Bridging Temporal Gaps Between Cs and Us in Autoshaping

Journal of Experimental Psychology:Animal Behavior Processes1982, Vol. 8, No. 2, 187-203

Copyright 1982 by the American Psychological Association, Inc.0097-7403/82/0802-0187J00.75

Bridging Temporal Gaps Between CS and US in Autoshaping:Insertion of Other Stimuli Before,

During, and After CSPeter S. Kaplan and Eliot Hearst

Indiana University

In four experiments pigeons were exposed to key-light illuminations separatedfrom food delivery by 12-60 sec. Approach to the key light did not develop onconventional trace-conditioning arrangements but occurred consistently when-ever some auditory or visual stimulus (a) filled the CS-US gap (serial condi-tioning) or (b) was always present except during the gap. Various comparisongroups showed that this enhancement of conditioning cannot be mainly attributedto similarity between the CS and the added stimulus, or to spread of specificresponses evoked by that stimulus, or to potentiation of CS's neural aftereffectsby the extra stimulus. However, modifications of condition b in the final exper-iment revealed that CS approach was strong only when the stimulus presentduring the intertrial interval remained on until the termination of CS; if thestimulus ended at CS onset, conditioning did not occur. Although discriminabilityof CS-US gaps from intertrial periods seems necessary for conditioning to occurin the absence of close CS-US contiguity, the outcome of the final experimentindicates that such discriminability is not sufficient for conditioning. The resultsare primarily interpreted in terms of (a) possible second-order conditioning ef-fects and (b) changes in the associative strength of the "local context" existingwhen CS appears, which may lead to superconditioning of CS.

Trace conditioning procedures, which in-volve CS presentation followed by a stimu-lus-free period before US delivery, are lesslikely to produce strong acquisition of con-ditioned responses to CS than if the gap be-tween CS offset and US onset is filled by adifferent external stimulus (serial condition-ing procedures) or by a continuation of theoriginal stimulus (delay conditioning pro-cedures). Facilitative effects produced byfilling the CS-US gap have been reported,for example, by Bolles, Collier, Bouton, andMarlin (1978), Kamin (1965), Kehoe, Gibbs,

This research was supported by National Institute ofMental Health Grant MH-19300. Portions of the datawere presented at the meeting of the Eastern Psycho-logical Association, Hartford, Connecticut, April 1980,at the meeting of the Psychonomic Society, St. Louis,Missouri, November 1980, and at the Harvard Sym-posium on Quantitative Analyses of Behavior, Cam-bridge, Massachusetts, June 1981, We thank SarahBottjer, Roberta Ewing, Dexter Gormley, Robert Lev-enson, Gary Lucas, Eileen Riffe, and William T. Wolfffor valuable advice and assistance.

Requests for reprints should be sent to Eliot Hearst,Department of Psychology, Indiana University, Bloom-ing ton, Indiana 47405.

Garcia, and Gormezano (1979), Newlin andLoLordo (1976), Pearce, Nicholas, andDickinson (1981), and Sears, Baker, andFrey (1979). Most previous attempts to ac-count for the ineffectiveness of trace condi-tioning have stressed the lack of contiguitybetween the US and either the CS itself orsome persisting neural-memorial aftereffectof the CS; a frequent statement is that fora particular CS and US, trace conditioningmay be impossible beyond some maximumgap between CS offset and US. Therefore,explanations of successful conditioning withfilled CS-US gaps have stressed nonassocia-tive and associative mechanisms that some-how compensate for the absence of conti-guity between CS and US or that involvedifferent contiguous relations (e.g., betweenCS and the gap-filler stimulus). Along theselines, such processes as stimulus generaliza-tion, temporal spread of responses acquiredduring the gap filler, sensory-sensory con-ditioning, higher-order conditioning, andsecondary reinforcement have been consid-ered candidates for strong involvement ineffective gap-bridging arrangements.

187

188 PETER S. KAPLAN AND ELIOT HEARST

Although we discovered later that Mowrerand Lamoreaux's (1951) account cannothandle the detailed results of our studies, theresearch reported here was initially guidedby an interpretation of trace conditioningfailures that was offered by them. Theypointed out that on trace procedures thebackground or situational cues that prevailduring the gap between CS and US are iden-tical to the stimulus conditions in forcethroughout most segments of the experi-mental sessions (the intertrial intervals, ITIs).This feature of the trace arrangement shouldproduce considerable generalization betweenthese two kinds of unfilled intervals andthereby hinder conditioning to the CS. Inother words, when the trace CS is off, sub-jects cannot easily discriminate whether theyare currently in an ITI or in a gap betweenCS and US. An obvious way to test Mowrerand Lamoreaux's account would be to com-pare standard trace conditioning with ac-quisition to the same CS when the gaps be-tween CS and US are clearly differentiatedfrom the intervals between trials. This goalcan be accomplished by inserting some extrastimulus either in the gapas has been suc-cessful in several prior studies of serial con-ditioning or in the ITI, which to ourknowledge was investigated here for the firsttime (but cf. Bolles et al., 1978).

The present series of experiments thusexamined approach and contact behaviordirected toward localized visual CSs (auto-shaping or sign tracking; see Hearst & Jen-kins, 1974) on standard trace conditioningprocedures, in comparison with the behaviordirected toward the same CS when certainauditory or visual stimuli were presentedbefore, during, or after the CS. Several CS-US gaps were studied, ranging from 12 secto 1 min, as well as the degree of overlapbetween the CS and the extra stimuli. Thegeneral findings are discussed in the light ofcurrent data and theories pertaining to com-pound conditioning, overshadowing andblocking, the role of contextual cues, simul-taneous versus successive associations, andsecond-order conditioning.

Experiment 1Mowrer and Lamoreaux's (1951) hypoth-

esis predicts that trace conditioning should

improve as the stimulus conditions in theCS-US gap and the ITI are made more dis-criminable from each other. An autoshapingexperiment performed, by Newlin andLoLordo (1976, Experiment 1) provides sup-port for this prediction. Each of the sessionsin their study can be described as consistingof a repeating cycle of events, composed offour successive segments: the ITI, the CS,the period between CS offset and US onset,and a period of grain delivery. One groupof pigeons was presented with a key lightthat was white throughout the ITI, greenduring the 4-sec CS period, and red duringthe 4-sec CS-US gap (serial conditioning).A second group was also presented with akey light that was white throughout the ITIand green during the CS period, but the lightturned white again during the CS-US gap(trace conditioning). Newlin and LoLordofound that birds in the serial "group, whichreceived ITIs that were clearly discriminablefrom CS-US gaps (white vs. red), reachedan autoshaping acquisition criterion muchfaster than birds in the trace conditioninggroup.

In our first experiment we sought to rep-licate and extend Newlin and LoLordo's re-sults and to examine further the applicabilityof Mowrer and Lamoreaux's hypothesiswithin the autoshaping paradigm. Fivegroups of pigeons were all presented with a12-sec green key light followed after a 12-sec period by brief access to grain. For onegroup, both the ITI and the period betweenCS1 and US were left unfilled (i.e., no ex-plicit stimulus was presented: a conventionaltrace conditioning procedure). For the qtherfour groups, the ITI was likewise left un-filled, but a different kind of auditory or vi-sual filler (white key light, red house light,clicker, or complete blackout of the cham-ber) was inserted during the gap between CSand US. These filler stimuli enabled us notonly to differentiate the CS-US gap fromthe ITI in the four groups but also to assess

1 For purposes of consistency and clarity we use the

term "CS" to refer to the stimulus separated from theUS by a time gap, even though it is of course possibleto conceive of the stimuli that fill this gap or the ITIas CSs, too. Therefore, in all the present experimentsthe green key light served as the CS; stimuli insertedin the CS-US gap are called "gap fillers," and stimuli,inserted in the ITI are called "ITI fillers."

