Parvocellular and magnocellular responses in peripheral vision

Direct psychophysical discrimination of the spatial and temporal characteristics of the retinal magnocellular and parvocellular pathways in the primate visual system has proved elusive. However, the results of the experiment reported here suggest that such discrimination is possible using stimuli tailored to the underlying physiology of the M and P retinal ganglion cells. To maximise the sensitivity, the visual stimuli used were tangential sinusoidal gratings with a linearly varying spacing matched to the retinal ganglion cell spacing. The results support the hypothesised dual-segment characteristic of the contrast sensitivity function and are consistent with the known contrast gain and spatial physiological characteristics of the P and M pathways in the primate visual system. The results are consistent with previous observations that the overall system response is that of the most sensitive of the two channels at a given contrast, rather than summing the two channel responses. In order to visualise how the two pathways transmit visual information to the lateral geniculate nucleus (LGN) and the higher visual areas, I developed a simple algorithm for determining local contrast as a function of scale in natural images, allowing the derivation of contrast heat maps. The algorithm was validated using simple sine wave images and the results on natural images were consistent with previous studies. I then used the same algorithm to plot contrast response images at the characteristic scales of the M and P pathways, which allowed visualisation of the spatial contrast responses of two pathways at an eccentricity of 23.5 degrees. The visualisation can be interpreted as indicating that the P and M pathways perform different filter functions of edge enhancement and contrast enhancement, respectively.