Journal of Comparative Psychology 1997, Vol. 111, No. 1, 76-81
Copyright 1997 by the American Psychological Association, Inc. 0735-7036/97/$3.00
Mental Rotation in Pigeons (Columba livia)? J e f f H a r e m , W i l l i a m R. M a t h e s o n , a n d W e r n e r K. H o n i g Dalhousie University Previous research has shown pigeons to be insensitive to the orientation of visual test stimuli both for response latency and for discrimination ratio. Discrimination of stimulus orientation has been more difficult to learn than discrimination of small arbitrary differences between stimuli. This has suggested that the visual processing of pigeons is orientation invariant, which would obviate the need for mental rotation such as is often observed in studies with human subjects. Contrary to previous findings, the current experiment obtained linear effects of orientation on response latency and discrimination ratio, with a go/no-go procedure. Pigeons (Columbia livia) first learned to discriminate among line drawings of similar objects and then were tested with rotated versions of the drawings. The pattern of data is similar to that found in studies of human recognition of rotated objects. One speculative explanation of this finding is the mental rotation of stimuli by pigeons.
concluded that discrimination between upright stimuli and stimuli rotated by 45 ° was more difficult than the discrimination of mirror images. Mirror image discrimination was, in turn, more difficult than discrimination of arbitrarily different shapes. The relative difficulty of discriminating stimuli on the basis of orientation supported the suggestion of Hollard and Delius (1982) that pigeons are relatively insensitive to stimulus orientation. The lower sensitivity to stimulus orientation was also shown by Lohmann et al. (1988, Experiment 2). In a successive conditional discrimination procedure, pigeons were required to peck 15 times at a sample stimulus. They were then given the opportunity to make the following binary decision: If the sample was the standard shape in an upright orientation, then the rewarded response was to peck a green (or red) key; however, responses to a red (or green) key were correct if the sample was rotated away from the upright (45 ° , 90 ° , 135 ° , or 180 ° clockwise), whether it was the standard shape or whether it differed from it either as a mirror image or by a small arbitrary change in shape. Effects of stimulus orientation were obtained when the rotated stimulus was identical to the standard shape. However, discrimination performance was unaffected by stimulus orientation when the rotated stimulus differed in shape from the sample, either by being the mirror image of the upright or by differing in an arbitrary detail of shape. The lack of an effect of orientation in these latter conditions was interpreted as demonstrating that, in pigeons, recognition based on shape is invariant of stimulus orientation. Performance was affected by stimulus orientation only when the shapes were identical, again suggesting that pigeons are insensitive to stimulus orientation. It was also shown that when the orientation discrimination used only upright stimuli versus identical stimuli rotated 180 ° , the orientation discriminations were slightly easier than discriminations based on mirror images 1 and no more difficult than discrim-
Studies using matching to sample and oddity from sample tasks have shown that pigeons are capable of discriminating simple shapes from their mirror images when the orientation of the comparison stimuli differs from the sample (Delius & Hollard, 1987; Hollard & Delius, 1982). The orientation of the comparison stimuli did not affect the response latency of a one-peck forced choice. Hollard and Delius (1982) concluded that this lack of sensitivity to the orientation of the comparison stimuli arises from ecological demands. "Pigeons operate visually predominantly on the horizontal plane where the orientation of objects is largely arbitrary, being relative to the position of the observer" (p. 806). This ecological explanation for the insensitivity of pigeons to stimulus rotation has recently been restated by Delius and Hollard (1995). Lohmann, Delius, Hollard, and Friesel (1988) examined the relative difficulty of discriminating stimuli along various dimensions using a successive go/no-go procedure. Pigeons were to discriminate a pair of stimuli, which differed along one dimension only (i.e., a small arbitrary difference in shape, mirror reflection along the axis of elongation, or orientation [upright or 45° rotation]). Relative difficulty was assessed by measuring the percentage of responses to the positive stimuli over the course of 10 sessions. More difficult discriminations would be expected to result in lower terminal performance or in a delay in reaching asymptotic performance levels. Using these criteria of difficulty, Lohmann et al. (1988) Jeff Hamrn, William R. Matheson, and Wemer K. Honig, Department of Psychology, Dalhousie University, Halifax, Nova Scotia, Canada. This research was supported by Natural Sciences and Engineering Research Council of Canada Grant 0102. We thank Brad Frankland and Patricia Cole for suggestions and comments throughout this work. Correspondence concerning this article should be addressed to Jeff Harem, Department of Psychology, Dalhousie University, Halifax, Nova Scotia, Canada B3H 4J1. Electronic mall may be sent via Internet to
[email protected].
