Seeing Through Sound: Dolphins (Tursiops truncatus ... - CiteSeerX

Journal of Comparative Psychology 1998, Vol. 112, No. 3,292-305

Copyright 1998 by the American Psychological Association, Inc. 0735-7036V98/$3.00

Seeing Through Sound: Dolphins (Tursiops truncatus) Perceive the Spatial Structure of Objects Through Echolocation Louis M. Herman

Adam A. Pack

University of Hawaii and Kewalo Basin Marine Mammal Laboratory

Kewalo Basin Marine Mammal Laboratory and The Dolphin Institute

Matthias Hpffmann-Kuhnt Kewalo Basin Marine Mammal Laboratory Experiment 1 tested a dolphin (Tursiops truncatus) for cross-modal recognition of 25 unique pairings of 8 familiar, complexly shaped objects, using the senses of echolocation and vision. Cross-modal recognition was errorless or nearly so for 24 of the 25 pairings under both visual to echoic matching (V-E) and echoic to visual matching (E-V). First-trial recognition occurred for 20 pairings under V-E and for 24 under E-V. Echoic decision time under V-E averaged only 1.88 s. Experiment 2 tested 4 new pairs of objects for 24 trials of V-E and 24 trials of E-V without any prior exposure of these objects. Two pairs yielded performance significantly above chance in both V-E and E-V. Also, the dolphin matched correctly on 7 of 8 1 st trials with these pairs. The results support a capacity for direct echoic perception of object shape by this species and demonstrate that prior object exposure is not required for spontaneous cross-modal recognition.

Several early studies demonstrated capabilities of bottlenosed dolphins (Tursiops truncatus) for discriminating echoically between different geometrically shaped objects: for example, cylinders versus cubes (Nachtigall, Murchison, & Au, 1980) and cylinders versus spheres (Au, Schusterman, & Kersting, 1980). However, the methods of these studies did not allow for a determination of whether the dolphins attained the shape discriminations through associative learning or direct perception. Recently, Herman and Pack (1992) and Pack and Herman (1995) developed a new technique for determining whether a bottlenosed dolphin might directly perceive shape through echolocation. The technique used a matching-to-sample (MTS), "cross-modal" recognition task in which a dolphin inspected a sample object through echolocation alone and subsequently chose between two alternative objects through vision alone. One of the alternatives physically matched the sample, and the other did not. A

response to the matching alternative was rewarded. The authors termed this task echoic-visual (E-V) matching, where the first letter indicated the sense used to inspect the sample object, and the second, the sense used to inspect the alternatives. The authors also tested visual-echoic (V-E) cross-modal MTS, in which the sample was exposed to the visual sense and the two alternatives to the echolocation sense. An object to be inspected through the echolocation sense alone was suspended inside of a customized anechoic box immersed in the dolphin's tank. The dolphin echolocated on the contents of the seawater-filled box through the front panel, a thin sheet of black, visually opaque Plexiglas that was acoustically transparent in water. To restrict inspection to the visual sense, the researchers presented objects either in air, a medium in which the dolphin's echolocation sense is ineffective, or on a television screen located behind an underwater window. These procedures allowed for immediate transition from one sensory condition to the other. E-V and V-E matching were tested in a counterbalanced order for each of eight pairs of abstract, complexly shaped objects. Each member of a pair was constructed of the same size and quantity of polyvinyl chloride (PVC) pipe and fittings, and each was of approximately the same cross-sectional area, so that the only variable in common across the senses was object shape. Matching performance over 48 trials was either errorless or greater man 89% correct for all eight pairs under V-E MTS and for seven of eight pairs under E-V MTS. Direct (first-trial) cross-modal recognition occurred on 14 of 16 first trials under V-E and on 13 of 16 first trials under E-V (there were two first trials for each of the eight pairs—the first trial with one member of the pair as sample and the first trial with the other member of the pair as sample). These results revealed a high degree of integration of

Louis M. Herman, Psychology Department, University of Hawaii, and Kewalo Basin Marine Mammal Laboratory, Honolulu, Hawaii; Adam A. Pack, Kewalo Basin Marine Mammal Laboratory, Honolulu, Hawaii, and The Dolphin Institute, Honolulu, Hawaii; Matthias Hoffmann-Kuhnt, Kewalo Basin Marine Mammal Laboratory, Honolulu, Hawaii. This project was funded in part by grants from Earthwatch and its Research Corps, by support from The Dolphin Institute and its members, and by donations from Apple Computer, Inc., and Mitsubishi, Inc. We thank all of the staff, students, interns, and Earthwatchers at Kewalo Basin Marine Mammal Laboratory who contributed to this project. Correspondence concerning this article should be addressed to Louis M. Herman, Kewalo Basin Marine Mammal Laboratory, 1129 Ala Moana Boulevard, Honolulu, Hawaii 96814. Electronic mail may be sent to [email protected].

292

SEEING THROUGH SOUND

visual and echolocation perception. Inasmuch as none of the simple echo attributes, such as echo amplitude or spectral composition, transfers directly to an attribute apprehended by the visual sense, the results strongly suggested that the echolocation sense yields a percept of the object that represents its shape and that is analogous to, or closely coordinated with, the shape percept developed through vision. In a sense then, the dolphin appears capable of "visualizing" an object's shape through sound alone. Harley, Roitblat, and Nachtigall (1996, Experiment 3), using procedures substantially different from those of Pack and Herman (1995) and exposing their dolphin subject to nine different matching conditions within each test session, obtained levels of performance in E-V and V-E matching tests well below those reported by Pack and Herman. No anechoic box was used. Instead, latex eyecups were placed over the dolphin's eyes when the sensory mode was echolocation. About 30 s were required to place or remove the eyecups, creating a long delay between presentation of a sample object and the subsequent opportunity to view the alternatives in either E-V or V-E matching. The first three cross-modal tests of their Experiment 3 used objects from the prior two experiments, during which the dolphin had ample opportunity to inspect the objects with both senses simultaneously. This opportunity made it possible for the dolphin to learn the associations between the sounds of echoes returning from objects and the visual appearance of those objects (as admitted by Harley et al., 1996), precluding any valid test of direct cross-modal perception. They used a three-alternative matching procedure in which chance performance was 33%. For E-V, for each of three sets of three objects, matching accuracy was 72%, 100%, and approximately 65% (the exact percentage was not stated) correct responses per set, respectively. For V-E, for the same three sets, accuracy was 22%, 61%, and 39%, respectively. These results suggest that the dolphin learned these associations asymmetrically—primarily favoring the association from sound to sight rather than the reverse. Harley et al. (1996) subsequently tested cross-modal performance using six sets of unfamiliar (novel) objects (three objects per set). Objects in these sets (which they labeled Sets 7-12) varied respectively in size only, brightness only, shape only, material + brightness + texture, texture only, and size + brightness + shape + material + texture. Each set was tested in a single 27-trial session with Trials 1-3 devoted to V-E testing and Trials 4-6 to E-V testing, creating a possible confounding of effect of the cross-modal condition with the order of testing (the additional 21 trials were used to test the seven remaining conditions of the total of nine). Data were only given for all six sets combined, a total of 36 trials (18 in V-E and 18 in E—V). Performance remained at approximately 39% correct responses (i.e., at a chance level) for both V—E and E-V. Clearly, these outcomes contrast sharply with those of Pack and Herman (1995) and are likely a result of several factors. The most critical of these is the dolphin's inability to discriminate the objects within modalities. Neither withinmodal echolocation matching nor within-modal visual matching performance with the novel objects (two of the nine

