Wednesday 13th July 2011
Seminar Room B10 (Basement)
Alexandra House, 17 Queen Square, London, WC1N 3AR
Temporal Correlations in Cortical Circuits
The spiking activity of single cortical neurons in vivo displays temporal irregularity and trial-to-trial variability. This seeming stochastic behavior is thought to be the result of strong temporal fluctuations in membrane potential due to the approximate cancellation between the large net excitatory and inhibitory drives to the neurons.
However, the population structure of these temporal fluctuations in recurrent networks is much less clear. Fluctuations in the activity of nearby neurons recorded simultaneously in primates is typically positively correlated - an important issue because such globally correlated network states pose significant constraints for decoding the activity of population codes. However, whereas it was believed that such correlations were caused by shared presynaptic inputs, the evidence in favor of this hypothesis was circumstantial, since a mechanistic understanding of the behavior of temporal correlations in recurrent circuits was lacking. Surprisingly, we have recently shown that the same mechanism responsible for generating strong temporal fluctuations in the activity of single cells causes these fluctuations to be, on average, effectively uncorrelated across neurons. Thus, we are now faced with the challenge of reconciling the irregular activity of cortical neurons with the global correlations which are typically observed experimentally. I will review our current theoretical understanding of temporally irregular activity in balanced networks of excitatory and inhibitory cells, discuss the theoretical and experimental challenges posed by these results, and describe preliminary data on the organization of temporal correlations in primary sensory cortex in the rat during spontaneous activity, and its relationship to evoked responses.