Color signals through dorsal and ventral visual pathways

Abstract

Explanations for color phenomena are often sought in the retina, LGN and V1, yet it is becoming increasingly clear that a complete account will take us further along the visual-processing pathway. Working out which areas are involved is not trivial. Responses to S-cone activation are often assumed to indicate that an area or neuron is involved in color perception. However, work tracing S-cone signals into extrastriate cortex has challenged this assumption: S-cone responses have been found in brain regions, such as MT, not thought to play a major role in color perception. Here we review the processing of S-cone signals across cortex and present original data on S-cone responses measured with fMRI in alert macaque, focusing on one area in which S-cone signals seem likely to contribute to color (V4/posterior inferior temporal cortex), and on one area in which S signals are unlikely to play a role in color (MT). We advance a hypothesis that the S-cone signals in color-computing areas are required to achieve a balanced neural representation of perceptual color space, while the S-cone signals in non-color-areas provide a cue to illumination (not luminance) and confer sensitivity to the chromatic contrast generated by natural daylight (shadows, illuminated by ambient sky, surrounded by direct sunlight). This sensitivity would facilitate the extraction of shape-from-shadow signals to benefit global scene analysis and motion perception.

Functional magnetic resonance imaging adaptation reveals a noncategorical representation of hue in early visual cortex

“…The color-tuning properties of neurons in the early visual cortex have been well studied (for reviews see Conway, 2014;Gegenfurtner, 2003;Shapley & Hawken, 2011;Solomon & Lennie, 2007). Relating this literature to our results is complicated by the hetero- geneity of findings across laboratories (Shapley & Hawken, 2011), by the fact that single units combine signals from cones nonlinearly (Horwitz & Hass, 2012;Solomon, Peirce, & Lennie, 2004), and by the fact that the response of individual neurons depends on the spatial and temporal structure of the stimulus (Shapley & Hawken, 2011;Solomon & Lennie, 2007).…”

Striking individual differences in color perception uncovered by ‘the dress’ photograph

“…Visual cortex shows a bias for colors associated with daylight [7, 8]; this bias may represent the brain’s internal model. We hypothesize that some brains interpret the surprising chromatic distribution (Figure 1B) as evidence that a portion of the spectral radiance is caused by a color bias of the illuminant [1] (Supplementary Discussion).…”

Asymmetries in blue–yellow color perception and in the color of ‘the dress’

“…This range far exceeds the differences required to tell two “whites” apart, and thus is not a failure of discrimination. The bias disappears when the test patch is equal in luminance to the background, differs from measures of threshold discrimination or standard metrics of suprathreshold saturation (Figure S1), and is selective for a chromatic axis to which cells early in the visual system are not tuned [6], arguing against sensitivity differences or an early nonlinearity in neural coding as a basis for the effect. This was further confirmed in studies of chromatic adaptation to alternating blue and yellow fields, which resulted in afterimages consistent with their linear average.…”