293

conditions tested) was different from chance levels. Obviously, if a subject is unable to discriminate objects within either or both of the senses tested, cross-modal recognition will not be successful (Bryant, 1968). Furthermore, if the previous successful cross-modal matching with familiar objects was based on associative learning (which seems to be the case in Harley et al., 1996), then novel objects may not be spontaneously recognized across the senses because the dolphin may be continuing to attempt to use an associative learning strategy. These factors, combined with so many different conditions, the use, in one case, of a dimension (brightness) that cannot be discriminated echoically and the putting on and taking off of eyecups, represent formidable obstacles to successful performance in a crossmodal task. In contrast, in Pack and Herman (1995), all eight pairs that were tested cross-modally were selected on the basis of the results of prior within-modal tests: The subject had an average of 89.1 % correct responses over 24 trials per pair for echolocation (E-E) matching and an average of 98.4% correct responses for 24 trials per pair for visual (V-V) matching. Additionally, elaborate measures were taken to guard against any cross-modal contamination during or from these within-modal tests. For example, objects were never available for both visual and echoic inspection simultaneously. Tests of a particular pair within one modality were separated from tests within the other modality by a minimum of 5 days, during which tests with other pairs were given. Also, a mean of 152 days (range = 79-394 days) intervened between the final day of within-modal testing and the 1st day of cross-modal testing with a particular pair (see Pack and Herman, 1995, for a complete description of these and other controls). However, it is still conceivable that the within-modal experiences somehow primed or prepared the dolphin for the subsequent cross-modal task, making possible the demonstration of a skill level that would otherwise not be revealed. One way an elevated skill level might have occurred is through enhanced familiarity with the pairs of objects or with particular objects as a result of the withinmodal experiences. In this article, we examine whether cross-modal recognition can take place even in the absence of prior within-modal matching experience. In the first experiment, 25 new pairings were made of a subset of the 16 objects (8 pairs) used by Pack and Herman (1995) in their cross-modal tests. These 25, unlike the previous 8 pairings, were tested directly in both E-V and V-E cross-modal MTS without first testing them unimodally. The individual objects, however, had been members of other pairings that had been tested unimodally and cross-modally by Pack and Herman. In Experiment 2, therefore, 8 new objects were constructed and organized into 4 pairs, which were then tested directly in both E-V and V-E MTS. Within-modal testing then followed rather than preceded cross-modal testing. If the dolphin proved capable of successfully matching the new pairings (Experiment 1) and at least some of the completely novel pairs (Experiment 2) at levels approaching or equaling those observed in die earlier study, including at least some instances of first-trial (direct) recognition, then the case for direct perception of

294

HERMAN, PACK, AND HOFFMANN-KUHNT

shape through echolocation (and through vision as well) would be given further substantial strength. Additionally, first-trial, cross-modal recognition of novel objects in Experiment 2 would demonstrate that cross-modal recognition can proceed without any prior experience with the particular test objects. Experiment 1 Method

Herman, 1995, for further details of the in-air strictures on the dolphin's echolocation sense).

Objects Test objects.

A subset of 8 of the 16 objects used by Pack and

Herman (1995) in cross-modal tests were selected (Figure 2), and 25 of the possible 28 unique pairings of these 8 objects were tested. Three pairings had been tested previously by Pack and Herman and were not retested. The remaining 25 pairings were never encoun-

Subject

tered previously by the dolphin. All objects were constructed of gray PVC pipe and fittings (schedule-40 or schedule-80 or both) ranging in diameter from 0.5 in. (1.27cm) to 1.5 in. (3.81 cm). We

The subject was a 9-year-old, female bottlenosed dolphin (Tursiops truncates) named Elele. She was also the subject in our

filled the pipes and fittings with dry sand to weight the objects, creating negative buoyancy, and to reduce internal reflections from dolphin echolocation sounds, thereby emphasizing reflections from

previous studies of cross-modal matching (Herman & Pack, 1992; Pack & Herman, 1995). Elele was housed with 3 other dolphins in

surface features. Figure 2 shows each object in the frontal plane, as it was oriented in the anechoic box and as it was displayed visually

two interconnected, circular seawater tanks (each 15.2 m in diameter and 1.8 m in depth) at the Kewalo Basin Marine Mammal

to the dolphin, and also shows the same object rotated 45° to the right in the Z-plane to show more clearly the three-dimensional aspects of the objects.

Laboratory in Honolulu, Hawaii. We conducted cross-modal tests in one of these tanks. Elele was fed approximately 9.1 kg of smelt and herring daily; she received a portion of this ration during test

Apparatus We exposed objects to Elele's echolocation sense alone by presenting them within a customized anechoic box (Figure 1). The box measured l . l m wide X 0.6 m deep x 1.0 m high. Its two sides and back consisted of redwood slats set at an angle to deflect sound. Redwood is also absorbent of sound (Johnson, 1967). The bottom of the box was covered by an opaque PVC sheet. The front of the box consisted of a 0.32-cm-thick black Plexiglas panel that precluded any visual inspection of the contents of the box. However, in water, Plexiglas of this type transmits sound well (e.g., 75% transmissibility at 60 kHz; see Pack and Herman, 1995, Footnote 1), allowing the dolphin to inspect the contents of the box echoically through the front panel. We suspended the box from the side of the tank so that the entire front panel and the adjoining sides and back were below the surface of the water in the tank. The front and side visual screens remained above the water surface and precluded any view into the box from above the surface and also prevented the dolphin from seeing any activities taking place behind the box. Seawater entered the box from a small opening at the back of the bottom panel, filling the interior to the water level outside the box. An object to be inspected echoically was suspended underwater inside the box by thin monofilament lines attached to a wooden bar. The bar was placed across two horizontal PVC outriggers attached to the box above the water surface; this arrangement held the object in a fixed orientation that was maintained for all trials. There were three anechoic boxes altogether. We used one of these boxes to present the sample object during E-V MTS, and we fitted it with a mirror at its top, angled so as to provide a person standing behind the box a downward view of the front of the box and of the dolphin when it was at the Plexiglas panel. We used the other two boxes during V-E or E-E MTS to present the two alternative objects. Each of these latter two boxes was fitted with an underwater response paddle that the dolphin pressed to indicate its choice of the object inside that box. No special apparatus was needed to present objects to the visual sense alone. Objects were simply presented in air, a medium in which the dolphin's echolocation sense is ineffective (see Pack and

Baseline objects. We used the same six objects used by Pack and Herman (1995) as a baseline set. Pairs of baseline objects and pairs of test objects were interleaved during each 24-trial testing session, each type of pair appearing 12 times. The baseline objects were highly familiar to the dolphin and almost always were matched errorlessly, ensuring that she experienced a reasonably high degree of success at each test session, regardless of performance with the test pairs. The six objects were a stainless-steel bowl, an orange ceramic terra-cotta flower pot, a green fiberglass grating, a black hard rubber roller, a sand-filled PVC cross, and a letter "I" constructed from aluminum I-beams.

