Jesse Macyczko¹, Camile Franke¹, Michael Crognale¹, Michael Webster¹; ¹University of Nevada, Reno

At early visual stages, information about color is carried by two “cardinal” dimensions comparing signals in the LvsM cones or SvsLM cones. Equating the strength of signals along these two dimensions is important for designing and interpreting many experiments, but is inherently problematic and task-specific, and can range in metrics from cone contrasts to multiples of threshold. We developed an objective measure of the equivalent contrasts along the two axes at early cortical levels, based on steady-state visual evoked potentials. Stimuli were 1 c/deg chromatic gratings defined by the LvsM and SvsLM axes, with equiluminance determined empirically with a minimum motion task. The two color axes were shown as onset patterns and alternated at 3 or 4 Hz , with responses recorded by a single electrode positioned at OZ. During presentation the contrast of the SvsLM grating remained fixed, while the LvsM contrast was varied over a wide range across successive trials, in order to estimate the relative contrast at which the alternation response (based on summing the to the fundamental frequency and its unique harmonics) reached a minimum. The procedure is thus similar in principle to nulling paradigms such as flicker photometry. Observers exhibited nulls that were well-fit by a modified Naka-Rushton function where the portion of the function below the cross point was mirrored. This allowed us to capture the contrast match point between the SvsLM and LvsM gratings, which was found to be generally consistent with a multiple-of-thresholds scaling of the axes. This direct measure of cortical contrast can be used to track color coding in different populations, such as during development or with color deficiencies. In additional measurements we are applying the technique to compare chromatic responses in color-normal observers and anomalous trichromats in order to test for potential compensatory neural gains in the anomalous observers.

Acknowledgements: Supported by EY010834