BRIDGING TEMPORAL GAPS IN AUTOSHAPING 189

possible contributions of (a) stimulus gen-eralization between CS and gap filler and(b) any response transfer dependent on thebehaviors evoked by the specific gap fillersthemselves (i.e., some of the fillers wouldprobably not evoke pecking, especially at thekey). As in the Newlin and LoLordo study,key pecks were recorded; however, our majordependent measure involved the index ofapproach to the CS developed by Wasser-man, Franklin, and Hearst (1974), whichhas generally proved more sensitive and re-liable than key-peck measures.

MethodSubjects. The subjects were 40 experimentally naive

female White Carneaux pigeons, 5-7 yr old, maintainedat 75% of their free-feeding weights. All birds were in-dividually housed, with water freely available.

Apparatus. Four standard Lehigh Valley Electron-ics three-key chambers were employed, but the middlekey was covered with tape and never used. The left andright keys were dark except during trials, at which timesone or the other key was illuminated by a miniprojectormounted behind it. Mixed grain was periodically ac-cessible for 3 sec in an aperture midway between thekeys and 11 cm above the floor of the chamber. A trans-lucent Plexiglas panel was installed 5 cm below the en-tire regular ceiling; two 2.8-W house lights were embed-ded in the regular ceiling near the center of the chamber.Presentation of either house light through the translu-cent panel was designed to decrease the localizabilityof these stimuli. Different house-light colors (red andwhite) were produced by use of a red plastic cover overone bulb and a clear plastic cover over the other bulb;only one bulb was ever illuminated at a given time. Theintensities of the red and white house lights were 7.5 Ixand 29,1 Ix, respectively, measured from the chamberfloor with a Photovolt Corporation (Model 210) pho-tometer. A loudspeaker was mounted behind the frontpanel and centered between and slightly above the twokeys. A 25-pps, 83-dB (SPL) clicking sound could bedelivered through this speaker, which was located ap-proximately 12 cm above another speaker that delivered

a constant 79-dB (SPL) white noise to the chamber.The latter stimulation, along with the continuous soundof the chamber's ventilating system, served to mask ex-traneous noises.

The location of a bird in the chamber (i.e., left orright side) was monitored by a microswitch beneath theteeter-totter floor (see Wasserman et al., 1974). Thenumber of key pecks during CS and ITI periods and theamount of time spent on the same side (left or right)of the chamber as the illuminated key were recordedby conventional electromechanical relay circuitry lo-cated in the control room adjacent to the experimentalcubicle. During selected sessions the subjects were ob-served directly by the experimenter or by videotapemonitoring and recording.

Procedure. Eight pigeons were randomly assignedto each of five groups. During magazine training, a pi-geon was placed in its experimental chamber with thewhite house light on and the hopper raised and filledwith grain. After the bird had eaten for about 20 sec,the hopper was lowered. Subsequent grain presentationsoccurred irregularly for brief periods (3-5 sec). Twentygrain presentations were given to each subject on 2 suc-cessive days.

On the third day of the experiment, conditioning be-gan. All birds in the experiment received presentationsof a 12-sec green key light followed after a 12-see in-terval by 3-sec access to grain. The type of stimuluscondition in effect during the 12-sec period betweengreen key light offset and grain delivery varied acrossthe five groups, whereas ITI conditions were the samefor all groups (a white house light was on and both keyswere dark). Table 1 displays the sequence of stimulusevents occurring on each trial for the different groups.In this experiment, of course, the only difference ingroup treatments occurred during the gap between CSoffset and grain delivery. Birds in different groups werepresented with either a white key light (Gap-Filled-Key-light), a change in the color of the house light fromwhite to red (Gap-Filled-Red-Houselight), a completeblackout of the chamber (Gap-Filled-Blackout), a click-ing noise (Gap-Filled-Clicker), or the same stimulusconditions as were present during the ITI (Gap-Un-filled).

Key-light presentations occurred on either the left orthe right side of the chamber. The exact order of leftversus right key illuminations varied irregularly fromtrial to trial, with the constraints that (a) the green key

Table 1Sequence of Events in Each Repeating Cycle of a Session for Different TreatmentGroups in Experiment I

Group label

Gap-FilledKeylightRed HouselightBlackoutClicker

ITI(VT 60 sec)

unfilledunfilledunfilledunfilled

CS(12 sec)

green keygreen keygreen keygreen key

Gap (12 sec)

white keyred house lightblackoutclicker

US(3 sec)

graingraingraingrain

Gap-Unfilled unfilled green key unfilled grain

Note. ITI = intertrial interval; VT = variable time.


1.00

90

.60

70

60

.50

.40

KL o BO UNF

HL o CL

r .24

DAYS

Figure 1. Mean approach-withdrawal ratios to CSacross sessions for birds in each of the five groups ofExperiment 1. (KL = key light; HL = house light;BO = blackout; CL = clicker; UNF = unfilled.)

light could not appear on the same side for more thanthree consecutive trials and (b) an equal number of leftand right key illuminations occurred daily. Birds in theGap-Filled-Keylight groups (green key light followedby white key light and then grain) experienced bothcolored lights on the same side of the chamber duringa given trial.

The total amount of time spent by a bird on the sameside of the chamber as the illuminated green key light(and white key light for the Gap-Filled-Keylight group)was recorded daily, and this sum was divided by thetotal amount of time that the key light was on to yielda performance measure called the approach-withdrawalratio. A ratio near 1.00 evidences strong approach be-havior toward the lit key, whereas a ratio near .00 in-dicates strong withdrawal from it; birds typically exhibitwithdrawal behavior in the presence of conditioned in-hibitory stimuli (e.g., Wasserman et al., 1974). A rationear .50 indicates that the bird's position in the chamberis not systematically controlled by the location of thelit key.

A variable time (VT) 60-sec ITI, with a range of 40-80 sec, was in force throughout the experiment for birdsin all groups. Each of the 24 training sessions, scheduled7 days per week, included 34 trials. All stimulus eventsand reinforcers occurred independently of the animal'sbehavior.

ResultsFigure 1 shows that birds in the Gap-Un-

filled (conventional trace conditioning) groupdid not acquire any tendency to approachthe CS; if anything, they displayed a slighttendency to withdraw from CS as trainingprogressed. In clear contrast, birds in all fourgap-filled groups rapidly attained high levelsof approach to the CS.

A mixed two-factor analysis of variancewas performed on the data of Figure 1. Themain effect of groups was highly significant,F(4, 35) = 26.45, p < .001, as was the maineffect of days and the Days X Groups inter-action, F(23, 805) = 22.09 and F(92,805) = 3.44, respectively, p < .001 in bothcases. Subsequent Newman-Keuls tests car-ried out on the data collapsed across all 24sessions revealed that birds in each of thegap-filled groups exhibited stronger ap-proach to CS than did birds in the gap-un-filled condition (p < .01 in each case). Inaddition, birds in the Gap-Filled-Keylightgroup displayed significantly stronger ap-proach to CS than did birds in the Gap-Filled-Clicker group (p < .01). No other in-dividual comparisons were statistically sig-nificant.

The mean numbers of CS key pecks persession over the 24 training days were 75.0,74.1, 61.1, and 106.0 for the Gap-FilledKeylight, Red-Houselight, Blackout, andClicker groups, respectively, all of whichdiffered significantly from the Gap-Unfilledgroup, in which only two subjects made anypecks to the CS during training (a total of19 and 7 pecks over 24 sessions). However,the amount of pecking in the gap-filledgroups was highly variable, and their meanlevels did not differ significantly from oneanother.

