The Journal of Experimental Biology 200, 1309Ð1316 (1997) 1309 Printed in Great Britain © The Company of Biologists Limited 1997 JEB0764

MENTAL ROTATION IN A CALIFORNIA SEA ( CALIFORNIANUS)

B. MAUCK1 AND G. DEHNHARDT2,* 1Universität Köln, Institut für Tierphysiologie, Weyertal 119, D-50931 Köln, Germany and 2Universität Bonn, Zoologisches Institut, Poppelsdorfer Schlo§, D-53115 Bonn, Germany

Accepted 25 February 1997

Summary Mental rotation is a widely accepted concept that previously shown sample. Both stimuli were rotated by a suggests an analogue mode of visual information- multiple of 30 ¡ with respect to the sample. The ’s processing in certain visuospatial tasks. Typically, these reaction time was measured by a computer-controlled tasks demand the discrimination between the image and touch-screen device, rewarding the animal for pressing its mirror-image of rotated figures, for which human subjects snout against the stimulus matching the sample. A linear need an increasing reaction time depending on the angular regression analysis of the animal’s mean reaction time disparity between the rotated figures. In pigeons, tests of against the angular rotation of the stimulus yielded a this kind yielded a time-independent rotational invariance, significant correlation coefficient. Thus, the present data suggested as being the result of a non-analogue can be explained by the mental rotation model, predicting information-processing that has evolved in response to the an image-like representation of visual stimuli in this horizontal plane that birds perceive from above while species. The present results therefore correspond well with flying. Given that marine use the water surface those found for human subjects, but are inconsistent with as the horizontal plane for orientation while diving, the the data reported for pigeons. ability of a California to mentally rotate two- dimensional shapes was tested. Using a successive two- Key words: mental rotation, mental representation, visuospatial alternative matching-to-sample procedure, the animal had information, mirror-image discrimination, , to decide between the image and mirror-image of a Zalophus californianus.

Introduction Mental representation is one of the central concepts of imagine the motion of objects retained in our memory comparative cognition. Many researchers have tried to (Jolicoeur and Cavanagh, 1992; Corballis and McLaren, determine how objects in our environment are represented 1982). mentally and the nature and structure of these Several studies have shown that factors such as the representations. The response behaviour of subjects in dimensionality of stimuli (Shepard and Metzler, 1988) or appropriate experiments could indicate the type and function weightlessness (Matsakis et al. 1993) may affect the response of the representations used in different tasks. In a classical characteristics of subjects, but the process of mental rotation study, Shepard and Metzler (1971) found that the time it took has always been confirmed. Nevertheless, the analogue nature human subjects to decide whether two figures were identical of the mental rotation process is still the subject of some debate or mirror-images of each other was a linear function of the (the so-called analogue-propositional debate; Pylyshyn, 1973, angular disparity between these figures. The interpretation of 1979a,b, 1981). In spite of these doubts, the idea of an image- these results was that the subjects used an analogue like representation has been suggested as the best explanation transformation process which Shepard and Metzler (1971) for such experimental results (Roitblat, 1987). called mental rotation, suggesting an image-like Accepting the model of mental rotation for humans, the representation of visual information. Furthermore, it was question remains whether the same mechanism underlies the argued that the process of imagining the rotation of an object visual information-processing of other organisms in such tasks. should be equivalent to the process of perceiving the physical Hollard and Delius (1982) tested the ability of humans and rotation of an existing object (Shepard and Cooper, 1982). It pigeons to discriminate between the image and mirror-image of was also supposed that the same neural substrate that is rotated two-dimensional stimuli in a matching-to-sample task. involved in the perception of real motion also enables us to Their results indicated that humans used mental rotation,

