ASA 130th Meeting - St. Louis, MO - 1995 Nov 27 .. Dec 01

4aPP7. Discharge patterns and amplitude modulation encoding in the torus semicircularis of the frog.

Nikolay G. Bibikov

Acoust. Inst., Schvernik st., 4, Moscow 117036, Russia

Responses to pure and sinusoidally 20-Hz amplitude-modulated tone bursts (612,5-ms stimulus on time at the rate of once per 2.2 s) were recorded in 202 units of the torus semicircularis (midbrain auditory center) in the immobilized grass frogs (Rana temporaria). Best frequency tones at 30 dB re: minimum threshold were used. The largest values of synchronization coefficients (SCs) to 80% amplitude-modulated stimuli were observed for same phasic units. The proportion of the responses with a good phase-locking ( SC > 0.5) was, however, higher in the tonic unit's population. Only in a few phasic units was the sustained response to 10% modulated stimuli observed. These particular units demonstrated, however, extremely high SCs. On the other hand, the phase-locking response to tones at 10% modulation depth was observed in 65% of the tonic units. In the majority of these neurons the partial SCs for successive modulation periods increased considerably from the initial to the terminal modulation periods. The psychophysical correlates of this physiological ``overshoot'' effect are discussed. [Work supported by ISF and RFFI.]