A further result seems worthy of mention.Approach behavior and key pecking wereexamined in the Gap-Filled-Keylight groupwith respect to the green key light (CS) ver-sus the white key light (gap filler). Meanapproach-withdrawal ratios toward the litkey over all sessions of training were .86during the CS and .94 during the gap filler;the latter value was greater than the formerfor every one of the eight birds. Similarly,the mean number of key pecks per sessionwas 75.0 during the CS and 143.6 duringthe gap filler, with six of the eight subjectsshowing more pecking during the gap fillerthan during CS.

Although birds in the other three gap-filled groups rarely pecked the (dark) keyduring the CS-US gap, videotapes and pe-riodic visual observation of all subjects inthose groups over the final 10 days of train-ing typically revealed consistent behaviors


directed at various parts of the front panelduring the gap (of course, since subjects inthe Gap-Filled-Blackout group were not vis-ible at that time, we could not monitor theirbehavior). During the gap birds in the Gap-Filled-Red-Houselight group engaged pri-marily in "goal-tracking" behavior (seeBoakes, 1977)movements and pecks di-rected at the area of the grain aperturewhereas some birds in the Gap-Filled-Clickergroup also pecked at the magazine, othersat the front wall, and still others generallybobbed their heads in a stereotyped manner.All these behaviors occurred much less fre-quently, if at all, during ITIs. Birds in theGap-Unfilled group did not peck at the darkkeys during the gap, nor were they observedto engage in any off-key pecking or goaltracking. Instead, these birds typically shut-tled back and forth in front of the stimuluspanel throughout the session. Pecking of anykind during the ITI was very infrequent forall 40 birds in the experiment.

DiscussionRegardless of the nature of the discrete

auditory or visual stimulus that filled the 12-sec gap between CS offset and US onset,clear-cut approach and pecking behaviorwere directed toward CS in all groups thatreceived such a filler stimulus. When no fillerwas presented during the gap (conventionaltrace conditioning), neither approach norpecking behavior toward CS developed. Be-cause (a) the strongest CS approach oc-curred when the CS-US gap was filled bya stimulus (lit key) that was similar to theCS and that itself evoked behavior verymuch like the responses directed at CS and(b) the weakest approach resulted when thegap filler came from a different sensory mo-dality (a clicker) and evoked a variety ofbehaviors besides pecking, there is appar-ently some contribution of stimulus gener-alization effects and/or response transfer tothe filled versus unfilled differences we ob-tained. However, the fact that strong CS-directed behavior emerged in all the gap-filled groups irrespective of the exact kindof gap filler or the behaviors emitted duringthe gap strongly suggests that such gener-alization and transfer effects cannot account

for the major finding of our experimentthespecific facilitation of CS responding pro-duced by insertion of other stimuli in theCS-US gap.

Kehoe et al, (1979) and Pearce et al.(1981) also dismissed simple stimulus gen-eralization as an adequate explanation forthe superiority of serial over trace condi-tioning obtained in their studies. Inci-dentally, along lines pertinent to theMowrer-Lamoreaux hypothesis, any dif-ferences between the effects of the differentgap fillers in our experiment might alter-natively be analyzed in terms of the degreeto which each kind of filler stimulus suc-cessfully differentiated the CS-US intervalfrom the ITI (rather than in terms of gen-eralization between filler and CS). Unfor-tunately, this alternative is hard for us toevaluate rigorously, because we possess noindependent measure of the discriminabilityof each type of filler from the ITI conditions.Another relevant point derives from recentexperiments indicating that similarity of CSiand CS2 facilitates Pavlovian second-orderconditioning (see Rescorla & Furrow, 1977).This type of effect implies that any transferof associative strength from gap filler to CSmay have been enhanced or promoted by thedegree of physical similarity between thesetwo stimuli.

A few birds in the Gap-Unfilled groupdeveloped withdrawal responses to the traceCS. This outcome is consistent with recentevidence suggesting conditioned inhibitionof the rabbit's nictitating membrane re-sponse by a trace CS (Hinson & Siegel,1980). Our result is an indication that traceCSs may acquire mild conditioned inhibitoryproperties under certain conditions. How-ever, of the approximately 40 pigeons thatwe have trained on procedures like the gap-unfilled arrangement of Experiment 1, fewerthan 25% exhibited any signs of withdrawalbehavior (see also Experiments 3 and 4 andFigures 3 and 4 of the present report, whichprovide no evidence of conditioned inhibitionto a trace CS, even though the CS-US gapwas considerably longer in Experiment 3than in this experiment). Therefore, we donot further discuss the possibility of inhibi-tory conditioning to the trace CS, althoughcertain temporal values of the ITI, CS, and


gap may produce such an effect in an au-toshaping situation.

As would be expected, all the gap fillersin the present experiment seemed themselvesto acquire conditioned excitatory value. Wepointed out that particular responses (signtracking, goal tracking, head bobbing, etc.)appeared consistently during the filler stim-uli and not during ITIs. Such an outcomesupports the possibility that second-orderconditioning occurred, with the filler as thefirst-order stimulus and the CS as the sec-ond-order stimulus; in other words, the CSmay have acquired its excitatory powerswholly or partially through contiguity withthe onset of the gap filler, rather thanthrough actual pairings with the US. Sec-ond-order conditioning to a key light CS hasbeen demonstrated with key lights, houselights, and auditory stimuli (but not black-outs) as first-order stimuli (see Rashotte,1981, for a review).

The findings of Experiment 1 are in gen-eral agreement with Newlin and LoLordo's(1976) trace versus serial conditioning re-sults for autoshaping and with predictionsfrom Mowrer and Lamoreaux's (1951) hy-pothesis concerning the improvement of con-ditioning that should result if the ITI andCS-US gap are clearly discriminable.Among other alternatives to the Mowrer-Lamoreaux account, second-order condi-tioning remains a potential mechanism bywhich this facilitation operates.

Experiment 2The crucial implication of Mowrer and

Lamoreaux's (1951) analysis of trace con-ditioning concerns the discriminability ofstimulus conditions prevailing during theCS-US gap in comparison with the condi-tions prevailing during the ITI. It should notmatter whether the distinctive filler is pre-sented during the gap or during the ITI,Consequently, in this experiment differentgroups of pigeons were treated as in Exper-iment 1, but with an important exception.For two groups an auditory or visual stim-ulus was presented only during the CS-USgap (a systematic replication of Experiment1), whereas for two other groups the audi-tory or visual stimulus was presented exceptduring the gap. Thus on the latter procedure,

a filler stimulus was presented throughoutthe ITI and did not terminate until CS off-set. Such an arrangement represents an in-teresting hybrid between conventional serialand trace procedures, because the conditionsin effect during the CS-US gap are identicalto those in force during standard trace pro-cedures (i.e., no external stimulus is pre-sented) but the stimulus conditions duringthe ITI and CS-US gap are not identical,as is also true of standard serial procedures.Will conditioning occur to a CS followed byUS after a long unfilled period, provided thatthe ITI contains a distinctive stimulus?

MethodSubjects. Twenty-eight female White Carneaux pi-

geons served as subjects, All housing, deprivation, andmaintenance conditions were the same as in Experi-ment 1. '

Apparatus. Slight modifications were made to thechambers used in Experiment 1. A 2.8-W bulb coveredby a red plastic cap was embedded in the center of thefront panel above the translucent Plexiglas ceiling. APlexiglas "diffuser," resting on top of the translucentceiling, was placed beneath the bulb. The red light cre-ated by illuminating the bulb appeared to the experi-menters as a patch of red light at the front center ofthe Plexiglas ceiling. This "signal light" was used as thevisual filler stimulus in Experiment 2 to roughly equatethe auditory and visual fillers in terms of magnitude ofchange from background conditions: The auditory filler,presentation of a 25-pps clicker, was the same as inExperiment 1, and consequently both kinds of fillersrepresent the addition of an external stimulus to generalsituational cues, whereas in Experiment 1 the visualfiller in the Keylight and Red-Houselight groups in-volved a change from one color to another and in theBlackout group a removal of all visual cues.