A 180° rotation is identical in its effect to two mirror image changes, one in the horizontal plane and one in the vertical plane. 76
MENTAL ROTATION IN PIGEONS inations based on an arbitrary shape change (Lohmann et al., 1988). Contrary to the conclusion of Hollard and Delius (1982), Lohmann etal. (1988) showed that the acquisition of mirror image discriminations was impaired if the orientation of the stimuli was allowed to vary between trials. If pigeons are insensitive to orientation in the recognition of shapes, then the acquisition of a mirror image discrimination should not be affected by varying the orientation of the training stimuli. One possible explanation for the difference between Hollard and Delius (1982) and Lohmann et al. (1988) is that variation along an irrelevant dimension, in this case orientation, will hinder the learning of a discrimination along a second dimension, in this case the difference between the normal stimulus and its mirror image. Although Lohmann et al. (1988) showed that pigeons can learn to discriminate orientation, as previously shown in other studies (e.g., Honig, Boneau, Burstein, & Pennypacker, 1963), it is of interest that the pigeons' ability to discriminate differences in shape has not shown any effect of stimulus orientation, except in discrimination acquisition. Increased difficulty in the acquisition of the discrimination may reflect confusion about the dimension upon which to base the discrimination. The suggestion that pigeons may discriminate shape on the basis of orientation invariant processes implies that pigeons should be able to ignore variation in stimulus orientation with little difficulty once a discrimination based on shape has been learned. Humans, on the other hand, appear to perform a mental transformation of rotated stimuli when they discriminate a given shape from its mirror image: This is known as mental rotation (Jolicoeur, 1990; Shepard & Cooper, 1982). When they are asked to name rotated line drawings of common objects (Jolicoeur, 1990), humans show an increase in naming latencies, which are linearly related to the orientation of the test stimulus, at least to 120 °. However, latencies for indicating 180 ° rotations are often less than would be predicted on the basis of extrapolation of the data between 0 ° and 120 °. Jolicoeur (1990) proposed a dual route model for human object recognition in which mental rotation explains the effects of orientation between 0 ° and 120 °, whereas the dip at 180 ° reflects a second, feature based route. A paper by Murray (in press) suggested that during naming of rotated stimuli, mentally "flipping" objects at 180 ° may be faster than mentally "rotating" them. This would explain the reduced effects of orientation between 120 ° and 180° without postulating a second "route." Such a systematic effect of object orientation on shape discriminations has not been shown in the pigeon when the stimuli are presented simultaneously (Delius & Hollard, 1987, 1995; Hollard & Delius, 1982). The current explanation for the differences between human and pigeon performance on rotated object recognition has focused on differences in ecological demands. Because the pigeons' flight will produce arbitrary alignments between themselves and their environments, it has been suggested that the pigeons' visual system has evolved to ignore variation in stimulus orientations (Delius & Hollard, 1995; Hollard & Delius, 1982). Contrary to this explanation, given the pigeons'
77
arbitrary alignment with their environment, it would be detrimental for pigeons to be insensitive to this alignment. It may be that the lack of systematic effects of stimulus orientation reflects an insensitivity to the tasks used rather than an insensitivity within the pigeon's visual processing. In simultaneous discrimination tasks, pigeons need only to respond to the stimulus that most closely matches the rewarded stimulus. As it has been shown that rotating a stimulus does not render it unrecognizable (Lohmann et al., 1988), then a rotated version of the positive stimulus will still facilitate a response to a greater degree than a rotated negative stimulus. Furthermore, a rotated negative stimulus would still inhibit responding. With one stimulus tending to attract responding