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The Journal of Neuroscience, July 18, 2007, 27(29):7673-7683; doi:10.1523/JNEUROSCI.1048-07.2007

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Behavioral/Systems/Cognitive
The Operating Point of the Cortex: Neurons as Large Deviation Detectors

Dario L. Ringach and Brian J. Malone

Department of Psychology and Neurobiology, Jules Stein Eye Institute, David Geffen School of Medicine, University of California, Los Angeles, Los Angeles, California 90095-1563

Correspondence should be addressed to Dario L. Ringach at the above address. Email: dario{at}ucla.edu

Spiking neurons translate analog intracellular variables into a sequence of action potentials. A simplified model of this transformation is one in which an underlying "generator potential," representing a measure of overall neuronal drive, is passed through a static nonlinearity to produce an instantaneous firing rate. An important question is how adaptive mechanisms adjust the mean and SD of the generator potential to define an "operating point" that controls spike generation. In early sensory pathways adaptation has been shown to rescale the generator potential to maximize the amount of transmitted information. In contrast, we demonstrate that the operating point in the cortex is tuned so that cells respond only when the generator potential executes a large excursion above its mean value. The distance from the mean of the generator potential to spike threshold is, on average, 1 SD of the ongoing activity. Signals above threshold are amplified linearly and do not reach saturation. The operating point is adjusted dynamically so that it remains relatively invariant despite changes in stimulus contrast. We conclude that the operating regimen of the cortex is suitable for the detection of signals in background noise and for enhancing the selectivity of spike responses relative to those of the generator potential (the so-called "iceberg effect"), but not to maximize the transmission of total information.

Key words: spike threshold; nonlinearity; generator potential; feature detector; large deviation; tuning selectivity; sparseness

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Received March 8, 2007; revised May 29, 2007; accepted May 30, 2007.

Correspondence should be addressed to Dario L. Ringach at the above address. Email: dario{at}ucla.edu

This article has been cited by other articles:

				J. W. Middleton, A. Longtin, J. Benda, and L. Maler Postsynaptic Receptive Field Size and Spike Threshold Determine Encoding of High-Frequency Information Via Sensitivity to Synchronous Presynaptic Activity J Neurophysiol, March 1, 2009; 101(3): 1160 - 1170. [Abstract] [Full Text] [PDF]

				N. J. Priebe The Relationship between Subthreshold and Suprathreshold Ocular Dominance in Cat Primary Visual Cortex J. Neurosci., August 20, 2008; 28(34): 8553 - 8559. [Abstract] [Full Text] [PDF]

				M. Diaz-Quesada and M. Maravall Intrinsic Mechanisms for Adaptive Gain Rescaling in Barrel Cortex J. Neurosci., January 16, 2008; 28(3): 696 - 710. [Abstract] [Full Text] [PDF]

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