Procedure. Pigeons were randomly assigned to fourgroups of seven subjects. Magazine .training proceededas in the previous experiment. During the training phase,all birds received 12-sec presentations of a green keylight followed after 12 sec by 3-sec access to grain, asin Experiment 1. A summary of the four treatments issupplied in Table 2. In a systematic replication of Ex-periment 1, two groups were presented with either thered signal light or the clicker during qnly the CS-USgap (Gap-Filled-Signal-Light and Gap-Filled-Clickergroups). For the remaining two groups, the signal lightor clicker was always present except during the CS-USgap (ITI-Filled-Signal-Light and ITI-Filled-Clickergroups). All other details of training were the same asthose in Experiment 1.

ResultsOne bird in the ITI-Filled-Signal-Light

group had to be dropped from the experi-ment after three conditioning sessions forfailure to eat reliably. Mean CS approach-


Table 2Sequence of Events in Each Repeating Cycle of a Session for Different TreatrnentGroups in Experiment 2

Group label

Gap-FilledSignal LightClicker

ITI-FilledSignal LightClicker

ITI(VT 60 sec)

unfilledunfilled

signal lightclicker

CS(12 sec)

green keygreen key

green key"green key'

Gap(12 sec)

signal lightclicker

unfilledunfilled

US(3 sec)

gramgram

grain"grain"

Note. ITI = intertrial interval; VT = variable time." All ITI fillers overlapped in time and coterminated with the CS. They came on again during the US.

withdrawal ratios for the remaining pigeonsin each group of Experiment 2 are displayedin the acquisition curves of Figure 2. Birdsin all groups developed a strong tendency toapproach the CS, although asymptQtie per-formance levels were generally lower thanthose attained by the groups in Experi-ment 1.

A mixed two-factor analysis of variancecarried out on these data indicated that themain effect of groups was insignificant,F(3, 23) = .59, whereas the main effect ofdays and the Days X Groups interactionwere significant, F(23, 529) = 11.87 andF(69, 529) =1.96, respectively, p< .001 forboth. Thus, in terms of overall approach-withdrawal ratios, it did not matter verymuch whether the signal light or the clickerwas used as a filler* or whether the filler wasinserted only during the CS-US gap or ex-cept during the gap. The conditioning levelsdisplayed by all four groups were substan-tially above the level shown by the conven-tional trace conditioning group in Experi-ment 1.

The mean numbers of CS key pecks persession over the 24 training days were 6,3,20.1, 6.5, and 8.6 for the Gap-Filled-Signal-Light, Gap-Filled-Clicker, ITI-Filled-Sig-nal-Light, and ITI-Filled-Cljcker groups,respectively. However, the amount of keypecking in these groups was highly variable,and mean levels did not differ significantlyfrom each other.

Despite the fact that little pecking of thedark keys" was recorded during the CS-USgap for any of the birds in the experiment,periodic observations of all birds over the

last 10 days of training revealed regular be-haviors during the gap; some subjects in eachgroup pecked in the area of the grain ap-erture, keys, or other places on the frontpanel, whereas some birds exhibited stereo-typed head-bobbing responses. The domi-nant behavior in a given bird seemed to befairly idiosyncratic, although several birdsin the Gap-Filled-Signal-Light group di-rected their pecks toward the signal lightitself. Pecking directed at the dark keys, orany other forms of consistent behavior be-sides general shuttling or circling, was ob-served very infrequently during ITIs.

DiscussionIn this experiment consistent approach

behavior emerged during a CS separatedfrom US by 12 sec, in strong contrast to the

D SL IN GAP o CL IN SAP

SL IN ITI CL IN ITI

12

DAYS

24

Figure 2. Mean approach-withdrawal ratios to CSacross sessions for birds in each of the four groupsof -Experiment 2. (SL =? signal light; CL = clicker;ITI = intertrial interval.)


lack of conditioning that occurred to sucha CS when the ITI and CS-US gap wereboth unfilled in the standard trace condi-tioning group of Experiment 1. As in theprior experiment, filling only the CS-US gapwith an auditory or visual stimulus removedthe conditioning deficit, but Experiment 2went further by demonstrating that a facil-itation of conditioning would also occur ifthe same auditory or visual stimulus filledall segments of the experimental session ex-cept for the CS-US gap. The findings areconsistent with Mowrer and Lamoreaux's(1951) suggestion that increases in the dis-criminability of the CS-US gap from theITI will improve conditioning to a CS sep-arated by several seconds or more fromthe US.

Coupled with the findings of Experiment1, our results argue convincingly against amajor role for stimulus generalization fromgap filler to CS in accounting for the facil-itative effects obtained; stimulus conditionsin the gap for subjects in the two ITI-filledconditions of Experiment 2 (i.e., no discreteexternal stimulus in the gap) were identicalto those in force on the standard trace con-ditioning procedure of Experiment 1, whichhad yielded no evidence of conditioned ap-proach to CS. Accounts based primarily onsimple spread or transfer to CS of antici-patory responses developing during the gapseem implausible, too, since a variety of dif-ferent responses were emitted during the gapby individual subjects in the separate groups,and yet subjects in all groups exhibited ap-proach and pecking to CS.

However, the differential behaviors (off-key pecking, goal tracking, head bobbing,etc.) observed during the CS-US gap, ascompared with the fairly nonspecific behav-iors occurring during ITIs, clearly indicatedthat stimulus conditions during the gap be-came excitatory in Experiment 2 regardlessof whether they involved presence or absenceof some auditory or visual stimulus. Becausethe CS immediately preceded this stimuluscondition, the possibility of an interpretationbased on second-order conditioning remainsviable. Although, to our knowledge, second-order conditioning has never been demon-strated with the absence of some explicitstimulus as the first-order excitor, we have

every reason to believe that attempts at sucha demonstration would succeed. Obviously,for "absence" to serve as an effective gapstimulus it must be differentiated or markedoff from other segments of the experimentalsession, because otherwise simple trace con-ditioning should generally be attainable, too.Thus the absence of an explicit stimulus dur-ing a relatively long CS-US gap does notpreclude conditioning to that CS. Inciden-tally, both Kamin (1965) and Pavlov (1927,p. 39) obtained successful first-order con-ditioning to a CS that involved the tempo-rary cessation or disappearance of a contin-uous auditory stimulus (see also Konorski,1967, p. 69), although Kamin concluded (p.142) that "CS termination appears to pro-duce a much shorter-lived trace than doesCS onset."

Experiment 3In our first two experiments, conditioning

to a CS separated by 12 sec from US wasobtained when either the CS-US gap or allother segments of training sessions weremarked or filled by some distinctive externalstimulus. Before attempting a further anal-ysis of these effects, we wanted to determinewhether strong conditioning would likewisedevelop over appreciably longer intervalsthan 12 sec, provided that an external stim-ulus was continuously present except duringthe CS-US gap. Pigeons were trained asbefore with a 12-sec green key light as theCS, but US followed after either a 30-sec(two groups) or a 60-sec (two groups) un-filled interval. Half of the birds at each gapduration were exposed to a conventionaltrace conditioning procedure (no fillers),whereas the remaining birds experienced anauditory stimulus at all times except duringthe gap (like the ITI-filled groups in Exper-iment 2).Method

Subjects. Thirty-two female White Carneaux pi-geons served. All general maintenance conditions wereidentical to those for Experiments 1 and 2.

