Coding of small sinusoidal frequency and amplitude modulations in the inferior colliculus of 'CF-FM' bat, Rhinolophus ferrumequinum.

Single neurons in the inferior colliculus of the Greater Horseshoe bat, Rhinolophus ferrumequinum, showed two broad categories of response patterns to sinusoidally frequency (SFM) or amplitude (SAM) modulated stimuli. Tonic responding cells (best excitatory frequency (BEF) between 10 and 90 kHz) showed a rough sinusoidal modulation of the discharge pattern to SFM. Transient responding neurons generally showing on- or off-responses to pure tones, (BEF between 65 and 88 kHz), displayed highly synchronized discharge patterns to SFM-cycles (Fig. 1). Modulation rates between 20 and 100 Hz were most effective and some neurons encoded modulation rates up to 350 Hz (Figs. 2 and 3). The SFM responses were best synchronized to the modulation envelope for center frequencies in the upper portion of the tuning curve (Figs. 4 and 5). Sharply tuned neurons with BEF around 80 kHz had the lowest threshold for modulation depth (+/- 10 Hz or 0.025%) (Fig. 6). In general, SAMs evoked the same type of response patterns and were encoded down to modulation index of 3% (Fig. 7). The fine frequency and amplitude discriminations for periodical modulations by collicular neurons is discussed as related to the detection and discrimination performance of bats, when preying on flying insects in clustered surroundings.