Back 28 Perception of Motion Depth and Form Robert H Wurtz Eric R Kandel IN 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 549 The Parvocellular and Magnocellular Pathways Feed Into Two Processing Pathways in Extrastriate Cortex In 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 the inferior 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 550 regions 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 4C 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 4C 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 551 to 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 between the blob and interblob compartments In V2 cross connections exist between the stripe compartments Thus the separation is not absolute but
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