Processing of the S-cone signals in the early visual cortex of primates

Abstract

The short-wavelength-sensitive (S) cones play an important role in color vision of primates, and may also contribute to the coding of other visual features, such as luminance and motion. The color signals carried by the S cones and other cone types are largely separated in the subcortical visual pathway. Studies on nonhuman primates or humans have suggested that these signals are combined in the striate cortex (V1) following a substantial amplification of the S-cone signals in the same area. In addition to reviewing these studies, this review describes the circuitry in V1 that may underlie the processing of the S-cone signals and the dynamics of this processing. It also relates the interaction between various cone signals in V1 to the results of some psychophysical and physiological studies on color perception, which leads to a discussion of a previous model, in which color perception is produced by a multistage processing of the cone signals. Finally, I discuss the processing of the S-cone signals in the extrastriate area V2.

Fig. 1

Anatomical pathways associated with the S-cone signals in V1. (A) A schematic diagram of the cone-specific projections from the LGN to V1. Terminals that carry S-off signals innervate layer 4A, whereas those carrying S-on signals innervate both layers 4A and 3. Terminals that carry opponent and nonopponent L/M-cone signals innervate layers 4Cβ and 4Cα, respectively. It was hypothesized that the S-on terminals in layer 3 and the S-off ones in layer 4 are concentrated in small patches. M, P, and K denote projections from the magnocellular, parvocellular, and koniocellular layers of the LGN, respectively. Reprinted by permission from Macmillan Publishers Ltd: Nature (Chatterjee and Callaway, 2003), copyright 2003. (B) A schematic diagram of the distribution of axonal terminals originating from cells in layer 4A. Most of these terminals in layer 3 are clustered in the CO blobs, although they are missing in a minority of the blobs (marked with *). There are also extensive connections between cells within layer 4A. From Yoshioka et al. (1994). Copyright 1994, Cambridge University Press.

Fig. 2

Distributions of the relative S-cone weights across LGN cells and V1 cells. The LGN contains two distinct groups of cells, one with small S-cone weights and the other with large S-cone weights. There are few cells with intermediate weights. The fi rst group corresponds to cells in the parvocellular and magnocellular layers of the LGN, which constitute the majority of LGN cells. The second group corresponds to cells in the koniocellular layers. V1 cells cannot be divided into distinct groups according to their S-cone weights, and the median weight is twice as large as that found in LGN cells. From De Valois et al. (2000). Copyright 2000, National Academy of Sciences, USA.

Fig. 3

The transformation from cone-opponent signals in the LGN to a continuous representation of color in visual cortex. The S-cone signals from the LGN are amplifi ed in V1 relative to the L/M-cone signals, as symbolized by the change in the line width that denotes the signal strength. According to the multistage model by De Valois and De Valois (1993), the S-cone signals play a modulatory role on the opponent L/M-cone signals in V1. As a result, chromatic preferences of V1 neurons cover the full gamut of color.