Procedure The cross-modal procedures used for E-V and V-E MTS were identical to those of Pack and Herman (1995). E—V matching. A single anechoic box was used. Before the start of each trial, we suspended a sample object inside the box, taking precautions to avoid any passive listening cues associated with object entry into the water by simultaneously introducing two masking objects into the box together with the sample object and then quickly withdrawing the two masks. For each of the 12 test trials, one of the masks was identical to the nonmatching alternative object; the other mask was selected from the baseline set such that over the 12 test trials, each baseline object appeared as a mask two times. We used a similar masking procedure for the remaining 12 trials when baseline objects appeared as samples; The two masks consisted of the baseline object appearing as the nonmatching alternative and one of the four remaining baseline objects, the latter presented in a balanced order over trials and sessions. On a signal from her trainer, Elele swam from her remote station to the anechoic box and touched her rostrum to the Plexiglas panel at the location of the object inside, which was suspended either toward the left or right side of the box. After verifying through the mirror that the object's location was identified correctly (which was always the case), an assistant positioned behind the anechoic box vocally signaled two other assistants, one located to each side of the anechoic box at 3-m distance, to stand erect and hold an object in the air over the tank wall and about 30 cm above the water surface. The walls of the tank rose 1.22 m above the surrounding deck, allowing the assistants to remain out of view crouched behind the wall on raised platforms until it was time to present the alternative objects in the air. Both assistants wore opaque goggles to obscure


295

Figure 1. Computer schematic (not to scale) of the anechoic box used for displaying sample objects to the dolphin's echolocation sense in echoic to visual (E-V) and echoic to echoic (E-E) matching. The front Plexiglas panel is shown open here to reveal the object suspended inside. Visual screens surround three sides of the box, but two are shown removed so as to reveal the box interior. The mirror for viewing the dolphin when it was at the front panel is shown at the top center of the box. Further details of the materials and dimensions of the anechoic box can be found in the text. Anechoic side boxes (not shown here) used to display the alternative objects in E-E and V-E matching were identical to the one depicted here, with the exception that no mirror was present. Also, an underwater response paddle that had an arm extending into the air and ended in a highly visible red foam float was attached to the outer side of each side box.

any view of the dolphin, and neither was aware of the identity of the sample. Also, the assistant behind the anechoic box was unaware of the location of the matching alternative. Exposure of the alternative objects signaled the dolphin to leave the anechoic box and choose one of the two alternatives. The response criterion, judged by an observer having no knowledge of the correct alternative, was that the dolphin remain stationary for 3 s in front of one of the alternatives. Correct responses were followed by the sounding of a whistle and then by a fish reward and social praise. These rewards were omitted after incorrect responses. V-E matching. The two anechoic boxes for presenting the alternative objects were separated from each other along the tank

wall by 6.2 m. On a signal to begin a trial, Elele positioned herself in front of an assistant standing on a 0.7-m-high platform located centrally between the two anechoic boxes and just outside the tank wall. The assistant wore opaque goggles to preclude any view of the dolphin's behavior or any activities of the individuals immersing the alternative objects in the anechoic boxes. Elele positioned her body vertically with her ventral side toward the assistant, her head out of the water with rostrum raised approximately 75°-90° above the horizontal plane, and her eyes facing forward, a favorable position for in-air viewing (see Pack & Herman, 1995). As soon as Elele assumed this position, a supervisor located in an observation tower directed the assistant to present the sample

296


B

C

H

I

I

I

I

60cm

30

Figure 2. Computer scans of photographs of the eight polyvinyl chloride objects used in cross-modal matching tests in Experiment 1. Each object is shown from the frontal view (0°—upper figure) and rotated 45° counterclockwise in the Z axis (lower figure). Letter codes associated with each object are shown above the computer scan of that object. All objects are shown to the same scale.

object. The assistant presented the object by bringing it forward from behind the tank wall and holding it stationary about 30 cm above the water surface where it could be seen by the dolphin. The sample object remained exposed in this position throughout the trial. After 5 s, the supervisor signaled for the two alternative objects to be immediately immersed in the side boxes by assistants positioned at each side box. Immersion in the left and right side boxes was simultaneous. The assistants had no knowledge of which alternative was correct. As was the case with the central box during E-V matching, two masking objects were immersed in each side box simultaneously with the alternative object and then immediately withdrawn. One mask was always identical to the alternative object in the opposite box. The second mask was the same for both boxes and was selected from the six baseline objects, with the constraint that across the 12 test trials each baseline object serve as a mask twice. Masks for the baseline trials were selected in a

similar manner, but consisted of other baseline objects. For both test trials and baseline trials, the alternatives were suspended by thin monofllament lines in the same manner as described for the sample object in E-V matching. On hearing the splashing sound of the objects being immersed, Elele placed her head underwater, echolocated, and then approached one of the side boxes, pressing the response paddle at that box to signify her choice of alternative object. The reward for choosing the matching alternative was as described for E-V MTS.1 1

After the 26th V-E test session (the 51st session altogether as an additional 25 sessions had been devoted to E-V testing), Elele became unwilling to approach the area between the two anechoic boxes where the sample object was being visually displayed. After several unsuccessful attempts at remedial training using baseline

297


Testing protocol. The 25 different pairings of the eight test objects were organized into eight sets of 3 pairs each. The one remaining pairing was tested alone. During each session of 24 trials with a given set, each of the 3 test pairings appeared four times for a total of 12 test trials. The remaining 12 trials consisted of pairings of the six baseline objects. The first 2 trials of each session were always baseline trials, a warm-up to ensure that the dolphin was responding well in a familiar situation. The order of appearance of the test pairings within a session was randomized with the constraint that each pairing had to appear twice in each half of the session. Baseline trials were interleaved among the test trials. Each set of 3 test pairs was tested for 12 sessions altogether, 6 sessions of E-V matching followed by 6 sessions of V-E matching, or the reverse, with the order of testing of E-V and V-E counterbalanced across sets. The first set of 3 pairs was tested first in E-V and then in V-E, the second and third sets were tested in the opposite order, and the fourth set again in the order E-V followed by V-E. We reversed this progression for the next group of four sets, and we tested the final pair first in V-E and then in E-V MTS. Thus, a complete testing of a set resulted in a total of 24 trials in E-V and 24 in V-E for each novel pairing. The 15 possible pairings of the six baseline objects appeared equally often or nearly so in a randomized but balanced sequence over the course of testing of a set of 3 novel pairings of test objects. At each session, each member of a test pair served twice as the sample and twice as the nonmatching alternative object. The matching alternative appeared on the left side or right side in a random order across tests of different pairs, with the constraint that the matching alternative for a particular pair not appear on the same side for more than three successive trials. To obtain an archival record of performance, we videotaped all sessions. Results Test Pairs Figures 3 and 4 show, respectively, performance on E—V and V-E MTS for each pairing of the eight objects pictured in Figure 2. The top of each cell shows performance on the 1 st trial given with a particular member of the pair as sample and the 1st trial given with the second member of the pair as sample. Performance on these trials provides a measure of spontaneity of cross-modal recognition. A plus indicates a correct response and a zero an incorrect response. Thus, a

