Tracking and capture of constant and varying velocity stimuli: a cross-species comparison of pigeons and humans

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1 Anim Cogn (2) 4:59 7 DOI.7/s ORIGINAL PAPER Tracking and capture of constant and varying velocity stimuli: a cross-species comparison of pigeons and humans Anna Wilkinson Kimberly Kirkpatrick Received: 6 May 2 / Revised: 5 July 2 / Accepted: 9 July 2 / Published online: 25 July 2 Springer-Verlag 2 Abstract The mechanisms underlying tracking and capture of moving objects in non-human animals are poorly understood. This set of experiments sought to further explore aspects of anticipatory tracking in pigeons and to conduct comparisons with human participants. In Experiment a, pigeons were presented with two types of varying velocities (fast-slow-fast or slow-fast-slow) in separate phases. They were readily able to track and anticipate both of these motion types. To examine the evects of predictability on anticipatory tracking, Experiment b presented the pigeons with the same two varying velocities randomly intermixed within a session. This resulted in reduced capture success, later capture, and errors that no longer anticipated ahead of the motion, suggesting that the anticipatory mechanism had been disrupted. This implies that the mechanisms involved in pigeon tracking are diverent from the predictive extrapolation mechanism proposed in humans. Experiment 2 tested this by presenting adult humans with a tracking task that was similar to tasks previously received by the pigeons. The capture behavior of humans was similar to the pigeons, but the errors revealed diverent processes underlying their tracking behavior. A. Wilkinson K. Kirkpatrick Department of Psychology, University of York, York YO 5DD, UK A. Wilkinson (&) Department of Cognitive Biology, University of Vienna, Althanstr. 4, 9 Vienna, Austria anna.wilkinson@univie.ac.at K. Kirkpatrick Department of Psychology, Kansas State University, 492 Bluemont Hall, Mid-Campus Drive, Manhattan, KS , USA Keywords Motion perception Visual perception Visual cognition Pigeon Columba livia Human Introduction Successful capture of a moving object is a complex behavior that requires an organism to process information about the object and its motion, plan object-directed actions and move the capture device (e.g. claw, hand or beak) to the correct location (Von Hofsten 987). Anticipation ahead of the actual position of a moving object, such as a prey item, is essential for eycient capture (which stands in contrast to other approaches such as chasing prey to the point of exhaustion or some forms of ambush predation). Anticipating motion requires tracking the current motion and extrapolating the future position of the object. Tracking behavior is commonly seen in the natural environment and occurs in predator prey interactions, in mating displays and shoaling behavior. Despite its prevalence, the mechanisms underlying tracking and capture have been experimentally observed in only a handful of animal studies (Byers 22; Lanchester and Mark 975; McVean and Davieson 989; Neiworth and Rilling 987; Pisacreta 982; Rilling 992; Rilling et al. 993; Rilling and LaClaire 989; Rilling and Neiworth 987; Ristau 99a, 99b). The most popular non-human species for investigating tracking and extrapolative behavior is the pigeon (Columba livia; McVean and Davieson 989; Miyata and Fujita 28; Miyata et al. 26; Pisacreta 982; Rilling 992; Rilling and LaClaire 989). They are highly sensitive to motion, and electrophysiological studies have shown that around 6% of the neurons in the pigeon visual system respond more to moving than to stationary stimuli (e.g., Frost and DiFranco 976; Frost and Nakayama 983). Rilling and LaClaire

2 6 Anim Cogn (2) 4:59 7 (989) demonstrated that pigeons can track and capture a moving object. They delivered a complex motion that moved at a sinusoidal velocity along a sinusoidal path and found that the pigeons successfully intercepted the stimulus and also generalized to a novel motion direction, but the birds tended to chase behind the stimulus rather than anticipate the motion. These Wndings suggest that pigeons are unable to extrapolate. However, as the task required complex non-linear extrapolation of both direction and velocity, it is possible that pigeons would be able to anticipate a simpler motion type. Therefore, Wilkinson and Kirkpatrick (29) presented pigeons with a simple linear rightward motion. Their stimulus travelled at a constant velocity on each trial within a session, but the velocity and stimulus size were manipulated across experimental phases. The Wndings revealed that the size of the stimulus determined capture success, whereas velocity determined error position. The majority of errors were ahead of the stimulus, suggesting that the pigeons were able to anticipate the motion. A second experiment presented the pigeons with four diverent motion types (upward, downward, rightward and leftward) and revealed that the pigeons had in fact learned to peck to the right of the stimulus. In a third experiment, naïve pigeons were trained with the four motion directions from the beginning. They learned to track all four motions, but anticipation was poorer. However, there was no rightward bias in their pecking. This suggests that predictability might be an important factor in determining whether pigeons anticipate ahead of the stimulus. It may also be an important factor in explaining the diverence in Wndings between Wilkinson and Kirkpatrick s (29) results and those of Rilling and LaClaire (989) as the studies divered in the degree of predictability of both velocity and direction of motion. The importance of predictability is supported by evidence from human infants who were presented with diverent levels of motion complexity; the infants were able to represent constant velocity motions over a period of occlusion at an earlier age than motions in which the velocity changed over the trial (Rosander and Von Hofsten 24). Infants of 2 weeks start to acquire representations of objects over occlusion, but it is not until they reach 2 weeks that they are able to adapt to dynamic velocities when predicting the reappearance of a stimulus (Rosander and Von Hofsten 24). This suggests that the process involved in anticipating varying velocities is more complex than constant velocities. In some ways, this Wnding is unsurprising given that varying velocities require learning of the pattern of velocity change combined with a continual update in the extrapolation parameter for anticipation, which depends on the current velocity and the projected change in velocity. It has been suggested that humans possess a sensorimotor system for anticipatory tracking, the predictive extrapolation mechanism (Von Hofsten 987). Predictive extrapolation involves the use of previous velocity and trajectory information to predict the future location of an object. Von Hofsten et al. (998) found that infants can anticipate approximately 2 ms ahead of the actual position of an object moving at a constant velocity. Their predictive reaches appeared to be based on linear components of the motion direction. When presented with sudden, non-linear changes in trajectory, the infants were unable to learn to predict the non-inertial motion even when they received the same non-linear motion for six trials in a row. This suggests that planning of predictive actions is avected by the observed object motion early in the trial more so than by the remembered trajectory. However, more recent experiments (Kochukhova and Gredebäck 27; von Hofsten et al. 2) have shown that 6-month-old infants were able to learn to predict the nonlinear reappearance of an object from an occlusion. Von Hofsten et al. (2) found that infants needed much experience to do so. However, Kochukhova and Gredebäck (27) used a more sensitive measure (eye tracking rather than head tracking) and found that infants learned to anticipate the appearance of the non-linear motion rapidly after only two passes of the occlusion. These Wndings show that infants can predict both linear and non-linear reappearance from behind an occluder but have a predisposition toward linear extrapolation. This could be the result of experience from real-life events. However, the ability to learn to predict non-linear motion has, to our knowledge, only been demonstrated when an occluder is present. In contrast to the velocity-based extrapolation observed in human infants, Wilkinson and Kirkpatrick (29) found that pigeons appear to anticipate the object position by aiming their peck a constant spatial amount ahead of the stimulus, rather than a constant temporal amount as was observed in human infants. Furthermore, the pigeons tracking appears to be avected by longer-term history evects (Wilkinson and Kirkpatrick 29); this is diverent from infants whose prior knowledge about a motion appears to be overridden by the current motion information if the object is not occluded (von Hofsten et al. 998). However, the pigeons tracking is also controlled by shortterm history evects (Wilkinson 28). These evects appear more similar to those observed in infants (although the underlying mechanisms controlling this might still be very diverent). Wilkinson and Kirkpatrick (29) proposed a simple model that contained two factors that could account for the pigeon Wndings. The Wrst factor of import was lag time; this is the time between Wxation and delivery of the peck and this factor is multiplicative with stimulus velocity. As a result, the lag time will have a greater negative evect on the accuracy of pecking at faster velocities (if a stimulus velocity is doubled, then the error in pecking the stimulus

3 Anim Cogn (2) 4: will also double due to lag time). The second factor, bias, is additive and not avected by stimulus velocity. It allows the bird to anticipate a Wxed spatial distance ahead of the stimulus regardless of the velocity of motion. To further investigate the nature of the pigeons tracking abilities and to conduct additional comparisons with both Rilling and LaClaire s (989) study and the human infant research (e.g., Gredebaak et al. 22; von Hofsten et al. 2; von Hofsten et al. 998), the present set of experiments focused on the evect of varying velocity within a trial on anticipatory tracking in the pigeon and in adult humans. In Experiment a, pigeons received two motions that moved along a linear path but at a varying velocity; these motions were presented in separate phases of the experiment. If varying velocities disrupt the anticipatory component of tracking, then errors should be predominantly lagging, as was found by Rilling and LaClaire (989). In Experiment b, the pigeons were presented with the same varying velocity motions as in Experiment a, but the velocity types were randomly intermixed within a session; this was to determine the impact of between-trial predictability on tracking. Experiment 2 presented adult humans with constant and varying velocity motions using tasks that were similar to previous experiments with pigeons and Experiment a. This allowed comparison of the tracking behavior of humans to the current and previous Wndings with pigeons. Experiment a The aim of the present experiment was to examine whether pigeons could anticipate a varying velocity stimulus that moved on a linear path. In phase, the birds were presented with a stimulus that started fast, became slower and then sped up (fast-slow-fast). In phase 2, they were presented with a stimulus that started slow, sped up and then slowed down (slow-fast-slow). Anticipation of a varying velocity would not only require the birds to process the current stimulus position and the current velocity, but also the change in velocity over time. Materials and methods Animals Three captive-bred pigeons (Columba livia) served as the experimental subjects: B7, O29 and Y2. The pigeons were approximately 2 and a half years old and experimentally naïve at the onset of the study. The birds were housed in individual cages in a colony room on a 2:2 light dark cycle with light onset at 8 a.m. Each bird was maintained at 85 9% of its free-feeding weight through the delivery of individual Noyes pigeon pellets in the experimental apparatus and supplementary access to grain in the home cage, ranging from 5 to 2 g per day. They were allowed free access to grit and water in the home cages. The birds were regularly placed in a Xight cage to receive exercise and a bath; they did not participate in the experiment while they were in the Xight cage. During this time, the birds received free access to grit and water and a once-daily feeding of grain scattered among the bedding at the bottom of the cage. Apparatus The pigeons were trained and tested in two cm operant chambers housed inside a soundand light-attenuating box (Med Associates, St. Albans, VT). One wall of the chamber was Wtted with an 2.