Paper Available: Emergent Model of Orientation Selectivity

[David Somers]

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An Emergent Model of Orientation Selectivity 
In Cat Visual Cortical Simple Cells (41 pages)

	David Somers, Sacha Nelson, and Mriganka Sur,
	MIT,	Dept. of Brain & Cognitive Sciences


To appear in: The Journal of Neuroscience

ABSTRACT
It is well known that visual cortical neurons respond vigorously 
to a limited range of stimulus orientations, while their primary 
afferent inputs, neurons in the lateral geniculate nucleus (LGN) 
respond well to all orientations. Mechanisms based on intracortical 
inhibition and/or converging thalamocortical afferents have 
previously been suggested to underlie the generation of cortical 
orientation selectivity; however, these models conflict with 
experimental data.  Here, a 1:4 scale model of a $1700\mu\mbox{m}$ 
by $200\mu\mbox{m}$ region of layer IV of cat primary visual cortex 
(area 17) is presented in order to demonstrate that local
intracortical excitation may provide the dominant source of 
orientation selective input.  In agreement with experiment, model 
cortical cells exhibit sharp orientation selectivity despite 
receiving strong iso--orientation inhibition, weak cross-
-orientation inhibition, no shunting inhibition, and weakly tuned 
thalamocortical excitation. Sharp tuning is provided by recurrent
cortical excitation. As this tuning signal arises from the same 
pool of neurons that it excites, orientation selectivity in the model 
is shown to be an emergent property of the  cortical feedback circuitry.  
In the model, as in experiment, sharpness of orientation tuning 
is independent of stimulus contrast and persists with silencing of 
ON--type subfields. The model also provides a unified account 
of intracellular and extracellular inhibitory blockade experiments 
which had previously appeared to conflict over the role of inhibition.  
It is suggested that intracortical inhibition acts non-specifically 
and indirectly to maintain the selectivity of individual neurons by 
balancing strong intracortical excitation at the columnar level.


David C. Somers
Dept. of Brain & Cognitive Sciences
MIT, E25-618
45 Carleton St.
Cambridge, MA 02139

ftp://ftp.ai.mit.edu/pub/users/somers/orient-jneurosci.ps.Z