objects only, we modified the procedure by having Elele's trainer display the sample object at Elele's remote station, located clockwise approximately 9.8 m from the right-side box. The two anechoic boxes remained in the same location as during the previous 26 sessions. After the normal 5-s visual sample exposure and after the alternative objects were immersed in the two boxes, Elele swam toward the boxes to make her choice. She approached the boxes from the right, swimming counterclockwise, thereby encountering the right box first. Elele had no difficulty with this modified procedure and immediately adopted it. Thereafter, beginning with Session 27, the sample was always shown to Elele by her trainer at the remote station, in the same manner as previously. The trainer had no knowledge of which box contained the matching alternative. Also, as before, the assistants immersing the alternative objects (and their masks) in the side boxes had no knowledge of which sample was displayed to Elele, and Elele's responses to the right or left anechoic box were judged by an observer having no knowledge of the location of the matching alternative.

Echoic-Visual (E-V) Matching Objects

H

G

F

E

D

C

B

A

24

24

22

24

24

24

18

24 + + 24

24

24

24

24

24

24 + + 24

23

24 + + 23 + + 19

B C D E F G

+

+

24 + + 23 + + 24

24 + + 24

23

Figure 3. The number of correct E-V matches over the total of 24 trials (errorless performance = 24) given with each unique pairing of the eight test objects (letter codes, as in Figure 2). Performance on the first 2 unique trials with each pair is shown at the top of each box, where a plus indicates a correct match and zero an incorrect match. The three lightly shaded squares represent results from the original pairings reported in Pack and Herman (1995). plus followed by a zero (from left to right) indicates a correct match on the 1st trial with one member of the pair as sample but an incorrect response on the 1st trial with the second member of the pair as sample. The number that appears at the bottom of each cell indicates the number of correct responses over the total of 24 test trials with a given pair. Results are shown for the 25 novel pairings (unshaded Visual-Echoic (V-E) Matching

Figure 4. The number of correct V-E matches over the total of 24 trials (errorless performance = 24) given with each unique pairing of the eight test objects {letter codes, as in Figure 2). Performance on the first 2 unique trials with each pair is shown at the top of each box, where a plus indicates a correct match and zero an incorrect match. The three lightly shaded squares represent results from the original pairings reported in Pack and Herman (1995).

298


boxes) and also for the 3 pairings (lightly shaded boxes) that had been tested previously by Pack and Herman (1995). The previous pairings had each been tested for 48 trials, but to maintain comparability with the new pairings, results are shown only for the first 24 trials. The one difficult old pairing, A versus B, although yielding only 13 correct responses in the first 24 trials, improved to 19 correct responses (79.2%, p = .003, by the cumulative binomial test) in the second 24 trials. Elele chose correctly on 49 of the 50 first trials (p < .001 by the cumulative binomial test) with the novel pairings during E-V matching (Figure 3) and on 44 of 50 first trials (p < .001 by the cumulative binomial) during V-E matching (Figure 4). These results demonstrate an impressive capability for direct cross-modal shape recognition. The difference between E-V and V-E performance levels was not significant, X2(100, N = 1) = 2.46, p > .05. Overall performance was also excellent. Significant levels of performance occurred with 17 correct matches or greater over the 24 test trials (p < .032 by the cumulative binomial test). For E-V matching, Elele was errorless on 20 of the 25 novel pairings and scored 19 correct matches or better on the remaining 5 pairings. In V-E MTS, she was errorless on 13 of 25 pairings and scored 20 correct matches or better on 11 of the remaining 12 pairings. Only 1 of the new pairings, F versus G, offered any relative difficulty. For this pairing, the dolphin made 16 correct matches in V-E tests (p = .076 by the cumulative binomial test) and 19 correct matches in E-V tests. There was neither facilitation nor decrement associated with the order of testing. When a procedure was the first given (E-V and V—E combined), Elele was correct on 585 of 600 trials (97.5%). When it was second (E-V and V-E combined), Elele was correct on 579 of 600 trials (96.5%). These differences were not significant, x2(1200, N = 1) = 0.72, p>. 05. Baseline Pairs Performance with baseline-object pairings was nearly errorless. In E-V MTS, Elele matched baseline objects correctly on 605 of 606 trials (99.8%), and in V-E MTS she matched correctly on 607 of 613 trials (99.0%). Decision Time Throughout the first 26 sessions of V-E testing, we showed the visual sample to the dolphin from a central point between the two anechoic boxes (see footnote 1 above). This procedure, together with the existing videotape record of sessions, made possible an estimate of the time taken by the dolphin, through echolocation, to decide which of the two anechoic boxes contained the object matching the visual sample. The videotape records of the first 24 sessions, each of 24 trials, were examined trial by trial to obtain estimates of decision time. These 24 sessions consisted of four blocks of 6 sessions, each block testing a different set of three unique pairings of the eight objects shown in Figure 2 (Set 1 pairings: C-H, A-F, B-G; Set 2: C-A, D-E, B-F; Set 3: G-D, E-H, C-B; and Set 4: D-F, G-H, E-A). Additionally,

during each block, all possible pairings of the six baseline objects were tested. Sessions 25 and 26 were an incomplete block and were therefore not included in this decision-time analysis. Estimating decision time was made possible by the reliability of the dolphin's behavior when inspecting the contents of the anechoic boxes after viewing the visual sample. As was described previously, during these early sessions the dolphin positioned herself vertically in front of the visually displayed sample object, head and upper body out of the water, ventral portions of the body facing towards the object, with the laterally placed eyes therefore viewing the object from the ventral aspect. On completion of the 5-s viewing time by the dolphin, the experimenter gave a verbal command to the assistants to immerse the two alternative objects in their boxes, together with their masks (which were then quickly withdrawn, as described earlier). In response, the dolphin lowered her head beneath the surface and oriented toward the box to her left, a strategy she adopted and used almost exclusively throughout the course of these early sessions. She then either gave a downstroke of her tail flukes and headed toward the left box to press the attached paddle or else, without any swimming movement to the left, spun around clockwise and headed toward the right box to press the paddle there. Decision time was defined as the time in seconds from the dolphin's lowering her head into the water (when echolocation could first begin effectively) until either the first downstroke of the fluke initiating movement toward the left box or the beginning of the clockwise rotation toward the right box. Relatively infrequently (n = 33 trials), the dolphin changed her pattern of response by beginning to swim toward the left box, apparently interrogating its contents further echoically, and then reversing direction to swim to the right box, either pressing the paddle there or swimming back again to the left box and pressing its paddle. On an additional 3 trials, she either hesitated en route to the left box and then continued toward it or looked left then right and then left again before finally going to the right box. These 36 occasions were called delayed-decision trials and were analyzed separately from the undelayed-decision trials. Altogether, the first 24 sessions consisted of 576 V-E trials. Half the trials were pairings of the test objects, and half were pairings of the baseline objects. Of these 576 trials, 64 trials (1 from Set 1; 12 from Set 2; 7 from Set 3; 14 from Set 4; and 30 from the baseline pairings) had no usable videotape record because of inadvertent failure to record or faulty recording. Of the remaining 512 trials, 476 were characterized as undelayed-decision trials and the remaining 36 as delayed, as noted earlier. Elele responded correctly on 472 (99.2%) of the undelayed trials, her four errors all occurring to the left anechoic box, three with Pair D versus E and one with Pair A versus E. She made no errors on the 36 delayed-decision trials. Figure 5 shows, for each unique pairing of test objects and for all baseline pairings combined, the mean undelayeddecision time, the range, and the number of trials analyzed for responses to the left anechoic box (white bars) and to the right anechoic box (gray bars). For Pair D versus E, all

