Current source density analysis of frequency coding in the inferior colliculus

Abstract

Current source density (CSD) analysis is a technique that provides information about the time course and spatial location of transmembrane currents derived from a laminar collection of evoked potentials. Restrictive conditions for acquisition of evoked potentials are required in order to yield a one-dimensional CSD analysis that is an accurate estimate of synaptic activity. Satisfaction of these conditions was assumed in recordings of tone-burst-evoked potentials along the axis of symmetry (tonotopic) in the inferior colliculus of adult mongolian gerbils. Off-line these data were converted into distance/voltage functions. The second spatial derivatives of these functions gave a family of profiles of the spatial distribution of current source and sink densities at discrete latencies relative to the stimulus. Results indicate a frequency-dependent spatial shift in the evoked current sink of about 280 μm octave. This sink, indicative of local excitatory synaptic drive, is surrounded, both spatially and temporally, by current sources. The spatial extent of excitation as estimated from CSD analysis, compares well quantitatively with predictions from an across-neuron model that is based on single unit data.