Electroencephalogram Analysis Regarding Visual Information Processing in a Grapheme-color Synesthete

Abstract

In order to explore characteristics of visual information processing in grapheme-color synesthesia, we examined behavior (response correctness and reaction time) and temporal activities of EEG during the performance of two kinds of "embedded shape tasks" in one synesthete and 16 nonsynesthetic subjects. We used three black capital letters, including one letter which a synesthetic subject perceived in color. The target grapheme was made of a letter which a synesthetic subject perceived in color (TASK1), and one that was not perceived in color (TASK2). There was a significant difference in reaction time between the two tasks. Measuring the difference in amplitude of EEG activity at P4 between the two tasks, biphasic activity change was observed. At 232.5 ms in the late phase, the bilateral occipital and parietal lobes, and the left frontal lobe were activated. These results suggest that biphasic activity change is related to different visual information processing in synesthesia; the early phase is related to directing attention to a shape with color, while the late phase to the recognition of a shape with color. It is also suggested that activated areas of the brain in the late phase function separately in causing grapheme-color synesthesia