*Author for correspondence. 1310 B. MAUCK AND G. DEHNHARDT whereas the reaction times of pigeons showed no linear increase Stimuli and test apparatus with the angle of rotation. On the one hand, Hollard and Delius Twelve two-dimensional stimuli similar to those of the study (1982) explained this rotational invariance by the pigeons’ by Hollard and Delius (1982) were used for testing reaction ability to distinguish image and mirror-image as easily as times, and ten additional stimuli were used in the course of arbitrarily different forms. While Lohmann et al. (1988) could acquisition (Fig. 1). A test stimulus consisted of nine black not confirm this hypothesis, Delius and Hollard (1995) found squares (27 mm×27 mm) attached to each other at their sides evidence that pigeons discriminate mirror-images, relative to and forming an asymmetrical shape. These shapes and their arbitrary shapes, more easily than do humans. On the other mirror-images were computer-designed, rotated by various hand, Hollard and Delius (1982) as well as Delius and Hollard multiples of 30 ¡, and printed in the centre of a white stimulus (1995) argued that the pigeons’ rotational invariance could have card (21 cm×25 cm). A diameter of 17 cm was not exceeded evolved in response to ecological demands. Birds use the when shapes were rotated, and each shape occupied 12 % of ground as the horizontal reference plane while flying. This the area of the stimulus card. The stimulus cards were shrink- could have favoured the evolution of a special visual wrapped in foil to protect them from water. In contrast to the information-processing system that allows birds to recognise study of Hollard and Delius (1982), stimuli were rotated both the landscape from the bird’s-eye view in any orientation clockwise and counter-clockwise with respect to the previously without any delay. Conversely, it is speculated that hominids shown upright sample shape; rotations of more than 180 ¡ are may have secondarily lost this ability of efficient visual therefore regarded as smaller angles of the opposite direction. recognition regardless of relative orientations. The computer-controlled experimental device was designed Species living in an aquatic environment could use the water to present stimuli in a successive two-alternative matching-to- surface as a horizontal reference plane while diving. , sample procedure. The apparatus was positioned on a dry for example, can be observed making use of the dark platform at the edge of the pool and could easily be reached silhouettes of prey contrasting with the bright water surface by the animal (Fig. 2). At a distance of 1.5 m from the platform, while hunting (Hobson, 1966). Furthermore, in visual a stationing hoop was installed immediately above the water discrimination experiments, seals and sea are often surface. The test apparatus consisted of a black described as swimming upside down and on their side, which polyvinylchloride (PVC) board with three Perspex windows does not affect their choice accuracy. On the basis of such side by side in the lower part. The bottom edge of each window observations, Schusterman and Thomas (1966) hypothesized was approximately 10 cm above the water surface. Behind each that visual perception in these marine mammals might be window was a halogen light (75 W) installed in a box different from that of terrestrial mammals. We therefore constructed of opaque PVC. Stimulus cards could be mounted wondered whether, during the course of evolution, ecological reversed in front of the windows. A stimulus was only visible demands could have triggered an information-processing when the respective stimulus card was illuminated from system similar to that suggested for pigeons, resulting in an behind. With the light in the box turned off, the window equivalent rotational invariance in mental rotation tasks. appeared to be a white sheet. Above the right-hand and left- Results that coincided with those found by Hollard and Delius hand windows, light barriers were installed, with their infrared (1982) would indicate a convergent evolution of information- beams running vertically at a distance of approximately 1 cm processing systems. in front of the stimulus cards. Breaking of the light barrier was detected by the computer and was used during a trial to switch off the stimuli and to record the animal’s reaction time and Materials and methods choice, thereby functioning as a touch-screen device. The test Subjects apparatus was connected to a control unit sited in the vicinity Owing to the scant availability of suitable test as well of the experimenter. The control unit was used to start a trial as the lengthy preparation period required for such animals, the and revealed information about the animal’s response (correct present investigation is a case study. It was conducted at the choice or error). The control unit was connected to a portable Dolphinarium Münster, Germany, using a 6-year-old male PC set up in a room next to the experimental chamber. Custom- California sea lion (Zalophus californianus Lesson). The designed software was used to control the hardware and to animal was experimentally naive but, apart from the record the animal’s reaction time and response. Reaction time experiments, it was trained for the daily shows at the could be measured with an accuracy of 0.05 s. dolphinarium. Experiments were performed during the morning hours in two sessions, before and after the first show. Because Experimental procedure the animal was fed for the last time each day