Responses to linear and logarithmic frequency-modulated sweeps in ferret primary auditory cortex

Multi-unit responses to frequency-modulated (FM) sweeps were studied in the primary auditory cortex of ferrets using six different stimulation paradig ms. In particular, the differences between the responses to linear FM sweep s (where frequency changes linearly with time) and logarithmic FM sweeps (w here frequency changes exponentially with time) were emphasized. Some gener al features of the responses to FM sweeps are independent of the exact deta ils of the frequency trajectory. Both for linear and for logarithmic FM swe eps, a short burst of spikes occurred when the sweep reached a triggering f requency close to the best frequency of the cluster. The neuronal preferenc e for FM velocity was also independent of frequency trajectory. Thus, clust ers that responded best to slow logarithmic FM also preferred slow linear F M and vice versa. Consequently, topographic distributions of velocity prefe rence were roughly independent of the stimulation paradigm. Other character istics of the responses, however, depended on the exact details of the freq uency trajectory. A significant number of clusters showed large differences in directional sensitivity between linear and logarithmic FM sweeps; these differences depended on the velocity preference of the clusters in some pa radigms but not in others. Consequently, topographic distributions of direc tional sensitivity differed between linear and logarithmic paradigms. In co nclusion, some characteristics of cluster responses to FM sweeps depend on the exact details of the stimulation paradigm and are not 'invariants' of t he cluster.