Conscious visual abilities in a patient with early bilateral occipital damage

Abstract

MM is a young adult who experienced bilateral strokes at birth that resulted in loss of left Brodmann areas 17, 18, 19 and right area 17, with extensive damage to right area 18. In spite of the total absence of primary visual cortex, he plays video games and can ride a bicycle in traffic (Jan et al., 1986, Ped Neurol). This is a preliminary report on his preserved vision. Horizontal OKN could be elicited from either eye. Smooth pursuit showed saccadic intrusions. The color of red, green and blue targets could be named, but Farnsworth color discrimination performance was at chance. Binocular grating acuity using Teller acuity cards was 10 cpd, providing MM was allowed to move his head and eyes. This is within normal limits for a 3 year-old child. The direction of motion in 100% coherent random dot kinematograms could be identified with perfect accuracy at speeds above 2 deg/s. MM reported he could “see” the moving dots and was not guessing. At speeds below 2 deg/s, he could not perceive motion and showed no evidence of better than chance guessing. These abilities are different from those termed “blindsight” in patients with homonymous hemianopia because MM appears to be aware of the stimuli he can discriminate. His vision may be similar to the residual vision reported in patient GY (eg. Morland et al., 1999, Brain). MM's motion perception was further studied using functional MRI. Fast motion activated several cortical regions: anterior and posterior left superior temporal gyrus (BA 22, 38), right premotor cortex (BA 6), right precuneus (BA 31) and right posterior cingulate (BA 23). Although the right area V5/MT+ appeared to be present based on anatomic landmarks, no activation was observed in this region. No activation occurred with slow motion, stationary dots or full-field flashes of light. This case demonstrates remarkable plasticity in the human visual system and may have implications for understanding the functional organization of the motion pathways.