|MadSci Network: Neuroscience|
Dear Mr. Wagner, You ask an interesting question and for some reason they sent it to me, a biochemist when they should have sent it to a neurobiologist. Ah well, the world isn't perfect so I'll take a shot. Incidentally, I spent many a happy hour in Fergus when I was in university! Basically, the retina in your eye is an extension of the brain, i.e. it is a part of the central nervous system. The neurons that make up the different cell layers in the retina are responsible for the perception of either stationary light (i.e. picking out the salient features of objects we look at) or motion. One group of cells, called retinal ganglia, contains two basic types of cells which can be distinguished on the basis of their responses to stationary and moving patterns of light. One class, called X cells responds to stationary spots or gratings in a particular manner. The other main class, Y cells, responds strongly to changes in illumination or to moving stimuli without clear spatial summation. X cells can distinguish the fine grain of stationary patterns and Y cells detect objects moving across the visual field or changes in intensity. Signals are then sent from these neurons to structures located in the brain including the lateral geniculate nucleus, the superior colliculus and ultimately the primary visual cortex. These signals are processed at each of these locations and the information is also disseminated to other sensorimotor areas. Thus, an initially simple analysis of edge detection by cells in the retina is converted into a sophisticated and integrated response which allows you to estimate the speed and direction of moving objects and make the necessary motor responses (such as jumping out of the way of a moving car!). I hope this helps.
Try the links in the MadSci Library for more information on Neuroscience.