Neurology and Neurosurgery Essay -- Medical Brain Health Essays

Neurology and Neurosurgery Neurology Overview Although our primary interest is with the Medial Temporal Lobe, also called the V5 area, a discussion of the entire motion perception pathway is instructive. Motion perception actually begins with the specialized visual receptors in the retina known as M-cells (from the Latin word magnus, for large). As the name implies, the M-cells are relatively large, located in the peripheral retina, and respond quickly to transient visual stimulation making them ideally suited for motion detection. By contrast, P-cells are smaller, located in the fovea, react more slowly to stimuli, and are suited to fine-detail vision. Impulses from the retina then travel via the optic nerve to the optic chiasm where fibers of the optic nerve from the inner (nasal) half of each retina cross while those from the outside (temporal) half of each retina stay on the same side. This partial crossing is a feature of mammals, whereas for most vertebrates below mammals, all the fibers cross. It must be pointed out that no motion processing is actually done in the optic chiasm. About 20% of the axons leaving the optic chiasm go to the Superior Colliculus, which is responsible for certain eye movements and spatial localization. The remaining 80% of the axons go to the Lateral Geniculate Nucleus, LGN (Schiffman, 2000, p. 71-73). The LGN represents the next motion processing step after the M-cells in the retina. The Magnocellular Division of the LGN specifically processes the impulses from the M- cells in the retina and is uniquely suited to distinguishing small contrasts between light and dark areas thereby enhancing three-dimensionality and motion ef... ..., J. W. (2004). Biological Psychology (8th ed.). Belmont, CA: Thompson-Wadsworth. Naikar, N. (1996). Perception of apparent motion of colored stimuli after commissurotomy. Neuropsychologia, 34(11),1041- 1049. Nawrot, M., Rizzo, M., Rockland, K.S., Howard, M. (2000). A transient deficit of motion perception. Vision Research, (40),3435-3446. Schiffman, H.R. (2000). Sensation and Perception (5th ed.). New York: John Wiley & Sons. Ulbert, I., Karmos, G., Heit, G., & Halgren, E. (2001). Early discrimination of coherent versus incoherent motion by multiunit and synaptic activity in human putative MT+. Human Brain Mapping, 13(4),226-238. Vaina, L.M., Cowey, A., LeMay, M., Bienfang, D.C., & Kikinis, R. (2002). Visual deficits in a patient with kaleidoscopic disintegration of the visual world. European Journal of Neurology, (9),463-477.