-in touch screen (Elotouch Systems, Accutouch) that was situated in front of a 5-in TFT monitor that was turned on its side (resolution = pixels). On the opposite wall of the chamber was a magazine pellet dispenser (Med Associates, ENV-23) and clicker (Med Associates, ENV- 35 M). Individual 45-mg pigeon pellets were delivered through a rubber tube into a food cup (Med Associates, ENV-2-R M) that was located 2 cm above the grid Xoor. A houselight (Med Associates, ENV-227 M) was located on the top-right wall above the food cup; this delivered divuse illumination to the pigeon chamber at an intensity of approximately 2 lux. A speaker, which was positioned outside the pigeon chambers, emitted a divuse 6-dB white noise to mask sounds outside the room. Responses were recorded from the touch screen via a USB touch screen controller (Elotouch Systems, 3U USB controller). Control of the feeder and houselight was accomplished by a digital I/O card (National Instruments, PCI-653). A video splitter (Rextron, BSA2) allowed simultaneous presentation of images to the control room and operant chamber. Two Viglen Genie P4 computers located in an adjacent room delivered the experimental procedures and recorded data using procedures written in MATLAB (The MathWorks, Inc.). At the time of each peck, the location of the peck and the position of the stimulus were recorded in the form of XY coordinates with a time tag. Procedure Pre-training Following initial hand shaping, the birds were presented with a yellow oval that decreased in size across sessions from cm to.8.4 cm. In the Wnal stage of

4 62 Anim Cogn (2) 4:59 7 pre-training, the stimulus could appear in any one of nine places on the screen to encourage the pigeons to peck the whole of the touch screen area. Training, phase A trial began with the presentation of a 2-cm-diameter white circle on a black background. A single peck on the white circle resulted in the circle disappearing and the immediate presentation of a.5-cm-diameter yellow circular stimulus. The stimulus could appear from one of Wve positions along any of the four sides of the active portion of the monitor, which measured cm. Both the leftward and rightward motions could appear at 6., 8.5,., 3.5, or 6. cm (measured from the top of the screen) from the left or right side of the active portion of the screen. The upward and downward motions started at 6., 8.5,., 3.5, or 6. cm from the left side of the active portion of the screen. The starting position of the stimulus was pseudo-randomly chosen on each trial, with the restriction that there were three trials for each starting position for each motion direction within a session. The stimulus moved at a varying velocity, with a mean of 3.4 cm/s. At its fastest point, the stimulus moved at 6.8 cm/s, and at its slowest point, it moved at.7 cm/s. The velocity changed at a constant rate during acceleration and deceleration periods. The equation used to calculate the target motion was a =(v t v )/t. With a being acceleration, t the time point of the trial, v t the Wnal stimulus speed and v the starting speed of the stimulus. When a was positive, the stimulus accelerated, when negative it decelerated. The birds received a motion that started fast, then gradually became slower (until it reached its slowest point in the middle of its path), and then accelerated to the other side of the screen (fast-slow-fast). In all cases, the object moved in a straight path from the starting point toward the opposite side of the screen. If a peck occurred anywhere within the stimulus boundary it counted as a catch, the stimulus disappeared, a clicker was sounded, the pigeon was rewarded with three food pellets, and the trial ended. If the bird did not successfully capture the stimulus, it moved ov screen when it reached the opposite side. In this case, the bird was not rewarded and the trial ended. Each training session consisted of 6 trials that were separated by an intertrial interval of 5 s, during which the screen was dark. The birds were trained for a minimum of 6 sessions. If they met a criterion of two consecutive sessions with at least 7% of trials ending in a capture response they were moved to phase 2. Birds B7 and O29 were tested for 6 days, and bird Y2 was tested for 4 days. Training, phase 2 This was the same as the Wrst training phase, but the motion started at its slowest velocity, sped up and then became slower again (slow-fast-slow), with a mean velocity of 3.4 cm/s. Birds B7 and O29 were tested for 6 days, and bird Y2 was tested for 33 days. Data analysis All pecks were recorded by their location on the screen (X, Y coordinates) and were marked with a time stamp with -ms resolution. Pecks within the peck-sensitive area were designated as capture responses, and pecks that occurred outside of this area were marked as errors. Within a trial, the bird could make both error and capture responses and these were analyzed separately. Capture responses were assessed with three diverent measures. The proportion of trials ending in a capture was the number of trials ending in a capture divided by the total number of trials in a session. The proportion of path traversed was determined by computing the distance travelled from the start of the path at the time of capture and dividing by the total possible path length. The number of pecks to capture was also calculated on capture trials by determining the total number of pecks emitted on that trial, including the capture response. Error pecks were assessed by determining the peck position relative to the center of the stimulus. Errors were expressed as lagging behind or leading ahead of the stimulus. Only the motion-relevant errors (errors that occurred in the same plane as the target movement) were analyzed. However, the mean position of the motion irrelevant errors are also presented. Only the Wrst 6 days of training were analyzed, after which two of the birds had reached criterion. Results Capture responses Figure presents the three measures of capture responding. The percentage of trials ending in a capture appears in panel A. A repeated-measures ANOVA revealed no diverence in capture success between the velocity types (fast-slow-fast vs. slow-fast-slow) F,2 =3.6, P =.2. A correlation examining whether capture success changed across sessions revealed that it signiwcantly increased over sessions for the motion (r =.35, P =.2), but not for the motion (r =.2). Panel B contains the mean number of pecks to capture over training. There was no signiwcant diverence in capture