and one stimulus tending to inhibit responding, simultaneous presentations may be insensitive to detecting orientation effects in pigeons. If pigeons are capable of discriminating shape on the basis of orientation invariant information, then a rotated version of the stimuli will not affect performance during a go/no-go sequential discrimination task. On the other hand, if performance shows systematic effects of stimulus orientation, then we shall conclude that pigeons do not make shape discriminations on the basis of orientation invariant information. In the present experiment, the stimuli were presented successively. Eight White Carneau pigeons were trained to discriminate six pairs of visually similar though distinct asymmetric stimuli, selected from Snodgrass and Vanderwart (1980). These stimuli are often used in studies of human object identification (Jolicoeur, 1990; McMullen, Hamm, & Jolicoeur, 1995; McMullen & Jolicoeur, 1990). Pigeons were trained to discriminate the individual stimuli presented in an upright orientation, and they were then tested for orientation invariant recognition by introducing nonrewarded rotated versions of the stimuli. If the rotated versions are effectively equivalent to the upright versions, then no systematic effect of stimulus orientation is predicted. However, a response gradient across stimulus orientations would not support an orientation invariant recognition process. Three dependent measures were used: the discrimination ratio (DR) calculated as the number of responses to S+ divided by the total number of responses to S + and S - , the number of responses to each stimulus, and response latency until first peck. Each of these measures examines a slightly different aspect of the response pattern. The DR should indicate the relative similarity between the S+ and the S - . The mean number of responses (MNR) allows us to examine the components of the DR more closely: Is no change in the DR really no change in the response pattern? Which member of the stimulus pair ( S + / S - ) is responsible for a change in the DR? The response latency (RL) measure allows us to examine whether pigeons begin to respond and then inhibit further responding on S - trials or whether the initiation of responding reflects a positive "decision." An orientation invariant recognition system would predict no significant differences across orientations for any of these measures. If rotated stimuli are less identifiable when
78
HAMM, MATHESON, AND HONIG
rotated, then discrimination ratios should approach 0.50 as a function o f stimulus orientation. Responding to the S + stimuli should decrease as a function of orientation, whereas responses to the S - should increase. The increase in responses to the S - is necessary to demonstrate that the effects shown are not due to the removal o f the reward contingency for the S + . Finally, response times to the S + should increase as a function o f stimulus orientation, implying that recognition o f a rewarded stimulus was delayed. Similarly, response times to the S - should decrease, implying that recognition o f the nonrewarded, rotated stimuli was delayed.
Method
Subjects Eight male White Cameau pigeons (Columbia livia) maintained at 80% of their free-feeding weight were used as subjects. The subjects had no experience with discriminations related to the present task. Two birds showed no evidence of learning the discriminations after 52 training sessions and were dropped from the experiment.
Apparatus The slide discriminations were conducted in an operant behavior chamber, 37 cm wide × 30 cm high × 30 cm long. The chamber contained a Plexiglas vertical response panel, 5 cm square, mounted 16 cm above the floor, and centered on the front wall. Six pairs of objects selected from Snodgrass and Vanderwart (1980) 2 were photocopied onto transparent sheets and placed in 2" × 2" slide mounts. With a Kodak carousel slide projector, the images were rear projected onto the response screen. The panel was hinged so that pecks broke the contact between two leaves of a microswitch. The front wall provided access to a food hopper through a 5 × 5 cm opening centered 8 cm above the floor. Presentation of the stimuli and the recording of data were controlled by an Apple He computer.
