Ilgin Cebioglu¹, Jasna Martinovic², Laura K. Young¹, Gabriele Jordan¹, Quoc C. Vuong¹, Anya Hurlbert¹; ¹Newcastle University, ²University of Edinburgh

Sorting socks, picking peaches, or following footballers are tasks that typically require differentiating colours. Chromatic discrimination ability is constrained by low-level factors, e.g. spectral sensitivities of the retinal cone types, yet influenced by higher-level cortical processes. At which level and to what extent these might compensate for sensory weaknesses are open questions. Here we determine whether differences in behaviourally measured chromatic discrimination thresholds along the L-M (‘red- green’) and S–(L+M) (‘blue-yellow’) cone-opponent axes are captured by differences in cortical visual evoked potentials (VEP). Seventeen participants with normal trichromacy (aged 18-32; 11 females) were assessed on the Colour Assessment and Diagnosis (CAD) test. Separately, participants completed an EEG task, in which they viewed chromatic sinusoidal gratings modulated along one of four unipolar directions (red, green, blue and yellow) at four chromatic contrast levels, at individually-determined isoluminance levels. Participants completed 60 trials per direction in randomised blocks. The amplitude and latency of VEPs were determined from the Oz, O1 and O2 electrodes. CAD thresholds varied with colour direction (p < .001), with higher thresholds for blue and yellow vs. red and green, and for blue vs. yellow. The amplitude and latency of the N1 component varied with chromatic contrast, in line with previous literature; the cortical contrast response gain was determined from the slope of these changes. Both amplitude and latency slopes varied significantly with colour direction (p < .001 and p = .006), with higher slopes for red and green vs. blue and yellow. Unlike CAD thresholds, slopes did not differ significantly between blue and yellow directions. The results suggest that changes in amplitude and latency of the N1 component capture asymmetries in suprathreshold chromatic sensitivity between the two cone-opponent axes, corresponding to asymmetries observed at threshold, supporting their future use in studies of potential cortical compensations for sensory weaknesses.