5 Anim Cogn (2) 4: Fig. Capture responses in Experiment a with separate exposure to the fast-slow-fast () and slow-fast-slow () velocities. a The proportion of trials ending in capture as a function of sessions of training for the two velocity types. b The mean number of pecks emitted before capture as a function of sessions of training for the two velocity types. c The proportion of the path traversed before capture as a function of sessions of training for the two velocity types. d The probability distribution of the number of pecks emitted before successful capture. e The probability distribution of the proportion of the path traversed before capture for both velocity types PROPORTION OF TRIALS ENDING IN A CAPTURE C A PROPORTION OF PATH TRAVERSED BEFORE CAPTURE PECKS TO CAPTURE B D PROBABILITY PECKS TO CAPTURE E PROBABILITY PROPORTION OF PATH TRAVERSED eyciency between the two velocity types, F,2 <. Correlations examining whether this changed over sessions revealed that there was no evect of session on capture eyciency for either the (r =.23) or the (r =.7) velocities. An examination of the distribution of pecks to capture in panel D indicates that the birds captured the stimulus most often on the Wrst peck, and the vast majority of captures occurred within or fewer pecks for both motion types. Thus, capture was eycient, but this eyciency did not improve over training. The average location where the stimulus was captured (see panel C) revealed no diverence between the motion types F,2 <, despite the slow-fast-slow motions starting much slower than the fastslow-fast motions. However, the distance travelled before capture did decrease across sessions for the velocity (r =.5, P <.), but not for the motion (r =.5). This Wnding is supported by data from panel E, which reveals that the capture location peaked later for the fast-slow-fast motion than the slow-fast-slow motion, even though the means did not diver signiwcantly. Errors Figure 2 displays the lag-lead error for both the fast-slowfast and slow-fast-slow conditions over sessions. The birds errors anticipated the stimulus on average by.49 cm for the velocity and.5 cm for the. This suggests that despite the complex changes in velocity, the pigeons were able to anticipate ahead of the stimulus motion. Furthermore, the error position did not change over sessions for either the (r =.2) or the (r =.22) velocities.

6 64 Anim Cogn (2) 4:59 7 LAG/LEAD ERROR (CM) The mean position of the motion-irrelevant errors for the motion was.4 cm (.3 SEM) above the stimulus for the horizontal motions and.36 cm (. SEM) to the left of the stimulus for the vertical motions. A very similar pattern was seen in the motion-irrelevant errors to the motion. These were.8 cm (.2 SEM) above the stimulus for the horizontal motions and.27 cm (. SEM) to the left of the stimulus on the vertical motions. This suggests that the pigeons closely tracked the stimulus on all axes and behaved in a similar manner to both motion types. Discussion Fig. 2 Lag-lead error over sessions for each velocity type in Experiment a When two diverent velocities were presented in separate phases, there was little diverence in the capture behavior or the errors to the two motion types. In terms of capture, both the fast-slow-fast and the slow-fast-slow velocities were intercepted eyciently. In fact, the capture eyciency measures revealed a similar level of performance to birds trained on the same four-motion task with a constant velocity (Wilkinson and Kirkpatrick 29). Capture success increased over sessions for the motion but not the (Fig. a). This is likely to be due to the being presented Wrst to all birds. Initial presentation of this was the Wrst time that they had ever tracked motion; nevertheless, they were still able to successfully capture the stimulus (in the Wrst half of the path) on over 6% of trials in the Wrst session, and there was no overall diverence in capture success between the two motion types. The present results indicate that a varying velocity on its own does not lead to lagging errors, suggesting that the previous Wndings of Rilling and LaClaire (989) were not solely due to the varying velocity. All of the birds anticipated ahead of the object (Fig. 2), and they anticipated by a similar amount for the two velocities. The degree of leading error was greater for pigeons trained on the varying velocity stimulus than was observed when birds were trained with a constant velocity of 3.4 cm/s where the errors only slightly led the stimulus (Wilkinson and Kirkpatrick 29), though 3.4 cm/s was also the mean speed of the varying velocities of this experiment. Here, the errors were close to.5 cm in front of the stimulus. Thus, the birds may have overcompensated for the varying velocity conditions by increasing their peck bias. This Wnding cannot be accounted for by the varying velocity motions having a greater starting velocity as the bias was similar when the varying velocity motion started with a slow velocity compared to when it started with a fast velocity. The similar degree of anticipation for both varying velocities provides further evidence that the anticipatory bias is an additive spatial distance factor. If bias were multiplicative, then a slower motion would produce a smaller error, so the slow-fast-slow motion would have smaller anticipatory error than the fast-slowfast motion because it was caught earlier in its path. The error position remained constant