Single unit responses to linear frequency-modulations in the inferior colliculus of the Greater horseshoe bat,Rhinolophus ferumequinum

Abstract

1. Recordings were made from 135 single inferior colliculus neurons of the Greater horseshoe bat,Rhinolophus ferrumequinum. These bats emit sonar signals which are characterized by a long constant frequency (CF-) component and a terminal frequency modulated (FM-) component. The responses of the units to CF- and FM-stimuli with different modulation heights, durations and directions were studied. 2. The majority of single units studied had minimum thresholds that were either equal for CF- and FM-signals of the same duration or lower for the CF-stimulus. Only rarely was the sensitivity for FM-signals higher than for a CF-signal. 3. With FM-signals the response properties of units with best frequencies between 65–81 kHz, i.e. in the frequency range of the FM-component of the echolocation call, did not differ significantly from units with best frequencies in lower frequency ranges. Relatively many units with best frequencies in the filter region of the audiogram (81–88 kHz) required FM-signals with particular modulation heights and rates to elicit excitatory responses. 4. Response patterns and spike count functions were found to differ with CF- vs. FM-signals. The most common difference was a smaller number of impulses per stimulus and a temporally more restricted (‘phasic’) discharge pattern to FM-signals than to CF-signals. Greater discharge activity to FM-signals occurred in only a few units. ‘Latency constant’ and ‘FM-specialized’ neurons, as reported for other species were also found. 5. Inhibition was frequently observed with FM-stimuli in spontaneously active neurons. An accurate prediction of a neuron's response to FM-signals from knowledge of the excitatory and inhibitory response areas to CF-signals was not always possible. 6. The responses of ‘FM-specialized’ neurons to the FM-component of a CF-FM-stimulus was not significantly altered by the presence of a CF-component in the filter region of the audiogram. 7. Responses of single units to signal and noise combinations are described. 8. Data are discussed in relation to results on the processing of FM-signals in other bats.