Neuronal assembly dynamics in the rat auditory cortex during reorganization induced by intracortical microstimulation.

Single neurons, acting alone, cannot account for the complex and rapid computations that are routinely accomplished by the behaving nervous system. Recent studies with separable multineuron recordings are showing that neuronal assemblies can indeed be detected and that their organization is very dynamic, depending on variables such as time, physical stimulus, and context. Here we explore both single-neuron and assembly properties in the rat's auditory cortex. Acoustic stimuli are used as a normal, physiological input, and weak electrical intracortical microstimulation (ICMS) as a perturbation that forces a rapid cortical reorganization. In this setting, various aspects of neuronal interactions are changed by the ICMS. We found that cortical neurons exhibited highly synchronous oscillatory firing patterns that were enhanced by ICMS. Cross-correlation studies between two spike trains showed that statistically significant correlations depended on the anatomical distance between the two neurons. ICMS changed the strength and the local number of such correlations. Joint petristimulus analysis and gravity analysis showed that the correlation between neuronal activities varied dynamically at several time scales. We have identified neuronal assemblies in two ways, defined through similarity of receptive field properties and defined through correlated firing. Close anatomical spacing between neurons was conducive to, but not sufficient for membership in, the same assembly with either definition. ICMS changed cortical organization by altering assembly membership. Our data show that neuronal assemblies in the rat auditory cortex can be established transiently in time and that their membership is dynamic.