forcement) were presented at low probabilities in an effort to elicit endogenous potentials similar to those which occur in humans following the presentation of infrequent or surprising stimuli.
Three classes of responses were recorded. Middle-latency components (P25- N60) showed short refractory periods and were maximal in amplitude to brief click stimuli similar to echolocation pulses. Long-latency components (N200- 450) showed comparable amplitudes for click and tone stimuli. When stimuli were repeated, the N200-P450 was markedly reduced in amplitude at all intervals tested (up to 6.0 sec). This refractory process was specific to the stimulus because conditioning tones of one frequency did not reduce N200-P450 amplitudes to probe tones of another frequency. The dolphin N200-P450 showed more marked and specific refractory effects than the human N100-P200 recorded in a comparable paradigm. The differences may reflect a more precise representation of auditory stimuli in dolphin short-term acoustic memory.
Deviant stimuli produced an enhanced long-latency positive component (P550) in the dolphin, similar in some respects to the "decision-related" P300 wave in humans.
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