Visual motion thresholds mapped to midget and parasol ganglion cell topography in the human retina

Motion in visual images can be described in terms of changes in phases of Fourier components (phase cues), or displacements in the position of specific features (position cues) over time. Human observers are able to perceive motion using both cues, where perceived direction of motion is biased in favour of phase cues at higher spatial and temporal frequencies, and in favour of position cues at lower spatial and temporal frequencies. This suggests the existence of separable mechanisms for processing phase and position cues. We propose that these mechanisms receive separate inputs from the parasol (magnocellular) and midget (parvocellular) retinal ganglion cells. Using two-frame apparent motion Gabor stimuli that isolated phase and position cues, we measured displacement thresholds for motion direction discrimination across the visual field (from 0 to 15 degrees eccentricity) for 7 observers. Thresholds for positional displacements decreased significantly more steeply with eccentricity than those for phase displacements, mirroring precisely the decline with increasing eccentricity of the linear densities of the midget and parasol retinal ganglion cell populations respectively. These results suggest that the magnocellular and parvocellular visual pathways could constitute separable neural substrates for first-order (Fourier) and third-order (feature-tracking) motion perception.