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Chroma for hue2/19/2023 ![]() ![]() Munsell color space consists of a central axis representing achromatic lightnesses between N 0 to N 10, with hue represented in the plane orthogonal to the lightness axis, and color chroma indicated with increasing distance from the central axis. Thus, to establish a suitable correspondence between V1 responses in macaques and perceptual responses in human participants, we adopted a Munsell color space approach ( Munsell, 1919), which is often employed in human color experiments. Notably, such methods are seldom used in human vision studies. Some studies have constrained lightness levels without controlling chroma, which could reduce response efficacy as a function of hue ( Bohon et al., 2016). In addition, most physiological studies of V1 have focused on equiluminant hue responses. Color gratings are often displayed to activate cells, especially as there has been great interest in understanding the relationship between orientation and color. Various color systems have been used to study V1 responses, but near-uniform perceptual color spaces are rarely employed. We thus initiated this project to determine V1’s precise contribution to the organizing principles underlying color perception. In sum, previous studies indicate that V1 may contribute to the representation of all three perceptual dimensions of color. V1 moreover encodes hue and lightness in parallel ( Hass and Horwitz, 2013 Johnson et al., 2001 Lennie et al., 1990 Livingstone and Hubel, 1988 Peng and Van Essen, 2005 Yoshioka et al., 1996) meaning that there could be a map of how different hue and lightness combinations are processed, with some V1 cells responses varying as a function of increased saturation ( Hanazawa et al., 2000). This suggests that neural populations in this area process different colors with segregated circuits. Anatomical evidence indicates that V1 layers II/III receive all three kinds of cone information ( Sincich and Horton, 2005), and intrinsic signal optical imaging (ISOI) has shown that different hues activate organized cortical representations in V1 ( Xiao et al., 2007). ![]() ![]() Subcortical opponent color processes undergo a nonlinear transformation in V1 ( Cottaris and De Valois, 1998 De Valois et al., 2000 Horwitz and Hass, 2012 Stockman and Brainard, 2010), which instantiates the initial organizing principles of the primary three dimensions of perceptual color space processing in the brain. The representation of perceptual color space appears to have developed gradually along the ventral visual pathway ( Liu et al., 2020). ![]() In the inferotemporal cortex (IT), cells with sharp tuning for saturation are also found ( Komatsu et al., 1992), but whether chroma is represented by a pattern in a cortical map is unknown. Downstream, V4 and the posterior inferotemporal cortex (PIT) are thought to fully represent perceptual color dimensions ( Bohon et al., 2016 Conway and Tsao, 2009 Li et al., 2014). In V2, hue-selective columns form band-like patterns in relation to the perceptual color wheel ( Xiao et al., 2003). In V1, the subset of neurons that are selective to colors ( Cottaris and De Valois, 1998 Lennie et al., 1990 Livingstone and Hubel, 1984, 1983 Thorell et al., 1984 Wachtler et al., 2003) concentrate within cytochrome oxidase (CO) blobs, in which cells preferring similar colors are often located in clusters ( Garg et al., 2019 Landisman and Ts’o, 2002 Ts’o and Gilbert, 1988). Within the ascending retino-geniculo-cortical pathway, interactions between cone signals take place in both the retina ( Dacey, 1996 Gouras, 1968) and in the lateral geniculate nucleus (LGN) ( De Valois et al., 1966 Derrington et al., 1984). Three types of cone photoreceptors are differentially selective to overlapping bands of wavelength within the retina. The primary neural pathway of color signal processing is known ( Conway, 2014 Shapley and Hawken, 2011). Most of them used dimensions of hue, lightness, and chroma, calling into question how human brains organize neural responses to produce these distinct dimensions of color perception. For more than one hundred years, artists and scientists have created various systems to describe the human perception of color. How the brain achieves the richness of color perception remains a mystery. Different combinations of electromagnetic frequencies and intensities within the visible spectrum result in the colors we see, but colors are not seen as a linear function of wavelength and intensity. Trichromatic color vision in primates is advantageous to seeking food ( Osorio and Vorobyev, 1996) and during social interactions ( Chang et al., 2017 Freiwald, 2020a, 2020b Hasantash et al., 2019 Hiramatsu et al., 2017 Shepherd and Freiwald, 2018 Sliwa and Freiwald, 2017). ![]()
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