The Dolphin Institute - Dolphin Research Summaries

(C) 1976 University of Hawaii

The first study, on delayed auditory sequence discriminations, tested the ability of the bottlenose dolphin (Tursiops truncatus) to remember sounds for which no explicit unambiguous retrieval cues were provided. The animal was trained to press a left-hand paddle after hearing the two-sound sequences AC or BD and to press a right-hand paddle after hearing the sequence AD or BC. Increasing the delay interval between the two sounds in a sequence beyond 2 to 3 sec resulted in an abrupt loss of discrimination characterized by stereotyped responding to one paddle after hearing Sound C, and to the other paddle after hearing Sound D, regardless of the initial sound, A or B. Testing with a variety procedures established that the loss of control was not an artifact of any one method of incrementing delays, nor was it due to the forgetting of the initial sounds over the delay interval. It was concluded that the dolphin had encoded the four sequences as four unique compound sounds, or configures, two were associated with the left-hand paddle and two with the right-hand paddle rather than as a conjunction of two individual sounds conditionally related to each of two possible spatial responses. Increasing the delay interval beyond approximately 2 to 2 sec. Apparently “destroyed” the percept of the configure, yielding two discrete sounds, neither of which was uniquely associated with either spatial response.

In the second study the effects of nonauditory spatial retrieval cues on the ability to remember nonspatial auditory signals was measured in both indirect “conditional” and direct forms of delayed auditory successive discrimination task. In the indirect condition one pair of paddles was located to the left of the centrally located underwater speaker, and another pair to the right. At each trial either sound A or B was played briefly from the center speaker. After prescribed delay interval, which was progressively increased over sessions, the dolphin was cued to go to either the left-hand or right-hand paddle-pair by a neutral sound played form a speaker located between each paddle pair. The dolphin was rewarded for pressing the outer paddle of a pair following sound A and the inner paddle of a pair after hearing sound B. The four paddle configuration guarded against the development of overt mediational responses during the delay interval since the animal could not predict on which paddle-pair to respond. In the direct delayed condition only one paddle-pair was used during a session, the other being withdrawn from the water. The animal could therefore “bridge” the delay interval by overtly orienting to either paddle following presentation of the discriminative stimulus. The dolphin failed to meet a predetermined performance criterion at delays greater than 26 sec. on the indirect task and at delays greater than 60 sec on the direct task. The performance differences across tasks seem attributable the development of overt mediational responses during the longer delays in the direct condition. In both tasks the decreasing performance level with increasing delays was interpreted as due to increasing difficulty in remembering the relative recency of the two highly familiar sounds. The delayed discrimination results, though evidencing some performance decrements relative to the high levels achieved in earlier reported delayed matching studies in which pre-delay and post-delay stimuli were always in the same modality, nevertheless demonstrated an ability to use post-delay retrieval cues occurring in a different modality than the pre-delay stimuli.

Thompson, R. K. R. (1976). Performance of the bottlenose dolphin (Tursiops truncatus) on delayed auditory sequences and delayed auditory successive discriminations. Unpublished doctoral dissertation, University of Hawaii, Honolulu.

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