across sessions except for the Wrst presentation of the slow-fast-slow motion where the mean error position was nearly double that of the errors for the last session of the fast-slow-fast motion (which directly preceded it). However, after only one session, the slow-fast-slow motion errors were indistinguishable from those of the fast-slow-fast velocity (see Fig. 2). Prior to this session, the pigeons had only experienced a motion that started fast and then slowed down. Their pecks in the Wrst session of the slow-fast-slow motion consistently overanticipated the stimulus, revealing the strong role that prior experience played when tracking a novel velocity. This indicates that they had learned at least some elements of the velocity pattern. Wilkinson and Kirkpatrick (29) found a similar evect of training history on tracking behavior. Birds that were initially trained on a rightward motion consistently pecked to the right of the stimulus on all motion types when they were then presented with motions that travelled leftward, upward and downward. However, in the current experiment, the birds were able to rapidly adapt, suggesting that the prior evect of varying velocities was transient. Perhaps the degree of transfer evect could be determined, at least in part, by the predictability of the motion that is initially experienced, thus making transfer from one varying velocity motion to another easier than adapting to a totally novel motion direction. Experiment b The birds in Experiment a anticipated ahead of a stimulus moving at a varying velocity; however, the velocity changed at a gradual and predictable rate, and the stimulus moved along a predictable, linear path. These Wndings

7 Anim Cogn (2) 4: diver from those of Rilling and LaClaire (989), where the pigeons predominantly chased the stimulus. In their experiment, both the velocity and direction of the stimulus varied over time, making the motion inherently less predictable than in Experiment a. If predictability of motion direction and/or velocity is the key to anticipation in the pigeon, then reducing predictability should increase the frequency of lagging errors. Experiment b examined the evect of decreasing between-trial predictability of the motion by presenting the two varying velocities from Experiment a randomly intermixed within a session. Materials and methods Animals The birds from Experiment a participated in Experiment b. Apparatus The apparatus was the same as in Experiment a. Procedure The training procedure was identical to Experiment a, but trials of the fast-slow-fast and slow-fast-slow velocities were intermixed, with 3 trials of each velocity type in a 6-trial session. Birds B7 and O29 were tested for 32 days, and bird Y2 was tested for 5 days. Bird Y2 failed to meet criterion of 7% of trials, ending in a capture response for two consecutive days and his training was terminated after 5 days. Data analysis Analyses of capture responses and motion-relevant errors were conducted in the same manner as in Experiment a. Only the Wrst 32 days of training were analyzed, after which two of the birds had reached criterion. Results Capture responses Panel A of Fig. 3 presents the percentage of trials ending in a capture. As can be clearly seen, there was no diverence in capture success between the velocity types, F,2 =.2. Correlations revealed that there was also no change in capture success across sessions for either (r =.7) or (r =.7). The number of pecks to capture (panel B) also revealed no diverence between the velocity types F,2 < and no evidence of change over sessions for either (r <.) or (r =.8). An examination of the distribution of pecks to capture in panel D indicates that for both motions, the birds captured the stimulus around 2% of the time on the Wrst peck, but there was also a second peak in captures at around 5 pecks. Thus, capture was reasonably eycient, and there was no change in eyciency over training. There was no diverence between the two velocity types in the location where the stimulus was captured (panel C), F,2 = 3.95, P =.9. This also did not change across sessions for either (r <.) or (r =.7) motion types. Examination of panel E reveals that when the motion types were intermixed, the probability of capture was broadly distributed, and the two velocity types were similar. Errors Figure 4 reveals that when the same two velocities as Experiment a were presented in an intermixed fashion, the birds no longer anticipated ahead of the stimulus, but error locations closely centered around zero ( =. cm; =.4 cm). Although small, this diverence in mean error position between the velocity types was signiwcant F,2 = 38.3, P =.3, and a correlation examining the evect of session on error position was positive for both the (r =.2, P =.5) and (r =.53, P <.) velocity types. Errors around both motions were initially lagging and moved toward zero over sessions. The mean position of the motion-irrelevant errors for the motion was.2 cm (. SEM) above the stimulus for the horizontal motions and.6 cm (.4 SEM) to the left of the stimulus for the vertical motions. A very similar pattern was seen in the motion-irrelevant errors to the motion. These were.9 cm (. SEM) above the stimulus for the horizontal motions and.5 cm (. SEM) to the left of the stimulus on the vertical motions. This reveals a similar pattern to the motion-irrelevant errors observed in Experiment a. The data from bird Y2 (the bird that did not reach criterion) were included in all analyses reported above. Examination of this bird s individual data revealed a trend toward lower capture success. However, there was no diverence in the probability distribution for the proportion of path traversed before capture, or the proportion of path traversed over sessions, even though capture success was lower. This bird made fewer pecks to capture, which rexected less responding rather than better capture, a pattern of behavior exhibited throughout training. The errors made by this bird were distributed in the same way as those of the other birds as there was no diverence in error position. Thus, the inclusion of bird Y2 did not appear to have caused any anomalies in the results, apart from a lowered capture success rate.

