Responses of inferior collicular neurones to acoustic stimuli in certain FM and CF-FM paleotropical bats

Abstract

1. Electrophysiological properties of auditory units of six species of paleotropical echolocating bats, including two FM bats and four CF-FM bats were studied by recording the responses to tone bursts of single units, multiple units and evoked potentials from their inferior colliculi. The orientation pulses of these bats are also described. 2. The peak latency of single collicular units of these six species ranged from 3.5 to 49.5 ms, but their mean latency was between 6.4 and 15.3 ms. 3. Most of the units discharged either tonically or phasically. Six units recorded from the CF-FM bats showed complex response patterns which sequentially exhibited a phasic, tonic and phasic discharge pattern as the stimulus intensity increased. 4. Whereas collicular units of FM bats were sensitive throughout the frequency range of their emitted signals, collicular units of the CF-FM bats were much more sharply tuned to a narrow band of frequencies near that of the CF component of orientation sounds. 5. The minimum thresholds of single units ranged between 0 and 78 dB SPL. The Q10-dB values were between 1.3 and 179. Sharply tuned threshold curves with very high Q10-dB values were only obtained from collicular units of CF-FM bats. 6. In the CF-FM bats, an off response appeared upon cessation of stimulus when the stimulus frequency was at that of the predominant CF component in the orientation pulse. 7. The characteristics of the auditory sensitivity and the frequency composition of the orientation signals of each species of bat are discussed.