Parvocellular neurons limit motion acuity in human peripheral vision

It is generally believed that the perception of moving targets is mediated by the magnocellular (M) pathway in primate vision, but evidence is emerging that the parvocellular (P) pathway may also play a role in motion perception. Human peripheral vision is susceptible to anomalous motion perception because of spatial aliasing, and in this study we used this fact to determine if the P pathway can mediate information about low- and high-velocity stimuli. Psychometric functions relating visual performance to stimulus spatial frequency were measured for the directional discrimination of drifting sinusoidal gratings presented at 40 degrees eccentricity. Applying the sampling theorem to our results, we estimated that the Nyquist frequency of the limiting sampling array for directional discrimination is 1.7 cycles per degree. This result was compared with the Nyquist limit and spatial filtering properties of M and P ganglion cells in the human peripheral retina, calculated from histological data on their density and dendritic field size. Our results provide evidence to suggest that the reversed motion illusion in human peripheral vision is due to spatial aliasing by the P ganglion cell mosaic. We conclude that the sampling density of P ganglion cells limits veridical motion acuity in human peripheral vision, even for high-velocity targets. This provides further evidence that the P pathway is involved in processing information about motion.