in the late At the beginning of a trial, the stimulus cards were mounted afternoon, there was a natural food deprivation of reversed in front of the three windows of the apparatus. approximately 16 h before the first session started. Experiments Following a hand gesture, the animal positioned its head in the were performed in a resting pool that allowed the separation of stationing hoop, facing the apparatus (Fig. 3A). In order to the subject from the other sea lions and dolphins kept at the avoid giving the animal any cues, the experimenter hid behind dolphinarium. the apparatus as soon as the animal reached its position in the Mental rotation in a sea lion 1311 stationing hoop. A trial was started by the experimenter by pressing a button at the control unit, whereupon the computer switched on the light behind the sample in the middle of the apparatus. After the sample had been shown for 5 s, the 1 234 computer switched it off and the two comparison stimuli were presented without delay. One comparison stimulus represented the image of the previously shown sample, while the other was its mirror-image. In mental rotation trials, both comparison stimuli were rotated in the same direction by a multiple of 30 ¡. 56789 The sea lion was rewarded for responding to the image of the sample. The appearance of the two comparison stimuli was the signal for the sea lion to choose. At the same time, the computer started to measure the reaction time. After leaving the hoop, the animal was asked to approach the apparatus and 10 11 12 13 14 to press its snout against one stimulus card, thereby breaking the infrared light beam (Fig. 3B). The computer switched off all stimuli and recorded the animal’s reaction time, while the control unit showed whether the response was correct. Correct choices were rewarded by a fish (Sprattus sprattus), but there 15 16 17 18 was no punishment for incorrect choices. Sessions were composed according to pseudorandom schedules (Gellerman, 1933) concerning the presentation of comparison stimuli at both positions of the apparatus and the sequence of test stimuli. A session consisted of 24 trials. 19 20 21 22 Learning criterion was defined as the animal’s performance of at least 80 % correct choices (χ2-test, P<0.01) in at least two Fig. 1. The stimuli used in the course of acquisition and during testing. successive sessions. Nevertheless, in order to establish the sea lion’s performance during acquisition, more sessions were those used later in tests with rotated shapes. With these new sometimes conducted after the animal had reached the discrimination requirements, choice accuracy fell to chance criterion. Since the animal was experimentally naive, it had to level during the first two sessions, but then (with one become acquainted with the matching procedure as well as exception) showed a steady increase until the sea lion reached with image/mirror-image discriminations before tests with criterion in sessions 10/11 and, after a small decline, in rotated shapes could be performed. For this reason, the study sessions 13/14 (Fig. 5A). was subdivided into various phases of acquisition and testing. We then replaced shapes 4 and 5 by eight novel ones (nos 6Ð13, Figs 1, 5B), all being equally represented during a session. As before, these shapes were paired against their Results mirror-images and were shown only in their normal upright Acquisition of matching and image/mirror-image orientation. This increase in complexity of the discrimination discriminations task again led to a decline in choice accuracy (Fig. 5B). Only Training on the matching procedure was started by after the nineteenth session did the sea lion’s performance presenting the animal with two shapes (circle versus angular remain consistently above 80 % correct choices. shape, nos 1 and 2 in Fig. 1) that were judged to be highly Up to this stage of the experiment, the animal had seen all discriminable from each other. shapes and their mirror-images in their normal upright After a strong side bias during the first four sessions orientation. Before tests with rotated stimuli could be started, (resulting in 50 % correct choices, Fig. 4A), the sea lion the sea lion had to become acquainted with the discrimination showed a steady improvement in performance and exceeded of such stimuli. For this purpose, shape no. 6 was presented 80 % correct choices in the tenth session. After the twelfth not only in the 0 ¡ orientation but also rotated by 30 ¡ and 60 ¡ session, the animal’s performance remained stable at greater in both directions. The other seven stimuli (nos 7Ð13) than 85 % correct choices. The addition of a third stimulus (no. continued to be shown exclusively in their normal upright 3, triangle), which was paired against each of the initial stimuli, orientation. had only a minor effect on the animal’s performance (Fig. 4B). While performance in ‘non-rotation’ trials generally The results of the next phase of acquisition, in which the surpassed that of ‘rotation’ trials during the first five sessions, animal was required for the first time to perform image/mirror- this discrepancy in choice accuracy disappeared during the image discriminations, are shown in Fig. 5. Two asymmetrical next five sessions (Fig. 6). Finally, the sea lion performed at shapes (nos 4 and 5) and their mirror-images were used as greater than 87.5 % correct choices in both types of stimuli in their normal upright orientation, both resembling discrimination task. Subsequently, in addition to shape 6, 1312 B. MAUCK AND G. DEHNHARDT

