Magnocellular-parvocellular imbalance correlates with mirror image discrimination

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

Objectives. 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. In the brain, information from the P channel mostly flows to occipito-temporal « ventral stream » areas aimed at object recognition, and information from the M channel mostly flows to occipitoparietal « dorsal stream » areas involved in spatial vision. The objective of this study is to contribute to a better understanding of how these complementary streams interact to support visual experience. Research question. Recent neuropsychological studies have reported three individuals who couldn’t discriminate mirror images of high-contrast and high-spatial frequency shapes (biased toward the Pchannel), but had far less or no difficulty at all for low-contrast, low-spatial frequency shapes. Here, 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. Materials and methods. To test this hypothesis, we first determined participants’ P- and M- efficiency (N=40). We used a two-alternative forced-choice orientation discrimination task and a staircase design to measure their contrast sensitivity threshold for P-biased (8 c/°, square-wave, ramped) and M-biased (0.5c/°, sine-wave, gaussian mask, intermittent flashing) 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 planerotation (40°), a mirror reflection across a vertical axis, or a mirror reflection across a shape-based axis in a speeded same/different judgment task. Results. We found significant positive correlations between an index of P-M imbalance (P-efficiency – Mefficiency) and individuals’ ability to discriminate both types of mirror images. Conclusion. These results suggest that an adequate balance between M- and P- channels could be determinant in our ability to discriminate mirror images.