Pepping & Li, 2000a; Pufall & Dunbar, 1992) or overesti-mations (Carello et al., 1989; Mark et al., 1997; Pepping & Li,2000a, 2000b; Warren & Whang, 1987) of affordance boundaries.These latter findings are interpreted as reflecting the conservativenature of judgments, whereby actors attempt to maintain a “marginof safety” to minimize their exposure to potentially dangeroussituations (Jiang & Mark, 1994; Mark, Jiang, King, & Paasche,1999; Pufall & Dunbar, 1992). Mark and colleagues have alsosuggested that differences between the judged and the actualboundary of an affordance indicate the difference between abso-lute affordance boundaries—the absolute limit of an action sys-tem’s capability to perform a particular mode of action—andpreferred affordance boundaries—the point at which actualizing anaffordance in a certain way is no longer comfortable or efficient(for more details, see Mark et al., 1997).

The current findings, however, indicate another possible differ-ence between judged and actualized affordance boundaries.Namely, the dynamic patterning of judged affordance boundariesappears to be opposite to the dynamic patterning of actualizedaffordance boundaries. Although it is difficult to make any strong


claims about why this is the case (given that the experiments werenot designed to investigate this finding), it is possible that thisdifference further indicates that judgments do not provide accuratedata about the perception of affordances (Heft, 1993; Jiang &Mark, 1994; Mark et al. 1990; Oudejans, Michaels, Bakker, &Dolne, 1996). It is also possible that actualizing an affordanceprovides additional (and sometimes more appropriate) sources ofinformation about affordance boundaries and that this additionalinformation is responsible for the differences observed betweenthe judging and actualization experiments. Indeed, Mark et al.(1990) and Oudejans et al. (1996) have argued that activity (evenunrelated activity) provides more information about action capa-bilities and that these additional sources of information act to tunean individual’s perception–action system such that more “accu-rate” or functional behavior is observed.

It bears mention that our results cannot be easily explained bytraditional representation-based, judgment- and decision-makingperspectives. That is, traditional explanations for why judgmentsof psychophysical dimensions such as length, height, weight, andtemperature are affected by the adaptation level (Helson, 1964) orcontextual distribution (Wedell, 1995; Wedell, Parducci, & Geisel-man, 1987) posed by prior stimuli have argued that the phenomenaof one’s judgment being “assimilated” to the preceding judgments(as in hysteresis) or contrasted in a direction away from thepreceding judgments (as in enhanced contrast) can be explained bycomparable processes. The same explanation is used for socialjudgment models, which focus on noetic (semantic) processes toexplain how attitudes, person perception, social comparison andself-judgments are affected by the interpretative frame or on judg-mental processes that occur when previous stimuli prime semanticcategories (e.g., Herr, 1986; Herr, Sherman, & Fazio, 1983; Manis& Paskewitz, 1984; Stapel & Winkielman, 1998). Whether ex-pressed in judgments or in action, the key factor determiningwhether assimilation versus contrast occurs is how large a discrep-ancy there is between the preceding stimuli or primes and the newstimulus. With minimal difference between a new stimulus and thepreceding ones, assimilation or hysteresis should occur, accordingto such theories. Therefore, in the case of our studies, such theorieswould always predict that hysteresis would dominate, given thegradual nature of the change in plank length in the ascending ordescending conditions.

Cooperative Action and Affordances

Although finding comparable action-scaled shifts for solo and jointaction is interesting, there is no reason to expect that this will alwaysbe the case. What is unique about the particular affordance(s) studiedhere is that the grasping affordances were quantitatively nested. Fig-ure 6 illustrates the structure of 1H, 2H, TH, and 2P grasping affor-dances as defined by the results of the four experiments. Implied inthe action of moving an object with one hand or with two is the salientpossibility that something that extends one’s effectivities quantita-tively will allow for moving longer objects. However, another tenet ofdynamical systems approaches is that there are qualitatively newpossibilities of action as a new synergy or coordinative structureemerges. Although these affordances were not examined in the cur-rent experiments, hints of such differences are suggested by thedifferences between the random conditions of Experiment 2 andExperiment 4. Specifically, when the dynamical pressure of a previ-ous trajectory of action was not present, the pull to cooperative actionappears to have been stronger than the pull to solo action (Experiment4). This suggests that affordances defined at an interpersonal level arestronger attractors than self-similar affordances defined in relation toaction systems that involve the individual self with inanimate tools.

One reason why the 2P grasping may have functioned as astronger attractor than TH grasping is that a cooperative interper-sonal or social unit reflects a more substantial change in anindividual’s action system than does embodying a simple tool.Indeed, the former is not only more flexible and adaptive, it alsoallows for the actualization of an extremely broad range of affor-dances, including many that can only be actualized by an inter-personal or social unit. In social psychological terms, the 2P actionsystem is a situation in which a social unit temporarily emerges toform a “fragile we” within a “mutually shared social field” (Asch,1952). In other words, because other people (in contrast to tools)offer a more global change in the affordance and effectivitystructure of the animal–environmental system, the 2P conditionmoves the individual away from simple embodiment (acting witha tool) toward joint activity and socially embodied cooperation. Inthis sense, examining the affordances of a social unit or synergy,as well as the information that specifies social affordances, mightallow one to better understand and determine the point at whichmere behavioral coordination of movement with another becomesteamwork—where a new unit of action, a social synergy, emerges.If so, hysteresis might reflect the beginnings of a process that

Figure 6. The experimentally defined structure of the nested 1H (one-hand), 2H (two-hand), TH (tool), and 2P(two-person) grasping affordances. nASt � normalized action-scaled ratio.


yields an emergent social unit (dyad, group, team), and differentmagnitudes of hysteresis (and enhanced contrast) could provide ameasure of social unity or rapport (or the lack thereof).

Conclusion

The four experiments presented here demonstrate that individ-uals can perceive tool-based and interpersonally based affor-dances. These experiments also extend the role of action scalingfrom solo action systems to action systems that entail aneffectivity-extending tool, be it physical or social. The currentresults also highlight how individuals spontaneously employ theuse of a tool or another person when their own effectivities areinsufficient to utilize a given affordance. More broadly, the currentresults challenge the egocentric view of affordances by showinghow solo actions are but a special case of action within an envi-ronment that is filled with other organisms as well as things.Indeed, by demonstrating the similitude of body, body–tool, andbody–body systems, the current experiments highlight how affor-dances and effectivities are mutually nested and similarly con-strained at all levels of the animal–environment system.

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Received October 16, 2005Revision received June 14, 2006

Accepted June 15, 2006 �


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Judging and Actualizing Intrapersonal and Interpersonal ...pacherie.free.fr/COURS/MSC/Richardson_Marsh_Baron_JEP-HPP200… · der Kamp, Savelsbergh, & Davis, 1998; Warren, 1984)