Mirror A Control unit

Detector of infrared barrier

Reflector of Stationing infra-red beam hoop Presentation of the sample

Box with halogen lamp Computer B connection Fig. 2. Schematic drawing of the experimental apparatus. shapes 4, 5 and 14 were also rotated by 30 ¡, 60 ¡, 90 ¡ and 120 ¡. In the course of 20 sessions, the animal was presented with tasks of increasing difficulty with regard to the angle of rotation (results not shown). The final performance of 100 % correct choices in rotation as well as non-rotation trials was considered to be a good basis for the subsequent measurement of reaction times.

Measurement of reaction times Testing was subdivided into six test series of 8Ð12 sessions Fig. 3. The experimental procedure. During presentation of the each (Fig. 7). In each series, two stimuli were used which were sample, the animal was stationed in a hoop (A). After being presented unknown to the animal except in their normal upright with the comparison stimuli, the animal made its choice by pressing orientation. The animal was familiarised in a few sessions its snout against one of the shapes (B). Photograph taken by H. before testing started with the normal upright orientation of Müller-Elsner. novel stimuli, which had been presented neither at 0 ¡ orientation nor rotated (nos 12, 13, 17Ð22). Sixty-one test approximately 30 % at 90 ¡, before it decreased again at higher sessions were performed, each consisting of 26Ð28 trials. angles of rotation (Fig. 8A), performance was always Rotation of stimuli in both possible directions (clockwise and significantly above chance (χ2-test, P<0.05). counterclockwise) was well-balanced and the sequence of test Mean reaction times were calculated from correct choices in stimulus presentation was determined according to 439 trials (0 ¡), 126 trials (30 ¡), 125 trials (60 ¡), 126 trials pseudorandom schedules (Gellerman, 1933). In order to (90 ¡), 281 trials (120 ¡), 283 trials (150 ¡) and 88 trials (180 ¡). determine a mean reaction time as reference in every series, up Although data points indicate a sigmoid curve with a slight to 10 trials with stimuli in the normal orientation (0 ¡) were decline in reaction time for rotations of 150 ¡ and 180 ¡, linear interspersed in every session. regression analysis yielded a significant correlation (r=0.873, The animal’s performance was above chance (χ2-test, P<0.05) between mean reaction time and absolute angle of P<0.05) in 60 of 61 sessions; in 50 sessions, performance was rotation (Fig. 8B). This correlation was unaffected in separate 80 % or greater of correct choices (χ2-test, P<0.01, Fig. 7). In analyses of reaction times for clockwise and counterclockwise all but one test series, performance was worst in the first rotations (clockwise, r=0.76, P<0.05; counterclockwise, session, but clearly improved in the second session. No r=0.90, P<0.01). significant preference for one direction of rotation could be detected from the number of correct choices (χ2-test, P<0.05). Mean error rates and mean reaction times with standard Discussion deviations for correct responses were analysed for every test The first basic requirement for testing the model of mental series and summarised in an overall evaluation as a function rotation was the animal’s success in performing matching-to- of the absolute angle of rotation (Fig. 8A,B). In spite of the sample tasks. The sea lion mastered the initial training increasing mean error rate from less than 10 % at 30 ¡ to problems with little difficulty, and its performance remained Mental rotation in a sea lion 1313