8 66 Anim Cogn (2) 4:59 7 Fig. 3 Capture responses in Experiment b, where the birds received intermixed exposure to fast-slow-fast () and slowfast-slow () velocities. a The proportion of trials ending in capture as a function of sessions of training for the two velocity types. b The mean number of pecks emitted before capture as a function of sessions of training for the two velocity types. c The proportion of the path traversed before capture as a function of sessions of training for the two velocity types. d The probability distribution of the number of pecks emitted before successful capture. e The probability distribution of the proportion of the path traversed before capture for both velocity types. PROPORTION OF TRIALS ENDING IN A CAPTURE C A. PROPORTION OF PATH TRAVERSED BEFORE CAPTURE E PROBABILITY B PECKS TO CAPTURE D PROBABILITY PECKS TO CAPTURE PROPORTION OF PATH TRAVERSED BEFORE CAPTURE Discussion Presenting the two varying velocities randomly intermixed within a session produced diverent results from the separate presentation of the same velocities. Capture success dropped from an average of 7.5% ( Experiment a ) to 59.8% (Experiment b), and it decreased by a similar degree for both velocity types. In general, capture measures were similar for both velocity types in the intermixed condition, but overall performance was generally poorer than when the same velocity types were presented separately in Experiment a. The apparent diverence in the number of pecks to capture between the and velocities in Experiment b (Fig. 3b) though apparently large is not signiwcant (or even approaching signiwcance). Examination of the errors (Fig. 4) revealed that when the two motion velocities were randomly intermixed, the birds did not show the anticipation that was observed with separate presentation. Instead, their errors were centered on zero. However, there was a signiwcant diverence between the two motion velocities in terms of mean error position, with more lagging errors to the motion. Interestingly, the error position for both velocity types moved further toward zero throughout training. This suggests that the intermixed exposure avected the tracking mechanism in a similar way though it is possible that the was avected slightly more than the.

9 Anim Cogn (2) 4: LAG/LEAD ERROR (CM) Fig. 4 Lag-lead error over sessions for each velocity type in Experiment b The diverences between the separate and intermixed exposure in the pigeons could be due to strain on the anticipatory mechanism caused by processing two diverent varying velocities; however, the birds were only processing one motion at a time in both cases. Therefore, it is likely that the pecking behavior was partially controlled by learning of the velocity from previous trials within a session. The predictability of the motion velocity from one trial to the next may be an essential component for anticipation in the pigeon. It is possible that when the birds were presented with two velocity types intermixed, the anticipatory mechanism was, at least partially, disrupted. This is diverent from Wndings in the infant literature, in which infants revealed no evect of separate exposure of a speciwc motion type on prediction of the current motion velocity when the motion was not occluded (von Hofsten et al. 998). This lends further support to the idea that the anticipatory mechanism underlying tracking behavior in pigeons is diverent from the predictive extrapolation mechanism that has been proposed in humans. Experiment The results of Experiment a and b revealed diverent behavior in our pigeons to that observed in infants. Infants Wnd it more diycult to anticipate a varying velocity than a constant velocity and are able to represent constant velocities over an occlusion earlier in life than varying velocities (Rosander and Von Hofsten 24). Pigeons, on the other hand, appear to be as good at intercepting varying velocities when presented separately (Experiment a) as they are at constant velocities (Wilkinson and Kirkpatrick 29). Experiment 2 attempted to further examine these diverences by presenting the pigeon tracking task to adult humans to provide a comparison to both the pigeon and the infant research. The participants were presented with a stimulus and procedure that was nearly identical to that used in Experiment a. Two groups of human participants were presented with either a constant or a varying velocity stimulus to track; this allowed examination of whether they would show the same basic pattern as the pigeons, or a pattern more similar to that reported in infants on similar tasks. Materials and methods Participants Twenty undergraduate students from the University of York took part in this experiment. The group consisted of 7 women and 3 men with a mean age of 2.6 years (range 8 29 years). Apparatus A 5-in touch screen computer monitor (Tyco Electronics) was used to display the stimuli. The participants were provided with a chin rest that ensured an equal head height (3 cm) and distance (3 cm) from the monitor. They were asked to place their right arm on the table in a comfortable position so that they could easily touch the screen without having to make any large arm movements; all participants were right-handed. Individual diverences in participant height were controlled for using a height adjustable chair so that participants could comfortably position both their head and arm in the correct manner. A Dell Pentium IV computer presented the experimental procedures and recorded data using procedures written in MATLAB in the same format as the pigeon data. As with the pigeons, the time of each touch, the location of the touch and the position of the stimulus were recorded in the form of X, Y coordinates with a time tag. Procedure Participants were given an instruction sheet that briexy outlined the purpose of the experiment and described how to make responses using the touch screen monitor. The participants were randomly assigned to one of two groups, constant or varying. Group constant (n = ) were presented with a stimulus moving across the screen at a constant velocity of 3.4 cm/s; the starting position, size, movement and presentation of the stimulus were identical to phase of Experiment a, except that the stimulus velocity was constant throughout the trial. Group varying (n = ) were presented with a stimulus that moved across the screen at a varying fast-slow-fast velocity. The velocity, movement path, and presentation of the stimulus were identical to phase of Experiment a.

