The lateral geniculate nucleus is thought to represent color using two populations of cone-opponent neurons (L-versus-M; S-versus-(L+M)) which establish the cardinal directions in color space (reddish-versus-cyan; lavender-versus-lime). How is this representation transformed to bring about color perception? Prior work implicates populations of glob cells in posterior inferior temporal cortex (PIT, the V4 Complex), but the correspondence between the neural representation of color in PIT/V4 and the organization of perceptual color space is unclear. We compared color-tuning data for populations of glob cells and interglob cells to predictions obtained using models that varied in the color-tuning narrowness of the cells, and the color-preference distribution across the populations. Glob cells were best accounted for by simulated neurons that have non-linear (narrow) tuning and, as a population, represent a color space designed to be perceptually uniform (CIELUV). Multi-dimensional scaling and representational similarity analyses showed that the color-space representations in both glob and interglob populations were correlated with the organization of CIELUV space, but glob cells showed a stronger correlation. Hue could be classified invariant to luminance with high accuracy given glob responses, and with above-chance accuracy given interglob responses. Luminance could be read out invariant to changes in hue in both populations, but interglob cells tended to prefer stimuli having luminance contrast, regardless of hue, whereas glob cells typically retained hue tuning as luminance contrast was modulated. The combined luminance/hue sensitivity of glob cells is predicted for neurons that can distinguish two colors of the same hue at different luminance levels (orange/brown).
Significance Statement: This paper provides the first quantitative test of the correspondence between the neural representation of color in posterior inferior temporal cortex (the V4 Complex) and the organization of perceptual color space. fMRI-guided micoelectrode recording was used to target two subpopulations of neurons within PIT/V4, globs and interglobs. The results suggest: (1) that glob cells have narrow color tuning, and as a population have a uniform representation of color space with a bias for warm colors; and (2) glob cells provide a neural correlate for the psychophysical distinction between two colors that have the same hue but differ in luminance (e.g. orange/brown). The work also underscores the importance of carefully controlled stimuli in neurophysiological studies of color.
Authors report no conflict of interest.
Funding: NIH EY023322, NSF 1353571.