How do incoming sensory inputs interact with ongoing activity?

November 12, 2013

Neurons across the cortex differ considerably in the degree to which they exhibit persistent activity. Neurons in frontal areas might fire persistently for seconds even in the absence of a sensory stimulus, while neurons in early visual cortex (V1) are more tightly linked to incoming sensory input. Does this tight linking arise because V1 circuits simply lack the features that allow persistent activity, or might the tight linking arise as the result of an active process?

photoAmyAn intriguing poster from Kim Reinhold in Massimo Scanziani’s lab suggests the latter. She has been running experiments to determine the timescale over which cortical activity changes when she removes thalamic inputs (via an optogenetic strategy). She finds that the time constant for the decay of activity is super-fast: about 9 ms. Given that 9 ms is around the membrane time constant of the cell, it would seem at first that the membrane properties of individual cells define the time constant of persistent activity for the whole area. But the plot thickens: when Kim squelched cortical inhibition, the time constant got considerably longer. This observation suggests that inhibitory neurons actively squelch cortical responses thereby preventing persistent activity. Why might this be? Kim reasons that fast-acting inhibition would ensure that the visual cortex was always at-the-ready for new incoming stimuli. This suggests a tradeoff between the ability to maintain a persistent response, and the ability to response with high temporal resolution.

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