Neuropsycholq,m, Vol. 31, No 2, pp. 199-205, 1993. Printed in Great Britain

028 ~3932193 56 OO+O.OO C 1993 Pergamon Press Lfd

EFFECT OF SPATIAL ATTENTION ON MENTAL ROTATION

MICHAEL C. CORBALLIS and RACHEL MANALO

Department of Psychology, University of Auckland, Private Bag, Auckland, Hew Zealand

(Received 30 October 1990; accepted 24 September 1992)

Abstract-Normal right-handed subjects performed a task requiring mental rotation of the letter R, presented in varying angular orientations in the left or right visual fields. They were told to attend to the side indicated prior to each trial by a centrally located arrow, while maintaining visual fixation on the arrow. On 75% of trials (compatible trials) the arrow pointed to the side of the letter, while on 25% (incompatible trials) it pointed to the other side. Although overall RT was shorter on compatible than on incompatible trials, there was no evidence that spatial attention affected mental-rotation rate. However, estimated rate was higher when attention was to the left side of space, consistent with right hemispheric specialization for mental rotation. This effect was especially marked when presentation of the letter was to the right.

INTRODUCTION

IT IS WELL KNOWN that directing attention to a spatial input channel can enhance the processing of information presented on that channel [ 131. For example, in experiments on dichotic listening, subjects are much better able to detect and remember information presented to the attended ear than that presented to the unattended ear Cl]. Similarly, the detection of visual signals is faster and/or more accurate if they appear in an attended location, and slower and/or less accurate if they appear in an unattended location, compared with a neutral condition in which no directional information is given. That is, there are both benefits and costs associated with visual attention [17, IS]. In these experiments subjects typically maintain visual fixation on a central stimulus, so that the effects of spatial attention are independent of where the subjects are actually looking.

The present study is concerned with the question of whether directed spatial attention may have a priming effect that goes beyond the detection of signals to influence higher-level cognitive processing. In particular, the aim was to determine whether mental rotation of a pattern would be enhanced by attention to the side of space on which the pattern appears, relative to the condition in which attention is directed to the other side. Note that the comparison here is not that between attended and unattended stimuli, since stimuli in the unattended field should eventually capture attention, presumably at the expense ofdelay and perhaps some degradation of the percept. The question rather is whether directed attention to a spatial location can enhance higher-order processing (as well as detection) of stimuli that fall in that location.

There is some evidence that concurrent memory load may increase overall reaction time (RT) on a mental-rotation task, but does not influence the estimated rate of mental rotation [4, 1.51. This implies that the process of mental rotation itself is an automatic rather than a controlled process [20,21]. On these grounds we might also expect directed spatial attention to have little effect, at least on rate of rotation. However CORBALLIS and SIDEY [lo] reported

199

200 M. C. CORRALLIS and R. MANAL~

that working-memory load increased the rate of mental rotation of letters presented to the right visual field (RVF) relative to that of letters presented to the left visual field (LVF). A possible interpretation of this effect, suggested by the work of KINSBWRNE [16], is that concurrent load activated the left cerebral hemisphere, biasing attention to the right side of space, and that this enhanced the mental rotation of letters presented in the RVF at the expense of rotation of those presented in the LVF.

The technique we used to test this was that developed by POSNER and his coworkers (e.g. Refs [17] and [18]). Prior to presentation of the stimuli, a small centrally located arrow appeared. The subjects were told to attend to the side indicated by the arrow, while maintaining fixation on the arrow itself. On 75% of the trials, the stimulus letter was in fact on the side to which the arrow pointed, while on 25% of trials the letter appeared on the unattended side. If spatial attention can indeed influence cognitive processes such as mental rotation, then we might expect a higher rate of rotation when the stimulus to be rotated falls in the attended location than in the unattended location.