10 68 Anim Cogn (2) 4:59 7 A trial began with the presentation of a 2-cm white circle on a black background; centered on the top of the viewing screen was the instruction Please touch the circle to begin a trial (font size 8, Times New Roman, white text). As soon as a touch was made, the white circle disappeared and the target stimulus appeared from one of the four sides of the screen, as in Experiment a. At the end of each trial, the participants received feedback to mimic the receipt or omission of food for the pigeons. If the humans successfully captured the stimulus, the trial ended and they received the feedback catch (font size 8, Times New Roman, white text). If the trial ended without successful capture of the stimulus, the participants received the feedback miss. Both groups received 2 training trials. Apart from these minor variations, all other procedural elements were the same as Experiment a. Data analysis The data were analyzed using the same analysis programs as were used with the pigeons in Experiment a and b. However, as the humans only completed one experimental session, no analysis was made over sessions. All 2 trials were analyzed for each subject. Results Capture responses Figure 5 presents the three measures of capture responding for both experimental groups. Panel A shows the proportion of trials ending with a capture; examination of this Wgure shows that the rates of capture were very high for both velocity types. A one-way ANOVA indicated that there was no evect of velocity type on capture success F,8 <. An analysis of the touches to capture (panel B) indicated that the varying velocity was associated with less eycient capture, but this was not signiwcant, F,8 =3.9, P =.6. An examination of the distribution of touches to capture in panel D indicates that capture was relatively eycient for both motions with the humans capturing the stimulus most often on the Wrst touch, and all captures occurring within or fewer touches. Panel C reveals the proportion of path traversed before capture. Here, there was a signiwcant evect of velocity type on capture location F,8 = 9.3, P <., with the constant motion being caught earlier than the varying motion. Examination of panel E reveals that the peak in capture was shifted later for the varying velocity. Errors The mean lag-lead error for both the constant and varying velocity groups indicates that the participants tended to anticipate the motion in the constant velocity group (mean error position =.7 cm), but not in the varying group (mean error position =. cm). The errors for the constant motion revealed a signiwcant anticipatory bias to respond ahead of the motion, assessed by a one-sample t-test (vs. ), t 9 =2.7, P =.3, whereas the errors in the varying condition did not diver from, t 9 <. However, a one-way ANOVA assessing the diverence in error position between the two motion types revealed that this diverence did not reach signiwcance F,8 =3.5, P =.8. The mean position of the motion-irrelevant errors for the constant motion was.32 cm (. SEM) above the stimulus for the horizontal motions and.3 cm (.2 SEM) to the right of the stimulus for the vertical motions. A similar pattern was seen in the motion-irrelevant errors to the varying velocity motion. These were.26 cm (.2 SEM) above the stimulus for the horizontal motions and.37 cm (.2 SEM) to the left of the stimulus on the vertical motions. This represents a similar margin of error to that observed in the pigeons. Discussion The participants were highly successful at capturing the moving stimulus, with capture rates being close to % for both motion types. However, in terms of capture eyciency, there was a diverence between the velocities, with the varying velocity being caught later in the path than the constant velocity; this may be due to one of two possible sources: (a) the starting velocity of the varying motion was faster which may have impaired capture early in the trial; or (b) the varying velocity may have been generally more diycult to track due to the more demanding nature of having to learn the pattern of change in velocity. There was an indication of more touches to capture with the varying velocity, further suggesting a reduction in capture eyciency associated with this condition though this diverence was not signiwcant. The error measure indicated that the participants had more diyculty anticipating the varying than the constant velocity motion. The errors to the constant velocity were signiwcantly ahead of the actual stimulus position, while presentation of the varying velocity resulted in errors that slightly lagged the stimulus. These Wndings Wt well with those in human infants, who, when presented with diverent levels of motion complexity, were able to track constant motions over a period of occlusion at an earlier age than dynamic ones (Rosander and Von Hofsten 24). This again suggests that the process involved in anticipating varying velocities is more complex than constant velocities.

11 . Anim Cogn (2) 4: Fig. 5 Capture responses in Experiment 2 with presentation of the constant and varying fastslow-fast () motions. a The proportion of trials ending in capture for the two velocity types. b The mean number of touches emitted before capture for the two velocity types. c The mean proportion of the path traversed before capture for the two velocities. d The probability distribution of the number of touches emitted before successful capture. e The probability distribution of the proportion of the path traversed before capture for both velocities A PROPORTION OF TRIALS. ENDING IN A CAPTURE C PROPORTION OF PATH TRAVERSED BEFORE CAPTURE E CONSTANT VARYING B TOUCHES TO CAPTURE VELOCITY TYPE D.5.4 PROBABILITY 5 5 CONSTANT VARYING VELOCITY TYPE CONSTANT VARYING.3.2. CONSTANT VARYING VELOCITY TYPE TOUCHES TO CAPTURE PROBABILITY CONSTANT VARYING PROPORTION OF PATH TRAVERSED BEFORE CAPTURE Our Wndings further indicate that the varying velocity was also more diycult for adult humans to track and extrapolate than the constant velocity, suggesting that this is a result of a fundamental component of the anticipation mechanism and not due to a lack of development in the infants. General discussion The two experiments presented in this paper revealed interesting similarities and diverences in the processes underlying the tracking behavior of humans and pigeons. Both species appeared to track a constant velocity motion in a similar manner, and comparison of their errors revealed that they anticipated the motion to a similar degree (see Wilkinson and Kirkpatrick 29). However, presentation of the varying velocity motion had diverent evects on the two species. The pigeons showed strong anticipation of the varying velocity motion when the two varying velocities were presented separately (Fig. 2). This is diverent from the humans whose errors lagged slightly behind the stimulus when they tracked the same varying fast-slow-fast motion. A further point of interest is the change in the tracking behavior of the pigeons from Experiment a to Experiment b.

