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Chris Tailby, Samuel G. Solomon, Neel T. Dhruv, Najib J. Majaj, Peter Lennie; Habituation reveals cardinal chromatic mechanisms in striate cortex of macaque. Journal of Vision 2005;5(8):80. doi: 10.1167/5.8.80.
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© ARVO (1962-2015); The Authors (2016-present)
Psychophysical studies assign special status to three post-receptoral mechanisms of color vision: two color-opponent ones capturing red-green and yellow-blue variations, and a non-opponent one capturing achromatic variations. The distinctiveness of the cardinal mechanisms is most clearly revealed by habituation to chromatic modulation, but their locus has been unclear. Chromatically opponent neurons in LGN do not habituate, so the mechanisms must arise in cortex, yet the preferred color directions of striate cortex cells are very broadly distributed, with no clear preference for ‘cardinal’ directions. The present study exploited habituation to chromatic modulation to reveal the possible existence of very early cortical mechanisms tuned to the cardinal directions. In V1 of anesthetized macaque we characterized the chromatic tuning of neurons that responded to isoluminant modulation, before, during and after prolonged exposure to an isoluminant stimulus whose chromaticity was modulated in time about a fixed white point. Test stimuli were defined by modulation along eight color vectors within the isoluminant plane. For every neuron habituation to a stimulus modulated along its preferred color direction reduced responsiveness. For a neuron whose preferred color direction was on or near one of the cardinal axes, habituation to modulation along any direction brought about a proportional reduction in responsiveness to all directions of modulation. For a neuron whose preferred color direction lay between the two cardinal axes, habituation to modulation along either axis brought about a loss of responsiveness that was greatest on or near that axis. The specificity of habituation suggests that it occurs in V1 prior to combination of inputs from different classes of LGN neurons—in neurons in the input layers or in the thalamocortical synapse.
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