Response patterns to pure tones of cochlear nucleus units in the CF-FM bat,Rhinolophus ferrumequinum

Summary

• 1. In horseshoe bats temporal response patterns to pure tone stimuli (10–100 kHz, 20 ms duration, 0.5 ms rise/fall-time) of 149 cochlear nucleus units (DCN and PVCN) have been recorded.

• 2. Distribution of the units' Best Frequencies (BF): low frequency neurons 26% (BF 10–65 kHz); FM-frequency neurons 20% (BF 65–81 kHz, i.e. frequencies occurring in the FM-part of the bat's echo signal); filter frequency neurons 52% (BF 81–88 kHz, i.e. frequencies occurring in the CF-part of the bat's echo signal); high frequency neurons 2% (BF > 88 kHz) (Table 1).

• 3. According to PST-histograms the neurons were classified as: sustained responders (28%, Fig. 1D, E); transient responders (51%, Fig. 1A–C); negative responders (4%, Fig. 1F) and complex responders (17%, Fig. 2–4). In the latter class response patterns drastically change with stimulus frequency and intensity. These units have suppressory sidebands on one or both sides of the excitatory field, sometimes overlapping and enclosing the excitatory area (Fig. 2 and 4). Frequently excitatory response patterns display simultaneous inhibitory processes the latency and duration of which depend on stimulus parameters.

• 4. In a few complex responders two or more excitatory areas exist, the BF of which may be harmonically related (Fig. 3 and 4).

• 5. Tuning curves of four auditory nerve fibers are reported showing two separate excitatory areas: a broad less sensitive one from 37 to 79 kHz (low frequency tail) and a narrow, more sensitive one from 82 to 90 kHz (Fig. 5).