Cortical responses in rats to periodic frequency-modulated sounds

Abstract

Spectral variations over time are a common characteristic of naturally occurring sounds. They constitute a prevalent feature in communication signals in several species. Studies of mammals have shown that auditory cortex neurons respond to single frequency-modulated (FM) sweeps and that most responses are selective for sweep direction and/or rate. Researchers have also used trains of FM sweeps to estimate spectrotemporal receptive fields in auditory cortex. In the present study, microelectrode recordings were used to explore how the auditory cortex responds to trains of FM sweeps in anesthetized rats. Maps of 20–60 penetrations were made for each subject. Sweep frequencies ranged from 1–16 kHz with FM rates ranging from 4–24 octaves/s and repetition rates from 2–24 sweeps/s. Both down-sweeps and up-sweeps were presented. Neuronal responses were analyzed in terms of onset, offset, oscillatory and directionally selective properties. Several types of responses predominated. Most units responded to sound onset. In contrast, responses to signal offsets were rare. Oscillatory responses were typically evoked only for repetition rates less than 12 sweep/s. Directionally selective responses were limited to these rates. These results indicate that oscillatory responses in the auditory cortex are limited to low repetition rate stimuli, and that directional selectivity depends on repetition rate. [Work supported by NSF.]