( Courchesne, 1978) and monkeys ( Arthur & Starr, 1984). Habituation may have similarly reduced P550 amplitudes in the dolphin. Second, in P300 experiments requiring auditory discriminations, humans with perfect pitch have small P300s ( Klein, Coles, & Donchin, 1984). The small P550s in the dolphin may similarly reflect a highly evolved capacity for pitch discrimination.
The latency of the P550 component also deserves comment. In humans, P300 latencies are related to the time required to recognize ( McCarthy & Donchin, 1981), and respond to stimuli ( Woods, Courchesne, Hillyard, & Galambos, 1980). Insofar as the latency of the P300-equivalent is indicative of the time required for processing auditory stimuli in different species, the dolphin would at first glance appear to process at least some auditory stimuli more slowly than humans, cats ( Wilder et al., 1981), or monkeys ( Arthur & Starr, 1984). However, several other factors may also have contributed to the prolonged latencies of the P550 component which we observed. First, reflections and resonances in the small restraining tank may have made acoustic discriminations more difficult. In humans P300 latencies in difficult tasks may exceed 600 msec ( Pritchard, 1981). Second, human P300 latencies are longer than those of equivalent potentials in animals with smaller brains. Thus, there may be a relationship between the time of tranmission between certain neuroanatomical structures and the latency of the P300-like component. The prolonged latency of the P550 in the bottlenosed dolphin is consistent with the large brain size (1500 grams) of this species, and suggests that the P300 pathways may have increased in length in comparison with terrestrial mammals. Third, different species may vary in the extent to which stimuli are used for "context updating" ( Donchin, 1981). A deeper and more thorough updating may be associated with longer latency P300s. If so, the long latency of the P300 equivalent in dolphin would be consistent with a thorough analysis of acoustic inputs. Fourth, P300 latencies in man increase (by up to 120 msec) with aging ( Goodin, Squires, Henderson, & Starr, 1978). If a similar relationship holds in dolphins the relatively advanced age of our animal (22 years) may have contributed to the prolonged latencies of the P300-like potentials that we observed.
We recorded event-related potentials (ERPS) from the skull surface of a bottlenose dolphin (Tursiops truncatus) in response to a variety of auditory stimuli including pure and complex tones, FM sweeps, clicks, and dolphin calls. The effects of stimulus repetition and probability on ERPs were examined in three experiments. In each experiment, infrequent, deviant sounds (such as dolphin calls mixed with trains of tones) or task-relevant stimuli (associated with rein-