Response characteristics of single units in the inferior colliculus of mustache bats to sinusoidally frequency modulated signals

Summary

• 1. The discharge properties of single units in the inferior colliculus of the mustache bat,Pteronotus parnellii, were evaluated with tone bursts and sinusoidal frequency modulated (SFM) stimuli. The SFM signals were designed to mimic the modulation patterns imposed upon the echoes reflected from the beating wings of flying insects.

• 2. Two groups of neurons were distinguished on the basis of their best frequencies (BFs). The first group was called the filter neurons and had BFs between 60–64 kHz. These neurons are involved in processing the constant frequency component of the bat's echolocation calls. The second group, or non-filter, neurons, had BFs below 60 kHz, or above 64 kHz.

• 3. Ninety percent of the filter neurons and 80% of the non-filter neurons responded to SFM signals with discharges that were phase locked to the modulation waveform. However, the two groups of neurons differed markedly in their sensitivity to very low modulation depths. The majority of filter neurons displayed locked discharges to SFM depths as low as ±50 Hz, and some even locked to depths of ±10 Hz, while only two non-filter units (out of 71 tested) locked to depths of ±50 Hz, and none locked to ±10 Hz.

• 4. Most filter units displayed a marked selectivity, or tuning, for particular modulation depths, rates, and signal intensities. In these tuned or selective units, locking could be evoked only over a fairly narrow range of depths, rates, and/or intensities. When SFM parameters outside of the neuron's tuned range were presented, only phasic-on or on-off discharges could be evoked.

• 5. The influence of the SFM carrier frequency was also evaluated in filter units. In some units, the particular carrier frequency used was not critical for evoking locked discharges so long as the signal encroached upon the unit's tuning curve. In the majority of filter units, however, there was a marked asymmetry in that some carrier frequencies were much more effective in evoking locked discharges than were other frequencies. In some asymmetric units, frequencies below the BF were most effective and in other units carrier frequencies above the BF were most effective.

• 6. We examined the inhibitory surround patterns in 15 filter units under the assumption that a predominant inhibitory flank on one or another side of the BF might account for the asymmetry in the effective SFM carrier frequencies. Detailed analyses in eight filter neurons yielded no relationship between the position of the inhibitory side bands and the asymmetry of SFM frequencies evoking locked discharges.