Color perception is based on the differential spectral responses of the L, M, and S-cones and subsequent subcortical and cortical computations and may include the influence of higher-order factors such as language. Although the early subcortical stages of color vision are well characterized, the organization of cortical representations of color remain elusive, despite numerous models based on discrimination thresholds, appearance, and categorization. An underexplored aspect of cortical color representations is their dynamic evolution. Here, we compare the evolution of three different color representations over time using magnetoencephalography. We measured neural responses to 14 hues at each of three achromatic offsets (increment, isoluminant, and decrement) while participants attended either to the exact color of the stimulus or its color category. We used a series of classification analyses, combined with multidimensional scaling and representational similarity analysis, to ask how cortical representations of color unfold over time from stimulus onset. We compared the performance of “higher order” models based on hue and color category with a model based simply on stimulus cone contrast and found that all models had significant correlations with the data. However, the unique variance accounted for by each model revealed a dynamic change in hue responses over time, which was consistent with a “coarse to fine” transition from a broad clustering into categorical groups to a finer within-category representation. Notably, these dynamics were replicated across data sets from both tasks, suggesting they reflect a robust reorganization of cortical hue responses over time.

The purpose of our study was to investigate the ability to process achromatic and short-wavelength-sensitive cone (S-cone)-isolating (blue–yellow) stimuli in the blind visual field of hemispherectomized subjects and to demonstrate that blindsight is mediated by a collicular pathway that is independent of S-cone inputs. Blindsight has been described as the ability to respond to visual stimuli in the blind visual field without conscious awareness [Weiskrantz, L., Warrington, E. K., Sanders, M. D., & Marshall, J. Visual capacity in the hemianopic field following a restricted occipital ablation. Brain, 97, 709–728, 1974]. The roles of the subcortical neural structures in blindsight, such as the pulvinar and the superior colliculus, have been debated and an underlying neural correlate has yet to be confirmed. Using fMRI, we tested the ability to process visual stimuli that isolated the achromatic and short-wavelength-sensitive (S-)-cone pathways in three subjects: one control subject, one hemispherectomized subject with blindsight, and one hemispherectomized subject without blindsight. We demonstrated that (1) achromatic and S-cone-isolating stimuli presented to the normal visual hemifield of hemispherectomized subjects and to both visual hemifields of the control subject activated contralateral visual areas (V1/V2), as expected; (2) achromatic stimulus presentation but not S-cone-isolating stimulus presentation to the blind hemifield of the subject with blindsight activated visual areas FEF/V5; (3) whereas the cortical activation of the control subject was enhanced by an additional stimulus (achromatic and S-cone isolating) presented in the contralateral visual field, activation pattern of the subject with blindsight was enhanced by achromatic stimuli only. We conclude that the human superior colliculus is blind to the S-cone-isolating stimuli, and blindsight is mediated by an S-cone-independent collicular pathway.