Specialized characteristics of single units in inferior colliculus of mustache bat: frequency representation, tuning, and discharge patterns.

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Abstract
The mustache bat [P. parnelli parnelli] emits echolocation calls characterized by a long 60-kHz constant-frequency (CF) component. The activity of neurons from the inferior colliculus of the mustache bat was monitored with particular attention directed toward discharge characteristics that have implications for the processing of the 60-kHz component of the biosonar cries. The best frequencies (BF) were obtained from 438 collicular neurons recorded from 7 bats. Each bat devoted .apprx. 50% of its neurons to a band of frequencies only 350-450 Hz wide, called the bat''s overrepresented frequency band. Each of the bats had its own overrepresented band that ranged from .apprx. 61-63 kHz, depending on the individual. Discharge patterns of neurons in the overrepresented frequency band were recorded. Of the overrepresented neurons 25% had tonic firing patterns, whereas 75% had phasic patterns. The phasic units tuned to the overrepresented frequency band are particularly noteworthy because their patterns were unique and highly stereotyped. When tone bursts having frequencies at the unit''s BF were presented, these units responded with a phasic on-discharge pattern regardless of the intensity. When the stimulus was 20 dB or more above the threshold of the unit''s BF and the frequency was 150-200 below the unit''s BF, a phasic on-off discharge pattern was invariably elicited. The mustache bat''s cochlear microphonic (CM) audiogram is sharply tuned to a frequency around 60 kHz, which is close to the bat''s overrepresented frequency band. To determine how close the correspondence between the tuned and overrepresented frequencies actually is, CM potentials and the BF of single units from 2 bats were recorded. In each bat the frequency to which the CM potential was most sensitive and the frequency to which the largest number of neurons were tuned were in exact agreement. The large number of neurons whose BF were in the overrepresented frequency band were distinguished from other neurons by their very narrow tuning curves. The Q values of the overrepresented neurons averaged 122 compared to a much lower average Q value of 18 for neurons tuned to all other frequencies in the bat''s hearing range. When the Q values and CM threshold obtained from an individual bat were plotted as a function of frequency, the Q values followed the CM sensitivity function very closely. Such a close agreement between the tuning of the cochlear partition and the tuning of collicular neurons provides strong support for the hypothesis that many of the specialized features of the overrepresented neurons are established at the level of the cochlea.