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The magnocellular neurons are particularly responsive to movement oCones provide the majority of the input to the P layers, whereas rods provide the majority of the input to the M layers xThe parvocellular and magnocellular neurons project to different sites in the lower part of layer IV of the striate cortex xIn turn, these M and P portions of lower layer IV project to different parts of visual cortex Distributing prohibited | Downloaded by School Work ([email protected])
6.4 Seeing Edges xEdges are the most informative features of any visual display because they define the extent and position of the various objects in it Lateral Inhibition and Contrast Enhancement xThe nonexistent stripes of brightness and darkness running adjacent to the edges are called Mach bands; they enhance the contrast at each edge and make the edge easier to see xAlthough we are normally unaware of it, every edge we look at is high lighted for us by the contrast enhancing mechanisms of our nervous systems xIn effect, our perception of edges is between than the real thing (as determined by measurements of the physical properties of the light entering our eyes) xThe lateral eyes of the horseshoe crab are ideal for certain types of neurophysiological research xUnlike mammalian eyes, they are composed of very large receptors, called ommatidia, each with its own large axon xThe axons of the ommatidia are interconnected by the lateral neural network xTwo important aspects oIf a single ommatidium is illuminated, it fires at a rate that is proportional to the intensity of the light striking it; more intense lights produce more firing oWhen a receptor fires, it inhibits its neighbors via the lateral neural network; this inhibition is called lateral inhibition because it spreads laterally across the array of receptors The amount of lateral inhibition produced by a receptor is greatest when the receptor is most intensely illuminated, and the inhibition has its greatest effect on the receptor’s mediate neighbours xThe neural basis of contrast enhancement can be understood in terms of the firing rates of the receptors on each side of an edge xDraw figure 6.15/145 Distributing prohibited | Downloaded by School Work ([email protected])
xThe receptor adjacent to the edge on the more intense side (receptor D) fires more than the other intensely illuminated receptors (A, B, C), while the receptor adjacent to the edge on the less well illuminated side (receptor E) fires less than the other receptors on that side (F, G, H) xLateral inhibition accounts for these differences xReceptors A, B and C all fire at the same rate, because they are all receiving the same high level of stimulation and the same high degree of lateral inhibition from all their highly stimulated neighbours xReceptor D fires more than A, B, C because it receives as much stimulation as they do but less inhibition from its neighbours, many of which are on the dimer side of the edge x