Abstract
Aim. We have previously shown that sensitivity to photometric (colour and luminance) transformations in images of real scenes is lower for familiar compared to unfamiliar transformations. This suggests a normalization, or gain reduction, of familiar transformations. At what stage does this normalization occur? We tested whether it occurred before or after the stage of binocular combination by measuring dichoptic difference thresholds, or DDTs (the DDT is the just detectable between-eye difference in a binocularly superimposed image-pair), for photometrically-transformed real scenes, and comparing these with conventional image-difference thresholds. Methods. Stimuli were fifty calibrated color photographs of real scenes. The chromaticity and saturation of every image pixel was represented as a vector in a modified version of the MacLeod-Boynton color space, and could be translated, rotated, compressed or randomly repositioned within that color space. The dichoptic image pairs were presented via a modified Wheatstone stereoscope, while the conventional image pairs were presented with the stereoscope removed. All thresholds were measured using 2AFC. Results. DDTs, unlike the conventional image difference thresholds, were more or less constant, i.e. unaffected by familiarity. Conclusion. The result suggests that the normalization process happens after the stage of binocular combination.
Supported by Canada Institute of Health Research grant #MOP-11554 given to F.K.