299


Set 2 Object Pairings


C vs. H

A vs. F

B vs. G Baseline


C vs. A

D vs. E

B vs. F

Baseline


G vs. H

E vs. A

Baseline

Figure 5. Mean decision time (in seconds) to respond to the left anechoic box (white bars) and to the right anechoic box (gray bars) for each indicated pairing of objects during visual-echoic matching when the dolphin was stationed centrally between the two boxes. Objects are represented by letters. The number of trials on which the means are based are shown within the bars. Range bars (maximum and minimum decision times) are also shown. See text for additional details.

decisions to the right anechoic box that could be analyzed (n = 7) were characterized as delayed-decision trials. A two-way analysis of variance revealed that decision time across all sets was significantly shorter for baseline objects (M = 1.61) than for test objects (M = 1.88), F(l, 469) = 66.7, p < .001; and for decisions on the left (M - 1.58) than on the right (M = 1.92), F(l, 469) = 93.8, p < .001. It is likely that the latter difference largely reflects the "look-leftfirst" strategy of the dolphin and the additional time needed to spin about to the right. The interaction of object type and side was significant, F(l, 469) = 8.9,p < .01. Decision time averaged 0.42 s faster for baseline objects than for test objects when the response was to the right, F(l, 469) = 62.2, p < .001, but only 0.20 s faster when the response was to the left, F(l, 469) = 13.4,p < .001. These results suggest that, in general, somewhat more interrogation time was necessary to reach a decision about the less familiar, more complex, and more similar test objects than for the baseline objects. The finding that most of the delayed-decision trials occurred with test objects (30 of 36 trials) offers further support for this theory. Discrimination difficulty in V-E for particular test pairings can be judged to some extent by performance difficulty

(Figure 4) and by delayed decisions (averaging 2.63 s over the 30 relevant trials of test objects), which would seem to reflect some degree of uncertainty. By these criteria, relative V-E difficulty occurred for Pair D-E by the delayeddecision metric (Figure 5) but not by the performance metric (Figure 4). Pair F-G was more difficult than other pairings in Experiment 1 under both V-E (Figure 4) and E-V matching (Figure 3), but unfortunately that comparison was not available for estimating decision time during the first 24 V-E sessions. (Pairing A-B, which proved difficult under E-V only, was tested in the earlier experiment of Pack and Herman, 1995). There seem to be no obvious reasons for these few difficulties, and given the very few examples of pairings that appeared difficult, it would be speculative to attempt to connect any particular attribute, set of attributes, or similarities or differences in attributes to performance. This connection might be attempted, however, with a carefully designed study defining and contrasting specific attributes. Discussion The key to a strong inference of echoic shape perception in the cross-modal task lies in the ease with which recogni-

300


tion took place across the senses. Inasmuch as the dolphin might have learned over repeated cross-modal trials with a given pair of objects to relate some arbitrary echoic attribute to some arbitrary visual attribute, examination of performance on the dolphin's initial experience with objects is critical for assessment of the presence of a shape percept. In the Pack and Herman (1995) study, the dolphin subject was significantly above chance on the initial trials before associative learning could take place. This result allowed for an inference that an echoic shape percept was directly available to the echolocating dolphin. Likewise, in the present study, Elele achieved ceiling or near-ceiling levels of performance in the tests of immediacy of recognition with novel combinations of objects, both for E-V and V-E matching. The rapidity with which she made decisions about the location of the matching alternative during V-E tests, taken together with the positive results for tests of direct (first-trial) recognition for both V-E and E-V, adds substantial support to the inference that a shape percept is directly available to the dolphin through echolocation. The results also indicate that shape information is readily shared across the dolphin's senses of echolocation and vision. Importantly, the dolphin's excellent cross-modal performance with these novel pairings was not predicated on her first experiencing the pairings in within-modal tests. Thus, although it is important that objects that are to be discriminated cross-modally be discriminate within each modality (Bryant, 1968), the current results demonstrate that prior within-modal experience with novel pairings of objects is by no means a prerequisite for accurate cross-modal performance with those pairs.

Experiment 2 In Experiment 1, although the specific pairings tested cross-modally were not tested previously within modalities, the individual objects composing those parrs had each been members of other pairings that were tested both within and across modalities by Pack and Herman (1995). In Experiment 2, we examined for cross-modal transfer with eight new objects organized into four pairs. None of these objects had been encountered previously by the dolphin in E-E, V-V, E-V, or V-E matching or in any other task for that matter. If the dolphin proved capable of direct cross-modal recognition with at least some of these pairs, it would provide compelling evidence that within-modal matching experience with particular objects is not a necessary prerequisite or primer for accurate cross-modal matching. It would also allow for a strong inference that perception of shape is directly available to the echolocating dolphin.

Method Subject and Apparatus Elele continued to serve as the subject in this experiment. The anechoic boxes described in Experiment 1 were again used here.

Objects Test objects. Eight objects of abstract shape were newly constructed for this experiment and organized into four pairs

(Figure 6). We constructed all of them from schedule-40, 0.5-in. (1.27-cm), 1.0-in. (2.54-cm), or 1.5-in. (3.81-cm) gray PVC pipe and fittings. Objects within and across pairs were designed to be substantially different from each other in shape, according to our judgment. Variables unrelated to shape were made as similar as possible within each pair. We adjusted sizes of the members of each pair so as to fit within a 43-cm2 perimeter area, with parts of the object touching each side of the square. Also, we used the same PVC size or combination of PVC sizes in constructing each member of a pan* and equated pair members to within 4% for their three-dimensional frontal surface area (after Pack and Herman, 1995). Finally, as in Experiment 1, the pipes and fittings were filled with dry sand. Baseline objects. We continued to use the same six objects from the baseline set of Experiment 1, in all possible pairings.

