The contribution and mode of integration of parvocellular and magnocellular information in the course of shape orientation perception.

Visual information reaches the cortex via multiple pathways, most notably the magnocellular (M) and parvocellular (P) streams. How their inputs are integrated remains unclear. We previously documented the case of Davida, a young woman who perceives high-contrast, sharply bounded 2D shapes as rotated or mirror-reversed, while he perception normalizes when stimuli are blurred, low in contrast, or brief. Here, we first review previous findings and propose that her disorder may reflect abnormally strong P-on-M inhibition. We then present three studies supporting this account: two demonstrate abnormally strong P-on-M inhibition in Davida, and a third shows that the degree of P-M imbalance predicts mirror-image discrimination difficulty in neurotypical individuals. Finally, re-analysis of published data suggests that distinct aspects of shape orientation are differentially sensitive to P-M imbalance. Together, these findings highlight the crucial role of dynamic P-M interactions in shaping visual perception.