Response to broadband repetitive stimuli in auditory cortex of the unanesthetized rat

This study examines the ability of multi-unit clusters (MUCs) in layer IV/V of primary auditory cortex of the awake rat to respond to a series of broadband click trains. The data from 113 multi-unit clusters were analyzed for synchronous and nonsynchronized responses using several methods. Synchronous responses were measured using window analysis, circular statistics and spectral analysis. Nonsynchronous responses were measured during different time intervals during the click train (first 50 ms, 50-450 ms, and the entire click train). The results demonstrate that multi-unit clusters are capable of synchronizing to clicks at rates up to 166 Hz. The mean synchronization boundary (limiting rate) for the group was found to be 72 Hz. Mean peak response rate, mean response duration, and mean time-to-peak response decreased as the stimulus presentation rate (SPR) increased, resulting in a temporal sharpening of the population response. For fast SPRs (>50 Hz), 50% of MUCs exhibited nonsynchronous responses in which the firing rate increased with SPR, although this activity was most prevalent during the first 50 ms of the response. Sustained increases in firing rate with SPR were seen in 8% of the MUCs, while another 38% of MUCs exhibited sustained decreases during the click train.