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Variable Gain Control in Local Cortical Circuitry Supports
Context-Dependent Modulation by Long-Range Connections
David C. Somers,
Louis J. Toth, Emanuel Todorov, S. Chenchal Rao,
Dae-Shik Kim, Sacha B. Nelson, Athanassios G. Siapas,
and Mriganka Sur.
Department of Brain & Cognitive Sciences
Massachusetts Institute of Technology
Cambridge, MA 02139
{somers,emo,thanos}@ai.mit.edu, {ljtoth,chenchal,dskim}@mit.edu,
nelson@binah.cc.brandeis.edu,
msur@wccf.mit.edu
Abstract
Long-range horizontal connections within the primary visual cortex
link cells with similar orientation preference over
large regions of visual space. These connections are thought to underlie
orientation-specific response modulations from beyond
a neuron's classical receptive field. The nature of
such modulations appears paradoxical as similar surround stimuli can
both strongly facilitate and suppress responses in a cell population.
We describe a biologically detailed model of primary visual cortex that
reconciles these data by proposing that fixed-strength, long-range inputs
can have both facilitatory and suppressive effects on the same group
of neurons. The sign of the effect varies with the gain control state
of the central cortical circuitry. The gain state varies with
the level of local activation, with high gain for weak central drive, and low
gain for strong central drive. Computer simulations demonstrate that
long-range inputs, via interactions with the local circuitry,
facilitate responses when central stimulation is weak
and suppress responses when central stimulation is strong.
We present experimental evidence to support the idea of bi-phasic response
modulation. By imaging intrinsic signals in primary visual cortex of cats,
we demonstrate the presence of long-range influences that change dynamically
from facilitatory to suppressive when the level of local drive changes
from low to high. Such context-dependent control of cortical response
gain appears well suited to underlie a range of physiological and
psychophysical phenomena in the visual system.
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