Procedure Subjects were first trained to eat reliably out of the food hopper when it was raised and illuminated during a 3-s cycle. All birds consistently approached and ate from the hopper after a few sessions of training, at which point they were autoshaped to peck at the response screen. During autoshaping, a slide depicting a white circle with a central black dot was rear projected on the response screen. Reinforcement was provided for a single peck. The birds were then introduced to a fixed interval (FI) schedule by which reward was given for the first peck following the completion of the FI (initially 7 s) on the condition that at least one peck have occurred during the scheduled interval. If no response was made during the FI, then the trial terminated with no opportunity for reward. When five consecutive rewards were presented, the FI was increased by 3 s until a maximum of a 15-s FI was reached. Each training session consisted of 25 presentations of the stimulus. All birds reached the maximum FI on the first training session. The birds remained on FI training until they achieved a minimum of 22 of the 25 rewards on each of three consecutive days. All birds reached this criterion in
the first 3 days. The birds were then introduced to discrimination training. The 12 training slides (6 S+ and 6 S - ) were arranged randomly within each block of trials with the constraint that no more than two trials in a row involved the same reward contingency. This procedure was repeated for six blocks, resulting in 72 trials per session. The slides were rearranged dally. With this randomization procedure the same reward contingency could be in effect during as many as four successive trials at the boundary of two blocks, which minimized any tendency for a strategic performance based on trial order. Discrimination training used the FI (15 s) schedule used previously, with rewards being presented only on S + trials. Probe trials were introduced when performance reached an overall DR of 85% with a minimum DR of 80% for each stimulus "pair" during three successive sessions. The S + and S - stimuli sets were considered to contain six pairs on the basis of the visual similarity between items of a given category, such as birds (Rosch, Mervis, Gray, Johnson, & Boyes-Braem, 1976). However, only a single stimulus was presented during each trial. During probe trials, two slides per block were replaced with rotated versions of the stimuli. One stimulus was an S +, and the other was its paired S - . The rotated stimuli were presented at one of the following orientations: 0 °, 60 °, 120 °, 180 °, 240 °, or 300 ° clockwise. During a probe session, the positive and negative items of a pair of stimuli were each shown in the same orientation, although not necessarily in the same block. Moreover, no two pairs were shown in the same orientation during a session. Probe sessions were run for 12 days, during which every pair was shown at every orientation twice. Pair orientation was counterbalanced across day of testing. For example, the tiger/zebra pair was shown at a different orientation on testing Day 1 for each of the 6 subjects.
Results
Test Data In a preliminary analysis involving test sessions (12 levels), it was found that none o f the interactions involving test session and orientation were significant; therefore, the effects found in the following analyses are not attributable to changes in performance during testing. The following analyses were performed on the probe data after they had been collapsed over the 12 days of testing. If no responses were made during a session to either the S + or the S - for a given orientation, the chance expectation of the DR (0.50) was entered for that orientation for that day. This substitution was made for four data points in total. However, a zero value was entered for the dally calculation for the mean number o f responses. As such, D R values calculated from the presented mean number o f responses will not directly be exactly equal to the D R values presented. However, none o f the findings change if D R is calculated on the basis o f data from the collapsed MNR. In all o f the following analyses, it should be noted that for nonprobe trials, the stimulus was always upright, and the orientation coding in the figures reflects the orientation o f 2 Stimuli pairs were zebra/tiger; house/barn; chair/rocking chair; ant/beetle; train/bus; and rooster/duck. The stimuli were mounted such that both members of the pair were facing in the same left-right direction. For the house/barn pair, the chimney and silo were placed on the same side.
MENTAL ROTATION IN PIGEONS the corresponding probe slide on the day of testing to provide an appropriate comparison. For example, if the zebra was shown as a probe item at 60 ° then the upright nonprobe presentations of the zebra were coded at 60 °. Because none of the measures showed a significant difference between + 6 0 ° and - 6 0 ° or between + 1 2 0 ° and - 120 °, the trend analyses are performed using contrasts that collapse around 180 ° .