Apparatus. The conditioning chambers and appa-ratus details were the same as those in the prior exper-iments. The auditory filler stimulus was different, how-ever. It consisted of a 1000-Hz, 88-dB (SPL) tone,presented from the same speaker as the clicking noisein the earlier work.


Procedure. After random assignment into fourgroups of eight subjects, birds were magazine trainedas in Experiments 1 and 2, During subsequent condi-tioning, two separate groups were presented with a 12-sec green key light followed by US after a 30-sec unfilledinterval. For one of these groups, the tone filler was oncontinuously except during the 30-sec gap (ITI-Filled-30 group). For the other group, no tone was ever pre-sented (ITI-Unfilled-30 group, a standard trace proce-dure). The ITI averaged 2.5 min (range: 100-200 sec)for both groups. Birds in the remaining two groups werepresented with an identical 12-sec green key light fol-lowed by US after a 60-sec unfilled interval. One ofthese groups was presented with a continuous tone thatterminated at CS offset (ITI-Filled-60 group), whereasthe other group never experienced the tone (ITI-Un-fllled-60 group, a standard trace procedure). The ITIfor these two groups averaged 5 min (range: 200-40Qsec). Our reason for changing the absolute ITI durationfrom a mean of 60 sec in Experiments 1 and 2 to 150sec or 300 sec in this experiment was to hold constantthe ratio of gap duration to ITI duration (1:5)likelyto be important on the basis of results from other workon temporal parameters influencing acquisition and per-formance (see, for example, Gibbon & Balsam, 1981).All birds received 34 trials during each of the 24 dailyconditioning sessions.

ResultsFigure 3 displays training data for birds

in the 30-sec-gap groups and 60-sec-gapgroups. At both gap durations, birds receiv-ing the tone filler at all times except betweenCS offset and US onset exhibited strong CSapproach behavior, whereas birds trained onconventional trace conditioning proceduresshowed virtually no conditioning to CS. Thelevels of approach behavior achieved in thetwo tone-filled groups were at least as highas that obtained with an auditory filler anda much shorter CS-US gap (12 sec) in Ex-periment 2.

Separate mixed two-factor analyses ofvariance were carried out on approach-with-drawal ratios at the two gap durations. Forthe 30-sec-gap groups, the main effect offilled ITI versus unfilled ITI was highly sig-nificant, F(l, 14) = 21.75, p < .001, just aswas the effect for the 60-sec-gap groups,F(l, 14) = 13.21, p < .005. In both analysesthe main effect of days and the Days XGroups interaction were also significant, at-taining p values of at least .012.

The mean numbers of CS key pecks persession over the 24 training days were 23.5and 6.1 (the latter "high" value was attrib-utable largely to one bird) in the 30-sec-

1.00

.90

.80

.70

.60

.50

1.00

.90

.80

70

.60

.SO

30-MC jopr"~v

r^S^r*^^*^^^

L_ ITI FILLED

ITI UNFILLED

60-sec gap

LL.12

DAYS

24

Figure 3. Mean approach-withdrawal ratios to CSacross sessions for birds in the 30-sec-gap and 60-sec-gap conditions of Experiment 3. (ITI = intertrial inter-val.)

Filled and 30-sec-Unfilled groups, respec-tively, and 3.3 and 0.1 in the 60-sec Filledand Unfilled groups, respectively. Differ-ences in CS key pecking between filled andunfilled conditions were significant in the 60-sec groups, *(14) = 2.93, p < .02, but failedto achieve an acceptable level of significancein the 30-sec groups, f(14) = 1.61. Counterrecords for birds in all four groups over thelast 10 days of training revealed little or nopecking at the dark keys during the CS-USgap or the ITI. The incidence of off-keypecking, goal tracking, and other consistentbehaviors during the CS-US gap by birdsin the ITI-filled conditions over the last 10sessions of training was extremely low, incontrast to the regular behaviors of this kindobserved asymptotically with the shorter, 12-sec gap used in Experiments 1 and 2.

Discussion' (

When all segments of experimental ses-sions except the CS-US gap were filled bya tone, clear-cut excitatory conditioning to


CS occurred even though the unfilled gapswere as long as 1 min in duration. This find-ing dramatizes the power of the proceduresused in the present series of experiments forenabling pigeons to bridge long temporalgaps. Typically, acquisition of CS-directedbehavior is very weak when unfilled CS-USgaps of more than approximately 5-8 sec areemployed (e.g., Newlin & LoLordo, 1976;Wasserman, Deich, Hunter, & Nagamatsu,1977; but cf. Lucas, Deich, & Wasserman,1981). Part of our success may be due to thesensitive approach-withdrawal measure used.

Experiment 4Compared with standard trace condition-

ing, the manipulations performed in Exper-iments 1-3 were designed to increase thediscriminability of stimulus conditions pres-ent during the CS-US gap from those pres-ent during the ITI. As a result of such pro-cedures, the unfilled temporal gap acrosswhich reliable autoshaping could be ob-tained was extended further than would beexpected on the basis of most prior research.However, in Experiments 2 and 3, when adistinctive stimulus was presented during allsegments of the experimental session exceptthe CS-US gap, the stimulus not only filledthe entire ITI but also overlapped the CSand terminated simultaneously with it. Theobserved facilitation of acquisition to CS,compared with standard trace conditioning,could be attributed to (a) differentiation ofstimulus conditions present during the CS-US gap from conditions during the ITI,along the lines of Mowrer and Lamoreaux's(1951) analysis; (b) some potentiation of theneural trace of the CS produced by the si-multaneous occurrence and/or terminationof key light and filler stimulus; (c) the pre-sentation of the CS in a presumably inhib-itory (or minimally excitatory) local contextor background, on the assumption that thelong-duration ITI filler rapidly becomes asignal for nonreinforcement. Conversely, instandard trace conditioning, the CS occursin the presence of excitatory general back-ground stimulation, or (d) second-order con-ditioning, with filler absence the first-orderexciter and the green key light the second-order stimulus, as suggested above.

Our next experiment was designed to helpevaluate the relative power and generalityof these various kinds of explanations. Wereturned to the CS-US and ITI durationsused in Experiment 2; four groups of pigeonswere trained with a 12-sec green key lightfollowed after an unfilled 12-sec gap by 3sec of grain. For three of these groups, a tonefilled the entire ITI and terminated (a) whenthe key light came on (No Overlap), (b)when the key light had been on for 6 sec(Partial Overlap), or (c) when the key lightwent off (Complete Overlap, as in Experi-ments 2 and 3). If the crucial feature pro-ducing successful conditioning over long un-filled CS-US gaps in Experiments 2 and 3was simply differentiation of gap and ITIconditions, then the three groups in the pres-ent experiment should perform more or lesssimilarly. If the crucial feature was occur-rence of the key light in a nonexcitatory orinhibitory local context, then conditioningshould be better to the degree that the ITIfiller and keylight overlapped. A second-or-der conditioning account would also seem toanticipate the latter general result, since inthe Complete Overlap group the key lightpreceded and predicted filler offset whereasin the No Overlap group the illumination ofthe key did not predict but was coincidentwith filler offset.

The fourth group of subjects was trainedon a trace-compound procedure in which 12-sec illuminations of the key light were com-pounded with 12-sec occurrences of the tone;there was no external stimulus presented ineither the ITI or the gap, and the US wasdelivered 12 sec after the offset of the com-pound CS. If the mere addition of an au-ditory stimulus somehow potentiates theneural trace of^the CS so as to facilitate thebridging of a subsequent unfilled CS-USgap (in Experiments 2 and 3 the key lightCS was always accompanied by the auditoryITI filler and terminated simultaneouslywith it), then birds in the Trace Compoundgroup should display good autoshaping tothe key light CS, too.

