Psychophysical channels and ERP population responses in human visual cortex: area summation across chromatic and achromatic pathways

In the early stages of vision, information is transmitted through distinct physiologically defined pathways. These may be related with three post-receptoral detection mechanisms defined psychophysically in humans. Accordingly, the parvocellular pathway is very sensitive to L-M-cone contrast, processes mainly foveal information and underlies fine discrimination of visual features. The magnocellular pathway is most sensitive to luminance contrast and is important for visuo-spatial and motion processing. The less understood koniocellular pathway responds to S-cone modulation outside the foveola. As such, the three pathways process visual information in a different manner, with the L-M-cone psychophysical channel being more devoted to central vision and the two other channels responding significantly to peripheral information. We measured size response functions of these three processing channels using event related potential (ERP/EEG) recordings and stimuli with various sizes and contrasts with the aim of studying coding of stimulus properties within each of these channels. The effect of stimulus size was significantly smaller for the L-M-cone channel consistent with its dominance in the central visual field. Furthermore, for this pathway, the effect of size was not modulated by stimulus contrast. In contrast, both the S-cone and achromatic channels showed a strong effect of size that was significantly modulated by contrast. Interestingly, both the S-cone and achromatic channels responded proportionally to the area of cortex activated, suggesting that the S-cone channel represents space in a similar manner to the achromatic channel. In conclusion, a fundamental relation exists between previously identified psychophysical mechanisms and population responses in the visual cortex.