79
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The discrimination ratio was analyzed in a two-way analysis of variance (ANOVA) with probe orientation (6) and trial type (2; nonprobe/probe) as factors. Planned contrasts were used to examine the symmetry about 180 ° and the linear, quadratic, and cubic trends. When human subjects identify rotated objects, naming latency curves often show a linear increase for rotations up to 120 ° , whereas the value for 180 ° rotations is less than would be predicted (see Jolicoeur, 1990, for review). For this reason, the trend analysis was repeated to exclude the 180 ° value to examine whether the birds show a similar deviation between 120 ° and 180 °. The DR data are shown in Figure 1A. There was a significant difference in the DR between the probe (77.26) and nonprobe trials (91.89; F[1, 5] = 131.23, p < .01). Furthermore, there was a significant interaction between orientation and probe and nonprobe trials (F[5, 25] = 7.52, p < .01). Planned contrast analyses revealed that this interaction was due to an interaction in the linear (F[1, 25] = 27.61, p < .01) and quadratic (F[1, 25] = 7.89, p < .01) components of the orientation function between 0 ° and 180 ° . When the trend analysis was performed without the inclusion of the 180 ° rotation, only the interaction in the linear component remained significant (F[1, 25] = 31.60, p < .01).
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Figure 1. Effects of probe orientation for (a) discrimination Mean Number of Responses (MNR) The M N R was analyzed in a three-way ANOVA, with Orientation (6), S + / S - (2), and Nonprobe/Probe (2) as factors. The same set of contrasts was used as in the DR analysis. The data may be seen in Figure lB. The three-way interaction among orientation, S + / S - , and probes/nonprobes was significant (F[5, 25] = 17.83, p < .01). Planned contrast analysis revealed that this interaction was due to a three-way interaction in the linear (F[1, 25] = 49.39, p < .01) and quadratic components (F[1, 25] = 38.53, p < .01) when the 180 ° rotation was included. Dropping the 180 ° orientation revealed only a significant linear (F[1, 25] = 85.55, p < .01) component between 0 ° and 120 ° .
Response Latency (RL) The RL of the first peck was analyzed in a three-way A N O V A , with Orientation (6), S + / S - (2), and Nonprobe/ Probe (2) as factors. The same set of contrasts was used as
ratio, (b) mean number of responses, and (c) latency until first response. To avoid redundancy between (a) and (b), discrimination ratios were calculated for each of the 12 test sessions, with the chance expectation (0.5) entered on days on which no response was made to either the rewarded stimulus (S+) or the unrewarded stimulus (S-). Results based on discrimination ratios calculated from the data presented in (b) are identical in their interpretation.
in the analysis of the DR and the MNR. The response latency data may be seen in Figure 1C. The three-way interaction among orientation, S + / S - , and probes/nonprobes was significant (F[5, 25] = 3.24, p < .05). Planned contrast analysis revealed that this omnibus interaction was due to a three-way interaction in the linear (F[1, 25] = 6.87, p < .05) and quadratic components (F[1, 25] = 7.59, p < .05) when the 180 ° rotation was included. Dropping the 180 ° orientation revealed only a significant linear (F[1, 25] = 13.87, p < .01) component between 0 ° and 120 ° .
