Magnocellular-parvocellular imbalance hampers mirror image discrimination

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

From the retina, information is conveyed to the brain through two main complementary channels: a parvocellular (P) channel highly sensitive to stimuli of high spatial frequency, high-contrast and low-temporal frequency, and a magnocellular (M) channel mostly sensitive to stimuli with complementary characteristics. A longstanding issue in vision neuroscience concerns how information carried in these two streams contributes to conscious visual experience. Based on recent neuropsychological evidence, we tested the intriguing hypothesis that efficiency at discriminating mirror images may depend on the balance between the involvement of M- and P- channels in processing a visual stimulus. We first determined the P- and M- efficiency of 40 participants with a two-alternative forced-choice orientation discrimination task and a staircase design to measure their contrast sensitivity threshold for P-biased and M-biased gratings tilted 45° left or right from the vertical. Then, we tested participants’ ability to discriminate tilted asymmetrical shapes differing in terms of either a plane-rotation (40°), a mirror reflection across a vertical axis, or a mirror reflection across a shape-based axis in a speeded same/different judgment task. We found significant positive correlations between an index of P-M imbalance (P-efficiency – M-efficiency) and individuals’ efficiency at discriminating both types of mirror images, but not plane-rotations. These results suggest that an adequate balance between M- and P- channels could be determinant in our ability to discriminate mirror images.