The magnocellular pathway is one of three primary subcortical pathways (magnocellular, parvocellular, and koniocellular pathways) leading from the retina to visual cortex via the lateral geniculate nucleus (LGN). Cells in the magnocellular pathway (M pathway) are specialized for detecting contrast sensitivity, course features, and movement. In some ways, the M pathway can be considered the origin of the parietal or dorsal visual stream, as it has been shown that the M pathway dominates in the route leading from V1 to the parietal cortex. Experimental ablation of the M pathway results in reduced spatial contrast sensitivity and impairments in detecting rapidly moving or flickering stimuli, while visual acuity and color contrast sensitivity appear to be spared.
Of the three major visual streams in the primate visual system, much is known about the magnocellular and parvocellular (P) pathways, whereas less is known about the more recently discovered koniocellular (K) pathway. For the most part, the M and P pathways are thought to function as separate systems, each serving distinct visual processes (e.g., motion, color), working in parallel from the retina to visual cortex; however, findings suggest that some aspects of motion detection could be supported by combining information from the M and P pathways in primary visual cortex (De Valois and Cottaris 1998).
In the M pathway retinal ganglion cells called parasol cells, which are sensitive to movement, flicker, and contrast, project onto the magnocellular layers (layers 1 and 2) of the LGN. Layer 1 receives input from the contralateral eye and is called the contralateral layer; layer 2 is ipsilateral. Only about 10% of the cells in the LGN are in the magnocellular layers (the remaining 90% are divided between the P and K layers). Investigations of the monkey visual system have revealed that projections from the magnocellular LGN layers terminate primarily in V1 layer 4Cα; cells from this region then synapse with cells in V1 layer 4B. Magnocellular LGN cells also project to V1 layer 6, which then projects back to the LGN. From layer 4B, projections are sent to several regions that are primarily involved in motion processing, low spatial frequency detection, and low-contrast analysis, including V2’s thick CO stripe regions, and to dorsal V3 and the middle temporal area (MT, also referred to as V5). Projections from MT travel predominantly to the parietal cortex. Motion-perception deficits, or akinetopsia, have been reported in patients with V5 lesions as well as in neurologically healthy individuals who received transcranial magnetic stimulation to cause a temporary disruption of MT processing.
For Information About the M and P Pathways See
- Kaplan, E., Lee, B. B., & Shapley, R. M. (1990). New views of primate retinal function. In N. N. Osborne & G. J. Chader (Eds.), Progress in retinal research (Vol. 9, pp. 273–336). New York: Pergamon Press.Google Scholar
For Information About the K Pathway See
- Xu, X., Ichida, M. J., Allison, J. D., Boyd, J. D., Bonds, A. B., & Casagrande, V. A. (2001). A comparison of koniocellular, magnocellular and parvocellular receptive field properties in the lateral geniculate nucleus of the owl monkey (Aotus trivirgatus). The Journal of Physiology, 531, 203–218.CrossRefPubMedPubMedCentralGoogle Scholar