MethodSubjects. Thirty-two female White darneaux pi-

geons, maintained as in the foregoing experiments,served.


Apparatus. ' The apparatus was the same as that inExperiment 3.

Procedure. After random assignment to four groupsof eight subjects, each pigeon was trained to eat fromthe grain hopper as in Experiments 1-3. On the dayafter completion of magazine training, acquisition be-gan. All birds were placed on a procedure in which a12-sec green key light was followed after a 12-sec un-filled interval by 3-sec access to grain. For birds in theTrace Compound group, a 12-sec, 1000-Hz, 88-dB(SPL) tone was presented only during illuminations ofthe green key light. For subjects in the remaining threegroups, the tone was on throughout ITIs and USs butnot during the CS-US gap. For birds in the No Overlapgroup, the tone terminated at the moment of key-lightonset and did not come on again until US presentation(i.e., the tone-off period lasted 24 sec). For birds in thePartial Overlap group, the tone terminated 6 sec afterkey-light onset, and thus 6 sec before key-light termi-nation, and stayed off until US presentation (i.e., thetone-off period lasted 18 sec). Birds in the CompleteOverlap group were trained like the ITI-fllled birds inExperiment 2 except that the auditory filler was the toneused in Experiment 3 and in the other groups of thisexperiment; the tone remained on during the ITI andthe entire 12 sec of key-light presentation, terminatedat the same time as the key light,-and stayed off untilUS presentation (i.e., the tone-off period lasted 12 sec).Twenty-one daily training sessions of 34 trials weregiven to each bird. All other stimulus conditions, tem-poral parameters, and so forth were the same as thosein Experiments 1 and 2 (see Tables 1 and 2).

ResultsTwo birds in the Trace Compound group

had to be dropped from the experiment afterthree conditioning sessions for failure to eatreliably. Before being excluded, neither birdhad pecked the key light or showed any signsof approaching it.

The strength of conditioned approach forbirds in the three ITI-filled groups (Com-plete Overlap, Partial Overlap, and NoOverlap) was directly related to the amountof temporal overlap between the tone fillerand the key light CS. Figure 4 displays themean daily approach-withdrawal "ratios forbirds in all groups. High levels of approachto the CS were exhibited by subjects trainedon the complete-overlap procedurethe samearrangement as was successful in our earlierexperiments. Moderate levels of CS ap-proach were shown by subjects irr the PartialOverlap group, in which the tone filler ter-minated midway through CS presentations.Little or no CS approach developed in sub-jects trained on the no-overlap procedure, in

COMPLETE OVERLAP

-*- PARTIAL OVERLAP NO OVERLAP

---- TRACE COMPOUND

DAYS

Figure 4. Mean approach-withdrawal ratios to CSacross sessions for birds in the Complete Overlap, Par-tial Overlap, No Overlap, and Trace Compound groupsof Experiment 4.

which the tone terminated at the momentof CS onset.

A mixed two-factor analysis of variancewas performed on the approach-withdrawalratios of birds in the three overlap groups.The main effect of groups was highly sig-nificant, F(2, 21) = 13.82, p < .001, as wasthe main effect of days and the Days XGroups interaction, F(20, 420) = 5.01 andF(40, 420) = 3.94, respectively, p < .001 inboth cases. Subsequent Newman-Keuls testscarried out on the data collapsed across all21 sessions indicated that birds in the Com-plete Overlap group exhibited significantlystronger CS approach than did birds in thePartial Overlap group (p < .05) and in theNo Overlap group (p < .01). Birds in thePartial Overlap group showed significantlystronger CS approach than did birds in theNo Overlap group (p < .05).

The mean numbers of key pecks per ses-sion over the 21 training days were 11.9,35.0, and 16.8 for the Complete Overlap,Partial Overlap, and No Overlap groups,respectively. Variability was high, and noneof the differences between groups were sta-tistically significant. Once again, the ap-proach-withdrawal ratio proved a muchmore sensitive measure of between-groupeffects than did key-pecking frequency.

Occasional visual observation of the sub-jects over the last 10 days of the experimentrevealed that all pigeons in each of the over-lap groups exhibited some kind of regular


but idiosyncratic behavior during the CS-US gap. Among these behaviors were off-key pecking, dark-key pecking, goal track-ing, and head bobbing. However, there wasso much heterogeneity in each group withrespect to these responses that we could notreliably distinguish the groups on this basis.

Birds in the Trace Compound groupshowed virtually no approach or pecking toCS, although one subject began to approachand peck very late in training. The poor per-formance in this group resembled perfor-mance on conventional trace procedures (seeExperiments 1 and 3).

DiscussionStrong approach to a CS separated from

US by an unfilled interval of 12 sec was ob-tained only when an ITI filler completelyoverlapped presentations of the CS; acrossgroups, the strength of CS approach wasdirectly related to the degree of overlap be-tween tone filler and key light CS.2 Meredifferentiation of stimulus conditions pre-vailing during an unfilled CS-US gap fromthose in force during the ITI may be a nec-essary condition for successful "trace" con-ditioning, but it is apparently not a sufficientone. Therefore, the discrimination hypoth-esis offered by Mowrer and Lamoreaux(1951) cannot encompass the specific resultsof the present experiment.

The failure of the trace-compound pro-cedure to generate conditioned approach tothe CS suggests that the high approach lev-els shown by birds in the complete-overlapcondition in this and prior experiments can-not be explained by some sort of potentiationof the persisting neural aftereffects of theCS by the concurrent presence or cotermi-nation of the CS and the ITI filler. Conse-quently, Kamin's comment (1965, p. 123)that "presumably, trace conditioning de-pends upon the contiguity of a 'neural trace'of the CS with the US" and that "thus aweak CS might not produce a trace of suf-ficient magnitude to 'bridge the gap' betweenCS and US" might well apply in certain sit-uations, but it is unlikely to provide an ex-planation for the great superiority of thecomplete-overlap groups in our experiments.

The differences between the overlap groups

also demonstrate that the reappearance ofthe ITI filler at the time of US onset wasnot important in bringing about the effectsobtained in Experiments 2 and 3. This pro-cedural feature likewise was in force for allthree overlap groups in this experiment, butlarge differences still emerged between them.

The present results seem basically consis-tent with the other two general accounts(excitatory-inhibitory context in effect dur-ing CS presentation; second-order condition-ing) proposed in the introduction to this ex-periment. Fiist of all, presentation of the CSpresumably occurred in what soon becamea weakly excitatory, nonexcitatory, or eveninhibitory local context (filler-on) in theComplete Overlap group, whereas it waspresented in what soon became a presumablyhighly excitatory context (filler-off, whichimmediately preceded US) in the No Over-lap group; the Partial Overlap group wasexposed to a condition intermediate betweenthe two others. If we can extend the type ofanalyses offered by the Rescorla-Wagnermodel (1972; cf. Leyland & Mackintosh,1978) to elements whose onset and/or offsetare not closely contiguous with the US, thestimulation provided by the termination ofthe ITI filler ought to overshadow or blockcontrol by the key-light stimulus in the NoOverlap group, whereas the reverse wouldbe true in the Complete Overlap group.

