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J Neurophysiol (November 23, 2005). doi:10.1152/jn.00885.2005
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Submitted on August 23, 2005
Accepted on November 21, 2005

Direction selectivity of neurons in the striate cortex increases as stimulus contrast is decreased

Matthew R Peterson1, Baowang Li1, and Ralph D Freeman1*

1 Group in Vision Science, School of Optometry, Helen Wills Neuroscience Institute, University of California, Berkeley, Berkeley, CA, USA

* To whom correspondence should be addressed. E-mail: freeman{at}neurovision.berkeley.edu.

Various properties of external scenes are integrated during the transmission of information along central visual pathways. One basic property concerns the sensitivity to direction of a moving stimulus. This direction selectivity (DS) is a fundamental response characteristic of neurons in the visual cortex. We have conducted a neurophysiological study of cells in the visual cortex to determine how DS is affected by changes in stimulus contrast. Previous work shows that the integration time of a neuron is increased at low contrasts, causing temporal changes of response properties. This leads to the prediction that DS should change with stimulus contrast. However, the change could be in a counterintuitive direction, i.e. DS could increase with reduced contrast. This possibility is of intrinsic interest but it is also of potential relevance to recent behavioral work in which human subjects exhibit increased DS as contrast is reduced. Our neurophysiological results are consistent with this finding, i.e., the degree of DS of cortical neurons is inversely related to stimulus contrast. Temporal phase differences of inputs to cortical cells may account for this result.




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