12 7 Anim Cogn (2) 4:59 7 Presenting birds with varying velocity motion did not disrupt anticipatory behavior; however, the presentation of the same velocities randomly intermixed within a session resulted in a disruption of the anticipatory component of tracking. This was revealed in a loss of anticipation, lower capture rates and less eycient capture. The lack of anticipation exhibited when tracking the intermixed motions is surprising, given that the motions were identical for both experimental phases. The diverence therefore appears to be the result of the change in predictability of the motion caused by intermixed presentation. This is supported by evidence from Wilkinson and Kirkpatrick (29) who found that the ability to anticipate motion varied with the predictability of the motion trajectory. Examination of the humans performance revealed that the overall diverence between tracking of the constant and varying motion was small. The humans did have more diyculty in predicting the varying motion, as evidenced by less anticipation and more lagging errors. However, this did not avect capture success; the only capture measure avected was the proportion of path traversed before capture; this may have been due to the varying velocity starting faster than the constant velocity or to the generally more demanding nature of tracking the varying motion. These results Wt well with the evidence from human infants. If the human adults in this experiment were tracking using a spatial anticipatory bias, then they would not capture the varying motion later in the path as the spatial bias does not take into account the change in speed at the start of the trial (as seen in the pigeon results). Thus, the Wndings suggest that the adults are extrapolating the motion in a similar way to infants. It is likely that the same mechanism controls their behavior. The main diverence between the two species was observed in their ability to anticipate the separate presentation of the varying velocity motion (Experiment a fastslow-fast condition in the pigeons vs. Experiment 2 varying velocity in the humans). This diverence can be accounted for by diverences in their predictive mechanisms. The additive spatial component in the pigeon s anticipatory mechanism would allow the pigeon to anticipate by pecking a certain distance rather than a temporal amount ahead of the stimulus. This would result in less sensitivity to the changes in velocity within a trial; however, this was not costly to the pigeon until the varying motion types were intermixed with each other. This resulted in a less predictable motion change that appeared to disrupt the anticipatory mechanism, as was evidenced in later capture, lower capture success and a loss of anticipatory errors (Experiment b; Fig. 3). However, the birds did still capture the stimulus at a relatively high rate. It is thus likely that the anticipatory mechanism was only partially avected and still had some control over the birds tracking behavior. If the lag time component of the pigeon peck (the time taken to peck) alone was guiding behavior, then entirely lagging (temporally controlled) errors would be expected. The exact impact of the intermixed presentation of varying motion on the anticipatory mechanism remains unclear. Further manipulations are required to clarify what component of the anticipatory bias remained after the two motion velocities were presented in an intermixed fashion. The diverences between the humans and the pigeons that were observed in this set of experiments may be due to the diverent manner in which each species intercepts the use of the hand versus the beak. The pigeon beak is always in a Wxed position relative to the eyes; to successfully capture a stimulus, the bird does not need any further information than that required for general feeding. However, in humans, the distance between the hand and eye can vary enormously. As a result, they require a system that allows them to track and approach the target at the same time. In its simplest form, this system would require two coupled mechanisms, one of which provides continuous input about the distance to target and the other the distal motion (Von Hofsten 987). This additional component makes calculation for interception more complex in humans than in the pigeon; however, it has the advantage that it allows continuous observation of the moving object. Conversely, the pigeon must stop tracking the motion with its eyes to allow interception with the beak. Furthermore, pigeons rewarded with food (as our pigeons were) shut their eyes while pecking (Jenkins and Moore 973), thus further reducing their chance for observation. As a result, the pigeon must track the motion, Wxate upon the target position and emit a peck without any update of the stimulus position. Fortunately, the pigeon peck is extremely rapid, taking only ms from Wxation to contact (Zeigler 974), which is likely to minimize the evect of the loss of updated sensory information on capture. However, these diverences in interception technique cannot be discounted when interpreting the diverences in anticipatory tracking behavior in the two species. Another diverence between the pigeon and human experiments was the visual angle at which the stimulus was observed. As the birds captured the stimulus by pecking on the touch screen, which required them to actively move their head (and whole body), the visual angle at which they observed the stimulus may have divered on every presentation. This is diverent from the human setup that ensured that they were always the same distance away from the screen. We also did not control the size of the stimulus compared to the intercepting species (the stimulus was much larger compared to the size of the pigeon than it was compared to the human); this inevitably led to diverences in the movement required by the interceptor to track and intercept the stimulus. However, because the key comparisons