Color Input into Motion Processing in Grapheme-Color Synesthetes

Abstract

Background: It has been proposed that individuals with grapheme-color synesthesia have increased levels of connectivity, particularly between V4 and Visual Word Form Area (Hubbard and Ramachandran, 2005 ). To investigate whether increased connectivity may be a widespread phenomenon, we asked whether color and motion interactions are stronger in synesthetes than in controls. To this end, we used a paradigm that indexes the amount of chromatic (red/green) input to motion processing as compared to a benchmark of luminance (light/dark) input to motion processing. Methods: We used a MOT/DET paradigm, which obtains the ratio of contrast threshold for discriminating direction of a moving grating (MOT), to the contrast threshold for detecting that same moving grating (DET). Typical adults exhibit MOT/DET ratios near 1.0 for luminance gratings, but closer to 2.0-4.0 for chromatic gratings, suggesting that chromatic information provides weaker input to motion mechanisms than luminance information. Our stimuli were luminance and chromatic horizontal gratings (1.0cpd, 5.5Hz, subtending 2.0x2.0°). In the MOT task, a grating was presented and participants indicated whether it moved up or down. In the DET task, participants indicated in which of two intervals contained the moving grating. The relative contribution of chromatic versus luminance information for motion processing is calculated as the difference in log MOT/DET ratios for chromatic vs. luminance gratings (Diff-Ratio), with values > 0 indicating weaker chromatic input. If synesthetes have greater-than-normal color-motion interactions: 1) their chromatic MOT/DET ratios should be lower than controls, and 2) their Diff-Ratio should be lower than controls. Results: While both synesthestes (n = 7) and controls (n = 7) showed Diff-Ratios significantly greater than 0 (indicating weaker chromatic vs. luminance input to motion), there were no group differences in Diff-Ratio, nor chromatic MOT/DET. Conclusions: At current, we do not find evidence for increased interactions between color and motion in synesthetes.