DETECTION OF GAPS IN SINUSOIDS BY FROG AUDITORY-NERVE FIBERS - IMPORTANCE IN AM CODING

Physiological studies were carried out in the frog (Rana pipiens pipie ns) eighth nerve to determine: (i) whether the modulation rate or the silent gap was the salient feature that set the upper limit of time-lo cking to pulsed amplitude-modulated (PAM) stimuli, (ii) the gap detect ion capacity of individual eighth nerve fibers. Time-locked responses of 79 eighth nerve fibers to PAM stimuli (at the fiber's characteristi c frequency) showed that the synchronization coefficient was a low-pas s function of the modulation rate. In response to PAM stimuli having d ifferent pulse durations, a fiber gave rise to nonoverlapping modulati on transfer functions. The upper cut-off frequency of time locking was higher when tone-pulses in PAM stimuli had shorter duration. The fact that the cut-off frequency was different for the different PAM series suggested that the AM rate was neither the sole, nor the main, determ inant for the decay in time-locking at high AM rates. Gap detection ca pacity was determined for 69 eighth nerve fibers by assessing fiber's spiking activities to paired tone-pulses during an OFF-window and an O N-window. It was found that the minimum detectable gap of eighth nerve fibers ranged from 0.5 to 10 ms with an average of 1.23-2.16 ms depen ding on the duration of paired tone pulses. For each fiber, the minimu m detectable gap was longer when the duration of tone pulses comprisin g the twin-pulse stimuli was more than four times longer. When the syn chronization coefficient was plotted against the silent gap between to nes pulses in the PAR?I stimuli, the gap response functions of a fiber as derived from multiple PAM series were equivalent to gap response f unctions deriving from twin-pulse series suggesting that it was the si lent gap which primarily determined the upper limit of time-locking to PAM stimuli.