Auditory physiological properties of the neurones in the inferior colliculus of the big brown bat,Eptesicus fuscus

Summary

• 1. Electrophysiological properties of single units in the inferior colliculi of 9 big brown bats,Eptesicus fuscus, were studied by recording the responses of single units to pure tone pulses and frequency-modulated signals. Evoked potentials were also measured from 5 locations.

• 2. Single units are tonotopically organized with low frequency units located dorsally, high frequency units ventrally and intermediate frequency units interposed between them.

• 3. Response patterns of 230 units could be classified into six types but most units fired either tonically or phasically.

• 4. The peak latency of the evoked potential is betwen 3.5 and 4.0 ms. Those of the 188 on-responding units are mainly between 5.0 and 12.5 ms.

• 5. Threshold curves of the evoked potentials are broadly tuned to frequencies from 10 to 90 kHz. Those of single units are either broad, narrow or closed (upper threshold). The Q_10−dB values range between 1.3 (BF = 31.4 kHz) and 39.5 (BF = 79 kHz) but the majority are below 24. Threshold curves of 4 on-off responders and 3 inhibitory units were also measured.

• 6. The impulse-count functions of 109 units were either monotonic or non-monotonic. Among the 89 non-monotonic units, 17 had upper thresholds so that they stopped discharging action potentials when the stimulus became very intense (Fig. 10C). A series of the impulse-count function of 29 units to pure tone pulses and different FM stimuli were also measured.

• 7. Minimum thresholds of 112 units to pure tone pulses and FM stimuli with different rates were compared. Most units had equal thresholds or within ±5 dB difference in thresholds to both pure tone pulses and FM stimuli. However, 15 units exclusively responded only to pure tone pulses.

• 8. Responses of 35 units to different stimulus duration were studied. The number of impulses of single units either monotonically or non-monotonically increased with stimulus duration or maintained a rather constant level regardless of stimulus duration. Response patterns of some units changed drastically with stimulus duration.

• 9. Responses of 33 units to different stimulus repetition rate were studied. The number of impulses of 26 units reached to the maximum value at a specific stimulus repetition rate. Those of the remaining 7 units either reached to the maximum over a broad range or maintained a rather constant level regardless of the repetition rate of a stimulus.