Looked at in a second way, which em-phasizes the relation between CS and gapconditions rather than between CS and US,our findings are consistent with approachesbased on the relative predictiveness of theCS: In the Complete Overlap group theunique predictor of imminent filler absenceis key-light onset, whereas in the No Overlapgroup key-light onset occurs simultaneouslywith the start of the filler-absent period anddoes not "predict" it. Therefore, a form ofsecond-order conditioning could account forthe findings of Experiment 4. Tone-off pe-

2 This conclusion is presumably not specific to the use

of a tone as the ITI filler, because we have found inother work that separate groups of birds trained on theno-overlap procedure with a clicker, red house light, orcomplete blackout as the ITI filler (i.e., the gap fillersof Experiment 1) also fail to acquire approach or key-pecking behavior to the CS. To save space here, we donot report details of these confirmatory findings.


riods may have functioned as first-order ex-citors, and key illuminations as second-orderexciters. Pavlov (1927, e.g., p. 33 and pp.69-72) stated that sequential, nonoverlap-ping presentation of second-order and first-order CSs should produce an excitatory sec-ond-order CS whereas simultaneous presen-tation of the two generally yields conditionedinhibition. If true, such an assertion wouldbe compatible with the differences we ob-tained between the three overlap groups (ourComplete Overlap group involves lack ofcoincidence between CS and tone-off pe-riods). However, recent data do not com-pletely support Pavlov's statement. Maisiakand Frey (1977) found that second-orderconditioning was most likely to occur whensecond- and first-order CSs partially or com-pletely overlapped in rabbit eye-blink andgerbil conditioned emotional response prep-arations. Furthermore, Rescorla (1980, p.37) concluded that simultaneous sensorypreconditioning is frequently even more ef-fective than successive sensory precondition-ing when appropriate assays are employedfor comparing the two modes of presenta-tion.

In any event, it is difficult to evaluafe themerit of a second-order conditioning expla-nation, because most prior research has notinvolved delivery of USs after CS2-CS,trials. When it has (Cheatle & Rudy, 1978;Holland, 1980), the strength of second-orderconditioning is appreciably weakened.

General DiscussionAlthough little or no autoshaping was ob-

tained on conventional trace conditioningprocedures in which CS offset preceded fooddelivery by 12 sec, strong approach to thesame CS developed if some auditory or vi-sual stimulus filled the CS-US gap or waspresent during all segments of experimentalsessions except for the gap. Furthermore,high levels of approach to CS emerged evenwith unfilled gaps of 30 or 60 sec betweenCS and US, so long as some explicit externalstimulus was present at all other times. Wedemonstrated that this facilitation of con-ditioning to a CS temporally distant fromUS was not primarily produced by physicalsimilarity between CS and the added stim-

ulus, or by general spread to CS of the par-ticular CRs that developed just before USdelivery, or by potentiation of the neuralaftereffects of the CS owing to the concur-rent presence and/or cotermination of theextra visual or auditory stimulus. However,the amount of conditioning to CS did dependon the degree of temporal overlap betweenit and the ITI filler; when the CS overlappedthe final portion of that filler, strong con-ditioning to CS was obtained, but when theCS came on after the filler terminated, noconditioning to CS was evident.

It is clear that the absence of an explicitexternal stimulus in CS-US gaps lasting aslong as 1 min does not preclude acquisitionof strong behavior to the CS. Close temporalcontiguity between CS and US is apparentlyunnecessary for Pavlovian conditioning, pro-vided that various segments of conditioningcycles are filled, overlap each other, or aresequentially related in certain ways.

The present series of experiments wasmainly instigated by Mowrer and Lamo-reaux's (1951) hypothesis to account for fail-ures of trace conditioning with aversiye USs.Their interpretation proposed that on stan-dard trace procedures the subject has diffi-culty discriminating between the two kindsof unfilled intervals that it experiences, thetime periods between CS and US and thetime periods between US and the next CS(i.e., ITIs). The presumption is that a CSwill not come to serve a signaling functionunless it provides differential informationwith respect to events occurring in these twotime periods. Otherwise, conditioning to gen-eral situational or background cues will pre-vail and overshadow control by the CS. TheMowrer-Lamoreaux interpretation predictsthat clear differentiation of,the two unfilledsegments of trace conditioning sessionsbyinsertion of a distinctive external stimuluseither in the CS-US gap or in the ITIshould facilitate conditioning to the nominal(trace) CS. Taken collectively, our experi-ments indicate that such differentiation ofthe CS-US gap from the ITI is probablyessential for successful autoshaping to a CSseparated from US by a relatively long timeperiod. However, despite its utility in high-lighting some relatively neglected factors inPavlovian conditioning, the Mowrer-La-


moreaux hypothesis is incomplete becauseit cannot handle the differences we obtainedbetween the various overlap groups in Ex-periment 4, all of which received stimuli dif-ferentiating the ITI from the gap.

Besides its shortcomings in dealing withthe effects of ITI-filler-CS overlap, the dif-ferentiation account as suggested by Mowrerand Lamoreaux is rather simplistic; it doesnot propose any definite mechanisms or pro-cesses by which a trace CS should gainstrong control when the segments surround-ing it are marked off from each other in adistinctive way. We indicated earlier thatvarious contemporary theories of Pavlovianconditioning stress the context in which CSis presented, as well as the associative re-lation between CS and either the US itselfor the stimulus conditions in force during thegap, i.e., the processes of either first-orderor second-order conditioning. Unfortu-nately, these and related approaches (e.g.,Gibbon & Balsam, 1981; Mackintosh, 1975;Pearce & Hall, 1980; Rescorla & Wagner,1972; see also Dickinson, 1980, for a generalreview and summary) have little to say about(a) sequentially presented stimuli and (b)CSs that are not closely contiguous to theUSs with which they are positively corre-lated. However, their interpretations of therole of contextual stimulation, the phenom-ena of overshadowing and blocking, and as-sociative competition between elements ofstimulus compounds may provide a generalbackground for our attempt to integrate theresults of the present set of experiments.Rather than speculating on how each of sev-eral different theoretical approaches mightpossibly handle our findings, we instead offera relatively brief and nonquantitative inter-pretation that reflects the influences of suchapproaches but is not clearly an outgrowthof any single one of them.

We find it useful to subdivide the "con-text" in which a CS appears into two cate-gories, general and local; local contexts areembedded in general contexts.3 In contem-porary learning theory, on the other hand,contextual cues typically refer only to stable,unvarying, continuously present situationalstimuli, which are nevertheless consideredto have functional properties similar to thephasic, comparatively brief and infrequent

events that are usually employed as CSs instudies of conditioning (cf. Nadel & Willner,1980). However, in our and various otherprocedural arrangements, not only are suchgeneral situational cues available for poten-tial conditioning, but sessions are also seg-mented into distinct periods that surroundor overlap CS presentations, i.e., ITIs andCS-US gaps. Thus a "trial" in many, if notall, conditioning experiments may be mostaccurately described as a cycle or successionof events rather than merely as the presen-tation of a CS in relation to some US againsta general background. In any case, our find-ings highlight the probable importance oftwo factors: (a) the discriminability fromeach other of the local contexts surroundingand/or overlapping CSs, a factor that is pre-sumably linked closely to (i) the relative sa-lience of the specific external stimuli pre-vailing in the ITI versus gap periods and(ii) the relative temporal durations of ITIsand gaps (not systematically examined in theexperiments reported here, but cf. Lucas etal., 1981); in the latter case a CS may serveto demarcate two unfilled but unequal timeintervals, permitting a "temporal discrimi-nation" between them, and (b) the excita-tory or inhibitory state of the local contextin which a CS temporally remote from USoccurs.