Procedure All four test-object pairs were tested cross-modally before any within-modal testing took place. Thus, the first exposure of the dolphin to these pairs was during the cross-modal matching procedure. The procedure was identical to that described in Experiment 1. Throughout V-E matching the sample object was exposed to Elele at the remote station as was the case in Experiment 1 after Session 51 (see footnote 1 above). We tested within-modal matching after the cross-modal tests were completed to assess whether any difficulties encountered in matching particular pairs cross-modally could be attributed to difficulties in discriminating objects echoically, visually, or both. After the completion of these within-modal tests, we again tested the four pairs cross-modally to compare performance levels with the previous cross-modal tests before the within-modal experience. Within each procedure—the initial cross-modal tests, the subsequent within-modal tests, and the final cross-modal tests—the sequence of tests was counterbalanced across the type of test. For example, in the initial cross-modal procedure, Pair 1 was tested first in E-V, then Pair 2 in V-E, Pair 3 in V-E, and Pair 4 in E-V. The sequence was then completed by testing Pair 1 in V-E, Pair 2 in E-V, Pair 3 in E-V, and finally Pair 4 in V-E. Pairs were tested in the order shown to allow a substantial time period between testing in each condition, thereby avoiding any direct carryover effect from one condition to the next. Similar procedures were applied to the subsequent within-modal and final cross-modal tests. The E-E and V-V matching procedures were as described in Pack and Herman (1995). Briefly, for E-E matching, three anechoic boxes were suspended into the water from the tank wall. The central box was used for presentation of the sample object and was located in the same position as in E-V matching. The two other boxes were used to display the alternative objects and were located to each side of the central box in the same positions as in V-E matching. On each trial, a sample object and two masking objects were simultaneously placed in the central box and the masking objects were then immediately removed, as in Experiment 1. The sample object thus remained suspended underwater. The dolphin was then signaled to approach the box as in the E-V procedure. After the dolphin had touched the front panel in the location corresponding to that of the object inside, the alternative objects were immersed in the side boxes together with their two masking objects; the masking objects were then immediately removed, as in the procedure used with the central box (on each trial, the three objects initially immersed in each box were identical across the central and side boxes). As in V-E matching, the dolphin was then required to select through echoic interrogation either the object in the left or right side box, signifying her choice by pressing the paddle attached to that box. When she selected the box containing

301


Pair 2

Pair 4

60cm Figure 6. Computer scans of photographs of the four pairs of eight novel polyvinyl chloride objects used in cross-modal and within-modal matching tests in Experiment 2. Each object is shown from the frontal view (0°—upper figure) and also rotated 45° counterclockwise in the Z axis (lower figure). Pair numbers are shown between each pair. All objects are shown to the same scale.

the matching alternative, she was rewarded as in the cross-modal procedures. For V-V matching, all objects were exposed in the air to the dolphin's visual sense. Elele was signaled to approach an assistant at tankside displaying an object for Elele to view. After a 5-s viewing period, the supervisor signaled two other assistants, crouched behind the tank wall approximately 3 m to each side of the assistant exposing the sample object, to stand and present an alternative object. Elele's task was to select the alternative that was identical to the sample. Selection was as in E-V matching, with Elele required to station herself in front of the selected object for 3 s. As in cross-modal matching, an observer having no knowledge of the correct alternative judged the dolphin's responses, all assistants wore opaque goggles, and no assistant had knowledge of which alternative object was correct. Testing protocol For each paradigm, two 24-trial sessions were run with each pair. Each session consisted of 12 trials using the test pairs and 12 trials using the baseline object pairings, presented in a quasirandom, balanced sequence. In each session,

each member of a test pair served six times as sample and six times as the nonmatching alternative object. Also, the matching alternative appeared six times on the left side and six times on the right side in a random order, with the same constraints and controls as those used in Experiment I.

Results and Discussion Figure 7 shows overall performance for each pair on the initial cross-modal matching tests, the subsequent withinmodal tests, and the retests of cross-modal matching. Panel A, the initial cross-modal test, reveals substantial performance differences across the four pairs. Overall performance was significantly above chance for Pairs 2 and 4 (p < .02, cumulative binomial test) but not for Pairs 1 and 3. Performance with Pair 2 was especially noteworthy: 91.7% correct responses in E—V and 95.8% correct responses in V-E. For Pair 4, the corresponding values were 75.0% and 79.2%.

302


A: Initial cross-modal test

SE-V

DV-E

100-r

90-80 70

I

60

:;

w/

60 -

'm « i i %

1++ ^

1

^

ii

2

s 18

V/yv yy/y

1

50

^

'm%.

::%

Panel B of Figure 7 shows surprisingly good performance on within-modal matching for all four pairs. For example, although performance on cross-modal testing of Pair 1 was only 62.5% correct for each cross-modal procedure, withinmodal performance was errorless. Hence, cross-modal performance did not necessarily predict subsequent withinmodal performance. Panel C reveals the results of the retests of cross-modal performance. Performance levels were now significantly above chance for Pairs 1 and 3 as well as Pairs 2 and 4. The increases in performance level were significant for Pair 3 for both cross-modal tests, x2(l, 48) ^ 4.92, p < .05, and for Pair 1 in the V-E test, x2(l, 48) = 8.75, p < .01. Performance on Pairs 2 and 4 was now errorless over both cross-modal tests. Panels A, B, and C in Figure 7 also show performance on the 1st trial of the 24 total trials in which one member of a pair appeared as sample and the first trial in which the other member appeared as sample for each condition. Here, we were interested in the spontaneity with which recognition occurred in cross-modal or within-modal tests. The possible outcomes, shown within each bar, are that the dolphin chose correctly on both first trials (+ +), on only one trial (+0 or 0+), or on neither (0 0). It is clear from the figure that there were relatively few errors on these trials, and that errors decreased to zero over successive paradigms. To evaluate whether the dolphin demonstrated direct recognition, we summed the number of correct choices within each paradigm across all eight tests (four E-V tests and four V-E tests) for a total of 16 first trials. By the cumulative binomial test, 12 or more correct first responses were required for significance (p ^ .038). It is clear from Figure 7 that this criterion was reached for all three paradigms (12 of 16,14 of 16, and 16 of 16 correct first choices for initial cross-modal tests, within-modal tests, and final cross-modal tests, respectively). Of greater pertinence, perhaps, is whether Elele demonstrated direct cross-modal perception during the initial cross-modal tests for the two pairings that yielded significant performance levels overall (i.e., Pairs 2 and 4). Figure 7 shows that the dolphin chose correctly on 7 of 8 first trials for Pairs 2 and 4 combined (p = .035, cumulative binomial test). In contrast, for Pairs I and 3 combined, neither of which had yielded significant overall performance

~

3

Pair Number

::: :

;;:;:-

1 - i 4

Figure 7 (Left). The dolphin's performance accuracy with four novel pairs of objects (labeled Pairs 1-4 as shown in Figure 6). Panel A shows the percentage of correct matches during initial cross-modal echoic-visual (E-V) and visual-echoic (V-E) tests; Panel B shows the percentage of correct matches during subsequent within-modal E-E and V-V tests; and Panel C shows the percentage of correct matches during final cross-modal E-V and V-E tests. Each bar represents 24 trials. The dolphin's performance on her first exposure to the 2 unique trials in which each object of a particular pair appeared as sample in each condition is shown within each bar (plus = correct, zero = incorrect). The horizontal line shows the 70.8%-correct level for above-chance performance over 24 trials in a two-alternative test with chance probability per trial = .50 (cumulative binomial test).