It is also possible that any effect might depend simply on whether attention is to the left or the right side. As noted above, one interpretation of the results reported by Corballis and Sidey is that concurrent load primed mental-rotation mechanisms in the left hemisphere at the expense of those in the right. However evidence from neurological patients suggests that the right hemisphere is intrinsically more specialized for mental rotation than the left [7-9, 11, 191, although evidence on visual-field differences is equivocal on this point 12, 5,6,8, 12, 141. Assuming that attention to one or other side of space activates the contralateral hemisphere, we might therefore expect mental rotation to be enhanced by attention to the left, and perhaps inhibited by attention to the right.

METHOD

The subjects were six men and six women ranging in age from 19 to 25 years. All described themselves as right-handed, and as having normal or corrected-to-normal vision.

Srimuli and apparatus

The stimuli were generated by an Apple IIe computer and presented on the fast-fade amber screen. Only one stimulus was used, an uppercase R measuring 1.6 cm high x 1 .O cm wide, and presented in eight different angular orientations, ranging from 0 to 315’ clockwise from the normal upright in 45’ steps. and in both “normal” and “backward” form at each orientation. The 16 stimuli so formed were presented four times in each visual field, making a total of 128 presentations. Prior to each presentation an arrow measuring 0.6 cm wide x 0.6 cm high appeared in the centre of the screen, and two dots also appeared, one in each visual field at a distance of 6 cm from the centrc. These marked the possible locations at which the letter would be centred. On three of the four occasions on which a particular stimulus would appear in a given visual field, the arrow pointed toward that field, while on the fourth occasion it pointed to the opposite field. The 128 stimuli and arrow conditions were presented in random order; 96 of these trials were compurihle in that the arrow pointed toward the subsequent stimulus, while 32 were inconqmtihlr in that it pointed the other way.

The subjects sat with their chins in a chinrest such that their eyes were 57 cm from the screen. Each centimetre on the screen therefore subtended 1’ of visual angle. The subjects were told to press the N key with the forefinger of the right hand for normal letters and the B key with the forefinger of the left hand for backward versions, a task that 1s generally taken to require mental rotation of the letters to the upright 133. RTs were recorded in msec from stimulus offset. Prior to each trial a message appeared on the screen instructing the subject to “Press any key when ready”. After a key press, the arrow and two dots would appear for 500 msec. followed by the stimulus for 100 msec. If the subject made an error. the stimulus was repeated at the end of the sequence of trials.

Each subject was first given 10 practice trials, drawn at random from the different conditions. They were then given the 128 experimental trials. They were told that w*hen they saw the arrow they should focus their attention on

SPATIAL ATTENTION AND MENTAL ROTATION 201

the location marked by the dot on the side of the screen to which the arrow pointed, and that the letter would in fact appear in that location on 75% of trials. However they were also told to keep their eyes fixed on the central arrow, and that failure to do so might mean that they would fail to perceive properly the occasional stimulus that appeared on the other side. They were told to respond as quickly as possible without sacrificing accuracy.

RESULTS

Reaction times

For the purposes of analysis, the factor of compatibility was treated as having four levels, since each stimulus was actually presented four times; on three of these presentations the direction the arrow pointed was compatible with the side of presentation, and on the fourth it was incompatible. In programming the experiment, the four presentations were labelled 1,2, 3 and 4, and these labels were used to differentiate the four levels, but since the presentations were randomly ordered assignment of RTs to the first three levels is essentially arbitrary. We chose to enter RTs for the three compatible presentations separately rather than average across them to avoid any complications (e.g. heterogeneity of variance) introduced by including averaged with non-averaged data in the same analysis.

RTs for correct responses were then subjected to analysis of variance, in which gender was a between-subjects factor, and compatibility, visual field, version and orientation were within-subject factors.

The effect ofcompatibility was highly significant [F (3, 30)= 13.16, P<O.OOl]. RTs for the three compatible presentations (908, 897 and 926 msec) did not differ significantly [F (2, 30)=0.35], but they were significantly shorter on average than the RT for incompatible presentations (1088 msec) [F (1, 30) = 38.79, P < O.OOl]. This shows that the directing of attention was successfully achieved.

