Characterising contrast response functions using salience-matched achromatic and chromatic Gabor patches

The human visual system processes chromatic and achromatic information by adding or contrasting signals from short (S), medium (M) and long-wavelength (L) cones, through cone-additive (L+M) or cone-opponent (L-M, S-(L+M)) mechanisms. Previous work has shown that Visual Evoked Potentials (VEPs) to chromatic and achromatic sinusoidal gratings differ with respect to morphology (e.g., Porciatti et al., 1999). Here we characterize VEP contrast response functions to chromatic and achromatic Gabors in a relatively large sample of participants (n=27). Heterochromatic flicker photometry was used to ensure isoluminance of chromatic stimuli. Participants viewed luminance and chromatic Gabors at four different contrast levels while electroencephalograms were recorded. Cone-specific detection thresholds (using Cambridge Colour Test) and salience matching data across the four contrasts for colour and luminance were also collected to establish the range of individual differences in the sample. We replicate the findings of previous normative studies: achromatic-driven pattern-onset VEPs are characterized by a robust P1 component that saturates at higher levels of contrast and is followed by a non-contrast-dependent N1, while chromatic Gabors elicit a single, strong negative deflection whose amplitude and latency depend on stimulus contrast more linearly. The data we present are the beginnings of a comprehensive normative data set that will eventually be used for control comparisons in a large-scale study on visual function in individuals with bipolar disorder.