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Unformatted text preview: Cognitive Neuroscience Visual System: From eyetobrain 1. Info transfer from eye to V1 2. "Where" info Eye of origin: depth Spatial location 3. "What" info Receptive fields and their properties 4. What and Where beyond V1 Retina Photoreceptors: contain lightsensitive photopigments which decompose when a single photon is absorbed graded potential (only ganglion & amacrine cells produce action potentials) Rods: Color insensitive Respond to low intensity of light (useful at night) Distributed throughout the retina Cones: Color sensitive Require higher intensities to respond Concentrated near the center of the retinamacula Extensive signal processing in the retina: 260 million photoreceptors > 2 million ganglion cells Macula central 10 Fovea highest acuity (density of cones) central 2 no rods From visual fields (VF) to brain Light from RVF falls on the left side of each retina (physics) Info from the left side of each retina travels to the left LGN and then on to the left hemisphere Vice versa for LVF To accomplish this: Fibers from the right and left side of each retina must travel separately Some fibers from each eye must "cross over" to the other side (optic chiasm) At the LGN the RVF info is in the left LGN and the LVF info is in the right LGN From eye to brain: Temporal & nasal hemiretinas Optic nerve Optic chiasm Optic tract 1 Cortical pathway: LGN (Thalamus) Visual radiations V1 (Occipital Lobe) 2Subcortical pathway Saccade Control : 10% of retina fibers Superior colliculus Pulvinar (Thalamus) Parietal lobe Cortical pathway 1 Cortical pathway: Retinogeniculate pathway: Temporal & nasal hemi retinas Optic nerve Optic chiasm Optic tract LGN Geniculocortical pathway: Visual radiations V1 Cortical pathway 1. Info transfer from eye to V1 2. Where info Eye of origin Spatial location 3. "What" info Receptive fields and their properties 4. What and Where beyond V1 Stereopsis: binocular depth perception Many sources of info about the distance of an object--some are monocular and others binocular The 2.4 inch (6 cm) distance between the two pupils two slightly different images of the world. This displacement between the horizontal positions of corresponding images is called binocular disparity. By using two images of the same scene obtained from slightly different angles, it is possible to triangulate the distance to an object with a high degree of accuracy. If an object is far away, the disparity of that image falling on both retinas will be small. If the object is close or near, the disparity will be large. "geographic" preservation of eye of origin Info from right and left eyes is segregated: LGN: layers 2,3,5 ipsilateral retina layers 1, 4, 6 contralateral retina V1: ocular dominance columns Segregation by eye of origin in cortex Intrinsic optical imaging: Calcarine fissure Retinotopic Map Cortical magnification While viewing the stimulus, the monkey was injected with a radioactive agent. metabolically active cells absorb the agent. Retinotopic Map Note: cortical "magnification" What should be the results of damage to different parts of V1? 1. Info transfer from eye to V1 2. Where info Eye of origin Spatial location 3. "What" info Receptive fields and their properties 4. What and Where beyond V1 Receptive Fields The area of the world which a cell responds to Receptive field properties: Location Size Preferred stimulus properties General principle: As we move forward into the (visual) system, receptive fields become larger and preferred stimulus properties more complex Single cell recording: A method for investigating receptive fields Receptive fields increase in complexity "edge" detectors (V1) How can this happen? Cortical Modules Each point in visual space is analyzed by a circumscribed 2x2 mm patch of cells (they all have a common receptive field location) Containing (among other things): 2 sets of ocular dominance columns a full set orientation columns representing all orientations of an edge 1. Info transfer from eye to V1 2. Where info Eye of origin Spatial location 3. "What" info Receptive fields and their properties 4. What and Where beyond V1 Beyond V1: what/where Basic distinction between what and where What (ventral stream): color & form Where (dorsal stream): motion & location Evidence types: 1Single cell recording studies 2Animal lesion studies 3Neuroimaging studies 4Neuropsychological cases 1Single unit recording evidence: the what/where distinction Temporal neurons: color, shape, faces Parietal neurons: Direction of motion, velocity, control of visual attention, reaching What's the basis for the what/where distinction? Rods Cones MagnocellsLarge Parvocells Small Different Types of Retinal Ganglion Cells LGN Single cell recording in LGN reveals receptive field properties Sensitivity: Magno (layers 1,2) Color Contrast Form Temporal lo hi lo hi Parvo (layers 36) hi lo hi lo Dorsal route Ventral route 2Animal lesion studies: Location/identity distinction (Ungerleider et al.,) What/where: Color/motion 4Neuropsychological Cases: Object recognition vs. spatial processing What: Temporal/occipital lesions: Agnosia Object recognition deficit Prosopagnosia Face recognition deficit Achromatopsia Color perception deficit Alexia Reading deficit Where: Parietal lesions: Visuospatial neglect Failure to attend to a side of space Akinetopsia Failure to perceive motion Disorientation deficits Navigational difficulties Agraphia Spelling/writing deficit ...
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This note was uploaded on 07/29/2008 for the course NEUROSCIEN 70 taught by Professor Whitney during the Spring '08 term at Johns Hopkins.
- Spring '08