There was a significant main effect of orientation [F (7, 70) = 15.82, P <O.OOl], and RTs were significantly shorter for normal (882 msec) than for backward (1027 msec) letters [F(l, lo)= 17.10, P<O.OOl]. Although the men (874 msec) responded more quickly on average than the women (1035 msec), the difference was not significant [F (1, 10) = 1 ,101. The effect of visual field was negligible [F (1, 10)=0.27], and the interaction between field and orientation was also insignificant [P (7, 70) = 0.891. Nevertheless the estimated rotation rate was higher for the left (412”/sec) than for the right (368”/sec) visual field, although this difference also failed to approach significance [F (1, 70) = 0.703.

The interaction between compatibility and orientation was marginally significant [F (21,210)= 1.64, P~O.051, suggesting that spatial attention may have affected rate of mental rotation. However this interaction does not approach significance according to the conservative degrees of freedom (1, 10) recommended by WINER [22] for testing repeated- measurements effects. For a more exacting test, therefore, the orientation factor was reduced to an idealized mental-rotation contrast defined by the coefficients - 2, - 1 , 0, 1,2, 1,O and - 1 representing the orientations 0,45,90, 135, 180,225 and 270’, respectively. This revealed the estimated rotation rate to be actually slightly higher with incompatible presentation (446’/sec) than with compatible presentation (364,434 and 334%‘/see), but the difference was not significant [F (1, 210) = 1.881, and the three compatible presentations (as expected) also did not differ significantly [F (2, 210)= 1.581. RTs are plotted against orientation for each of the compatible and incompatible presentations in Fig. 1.

The triple interaction between visual field, compatibility and orientation approached significance [F (21, 210)= 1.45, P<O.lO], and was again assessed with the orientation factor reduced to the idealized mental-rotation contrast. When the compatibility factor was also

202 M. C. CORBALLIS and R. MANALO

1 v compatible 1 P compatible 2 . compatible 3

Fig. 1. Mean RT for compatible and incompatible presentations at each angular orientation.

reduced to the compatible vs the incompatible presentations, the triple interaction proved significant [F (1,210)=5.18,P<O.O5J. Thisinteractioncan beinterpreted asa main effect of the visual field to which the subjects attended on the rate of rotation, with faster rotation rates when attention was to the left visual field. In fact, when RTs are averaged for each orientation over all presentations for each direction of attention (left or right), the estimated rate of mental rotation was 456”/sec when attention was to the left, and 339”/sec when attention was to the right.

However this effect was itself somewhat asymmetrical, and depended to some extent on which field the stimuli were presented in. When presentation was to the left visual field, the estimated mental-rotation rate was indeed higher when attention was also to the left (429’isec) than when attention was to the right (369”/sec), but a test of simple interaction [22] showed this to be insignificant [F (1,414)= 1.261. The difference was much more striking when presentation was to the right visual field; with attention to the left, the estimated rotation rate was 564”/sec, compared with 330’/sec when attention was to the right, and this effect was highly significant [F(l, 414)= 13.61, P<O.OOl]. RT is shown as a function of orientation for each visual field and direction of attention in Fig. 2.

Errors

Compatibility was again treated as having four levels, with the first three corresponding to the compatible presentations and the fourth to incompatible presentations. The number of errors was then subjected to analysis of variance with the same factors as for the analysis of RTs.

There was a significant main effect of orientation [F (7, 70) = 3.44, P< 0.011; this effect roughly paralleled the RT function with the number of errors peaking at 12.33% at the 180’ orientation. The effect ofcompatibility was also significant [F (3, 30) = 5.20, P< O.Ol]. There were significantly more errors with incompatible (10.70%) than with compatible presenta- tions (5.88. 3.52 and 5.19%) [F(l, 30)= 14.16, P~O.0011, while the three means for compatible presentations did not differ significantly [F (2, 30) = 0.72]-not surprisingly,

SPATIAL ATTENTION AND MENTAL ROTATION 203

LVF RVF

1400

1300

1200

1100

: 1000

d .3 900

Fs 800

600 0 45 90 135180225270315360 0 45 90 135180225270315360

Degrees clockwise from normal upright

Fig. 2. Mean RT at each angular orientation in each visual field, shown separately for each direction of attention.

since the levels were essentially arbitrarily assigned. This result shows that the effect of compatibility on RT did not arise through a speed/accuracy trade-off.