A three-stage process seems to be involvedin the successful acquisition observed in thepresent experiments to a CS separated fromUS by 12-60 sec. Like many other experi-menters, we assume that excitatory condi-tioning takes place quickly to the generalcontextparticularly in the .case of auto-shaping, which typically entails several pre-liminary sessions of magazine training with-out any presentations of CSs or filler stimuli.Conditioning to the general context ob-viously has a head start under such an ar-rangement. If standard trace conditioningis then begun with unfilled ITIs and gaps,the rapidly established conditioning to thegeneral context should block or overshadowthe development of excitatory control by CS,which in any event stands in a relatively un-

3 In some cases, it may be worthwhile to view general

apparatus cues as local contexts, in relation to the entiredaily life of an experimental organism,- mainly spent inits home cage.


favorable contiguous relation with US. Pre-sumably, lack of CS-US contiguity operatesas weak CS salience does in the. Rescorla-Wagner model. In fact, using a techniquefor measuring contextual associations de-vised by Odling-Smee (1975), Marlin (Note1) showed that fear of the general contextis directly related to the length of the CS-US gap in aversive trace conditioningwhereas fear of the CS is inversely relatedto the length of the CS-US gap.

However, after this first stage involvingexcitatory conditipning to the general back-ground' has progressed, the presence of def-inite cues distinguishing the ITI and gap inour experiments should eventually lead tothe acquisition of inhibitory and excitatorystrength by the respective stimulus condi-tions in force during these local contexts.The degree to which such excitatory and in-hibitory strength develops will, of course,depend on the relative salience and durationof the two local contexts as well as the prob-ability of US delivery in each. Our "filler"experiments, which involved segmentationof sessions into cycles of two periods com-prising (a) an unreinforced initial interval(ITI) that was on the average five to sixtimes longer than the terminal interval (gap)and (b) a terminal interval that alwaysended with US delivery, would be expectedto produce relatively strong excitation dur-ing the terminal interval and probably somedegree of inhibition during the initial inter-val. Furthermore, excitatory conditioning tothe general context should gradually becomeweaker owing to the presence of a betterpredictor of US delivery times, i.e., stimulusconditipns in the gap.

After this second stage has progressed farenough so that the local contexts have ac-quired some above-threshold degree of ex-citatory and/or inhibitory power, it appearsthat even a brief CS temporally distant frombut positively correlated with US cart ac-quire excitatory strength if it is presented ina relatively nonexcitatory or inhibitory localcontext. Such was apparently the outcomein all our serial conditioning arrangements(Experiment 1) and all the ITI-filler ar-rangements with complete overlap of CS andITI filler (Experiments 2-4). On the otherhand, if this kind of CS is presented during

an excitatory local contextas was presum-ably the case for our No Overlap group inExperiment 4it will not normally gainmuch, if any, excitatory power of its own(depending, of course, on the CS's salience,duration, and distance from US).

According to this general interpretation,standard trace conditioning arrangements aswell as our no-overlap procedure fail to pro-duce strong conditioned responses to CS notonly because the CS stands in an unfavor-able temporal relation to US but, perhapsmore important, because the CS is also pre-sented against an excitatory background.Thus, the growth of positive associativestrength to CS in the third stage of our pre-sumed acquisition sequence depends on pro-cesses analogous to those underlying in-stances of "superconditioning" reported byRescorla (1971; see also Blanchard & Honig,1976; Dickinson & Dearing, 1979). Rescorlafound that a standard positive CS becomesmuch more powerfully conditioned if pre-sented in combination with an already in-hibitory rather than a neutral or excitatorystimulus. If our speculations are correct, thistype of process can override an otherwiseunfavorable degree of contiguity betweenCS and US and can operate in situations inwhich the CS is presented prior to and con-currently with the establishment of inhibi-tion or the removal of excitation to a localcontext.

Our use of filler stimuli in the gap or ITIproduced arrangements that fulfill many of

Jhe features of conventional second-orderconditioning procedures (see also Egger &Miller, 1962, 1963). However, the presentexperiments differed from standard studiesof second-order conditioning in our use ofstimulus absence as the presumptive first-order excitor (Experiments 2-4) and in ourpairing of CS (the presumed second-orderexcitor) with the first-order excitor and theUS on every trial from the outset of training.Usually, the first-order excitor is establishedbeforehand by pairings with US, and thenthe second-order stimulus is introduced,paired only with the first-order stimulus andnever with the US. It may be worth pointingout that, even on this usual procedure, pre-differentiation of two local contexts also oc-curs; stimulus conditions during the (empty)


Ills have presumably become inhibitorybefore the second-order stimulus is ever in-troduced during those conditions.

In contrast to our stress on the excitatoryversus inhibitory state of the local contextin which a CS temporally remote from USis presented, one could argue that the sim-plest account of our results would focus onthe contiguity and relative predictiveness ofthe CS with respect to a first-order exciter,the distinctive stimulus condition appearingduring the gapregardless of whether pres-ence or absence of some stimulus is involvedthere (in our experiments no clear-cut dif-ferences between "presence" and "absence"were detected, but it is possible that furtheranalysis will reveal important differencesalong these lines). Such a second-order con-ditioning account (see also Pearce et al.,1981) would grant that the gap conditionswould first have to be differentiated fromconditions during the ITI but that once thegap stimulus becomes excitatory, its powerto condition an immediately preceding CSdepends simply on that CS's relative abilityto predict it. In the serial arrangements ofExperiment 1 and the complete-overlap con-ditions of Experiments 2-4, the CS wouldserve as a unique predictor of the first-orderexcitor, whereas in the no-overlap arrange-ment of Experiment 4, CS onset coincidedwith the start of the presumptive first-orderexcitor and would not predict it, thus ac-counting for the lack of conditioning to CSon that arrangement. This type of interpre-tation stresses the relations between the CSand the gap conditions, rather than the roleof the US in creating inhibitory and exci-tatory local contexts for modulating the ef-fectiveness of CS. A definite choice betweenthese theoretical alternatives may not benecessary, since both could be operating tobring about the effects described here. Fur-thermore, the two types of explanation arelikely to be eventually integrated into a sin-gle unified approach, as we come to learnmore about the role of local contexts in se-quential conditioning.

In a recent study Rescorla (1982) usedwithin-subject designs to cbmpare serial andtrace autoshaping procedures. His resultsnicely complement the findings we obtainedin Experiment 1: The insertion of auditoryor visual stimuli in the CS-US gap facili-

tated pecking to a key light CS. Differentialconditioning to general background cuescannot easily account for such results, andRescorla also was dubious about second-or-der conditioning interpretations, on the basisof the outcomes he obtained from applica-tion of standard assays for higher-order con-ditioning (Rescorla, 1980). Rescorla specu-lated that the inserted stimulus serves a kindof "catalytic" function, promoting the as-sociation between the CS and the US or thatit acts in gestaltlike fashion to perceptuallylink the CS and US. Such views need to bespelled out in more detail before their valuecan be assessed; for example, it is hard tosee how they could specifically handle thedifferences we obtained between the variousoverlap conditions in Experiment 4.

Regardless of the theoretical approachone favors to handle our and related results,the present set of experiments indicates thatlack of contiguity between a CS and an up-coming US does not preclude conditioningto that CS. Contiguity does not seem to func-tion in the absolute fashion that is often im-plied in descriptions of its actionfor ex-ample, in terms of some decaying neuraltrace of the CS. Effects of the CS-US in-terval are clearly dependent on the stimulusconditions prevailing in various segments ofexperimental sessions as well as on the rel-ative durations of the ITI, CS, and gap. Thisgeneral conclusion should encourage stu-dents of Pavlovian conditioning to carefullyreexamine the term "contiguity" and to bemore analytical with respect to its explan-atory status.

Reference Note1. Marlin, N. A. Contextual cues in trace conditioning.

Paper presented at the meeting of the PsychonotnicSociety, St. Louis, Missouri, November 1980.

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Received June 11, 1981

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Bridging Temporal Gaps Between Cs and Us in Autoshaping