Magnocellular contribution to mirror image discrimination

Houbben, Marie [UCL] Vannuscorps, Gilles [UCL]

The human visual system processes visual information in two parallel streams with complementary specialization: the parvocellular (P) stream, which is highly sensitive to sharp-edged, fine, stationary, and highcontrast 2D stimuli and the magnocellular (M) stream, mostly activated by stimuli with complementary characteristics (coarse, large, moving, brief, 3D and low-contrast). A longstanding issue in vision neuroscience concerns how information carried in these two streams contributes to conscious visual experience. Recent neuropsychological evidence suggests that the M stream may play a critical role in the visual system’s ability to discriminate mirror images. To test this hypothesis we measured the efficiency of the M and P stream of 40 participants, as well as their ability to discriminate shapes that differed in terms of either a plane-rotation or a mirror reflection. In line with our hypothesis, we found a significant positive correlation between individuals’ M stream efficiency and their efficiency in discriminating mirror images (but not control stimuli differing by a plane-rotation). Unexpectedly, we also found an inverse correlation between the ability to discriminate mirror images and individuals’ P stream efficiency. These results suggest that unbalance between the efficiency of the M and P streams in the detriment of the magnocellular pathway is associated with difficulties discriminating mirror images. This finding may have implications for understanding the origin of mirror letter confusions in dyslexia.