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P. R. Martin, E. M. Blessing, P. Buzas, J. D. Forte; Eccentricity Dependence of Chromatic Sensitivity in Marmoset Parvocellular Neurons. Invest. Ophthalmol. Vis. Sci. 2009;50(13):3473.
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To understand how medium (M) and long (L) wavelength cones contribute to parvocellular receptive fields in the central and peripheral visual field.
We measured extracellular responses to M and L cone-selective, chromatic (L-M) and luminance (L + M) modulated gratings in the parvocellular layers of the lateral geniculate nucleus in sufentanil-anaesthetised marmosets (n = 13). Cone selective gratings produced close to 30% contrast in the modulated cone; chromatic and luminance gratings produced close to 15% contrast in M and L cones either out-of-phase (chromatic) or in-phase (luminance). Response metric was the first harmonic Fourier component of spike density at the stimulus frequency (4 Hz).
In trichromatic marmosets (n = 8), mean chromatic gain ratio (M - L) / (M + L) decreased from 4.34 to 2.09 between central (< 2.2 deg, n = 36) and peripheral (> 8.0 deg, n = 35) visual field. This fall (~ 50%) is less than the value (~85%) predicted by a binomial ("random wiring") model of receptor convergence. In central visual field the majority of cells (86%) showed opponent responses (gain ratio > 1) and many showed low-pass spatial frequency (sf) tuning for cone-isolating gratings. In peripheral visual field the proportion of opponent cells was lower (43%) and the tuning curves were more band-pass, indicating increasing contribution of the center-dominating cone type to the receptive field surround (low-to-peak sf amplitude ratio: central 0.71, sd 0.27, n=35; peripheral 0.45, sd 0.17, n = 32, p < 0.01, Wilcoxon test). The same trend towards band-pass sf tuning was seen for L+M gratings (p < 0.01, Kruskal-Wallis test) and was also a feature of responses of parvocellular cells (n=78) to L+M gratings in dichromatic marmosets (p < 0.01, Kruskal-Wallis test). This indicates that the eccentricity-dependent increase in surround strength is not a specialization for color vision.
Cone opponent receptive fields are present in peripheral visual field but at lower frequency than in the central field. Increased efficacy of the receptive field surround mechanism in peripheral visual field contributes to this deterioration in the quality of the M-L chromatic signal.
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