Selective damage to either the "M" or "P" cell pathway in human retinal disease: implications for visual processing.

Studies in primates have shown there to be 2 main visual afferent pathways (magno and parvo) operating in parallel with different functional roles. Disease state may selectively affect one or other of these, as shown by particular sensory losses. In cases of treated Cuban tropical "neuropathy" (a retinopathy also occurs) visual acuity and achromatic contrast sensitivity are preserved although colour vision is severely affected indicating selective preservation of the "magno" pathway. In Melanoma Associated Retinopathy, red-green colour vision is preserved although achromatic contrast sensitivity for low spatial frequencies, and the detection of motion is damaged suggesting preferential damage to the "magno" pathway. Our studies illuminate two controversial aspects of the P and M systems: the role of type II parvocellular cells in colour vision and the role of M cells in detecting stimuli of low spatial and low temporal frequency.