J Comp Physiol A �2000) 186: 923±937 DOI 10.1007/s003590000144 ORIGINAL PAPER

Integration and recovery processes contribute to the temporal selectivity of neurons in the midbrain of the northern leopard frog, Rana pipiens Abstract This study examined the mechanisms underlying amplitude modulation selectivity in the anuran auditory midbrain. Single units were recorded extracel-lularly in the torus semicircularis of the northern leopard frog, Rana pipiens. Two physiologically distinct classes of neurons were identi®ed, based on their response latencies and their selectivities to pulse repetition rates. Cells in one group had short response latencies (median ˆ 31 ms) and responded best to pulse repetition rates below 40 Hz. Tuning to low amplitude modulation rates was largely determined by recovery processes and phasic response properties. Cells in the second group had much longer latencies (median=81 ms) and were generally selective for pulse repetition rates greater than 40±50 Hz. Sensitivity to higher amplitude modulation rates resulted from integration processes; these units only responded when a threshold number of pulses were presented at a minimum pulse density (amplitude modulation rate). At amplitude modulation rates above their best rate, their responses decreased, apparently due to inadequate recovery time between pulses. Abbreviations AM amplitude modulation á BEF best excitatory frequency á pps pulses per second á PRR pulse repetition rate á PSP postsynaptic potential á SAM sinusoidal amplitude modulation á VDC variable duty cycle