100 A B 100 90 90 80 80 70 70 60 60 50 40 50

Correct choices (%) 30 40

Correct choices (%) ★ 20 30 10 20 0 1 5 10 16 1 6 10 Number of sessions 0 Fig. 4. The animal’s acquisition of the matching rule. (A) 15 10 Performance during training with two stimuli. (B) Performance after Number of sessions adding a third stimulus. Fig. 6. Performance after introducing a rotated stimulus. ᭜, correct ,ٗ ;choices in all trials; ᭝, correct choices in rotation trials only well above chance in sessions with novel stimulus correct choices in non-rotation trials only; ଙ, stimulus used in rotation configurations and comparable with that of the pigeons in the trials. study by Hollard and Delius (1982). Unlike Hollard and Delius, we do not conclude from this performance that the sea of a generalised matching concept and the use of stimulus- lion applied a concept-like matching rule (sameÐdifferent or specific rules are not mutually exclusive (Roitblat and von identity concept) for solving mental rotation problems. For the Fersen, 1992), so that the sea lion could have used both interpretation of such a generalised matching concept, a strategies simultaneously. Whichever discrimination rule a sufficiently large sample size of first-trial data is necessary subject applies to a mental rotation task, as long as it maintains (Thomas and Noble, 1988; Oden et al. 1988; Schusterman and a high level of performance, the nature of the rule should not Kastak, 1993), which most experiments on mental rotation do affect reaction times. not provide. Although California sea lions have been shown to The sea lion also complied with the second basic fulfil this first-trial criterion in concept formation experiments requirement for this study in reliably performing image/mirror- (Hille, 1988; Kastak and Schusterman, 1994), this ability is not image discriminations, which cannot be taken for granted in essential for solving mental rotation problems. On the contrary, animals. Pigeons have much more difficulty in discriminating the constant use of the images of the test shapes as sample and mirror-image patterns than discriminating arbitrarily different S+, while their mirror-images were always designated as S−, pairs of patterns (Lohmann et al. 1988). They find mirror- may have favoured the formation of stimulus-specific rules image patterns especially difficult to discriminate if the rather than a concept of same or different. Furthermore, the use patterns are reflected along their vertical axis (Todrin and Blough, 1983). However, Delius and Hollard (1995) found 100 A B evidence that pigeons Ð at least in comparison with humans Ð have less difficulty with mirror-images than with arbitrary 80 shapes. Regarding primates, bushbabies (Galago senegalensis) have been shown to be greatly confused by mirror-image 60 discriminations (Sanford and Ward, 1986), whereas baboons (Papio anubis) have little difficulty in solving such tasks 40 (Hopkins et al. 1993). Our sea lion also had little difficulty in performing mirror-image discriminations. Nevertheless, Correct choices (%) 20 during testing with rotated stimuli, which can be assessed as 0 an additional source of difficulty, performance was worst in 1 5 10 14 1 5 10 15 20 25 30 the first session in all but one series. According to D’Amato et Number of sessions al. (1985), low performance levels in first sessions in Fig. 5. Acquisition of mirror-image discriminations. (A) Performance matching-to-sample tasks can be explained by the so-called during training with two asymmetrical shapes. (B) Performance after novelty effect, a disruptive, neophobic response of subjects. replacing the two initial shapes with eight new ones. Transfer criterion is therefore often determined Ð as in the 1314 B. MAUCK AND G. DEHNHARDT

