Sensitivity to chromatic and luminance contrast and its neuronal substrates

Human sensitivity to changes in luminance or chromaticity differ as a function of temporal or spatial frequency. Luminance sensitivity is band-pass in shape, while chromatic sensitivity curves are low-pass, along both L-M cone opponent (red- green) and S-cone opponent (blue-yellow) axes, where L, M, and S refer to the long-wavelength middle-wavelength and short-wavelength sensitive cones. Temporal sensitivity curves are supported by the magnocellular pathway for luminance, the parvocellular pathway for L-M modulation and the koniocellular pathway for S-cone modulation. This division largely holds for spatial contrast sensitivity. But there are discrepancies. For example, parvo and konio cells respond to higher temporal and spatial frequencies than can be perceived. Also, the role of eye movements in spatial contrast sensitivity - and spatial vision as a whole - is now debated; the spatial and temporal properties of eye movements profoundly influence the spatiotemporal signal leaving the retina. Finally, human psychophysical thresholds for color and luminance seem largely independent. Yet recent cortical data show much combination of magno, parvo, and konio signals in the cortex. This oddity remains puzzling but may have to do with the segregation of different types of information into dorsal and ventral cortical streams.