Integration and recovery processes contribute to the temporal selectivity of neurons in the midbrain of the northern leopard frog, Rana pipiens.

This study examined the mechanisms underlying amplitude modulation selectivity in the anuran auditory midbrain. Single units were recorded extracellularly in the torus semicircularis of the northern leopard frog, Rana pipiens. Two physiologically distinct classes of neurons were identified, based on their response latencies and their selectivities to pulse repetition rates. Cells in one group had short response latencies (median = 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.

Long-term temporal integration in the anuran auditory system.

Analysis of the temporal structure of acoustic signals is important for the communication and survival of a variety of animals including humans. Recognition and discrimination of particular temporal patterns in sounds may involve integration of auditory information presented over hundreds of milliseconds or seconds. Here we show neural evidence for long-term integration in the anuran auditory system. The responses of one class of auditory neurons in the torus semicircularis (auditory midbrain) of frogs reflect the integration of information, gathered over approximately 45-150 ms, from a series of stimulus pulses, not stimulus energy. This integration process is fundamental to the selective responses of these neurons for particular call types.