Poster Session: Characterising colour processing in anomalous trichromacy with steady-state visually evoked potentials.

Colour vision is based on the capture of light by short (S), medium (M) and long (L) wavelength sensitive retinal cones. In postreceptoral colour processing, the outputs of the three cone types are first compared by two cone-opponent mechanisms, L/(L+M) and S/(L+M). In anomalous trichromacy, the separation between L and M cone peak spectral sensitivities is reduced compared to normal trichromacy, leading to decreased sensitivity for L/(L+M) colour differences. However, colour appearance is more similar to that of normal trichromats than cone-opponent models predict. Current evidence suggests this is due to postreceptoral compensation in the cortex, where reduced colour signals are amplified to use available neural resources. We devised a novel approach to investigate the site of postreceptoral compensation using steady-state visually evoked potentials (SSVEPs), captured by electroencephalography. We measured signals in response to flickering stimuli designed to isolate the S/(L+M) and L/(L+M) cone opponent mechanisms at both retinal and cortical sites. If compensation is cortical, we would expect any reduction for anomalous trichromats in retinal L/(L+M) SSVEP signals compared to S/(L+M) SSVEP signals to be rectified at the cortical site. Our study did not exclude the possibility of retinal compensation, in contrast to an existing fMRI study (Tregillus et al., 2021, Curr. Biol.). We present our novel method to address potential challenges in characterising these processes.