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Barry B. Lee, Hao Sun State, Joshua Kunken; Spatial characteristics of the frequency-doubled response of MC cells to chromatic modulation. Journal of Vision 2003;3(12):41. doi: 10.1167/3.12.41.
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© ARVO (1962-2015); The Authors (2016-present)
Purpose Cells of the magnocellular (MC) pathway respond to equiluminant red-green borders with an excitatory response independent of the direction of border movement, and respond to red-green chromatic modulation with a frequency-doubled (2F) response. The 2F response becomes smaller with small spot stimuli, and was attributed to the surround. We have now investigated the spatial characteristics of this 2F response in more detail. Results Equilumnant border responses are spatially crisp, with a width 1–1.5 times that of the response to a luminance edge. Area summation curves with luminance or chromatic sinusoidally modulated spots also indicate a 2F summation area 1–1.5 times the center size. Together with achromatic center-surround interaction, this can account for the small size of the 2F response with small stimuli. We also measured linearity of spatial summation of luminance and chromatic responses using counterphase modulated luminance and red-green gratings. Both responses showed a null position at a similar grating locus. Conclusions The results indicate the chromatic input responsible for the 2F response has a summation area not much larger than the center. There is a separate achromatic surround which is larger. We interpret the presence of a 2F response null position as indicating that it originates from a non-linear summation of +L−M and +M−L chromatic mechanisms rather than a subunit structure. Each opponent mechanism is spatially linear and they are concentric. If spatial summation occurs prior to the nonlinear summation of the two mechanisms, a null position for the 2F response would be expected.
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