Temporal coding of low-frequency amplitude modulation in the torus semicircularis of the grass frog

Single neuron responses to sinusoidal 20 Hz amplitude modulated tone bursts (612.5 ms stimulus-on time at the rate of once per 2.2 s) were studied in the auditory midbrain (torus semicircularis) of the immobilized grass frog (Rana temporaria temporaria). The characteristic frequency stimuli at 30 dB above the minimum threshold included 12 full modulation periods with fixed initial phase. Neurons generally showed good phase-locking to the envelope waveform. 160 of the 186 investigated neurons responded to 80% amplitude modulated stimuli with discharges synchronized to the modulation cycle. For this modulation depth the best phase-locking capability was observed for certain phasic and build-up units. The synchronous response to 10% modulated stimuli was observed in 104 units. Though a few (2 of 29) phasic units were capable of reproducing this modulation with very high fidelity, the general tendency was the increasing of phase-locking capacity for units with a substantial sustained activity. In this condition for 66 units (63% of the units displaying the synchronous response) we observed a significant improvement of phase-locking from the initial to the terminal periods of modulation. This effect could be interpreted as an initial stage of the enhancement of small amplitude changes in the course of the long-term adaptation.