UT PSY 394U - Perception of Motion, Depth, and Form

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Back28Perception of Motion, Depth, and FormRobert H. WurtzEric R. KandelIN VISION, AS IN OTHER mental operations, we experience the world as a whole. Independent attributes—motion, depth, form, and color—are coordinated into a single visual image. In the two previous chapters we began to consider how two parallel pathways—the magnocellular and parvocellular pathways, that extend from the retina through the lateral geniculate nucleus of the thalamus to the primary visual (striate) cortex—might produce a coherent visual image. In this chapter we examine how the information from these two pathways feeds into multiple higher-order centers of visual processing in the extrastriate cortex. How do these pathways contribute to our perception of motion, depth, form, and color?The magnocellular (M) and parvocellular (P) pathways feed into two extrastriate cortical pathways: a dorsal pathway and a ventral pathway. In this chapter we examine, in cell-biological terms, the information processing in each of these pathways.We shall first consider the perception of motion and depth, mediated in large part by the dorsal pathway to the posterior parietal cortex. We then consider the perception of contrast and contours, mediated largely by the ventral pathway extending to the inferior temporal cortex. This pathway also is concerned with the assessment of color, which we will consider in Chapter 29. Finally, we shall consider the binding problem in the visual system: how information conveyed in parallel but separate pathways is brought together into a coherent perception.Figure 28-1 Organization of V1 and V2.A. Subregions in V1 (area 17) and V2 (area 18). This section from the occipital lobe of a squirrel monkey at the border of areas 17 and 18 was reacted with cytochrome oxidase. The cytochrome oxidase stains the blobs in V1 and the thick and thin stripes in V2. (Courtesy of M. Livingstone.)B. Connections between V1 and V2. The blobs in V1 connect primarily to the thin stripes in V2, while the interblobs in V1 connect to interstripes in V2. Layer 4B projects to the thick stripes in V2 and to the middle temporal area (MT). Both thin and interstripes project to V4. Thick stripes in V2 also project to MT.P.549The Parvocellular and Magnocellular Pathways Feed Into Two Processing Pathways in Extrastriate CortexIn Chapter 27 we saw that the parallel parvocellular and magnocellular pathways remain segregated even in the striate cortex. What happens to these P and M pathways beyond the striate cortex? Early research on these pathways indicated that the P pathway continues in the ventral cortical pathway that extends to theinferior temporal cortex, and that the M pathway becomes the dorsal pathway that extends to the posterior parietal cortex. However, the actual relationships are probably not so exclusive.The evidence for separation of function of the dorsal and ventral pathways begins in the primary visual, or striate, cortex (V1). Staining for the mitochondrial enzyme cytochrome oxidase reveals a precise and repeating pattern of dark, peg-like regions about 0.2 mm in diameter called blobs. The blobs are especially prominent in the superficial layers 2 and 3, where they are separated by intervening regions that stain lighter, the interblob P.550regions. The same stain also reveals alternating thick and thin stripes separated by interstripes of little activity (Figure 28-1 in the secondary visual cortex, or V2).Figure 28-2 The magnocellular (M) and parvocellular (P) pathways from the retina project through the lateral geniculate nucleus (LGN) to V1. Separate pathways to the temporal and parietal cortices course through the extrastriate cortex beginning in V2. The connections shown in the figure are based on established anatomical connections, but only selected connections are shown and many cortical areas are omitted (compare Figure 25-9). Note the cross connections between the two pathways in several cortical areas. The parietal pathway receives input from the M pathway but only the temporal pathway receives input from both the M and P pathways. (Abbreviations: AIT = anterior inferior temporal area; CIT = central inferior temporal area; LIP = lateral intraparietal area; Magno = magnocellular layers of the lateral geniculate nucleus; MST = medial superior temporal area; MT = middle temporal area; Parvo = parvocellular layers of the lateral geniculate nucleus; PIT = posterior inferior temporal area; VIP = ventral intraparietal area.) (Based on Merigan and Maunsell 1993.)Margaret Livingstone and David Hubel identified the anatomical connections between labeled regions in V1 and V2 (Figure 28-1B). They found that the P and M pathways remain partially segregated through V2. The M pathway projects from the magnocellular layers of the lateral geniculate nucleus to the striate cortex, first to layer 4Cα and then to layer 4B. Cells in layer 4B project directly to the middle temporal area (MT) and also to the thick stripes in V2, from which cells also project to MT. Thus, a clear anatomical pathway exists from the magnocellular layers in the lateral geniculate nucleus to MT and from there to the posterior parietal cortex (Figure 28-2).Cells in the parvocellular layers of the lateral geniculate nucleus project to layer 4Cβ in the striate cortex, from which cells project to the blobs and interblobs of V1. The blobs send a strong projection to the thin stripes in V2, whereas interblobs send a strong projection to the interstripes in V2. The thin stripe and interstripe areas of V2 may in turn project to discrete subregions of V4, thus maintaining this separation in the P pathway into V4 and possibly on into the inferior temporal cortex. A pathway from the P cells in the lateral geniculate nucleus P.551to the inferior temporal cortex can therefore also be identified (Figure 28-2).Figure 28-3 Motion in the visual field can be perceived in two ways.A. When the eyes are held still, the image of a moving object traverses the retina. Information about movement depends upon sequential firing of receptors in the retina.B. When the eyes follow an object, the image of the moving object falls on one place on the retina and the information is conveyed by movement of the eyes or the head.But are these pathways exclusive of each other? Several anatomical observations suggest that they are not. In V1 both the magnocellular and parvocellular pathways have inputs in the blobs, and local neurons make extensive connections


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