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Dany V. D'Souza, Tibor Auer, Hans Strasburger, Jens Frahm, Barry B. Lee; An fMRI study of chromatic processing in humans: Temporal characteristics of cortical visual areas. Journal of Vision 2009;9(14):38. doi: https://doi.org/10.1167/9.14.38.
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© ARVO (1962-2015); The Authors (2016-present)
Psychophysical sensitivity to chromatic modulation declines steeply at high temporal frequencies but the relevant retinal ganglion cells (midget ganglion cells for red-green modulation, small bistratified cells for blue-yellow modulation) show robust responses to high temporal frequency chromatic modulation. It has been posited that low-pass filtering of chromatic information occurs at neural loci across the visual cortex. We conducted two functional magnetic resonance imaging (fMRI) experiments on human subjects to investigate the cortical representation of temporal frequency dependent chromatic information. In the first experiment, we employed standard retinotopic mapping methods to identify retinotopic visual areas. In the second experiment, fMRI responses to high cone-contrast chromatic and luminance grating stimuli at various temporal frequencies were measured in lateral geniculate nucleus (LGN) and cortical visual areas. An optimized, M-scaled spatial frequency profile was implemented in the stimulus. We provide fMRI evidence that high temporal frequency chromatic information is transmitted through LGN. In V1, blue-yellow information is subjected to low-pass filtering implying that a loss of psychophysical sensitivity to high temporal frequency blue-yellow information has neural substrates as early as V1. There was no filtering of high temporal frequency red-green information in V1. The findings suggested that ventral and dorsal visual areas have distinct specialization for temporal frequency-dependent chromatic information, the former showing low-pass tuning characteristics, whereas the latter show robust responses to high temporal frequencies. Besides the amplified luminance response in middle temporal area (MT), responses increased with increasing temporal frequency, suggesting MT's well established role in luminance-mediated motion processing. On the basis of the temporal frequency characteristic processing of luminance and chromatic information, we propose that visual areas can be clustered in at least three groups. We suggest that the cluster containing the dorsal areas V3d & V3a, in combination with area MT constitute a functional network for the coding of high temporal frequency information whereas the cluster comprising the ventral areas VP & V4 constitutes a functional network for processing low temporal frequency chromatic information, and hence might provide a neural substrate for the psychophysical sensitivity loss to high temporal frequency chromatic information.
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