Functional anatomy of the inferior colliculus and the auditory cortex: current source density analyses of click-evoked potentials.

In the auditory midbrain (inferior colliculus) and cortex (superior temporal gyrus) of awake squirrel monkeys profiles of click-evoked field potentials were recorded. The recording tracks were reconstructed anatomically. From the field potentials the one-dimensional current source density (CSD) distributions were calculated. By comparing the CSD profiles with the anatomical features of the reconstructed recording paths, the components of the CSDs could be attributed to certain anatomical sites. Thus a physiological method for the functional identification of recording sites was obtained. It permits the identification of depth locations of specific laminae in cortex. In the inferior colliculus it permits distinction between central and peripheral regions and between three depth segments. The CSDs in the central nucleus of the inferior colliculus lend functional support to the anatomical division into three distinct parts and, in addition, provide the temporal aspects of the main groups of synaptic activities. The CSDs in the auditory cortex permit determination of five different groups of excitatory synaptic activations. The spatio-temporal distributions of these components are very similar to those obtained in other neocortical areas and thus corroborate the hypothesis that afferent activity is relayed very similarly in all sensory areas of neocortex.