100

90

80

70 Correct choices (%) 60 0 Њ, 30 Њ 0 Њ, 30 Њ 0 Њ, 120 Њ 0 Њ, 120 Њ 0 Њ, 120 Њ 0 Њ, 120 Њ 60 Њ, 90 Њ 60 Њ, 90 Њ 150 Њ 150 Њ 150 Њ, 180 Њ 150 Њ, 180 Њ Fig. 7. Performance during the six test series. 50 10 30 50 60 Rotated test shapes and angles of rotation are shown 20 40 below the respective data curves. Number of sessions study by Hollard and Delius (1982) – neglecting the first the short way Ð that the subject takes to mentally rotate a session after a change of stimuli. Although our sea lion’s stimulus. Furthermore, a subject might change its direction of performance seems to be impaired by a novelty effect, choice mental rotation during a trial for two reasons: either when it accuracy differed significantly from chance level in all but one recognises that the other way round would be shorter, or when of the first sessions with novel stimulus configurations. While it recognises that mental rotation of a particular asymmetrical Schusterman and Thomas (1966) demonstrated, even before stimulus would be easier in one of two possible directions. the phenomenon of mental rotation had been described, that a Taking this into consideration, reaction times that are shifted California sea lion readily recognizes shapes irrespective of upwards are to be expected, especially for higher angles (below their spatial orientation, the present results show that this 180 ¡), while a switch in direction of mental rotation would ability remains unaltered when complex image/mirror-image seldom occur at 180 ¡, because at this angle it takes the same discriminations are required. As predicted by the model of mental rotation, the animal’s A mean reaction times increased linearly with angular disparity. 30 The overall reaction time is composed of both the time it takes 20 the animal to approach the apparatus and the time used in 10 Errors (%) decision-making. Since our sea lion had to cover a 0 comparatively long distance from its stationing hoop to the test board, its overall reaction times Ð compared with those of 11 human subjects or pigeons (Hollard and Delius, 1982) Ð were B rather long. However, since the time it took the sea lion to 10 approach the apparatus can be assumed to be relatively 9 constant, the increase in overall reaction time may be ascribed 8 to an increasing time required to make a decision. As in many 7 other studies on mental rotation (Just and Carpenter, 1976; 6 Shepard and Cooper, 1982; Folk and Duncan Luce, 1987; Kail, 5