during the initial cross-modal test, she was correct on only 5 of 8 first trials (p = .3633). The results of Experiment 2 support the findings of Experiment 1 as well as those of Pack and Herman (1995). Two pairs of novel objects yielded high levels of crossmodal performance on first trials as well as thereafter, but two others did not. These differences could not be attributed to differences in ability to match these objects within modalities. AH four pairs, regardless of the prior level of cross-modal performance, yielded high levels of performance during the subsequent within-modal tests, as well as during the final cross-modal retests. The most likely reason for this set of results would seem to be differences in the initial ease with which the shapes of individual objects could be mentally represented or discriminated from the shape of the alternative member of the pair. Recall that the shapes were complex and abstract and that variables other than shape were largely equated within pairs—including the perimeter area within which a shape was inscribed, the materials from which each shape was constructed, and the combined area of the reflective surfaces of each shape. Hence, encoding or representing shapes or shape differences could have been of varying difficulty within pairs. Nonetheless, Elele seemed to overcome any initial perceptual difficulties relatively quickly with added exposure of the objects to her over trials or paradigms, as evidenced by the high levels of performance she achieved or maintained during the subsequent within-modal tests and the final cross-modal retests.

General Discussion Our current results, taken together with those of Pack and Herman (1995), suggest that dolphins possess a fundamental ability for shape perception through their echolocation sense. Experiment 1 revealed that 24 of 25 new pairings (unique combinations) of eight individual objects that had been previously exposed to the dolphin in the Pack and Herman study were directly and easily matched crossmodally in the current study. Thus, the particular pairings contrasted were of little consequence. The rapidity with which echolocation response decisions were made under V-E (an average of 1.7 s for left responses and 2.2 s for right responses) and the overwhelming accuracy of those decisions, combined with the abstract, complex, and varied characteristics of the individual objects, argue against an echolocation strategy dependent on searching for, extracting, and encoding a particular feature or features differentiating particular pairings of objects. Instead, the behavioral data suggest that Elele perceived the objects holistically. In Experiment 2, which used four pairs of objects completely new to the dolphin's experience, direct crossmodal matching of two of the four pairs was demonstrated. These results demonstrate conclusively that within-modal experience is not a prerequisite for accurate cross-modal matching of novel objects. The current findings strongly suggest that the high levels of cross-modal performance reported by Pack and Herman (1995) were not the result of

303

any priming by prior within-modal experiences. The results also argue strongly that learning to associate particular characteristics of echoes returning from objects with the visual appearance of those objects was not the mechanism through which the dolphin in this study or the Pack and Herman study accomplished the cross-modal matching task (cf. Harley et al., 1996). Rather, the most compelling general explanation for the demonstrated cross-modal ability is that echolocation yields a direct percept of the object, in effect a representation of the object's shape that is analogous to or easily integrated with the percept developed through vision. In Experiment 2, we found that even the two pairs not yielding direct, successful, cross-modal matching could nonetheless be subsequently matched successfully within modalities as well as in subsequent cross-modal retests. This finding suggests that some shapes or shape differences are not as easily represented as others, but that dolphins can quickly overcome these difficulties with added exposure to the objects. Exactly which shapes or shape attributes lend themselves to ease of initial representation requires further detailed study, however. The findings underscore, nonetheless, that cross-modal recognition does not necessarily predict within-modal performance, although cross-modal recognition cannot proceed directly if within-modal recognition is lacking (Bryant, 1968; Pack,' 1994). Presently, the neural mechanisms and processes through which dolphins achieve shape perception using echolocation are speculative. The processes likely include both specialized peripheral processing as well as specialized central processing, hierarchically arranged and yielding a representation of the contours of the object (cf. Terhardt, 1987). Peripheral processing may include extraction of the spatial and temporal features of returning echoes to produce a spatial-temporal array that dolphins interpret through central mechanisms as an object shape in three dimensions. Research suggests that dolphins are capable of very fine range-difference resolution through echolocation (Au & Pawloski, 1989; Murchison, 1980) and very fine azimuthdifference (cross-range) resolution through passive listening (Renaud and Popper, 1975). Cross-range resolution through echolocation appears not to have been systematically tested thus far. The one study reported in the literature (Bel'kovich, Borisov, & Gurevich, 1970) appears to have had a temporal cue present, casting doubt on the validity of the very small threshold obtained (Au, 1993). In the echolocating bat (Eptesicus fuscus), the demonstrated capability for fine range-difference resolution has been theorized as a mechanism allowing for a representation of the spatial features of targets ensonified by the bat (Simmons, 1989; Simmons, Moss, & Ferragamo, 1990). According to these authors, echoes arising from the different reflecting points of a target create a range profile, which contributes to the development of a shape percept. The central mechanisms by which bats develop these spatial percepts are poorly understood at present, but researchers are studying them using recordings in the echolocating bat's auditory cortex as it responds to synthetic pulse-echo pairs (e.g., Dear, Simmons, & Fritz, 1993).

304


The central mechanisms supporting cross-modal perception are also speculative. Stein and Meredith (1990, 1993) have described multisensory neurons in the deeper layers of cats' superior colliculus (and possibly elsewhere) that support visual, auditory, and somatoscnsory input and may achieve coordination or integration among the different inputs arriving simultaneously or sequentially. These multisensory structures likely can be generalized to other mammalian species as well, although the senses involved would obviously depend on the particular species. Stein and Meredith (1993) also summarized alternative views for judging equivalence of percepts from different senses, including the following: (a) Some central process accesses representations from each sense held in modality-specific form; (b) the different representations for the different senses are translated into the reference scheme of one modality; and (c) the representations from the different senses are held in an amodal form. These authors note, however, that these alternatives need not be mutually exclusive, and all may coexist within different functional capabilities of individual neurons or in different areas of the brain and may even act in concert. The current behavioral findings cannot distinguish among these alternatives, but they do demonstrate that whatever the process is for dolphins, it is rapid, not dependent on specific learning, available symmetrically in either sensory direction, and easily accessed. The latter point cannot be overemphasized: The cross-modal matching task proved very easy for the dolphin. The surprising finding from the current study and from Pack and Herman (1995) is not so much that the dolphin is capable of cross-modal matching, but that the spatial structure of objects—their physical appearance—is so readily available through the echolocation system and so readily shared with the visual system. What form mental representations of object shape may take in the brain has long been a matter of debate (see, e.g., Kosslyn, 1995, for a recent review). Whatever the form, prepositional or depictive or some combination, the benefits of intersensory recognition for dolphins in their real world are apparent: in the least, enhanced perceptual monitoring of its real world, recognition of an object encountered with one sense, and, quite likely, when both senses are used in concert, a richer representation of the structure of the object and its attributes. In summary, the results reported here as well as those of Pack and Herman (1995) begin to construct a compelling case for a concept of echo imaging by the dolphin. The dolphin can apparently use returning echoes to construct a representation of an object that preserves the spatial structure of that object, to the extent that representation can be used to directly recognize that same object through the visual sense alone. The very short decision times used by the dolphin suggest that the object percept develops almost immediately and is of an integral whole, rather than limited to the examination and extraction of selected features (cf. Shepp, Barrett, & Kolbert, 1987). Furthermore, under the conditions tested, this cross-modal matching abiUty is clearly symmetric in that the dolphin is equally skilled at

matching from echolocation to vision as from vision to echolocation. This finding is unlike the results of several other cross-modal matching studies with human adults (e.g., Abravanel, 1973), human infants (e.g., Bushnell & Weinberger, 1987; Rose & Orlian, 1991), and rhesus monkeys (e.g., DiMattia, Posley, & Fuster, 1990) that used the senses of vision and touch (haptic sense). These studies show asymmetries in performance between haptic-visual and visual-haptic matching in one direction or the other. Which conditions, if any, may disrupt cross-modal symmetry in dolphins await further study.

