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Brian Schmidt; Variability in chromatic sensations from cones with the same photopigment. Journal of Vision 2017;17(15):8. doi: 10.1167/17.15.8.
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We probed individual, spectrally-identified cones with tiny spots of light using adaptive optics in human volunteers. Previously, using this technique, we reported a small percentage of L- and M-cones mediated either red or green sensations, respectively, while a majority produced white sensations (Sabesan et al. 2016. Sci. Adv.).
Here, we quantified the variability in sensations arising from individual L- and M-cones using three stimulus intensities and a finer response scheme. Cones were targeted 1–1.5° from the fovea in two subjects. After each stimulus (500 ms; 0.33 arcmin), subjects rated brightness on a scale from 0 to 5. Trials above threshold were additionally described with hue and saturation scaling. The stimulus wavelength (543 nm) equally activated L- and M-cones and the background was a neutral gray that adapted all cones uniformly. Detection thresholds for S-cones were significantly higher than L/M-cones, consistent with single-cone isolation. L- and M-cones, on average, produced reddish-yellow or greenish-blue hue sensations, respectively. Saturation was variable between cones of the same photopigment and in one subject we found evidence of a bi-modal distribution.
At the level of the synapse, activity from a single cone is insufficient to unambiguously identify the color of a light. Thus, our findings imply that, in the face of this ambiguity, the visual system utilizes prior information to assign chromatic sensations to single-cone stimuli in a manner consistent with cone type. The variability in saturation we observe suggests individual cones are more strongly associated with either chromatic or achromatic sensations.
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