An fMRI Investigation of a Patient with Cerebral Achromatopsia: Evidence for a Role of Chromatic Extrastriate Mechanisms in Motion Encoding

Abstract

We have previously reported psychophysical results of tests on a patient (WM) who suffered from cerebral achromatopsia but nevertheless showed strong signs of unconscious access to chromatic information. This took the form of (a) better performance when chromatic modulation was added to existing luminance modulation in the stimulus, and (b) evidence suggesting that the chromatic information was supplied by a fast, non-opponent set of channels (Troscianko et al, 1996 Current Biology6 200 – 210). The psychophysical findings led us to predict that WM should show strong potentiating effects of chromatic modulation, and that these should be evident in enhanced activity of extrastriate areas thought to respond to fast flicker and motion, particularly V5 (Brodmann area 37/39 in lateral occipito-temporal cortex), when a (luminance + colour) stimulus is presented as compared to a luminance-only stimulus. WM was tested with susceptibility-based functional magnetic resonance, by applying gradient-recalled echo-planar imaging (EPI) sequences, with and without visual motion stimulation. Anatomical and functional images were recorded with a 1.5 T Siemens Magnetom Vision scanner (TR 3000 ms, TE 70 ms, flip angle 90°, FOV 250 mm, matrix 112 × 128, 12 4-mm slices through visual cortex). The visual stimuli were drifting sinusoidal gratings with either luminance-only, or colour + luminance modulation, and with the addition of either static or 25 Hz dynamic luminance noise. After correction for head-motion artifacts, we found (a) a significantly enhanced extrastriate (V5/V5a) response when chromatic information was added, and (b) that fast dynamic noise affected the same areas. We conclude that colour information can activate (or modulate the activity of) motion-specific mechanisms usually thought to be colour-blind. This finding has implications both for the understanding of unconscious information processing in vision, and for considerations of neural efficiency when encoding redundant image information.