80
HAMM, MATHESON, AND HONIG Discussion
The current experiments examined the hypothesis that pigeons are insensitive to the orientation of stimuli when performing a discrimination on the basis of shape. During testing, the pigeons maintained the nonprobe discriminations at the levels obtained during training, whereas probe discriminations showed a systematic effect of stimulus orientation. The DR of the rotated probe slides decreased as a function of the degree of rotation. In all three measures, there were systematic effects of orientation on performance for stimulus rotations to 120 ° . These measures suggest that recognition of the stimulus as a positive or negative exemplar was increasingly difficult as a function of stimulus orientation. These findings are not consistent with the hypothesis that pigeons are insensitive to stimulus orientation. The day of testing did not affect the DR, which suggests that the orientation effects were not acquired over the course of testing. Furthermore, as the MNR to the negative stimuli increased linearly as a function of orientation, the decrease in responding to the rotated S + slides cannot be attributed to the absence of the reward during testing. If the effects of orientation were due to the absence of the reward, one would expect no change in the performance on the trials with negative stimuli, as these were never rewarded in either training or testing. Also, there would have been an interaction between the effects of orientation and of the day of testing. The lack of this interaction shows that the orientation effect was not learned during the course of testing. The separate analyses of the MNR and RL for the positive and negative stimuli reveal that each of these stimulus classes showed effects of orientation. The MNR decreased for the positive stimuli and increased for the negative stimuli, whereas responding began later for the positive stimuli and sooner for the negative stimuli. This suggests that the rotated stimuli were more poorly recognized as their orientation increasingly deviated from the upright. It is especially interesting that the measures involved only a linear component for orientations between 0 ° and 120 °, whereas the inclusion of the 180 ° point introduced a quadratic component. This finding parallels the human naming literature where naming latencies show only a linear trend between 0 ° and 120°; however, between 0 ° and 180 °, both linear and quadratic trends have been observed (Jolicoeur, 1985). The results of this study show that systematic effects of stimulus orientation can be obtained in pigeons, suggesting that pigeons do not base shape recognition solely on orientation invariant features. On the other hand, pigeons are capable of maintaining a discrimination when the stimuli are presented in unfamiliar orientations; the DR in our results never reached chance. Given that the rotated positive stimuli and negative stimuli continue to be discriminable in forced choice paradigms (Delius & HoUard, 1987; Hollard & Delius, 1982), the positive stimuli would tend to elicit responding, whereas the negative stimuli would tend to inhibit responding. Such tendencies would allow the pigeon to choose the "betted' of two stimuli, which may mask the effect of orientation.
Given this methodological explanation of the lack of orientation effects during simultaneous presentations, it may be premature to suggest fundamental differences in visual processing developing between humans and pigeons. The results of the current experiment do not suggest that orientation invariant shape-based recognition is used by pigeons. Interestingly, the initial analysis by Lohmann et al. (1988) of the data from their Experiment 2 indicated separate main effects of orientation and shape of the comparison stimuli, with no interaction between the two. In other words, orientation affected the discrimination, even with arbitrarily different comparison stimuli, which is consistent with the present results. However, Lohmann et al. conducted separate analyses that showed only the effects of orientation for physically identical shapes and based their interpretations on this second set of analyses. To restate the current findings, discrimination ratio decreased as a function of stimulus orientation. The decrease in the discrimination ratio cannot be explained by the removal of the reward contingency for the positive items. If the removal of the reward contingency caused the decrease in the discrimination ratio, then the following results would be predicted. First, this effect would have to have been learned over the course of testing. However, no interaction between day of testing and any measure involving orientation was found to be significant, indicating that the pattern was not a result of learning to inhibit responding to rotated stimuli. Second, the negative stimuli showed an increase in responding as a function of stimulus orientation. If the decrease in discrimination ratio