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PSYC 3530 (13)
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13. The Occipital Lobes.docx

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York University
PSYC 3530
Guy Proulx

13. The Occipital Lobes Tuesday, November 5, 2013 9:48 AM Anatomy of the Occipital Lobes  Lies beneath the occipital bone at the back of the skull  Distinguished from the parietal lobe by the parietal-occipital sulcus   Calcarine sulcus-- contains much of V1, divides the upper and lower halves of the visual world  Subdivisions of the Occipital Cortex o Areas V4 and V8 are color regions (mostly V4) o Area V1 has many layers, more than the usual 6 o Area V1 is functionally heterogeneous-- it has more than one distinct function (i.e., it represents color, form, and motion) o The visual cortex is sometimes called the striate cortex because it shows stripes when stained o "Thin stripes" - color perception o "Thick stripes" - form perception o "Pale stripes" - motion perception o Color vision is integral to the analysis of position, depth, motion, and the structure of objects (movement is not color-blind) o Color vision is evolutionarily advantageous-- being able to quickly spot a yellow fruit on a green background  Connections of the Visual Cortex o V1 (the striate cortex) is the primary vision area: receives the largest input from the lateral geniculate nucleus of the thalamus, and it projects to all other occipital regions  First processing level in the hierarchy o V2 also projects to all other occipital regions  Second level of processing o After V2, 3 distinct, parallel pathways emerge en route to the parietal cortex, superior temporal sulcus (STS), and inferior temporal cortex, for further processing o o The dorsal stream has a role in the visual guidance of movement o The ventral stream is concerned with object perception (including color) o The STS stream is probably important in visuospatial functions and in the perception of certain types of movements A Theory of Occipital-Lobe Function  Area V5 is specialized to detect motion  Area V3 detects "dynamic form"-- the shape of objects in motion  Selective lesions up the hierarchy in areas V3, V4, and V5 produce specific deficits in visual processing  A lesion in V5 produces an inability to perceive objects in motion  Area V4 also processes form, and thus damage to V3 is not enough to eliminate form from perception  Area V1 must function for the brain to make sense of what the more specialized visual areas are processing  Visual Functions Beyond the Occipital Lobe o More cortex concerns vision than with any other function in the primate brain  Total surface area for vision-related functions: 55% o o All ventral-stream regions are clearly responsive to some degree to all categories of stimuli o Vision is not unitary but is composed of many quite specific forms of processing; 5 specific forms of processing:  Vision for Action  Visual processing required to direct specific movements  Must be sensitive to movement of the target  A function of the parietal visual areas in the dorsal stream  Action for Vision  The viewer actively searches for only part of the target object and attends selectively to it  People with deficits in action for vision are likely to have significant deficits in visual perception  Visual Recognition  Recognizing objects and responding to visual information  Visual Space  Visual information comes from specific locations in space  Objects have location relative to an individual (egocentric space) and relative to one another (allocentric space)  Different aspects of spatial processing probably take place in both the parietal and temporal visual regions  Visual Attention  Independent actions of attention are probably required both for the guidance of movements (in the parietal lobe) and for object recognition (in the temporal lobe)  Visual Pathways Beyond the Occipital Lobe o Vision evolved first for motion, not for recognition o As primitive animals interact with their environment, they are adapted to learn more about it o The system of knowing what an object is includes the flow of visual information from area V1 to the temporal lobe in the ventral stream o The system controlling the visual guidance of movements includes the flow of information from area V1 to the parietal lobe in the dorsal stream  Patients with dorsal stream damage consciously report seeing objects but cannot reach accurately or shape the hand appropriately when reaching  Milner and Goodale propose that the dorsal stream be thought of as a set of systems for the on-line visual control of action  There are neurons in the posterior parietal regions that are active only when visual stimulation is combined with associated behavior (this can be seen as an interface between analysis of the visual world and motor action taken on it)  Most of the visual impairments associated with lesions to the parietal cortex can be characterized as visuomotor or orientational o Milner-Goodale model o o Dorsal stream-- guiding movements o Ventral stream-- identifying objects o The STS is part of the multimodal cortex and is characterized by polysensory neurons-- neurons that are responsive to both visual and auditory or both visual and somatosensory input  Imaging Studies of Dorsal and Ventral Streams o Neuroscientists identify brain regions associated with specific visual pathways by measuring regional blood flow as people perform visual tasks o Different regions take place in identifying faces vs. dots o The detection of shape activates regions along the STS and in the ventral region of the temporal lobe o Color activates V4 = lingual gyrus Disorders of Visual Pathways  2 key elements in the way the visual fields are organized: 1. The left half of each retina sends its projections to the right side of the brain, vice versa 2. Different parts of the visual field are topographically represented in different parts of area V1; thus injury to a specific region of area V1 produces a loss of vision in a specific part of the visual world   Monocular blindness-- destruction of the retina or optic
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