Task-Dependence of Spatial Sensitivity in Areas A1, DZ and PAF of Cat Auditory Cortex.

Abstract

Auditory cortex is essential for normal sound localization behavior. Reversible inactivation of primary auditory area (A1), dorsal zone (DZ) or posterior auditory field (PAF) in cats produces varying types and degrees of localization deficits. However, previous physiological studies in anesthetized animals have demonstrated rather small differences in spatial sensitivity across cortical fields. In the current study we characterized the auditory spatial sensitivity of neurons in A1, DZ and PAF in awake animals and investigated the influence of the attentional state on the spatial receptive fields. We recorded extracellular spike activity to broadband noise-burst probe stimuli from various azimuth locations when cats performed a behavioral task that either required evaluation of sound location or discrimination of sound timbre regardless the location of the sound.

Overall, we found that neurons in area A1 usually have broad spatial tuning, usually broader than a hemifield. In area DZ, neurons usually respond best to stimuli near the frontal midline. In area PAF, neurons with sharp spatial tuning exhibited best areas more evenly distributed across space. Neurons in A1 exhibited stronger task-dependent modulation of their responses than did neurons in DZ and PAF. During the localization task, many A1 neurons sharpened their spatial receptive field compared to the idle conditions by suppressing the responses to least-favored stimuli. Spatial sharpening occurred on a scale of tens of seconds and could be replicated multiple times within single ~1.5-hr test sessions. That and an observed increase in latencies suggest an important role for inhibitory mechanisms. In areas DZ and PAF, the effect of behavioral tasks usually were associated with an increase of tonic firing to the favored stimuli in addition to the suppression of responses to the least-favored stimuli. The location-related information conveyed by the temporal firing patterns among a small ensemble of DZ and PAF units was higher during the behavioral conditions than in the Idle condition. The differential task-dependent modulation of spatial sensitivity among these three areas, as well as the differences in distribution of spatial receptive fields, suggests that these three cortical areas might play distinct roles in auditory spatial processing.