Long-Latency "Subthreshold" Collicular Responses to the Constant-Frequency Components Emitted by a Bat

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Abstract
A previously undescribed response pattern has been observed in certain single units in the posterior colliculus of Pteronotus suapurensis . These units, constituting about one-third of those tuned to the region of the dominant constant-frequency (CF) components of the orientation sounds, respond to a tone pip with a burst of spikes at a latency of 3 to 6 milliseconds, within the frequency-intensity domain of a normal V-shaped response area. In these units, however, as intensity is dropped below threshold for this response, a response of 5- to 10-milliseconds longer latency appears and persists throughout another 10 to 30 decibels of attenuation. These late responses can be very vigorous, are sharply tuned to frequencies at or just above the CF components of the signal, and are often strongest and of lowest threshold at stimulus durations of 1.5 to 3 milliseconds—approximately the duration of the CF component. These properties imply that the late responses are concerned with analysis of the CF components of echoes, apparently in ways not as prominent in other bats.