Lagged cells in the inferior colliculus of the awake ferret

Neurons in the primary auditory cortex (AI) encode complex features of the spectral content of sound, such as direction selectivity. Recent findings of temporal symmetry in AI predict a specific organization of the subcortical input into the cortex that contributes to the emergence of direction selectivity. We demonstrate two subpopulations of neurons in the central nucleus of the inferior colliculus, which differ in their steady-state temporal response profile: lagged and non-lagged. The lagged cells (23%) are shifted in temporal phase with respect to non-lagged cells, and are characterized by an 'inhibition first' and delayed excitation in their spectro-temporal receptive fields. Non-lagged cells (77%) have a canonical 'excitation first' response. However, we find no difference in the response onset latency to pure tone stimuli between the two subpopulations. Given the homogeneity of tonal response latency, we predict that these lagged cells receive inhibitory input mediated by cortical feedback projections.