The interaction between compatibility and orientation was not significant [F (21,210)=0.69], nor was the triple interaction between visual field, compatibility and orientation [F (21, 210)= 1.091. In fact the only other significant effect was a triple interaction between gender, visual field and orientation [F (7, 70)=3.13, P<O.Ol]. Since this interaction was not significant according to the reduced degrees of freedom (1, 10) recommended by WINER [22] for testing repeated-measurements effects, and did not bear on any of the experimental questions or on any of the significant RT effects, it was not investigated further.

DISCUSSION

This experiment shows that spatial attention, directed to one or other visual field, does not increase the rate of mental rotation of stimuli presented in that field. If anything, the results are contrary to this expectation; that is, rotation rate was somewhat slower overall when the stimulus was presented on the attended side than on the unattended side, although the difference was not statistically significant. Moreover, estimated rotation rate was slowest of all when attention was directed to the right side of space, and the stimulus appeared on that side. This suggests that the effect of load in increasing mental-rotation rate for stimuli in the right visual field in CORBALLIS and SIDEY’S [lo] study cannot be attributed to a biasing of attention to the right side of space.

A second hypothesis anticipated in the Introduction did receive support from the data: Estimated mental-rotation rate was higher overall when attention was directed to the lej? visual field than when it was directed to the right visual field. This might be interpreted to mean that mental-rotation mechanisms are indeed localized primarily in the right cerebral hemisphere, as suggested by evidence from neurological cases [7-9, 11, 193. COHEN [2], in a study of visual-field differences in mental rotation, also argued for a right hemispheric specialization, on the grounds ofan overall LVF advantage in RT but no interaction between visual field and angular orientation. She concluded that stimuli presented to the RVF, and so

204 M. C. CORHALLIS and R. MAYALO

to the left hemisphere, were transmitted via the commissures to the right hemisphere for mental rotation, resulting in an overall increase in RT. Other studies of visual-field differences have yielded mixed results, sometimes favouring the RVF [S, 8, 121. and sometimes favouring neither field 16, 141.

However some of the variation in visual-field differences may depend on factors influencing the perception or preprocessing of the stimuli to be rotated. For example, CORBALLIS and MCLAREN [5] found a shift toward a relative LVF advantage when the stimuli to be rotated were changed from verbal to nonverbal ones. Similarly, COHEN [2] found that advance information as to the identity, or as to both the identity and the orientation of the stimulus, progressively introduced a relative RVF advantage, and she suggested that the preprocessing induced by advance information depended on left hemispheric specialization.

Our data suggest likewise that opposite hemispheric specializations may prevail in a single task, and indeed that the optimal condition for rotation may occur when the left hemisphere is involved in perception of the stimulus while the right hemisphere is primed for rotation itself. This might explain why rate of rotation was fastest when attention was to the left but presentation was to the right. However this optimality applies only IO the rotation component; overall RT was slowed under this condition by virtue of the fact that presentation was on the unattended side.

A final point to make is that the efl’ect of spatial attention on mental rotation is quite different from that on detection. For one thing, the effect on detection is bidirectional, whereas rate of mental rotation is increased only by attention to the left. More importantly, directed attention enhances the detectability of stimuli that fall within the focus of attention, whereas the enhancement of mental rotation was greatest when the stimuli fell in the visual field opposite the attended one-that is, when attention was to the left but presentation was to the right. Directed spatial attention may therefore provide a way of studying hemispheric specialization for higher-order processing that is independent of its effects on stimulus

detection.

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