1991; Corballis and Manalo, 1993; Corballis and Sidey, 1993), Time (s) the sea lion’s reaction times deviate from linearity at some 4 angles of rotation. Although, at first glance, a sigmoid curve 3 seems to provide a better fit to the data presented in Fig. 8B, 2 a decrease in reaction time at angles of 150 ¡ and 180 ¡ can be 1 explained by the model of mental rotation. Shepard and Cooper (1982) reported that, beyond 180 ¡, reaction times are 0 bimodally distributed, with the upper mode corresponding to 0 30 60 90 120 150 180 the linear extrapolation of the reaction-time function to angles Angle of rotation (degrees) larger than 180 ¡ and the lower mode corresponding to the linear extrapolation of that function backwards to a point at the Fig. 8. Overall evaluation of the animal’s performance and reaction same distance from 180 ¡. This suggests that reaction time is time. (A) Mean error rate as a function of the angle of rotation. not necessarily determined by the angular disparity between (B) Mean reaction times (±S.D., N=125Ð439) as a function of the angle two stimuli but rather by the particular direction Ð the long or of rotation. Mental rotation in a sea lion 1315 time to rotate the shape in either direction. Although these have produced inconsistent results (e.g. Ornstein et al. 1980; arguments could explain deviations from linearity for some Deutsch et al. 1988; Fischer and Pellegrino, 1988; Dittuno and angles, a case study such as this one cannot prove these Mann, 1990), the results from those that examined the considerations. involvement of both hemispheres in clockwise and The rotational invariance found in pigeons and the non- counterclockwise mental rotation are more consistent. Burton increasing reaction time function of baboons when they see et al. (1992) showed that counterclockwise rotation was shapes in their left optical hemifield (Vauclairs et al. 1993) are processed more efficiently by the left hemisphere/right visual assumed to be adaptations which have arisen phylogenetically field, coinciding with the results of Corballis and Sergent because of the demand to operate visually on a horizontal (1989). Cook et al. (1994) found a more efficient hemispheric reference plane (Hollard and Delius, 1982; Delius and Hollard, cooperation with the left hemisphere taking the part of active 1995). In spite of similar ecological demands on pinnipeds, manipulation of a mental image, while the reference role is which use the water surface as the horizontal reference plane, performed by the right hemisphere. During the past two we could detect no rotational invariance in our sea lion. In fact, decades, various studies have also shown a strong cerebral our data can be explained assuming a mental rotation of an lateralization in the visual system of pigeons. In most cases, a image-like representation of visual stimuli, a result that is dominance of the left hemisphere/right visual field was found normally obtained in experiments with human subjects. An for certain visual tasks (Güntürkün, 1985; Güntürkün and image-like representation of visual information in California Kesch, 1987; von Fersen and Güntürkün, 1990). However, sea lions has also been concluded from artificial language until now, there has been no study that examined the influence comprehension tests (Schusterman and Krieger, 1986), in of this visual lateralization in pigeons on the ability to perform which the animals transformed gestural signs referencing mental rotation tasks. Certainly, it would be interesting to specific object properties into image-like representations of determine whether the pigeons’ dominance of the left those objects. hemisphere favours performance in tasks that demand counter- Obviously, ecological demands such as a horizontal clockwise rotation, as might be supposed bearing in mind the reference plane are not necessarily correlated with the results of Burton et al. (1992). If this hypothesis were to be evolution of information-processing systems that facilitate a confirmed, it could explain the consistently fast reaction times rotational invariance. However, given the variety of methods found for pigeons. used in studies on mental rotation, it should be asked whether In the present study, we provide the first evidence for mental certain methodological details could trigger a rotational rotation in a non-primate species. The appearance of this invariance. Besides differences in the intensity of training, our mental rotation effect in spite of similar ecological demands study differed from that of Hollard and Delius (1982) in the for birds and pinnipeds could also be ascribed to the fact that way in which the stimuli were rotated. We demanded from our the horizontal reference plane is not as important to pinnipeds animal mental rotation in both possible directions by as has been concluded from their aquatic life-style. However, presenting it with stimuli which were rotated clockwise as well if ecological demands are responsible for the phenomenon of as counterclockwise with respect to the previously shown rotational invariance in pigeons, it should also be detected in sample shape. In contrast, the stimuli used in the study of other avian species. Appropriate experiments are currently Hollard and Delius (1982) were rotated only clockwise with being carried out. respect to the sample. Assuming that the comparison stimuli are to be rotated back to the normal upright position of the We are grateful to Martin Dambach for his support sample, the pigeons would have had to perform a throughout this study. We are especially indebted to Wolfgang counterclockwise mental rotation. Delius and Hollard (1995) Antpöhler for technical assistance. also varied the angle of the sample shapes, whereas the comparison shapes were presented in the normal upright orientation. At first glance, this suggests that subjects in this References case would have had to perform a clockwise mental rotation, BURTON, L. A., WAGNER, M., LIM, C. AND LEVY, J. (1992). Visual rotating the comparison stimuli back to the sample’s field differences for clockwise and counterclockwise mental orientation. Nevertheless, the use of a simultaneous testing rotation. Brain Cogn. 18, 192Ð207. procedure as well as the growing familiarity with stimuli in COOK, N., FRÜH, H., MEHR, A., REGARD, M. AND LANDIS, T. 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