References Abravanel, E. (1973). Retention of shape information under haptic or visual acquisition. Perceptual and Motor Skills, 36, 683-690. Au, W. W. L. (1993). The sonar of dolphins. New York: SpringerVerlag. Au, W. W. L., & Pawloski, J. L. (1989). Detection of noise with rippled spectra by the Atlantic bottlenosed dolphin. Journal of the Acoustical Society of America, 86, 591-596. Au, W. W. L., Schusterman, R. J., & Kersting, D. A. (1980). Sphere-cylinder discrimination via echolocation by Tursiops truncatus. In R. G. Busnel & J. F. Fish (Eds.), Animal sonar systems (pp. 859-862). New York: Plenum Press. Bel'kovich, V. M., Borisov, V. I., & Gurevich, V. S. (1970). Angular resolution by echolocation by Delphinus delphis. Proceedings of the 23rd Scientific-Technical Conference, Ministry of Higher and Secondary Specialized Education RSFSR, Leningrad, USSR, 66-67. Bryant, P. E. (1968). Comments on the design of developmental studies of cross-modal matching and cross-modal transfer. Cortex, 4, 127-137. Bushnell, E. W., & Weinberger, N. (1987). Infants' detection of visual-tactual discrepancies: Asymmetries that indicate a directive role of visual information. Journal of Experimental Psychology: Human Perception and Performance, 13, 601-608. Dear, S. P., Simmons, J. A., & Fritz, J. (1993). A possible neuronal basis for the representation of acoustic scenes in auditory cortex of the big brown bat. Nature, 364, 620-623. Dimattia, B. V., Posley, K. A., & Fuster, J. M. (1990). Cross-modal short-term memory of haptic and visual information. Neuropsychologia, 28, 17-33. Harley, H. E., Roitblat, H. L., & NachtigaU, P. E. (1996). Object representation in the bottlenose dolphin (Tursiops truncatus): Integration of visual and echoic information. Journal of Experimental Psychology: Animal Behavior Processes, 22, 164-174. Herman, L. M., & Pack, A. A. (1992). Echoic-visual cross-modal recognition by a dolphin. In J. A. Thomas, R. A. Kastelein, & A. Supin (Eds.), Marine mammal sensory systems (pp. 709—726). New York: Plenum Press. Johnson, C. S. (1967). Sound detection thresholds in marine mammals. In W. N. Tavolga (Ed.), Marine bio-acoustics Vol. 2. (pp. 247-260). New York: Pergamon Press. Kosslyn, S. M. (1995). Mental imagery. In S. M. Kosslyn & D. N. Osherson (Eds.), Visual cognition: An invitation to cognitive science (Vol. 2, 2nd ed., pp. 267-296). Cambridge, MA: MIX Press. Murchison, A. E. (1980). Detection range and range resolution of echolocating bottlenose porpoise (Tursiops truncatus). In R. G.

305

SEEING THROUGH SOUND Busnel & J. F. Fish (Eds.), Animal sonar systems (pp. 43-70). New York: Plenum Press. Nachtigall, P. E., Murchison, A. E., & Au, W. W. L. (1980). Cylinder and cube shape discrimination by an echolocating blindfolded bottlenosed dolphin. In R. G. Busnel & J. F. Fish (Eds.), Animal sonar systems (pp. 945-947). New York: Plenum Press. Pack, A. A. (1994). Cross-modal recognition of complexly-shaped objects by a bottlenosed dolphin (Tursiops tnmcatus) using vision and echolocation. Unpublished doctoral dissertation, University of Hawaii, Honolulu. Pack, A. A., & Herman, L. M. (1995). Sensory integration in the botdenosed dolphin: Immediate recognition of complex shapes across the senses of echolocation and vision. Journal of the Acoustical Society of America, 98, 722-733. Renaud, D. L., & Popper, A. N. (1975). Sound localization by the bottlenose porpoise, Tursiops tnmcatus. Journal of Experimental Biology, 63, 569-585. Rose, S. A., & Orlian, E. K. (1991). Asymmetries in infant cross-modal transfer. Child Development, 62, 706-718. Shepp, B. E., Barrett, S., & Kolbert, L. K. (1987). The development of selective attention. Holistic perception versus resource allocation. Journal of Experimental Child Psychology, 43, 159-180.

Simmons, J. A. (1989). A view of the world through the bat's ear: The formation of acoustic images in echolocation. Cognition, 33, 155-199. Simmons, J. A., Moss, C. F., & Ferragamo, M. (1990). Convergence of temporal and spectral information into acoustic images of complex sonar targets perceived by the echolocating bat, Eptesicus fuscus. Journal of Comparative Physiology A, 166, 449-470. Stein, B. E., & Meredith, M. A. (1990). Multisensory integration: Neural and behavioral solutions for dealing with stimuli from different sensory modalities. Annals of the New York Academy of Sciences, 608, 51-70. Stein, B. E., & Meredith, M. A. (1993). The merging of the senses. Cambridge, MA: MIT Press. Terhardt, E. (1987). Gestalt principles and music perception. In W. A. Yost & C. S. Watson (Eds.), Auditory processing of complex sounds (pp. 157-166). Hillsdale, NJ: Erlbaum.

Received October 2,1997 Revision received February 3,1998 Accepted February 5,1998

New Editors Appointed, 2000-2005 The Publications and Communications Board of the American Psychological Association announces the appointment of three new editors for 6-year terms beginning in 2000. As of January 1,1999, manuscripts should be directed as follows: •

For Experimental and Clinical Psychopharmacology, submit manuscripts to Warren K. Bickel, PhD, Department of Psychiatry, University of Vermont, 38 Fletcher Place, Burlington, VT 05401-1419.

•

For the Journal of Counseling Psychology, submit manuscripts to Jo-Ida C. Hansen, PhD, Department of Psychology, University of Minnesota, 75 East River Road, Minneapolis, MN 55455-0344.

•

For the Journal of Experimental Psychology: Human Perception and Performance, submit manuscripts to David A. Rosenbaum, PhD, Department of Psychology, Pennsylvania State University, 642 Moore Building, University Park, PA 16802-3104.

Manuscript submission patterns make the precise date of completion of the 1999 volumes uncertain. Current editors, Charles R. Schuster, PhD; Clara E. Hill, PhD; and Thomas H. Carr, PhD, respectively, will receive and consider manuscripts through December 31, 1998. Should 1999 volumes be completed before that date, manuscripts will be redirected to the new editors for consideration in 2000 volumes.