This project will examine the functional organization of small neuronal assemblies in the auditory cortex of cats. This will be done by simultaneously recording the individual extracellular spike trains of 15-30 neurons. Interactions among these neurons, as measured by cross-correlation methods, can be represented by a functional """"""""wiring diagram"""""""". In particular, we will examine the changes in the wiring diagrams that correspond to changes in the location of a sound source. Ablation studies have demonstrated that auditory cortex is necessary for accurate localization of sound. The auditory cortex in cats is known to be subdivided into smaller regions (columns or bands) characterized by frequency selectivity and differential response to ipsilateral and contralateral stimuli. Unlike visual cortex, no simple mapping of location has been found by single neuron studies in auditory cortex. This reserach seeks to demonstrate that neuronal assembly properties are involved in the representation of acoustic space in the auditory cortex. Cats are anesthetized and a bundle of 25-micron wires is inserted tangentially into the auditory cortex through a 24G guide tube. The bundle is advanced by a microdrive mounted on the cat's head. After several days for recovery, the cats are restrained and computer-controlled auditory stimuli are presented through matched earphones. Extracellular spike trains are separated with analog template-matching devices. A multiple computer network coordinates stimulus control, data acquisition and analysis. Preliminary studies in our lab have demonstrated the feasibility of this approach. The cortical representation of a sound's location probably involves many more neurons than we can sample simultaneously, however even with this limited sample, we will provide experimental data on the changes in the functional interconnections that do occur. This is a necessary first step in defining the fundamental principles by which small neuronal assemblies organize to perform complex functions.