was due to the removal of the reward contingency to the positive stimuli, then no effect should be shown for the negative stimuli. These stimuli were never associated with reward; therefore, if the rotated version was as identifiable as the upright version, no change in response rates would be predicted. The pattern of results for all of the independent measures are consistent with predictions derived from human rotated object recognition studies (Jolicoeur, 1990). In conclusion, previous studies may have shown that pigeons do not readily learn discriminations on the basis of orientation and that it is not a very salient dimension for them; instead, they recognize visual stimuli on the basis of orientation invariant information. In the current experiment, pigeons were able to recognize stimuli in novel orientations, as even the worst performance at 120 ° of rotation is above chance (DR = 0.68). However, the continued ability to recognize a rotated stimulus does not imply that orientation invariant processes are being used during recognition. In studies with humans, decision time increases and accuracy decreases as a function of stimulus orientation; these results are interpreted as evidence for mental rotation. Pigeons may well be capable of a similar process. Neiworth and Rilling (1987) presented pigeons with a clock hand that rotated at a constant speed. The clock hand would disappear during the rotation and then reappear after various delays. Pigeons were able to discriminate between trials in which the hand reappeared in the location predicted on the basis of the delay and speed of rotation and trials in which the hand reappeared in a nonpredicted location. Such
81
MENTAL ROTATION IN PIGEONS findings are consistent with the notion that pigeons use both imagery and mental rotation. However, such claims are speculative, and additional research is required. Humans continue to show effects of stimulus orientation in comparing stimuli during simultaneous presentations, whereas the performance of pigeons is not adversely affected (i.e., Delius & Hollard, 1995; Hollard & Delius, 1982). This suggests differences in processing strategies that need to be addressed. Finally, it appears that orientation effects on object discriminations in pigeons may best be examined through the employment of a go/no-go single presentation paradigm. With the examination of several dependent measures, the present method permits the development of a more complete picture of the effects of orientation on stimulus control. References Delius, J. D., & Hollard, V. D. (1987). Orientation invariance of shape recognition in forebrain-lesioned pigeons. Behavioral Brain Research, 23, 251-259. Delins, J. D., & HoUard, V. D. (1995). Orientation invariant pattern recognition by pigeons (Columba livia) and humans (Homo sapiens). Journal of Comparative Psychology, 109, 278-290. Hollard, V.D., & Delius, J.D. (1982). Rotational invariance in visual pattern recognition by pigeons and humans. Science, 218, 804-806. Honig, W.K., Boneau, C.A., Burstein, K.R., & Pennypacker, H. S. (1963). Positive and negative generalization gradients obtained after equivalent training conditions. Journal of Comparative & Physiological Psychology, 56, 111-116.
Jolicoeur, P. (1985). The time to name disoriented natural objects. Memory and Cognition, 13, 289-303. Jolicoeur, P. (1990). On the role of mental rotation and feature extraction in the identification of disoriented objects: A dualsystems theory. Mind and Language, 5, 387-410. Lohmann, A., Delius, J.D., Hollard, V.D., & Friesel, M.F. (1988). Discrimination of shape reflections and shape orientations by Columba livia. Journal of Comparative Psychology, 102, 3-13. McMullen, P. A., Hamm, J., & Jolicoeur, P. (1995). Rotated object identification with and without orientation cues. Canadian Journal of Experimental Psychology, 49, 133-149. McMuUen, P.A., & Jolicoeur, P. (1990). The spatial frame of reference in object naming and discrimination of left-fight reflections. Memory and Cognition, 18, 99-115. Murray, J. (in press). Flipping and spinning: Spatial transformation procedures in the identification of rotated natural objects. Memory and Cognition. Neiworth, J. J., & Rilling, M. E. (1987). A method for studying imagery in animals. Journal of Experimental Psychology: Animal Behavior Processes, 13, 203-214. Rosch, E., Mervis, C. B., Gray, W. D., Johnson, D. M., & BoyesBraem, P. (1976). Basic objects in natural categories. Cognitive Psychology, 8, 382-439. Shepard, R. N., & Cooper, L. A. (1982). Mental images and their transformations. Cambridge, MA: MIT Press. Snodgrass, J. G., & Vanderwart, M. (1980). A standardized set of 260 pictures: Norms for name agreement, image agreement, familiarity, and visual complexity. Journal of Experimental Psychology: Human Learning and Memory, 6, 174-215. Received October 17, 1995 Revision received June 28, 1996 Accepted July 22, 1996
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