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Chapter 13

PSYCH 2NF3 Chapter 13: PSYCH2NF3 - CHAPTER 13

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McMaster University
Ayesha Khan

2NF3 – CHAPTER 13 OCCIPITAL LOBES Divided from parietal lobe by the parietal-occiptal sulcus → no clear division on lateral hemispheres (temporal lobe or parietal cortex) → merges with medial and ventral temporal cortices → where lingual gyrus and fusiform gyrus are located → areas V2 and VP (on lingual gyrus) and areaV4 in the fusiform gyrus → ventral surface 3 clear (separate) parts → cuneate gyrus → calcarine sulcus → divides lower and upper halves of visual world → much of primary visual cortex (V1) → areas V2 and VP (on lingual gyrus) and areaV4 in the fusiform gyrus → ventral surface of each hemisphere → collateral sulcus SUBDIVISIONS OF THE OCCIPITAL CORTEX - Humans have additional visual processing capacity compared to monkeys, with areas going beyond V4 (from V1) - Area V1 has complex laminar organization → more than the typical 6 layers on a cortex → because cortical layer 4 itself has 4 distinct layers in the are V1 and looks as a thick stripe → which is why visual cortex is also called striate cortex - Area V1 is functionally heterogeneous → has more than one function → heterogeneity can be seen by staining cytochrome oxidase (an enzyme responsible for cell energy) → leads to viewing blobs, rich in cytochrome oxidase, which are separated by interblob regions (between the blobs) which are not so rich in cytochrome oxidase → blobs take part in colour perception → interblobs have a role in motion and form perception - Area V2 is also heterogeneus with stripes → thin stripes take colour perception → thick stripes take up form perception → pale stripes take up role of motion perception - Colour vision is important for perception of position, depth, motion and structure of objects - Major function of Area V4 is color processing → receives info from blobs CONNECTIONS OF THE VISUAL CORTEX - V1 is primary receptor → receives largest input from lateral geniculate nucleus (of the thalamus) and → projects to all other occipital regions - V2, second processing level → projects to all other occipital regions - After V2 → 3 different parallel pathways emerge to: 1→ parietal cortex → on dorsal stream → for visual guidance of movement 2→ superior temporal sulcus and 3→ inferior temporal cortex → both of these on ventral stream → responsible for object perception (colour, faces), certain types of movement A THEORY OF OCCIPITAL-LOBE FUNCTION - Area V4 → responds mainly to colour but also to form → receives information from blobs from area V1 → lesion to this area results in inability for colour cognition → even recalling the sensations of colours before lesion → can only see shades of gray and only remember gray - Area V5/area MT → responsible for detecting motion → receives info that is passed from Area V1 to Area V2 → lesion results in failure to perceive moving objects → they simply vanish once moving - Area V3 → receives info from Area V1 and V2 → responsible for dynamic form → the shape of objects in motion → would in principle affect form perception, but does not as Area V4 also deals with form - Area V1 (damage) → Area V1 must function for brain to make sense of what higher levels are processing → since it distributes information to area V2, V3, V4 and V5, damaging Area V1 leads to person being unaware of visual input but brain still perceives other inputs from as Area V2 → Area V2 receives information from lateral geniculate nucleus and through collicullus → to the thalamus → to the cortex VISUAL FUNCTIONS BEYOND THE OCCIPITAL LOBE - visual processing continues to parietal, temporal and frontal lobes → functions not yet assigned 5 forms of processing: - vision for action - action for vision - visual recognition - visual space - visual attention Vision for Action A function of parietal visual areas in the dorsal stream - Visual processing required to direct specific movements - Movement (of the body) guided by vision → such as reaching for something in a specific movement because you saw where and how it is located - Various visual areas guide all kinds of movement for vision for action →, a single region cannot guide all movements (head, neck, arms, fingers, etc) as the requirements are too different - must be sensitive of target's movement (shape, speed, location) Action for Vision - Viewer actively searches for only parts of the target object and attends to it selectively → we scan stimulus with numerous eye movements which focus on important/distinct features of the stimulus - When scanning a face we → tend to focus on eyes and mouth → also focus on left visual field of the face (right side of person's face) → only found when scanning faces and no other objects - Deficits in Action for vision are perception deficits - We also make a lot of eye movement, especially to the left, when asked to “visualize” (mental image of) certain objects → we also have such eye movement while trying to visualize things in the dark Visual Recognition - Some areas in temporal regions for faces, hands, other areas for objects and places - Ability to recognize objects and give them meaning Visual Space - Location of an object gives us a lot of information such as how far it is - egocentric space: location relative to the individual → central for controlling actions towards objects → visual space seems to be coded in neural systems related to vision for action - allocentric space: location relative to one another (one object/person/etc to another) → dependent on the identity of particular features of the world → likely to be associated with the regions of visual recognition Visual Attention - Selecting specific inputs of visual field and attending to them selectively - Such selection is dependent on time, places, or if particular movements are to be executed - Attention for guiding movement → in parietal lobe - Attention for recognizing objects → temporal lobe Visual Pathways Beyond the Occipital Lobe Vision evolved first for motion, not for recognition - Simple organisms can detect light → move from and to it - Recognition of what an object is → on temporal lobe in the ventral stream → receives info from area V1 - Control of visually guided movements → on parietal lobe in dorsal stream → receives info from area V1 - D.F. = patient with lesion to lateral occipital area → blind but shaped her hands appropriately reaching for objects → could unconsciously see location, size, shape as dorsal stream was intact - Patients with dorsal stream damage report seeing things but cannot reach accurately of shape the hand appropriately when reaching for objects Milner and Goodale: propose that dorsal stream can be seen as set of systems for visual control of action. Evidence: 1- Visual neurons in posterior parietal regions are only active when brain acts on visual info 2- Visual posterior parietal neurons therefore act as an interface between analysis of the visual world and motor action taken on it 3- Most visual impairments to the parietal cortex can be characterized as visuomotor or visuospatial Pathway of Action: Dorsal Stream V1 → V2 → V5(motion) + V3A(form) → Parietal Visual areas V1 → V2 → Parietal cortex Pathway of Object recognition: Ventral Stream V1 → V2 → V3(dynamic form) + V4(color form) → temporal visual areas Temporal and Ventral streams also exchange information through polysensory neurons in the STS stream → polysensory neurons: neurons that respond to both audio+visual or visual+somatosensory information → STS: superior temporal sulcus IMAGING STUDIES OF DORSAL AND VENTRAL STREAMS The identification of visual region is made by measuring regional blood flow as people perform visual tasks → facial stimuli tasks activated temporal regions → dot location tasks activated
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