Directional coding by binaural brainstem units of the CF-FM bat,Rhinolophus ferrumequinum

Summary

• 1. Monaural and binaural neurophysiological data were obtained from 264 units in the medullary and mesencephalic levels of the central auditory system in the CF-FM bat,Rhinolophus ferrumequinum, using earphones for acoustically separated stimulation of the two ears.

• 2. The units responded to pure tones delivered at their best frequencies with typical basic patterns (PST-histograms) which were classified according to the literature.

• 3. The latencies of the units' responses changed systematically with the recording depth from about 8 to 1.5ms when passing from the inferior colliculus at the brain's surface through the nucleus of the lateral lemniscus down to the superior olivary complex.

• 4. Best frequencies covered almost the whole hearing range from 2kHz up to 96kHz, but 50% of all units lay between 75 and 85kHz, the animal's best hearing range, the emission frequency of orientation sounds being between 82 and 84kHz (CF-part).

• 5. All lowest thresholds of single units put together demonstrate an almost perfect fitting with the behavioural hearing curve.

• 6. Monaural excitatory impulse count functions were mostly monotonic but many were very non-monotonic and intermediate.

• 7. The binaural classification (Table 1) confirmed the classical concept of the pathway and the theory of binaural interaction. The most frequently observed types were E/I (ipsilateral inhibitory, contralateral excitatory; 53 %) and E/E (both sides excitatory; 22%).

• 8. In all cases tested so far, best frequencies and tuning curves were the same for both sides regardless of the kind of binaural interaction.

• 9. The most interesting units found (13% of the total) showed together with binaural inhibition, strong facilitation in certain ranges of their dynamic range thus reinforcing the response to particular stimulus combinations. These units may be called directional specialists, useful for angular sensitivity.

• 10. The excitatory impulse count functions were usually steep covering a best dynamic range of 10–30dB, whereas inhibitory curves were much flatter but covered a wide dynamic range of 30–100dB.

• 11. Since therefore the excitatory influence is always predominant in any unit tested, coding of the sound direction is ambiguous when the absolute intensity is altered. Hence the response is not only a function of the interaural intensity difference as postulated by the theory, but also dependent upon absolute intensity levels. Neural mechanisms to compensate such ambiguities in presumably higher centers are postulated and discussed.