Fichas
Visual Cortex and Perception
Fichas del conjunto (14)
- 1The geniculo-striate pathway Projection targets of the optic nerve in descending order of sizeLateral geniculate nucleus
Superior colliculus (eye and head movements)
Accessory optic nuclei of pretectum (optokinetic reflex)
Olivary pretectal nucleus (pupil light reflex)
Suprachiasmatic nucleus of hypothalamus (entertainment of circadian rhythms) - 2Cortical magnificationNeed a lot of processing power for all the photoreceptors in the fovea
- 3Cells present in the visual cortex (V1) 9 layers with distinct cytoarchitecture and staining properties: I, II, III, IVA, IVB, IVCα, Ivβ, V, VIPyramidal cells - layers III, IVB, V and VI only
Only pyramidal cells send information out of the cortex to form connections with outer parts of the brain
Satellite cells (layer IVC) make only local connections - 41950s Hubel and Wiesel explored physiology of striate cortex in V1 in cats (later monkeys)Findings: Info from each eye was collected in ocular dominance columns. V1 contains neurons with a range of receptive field properties:
Binocularity
orientation selectivity
Direction sensitivity
Colour processing - 5Hubel and Wiesel's classic experiments on the role of environmental stimulation on the development of normal vision. Raise kittens from birth for 6 month. Double blindfold some, blindfold 1 eye of some and have a control group with no blindfold The same experiment in adult animals had little affect as the columns had already formedThere is a critical development for visual development where the cortex requires sensory visual stimulation from the eye itself. Lack of environmental stimulation severely and irreversibly affects visual development. After the window has closed, limited further changes are possible and the visual system cannot develop fully
- 6Hubel and Wiesel's simple and complex cells Simple cells - linear receptive fields formed from converging cumulative input from >3 LGN neurons with receptive fields aligned along one axis. There are obvious on and off regionsComplex cells - composed of several like orientated simple cells. Have no distinct excitatory or inhibitory regions. Large receptive field. Give responses to stimuli throughout the receptive field in the correct orientation
- 7Blobs: circular receptive field, double colour opponent centre surround. Involved in the analysis of coloutInterblobs: binocularity, orientation sensitivity, directional sensitivity. Can be simple cells or complex cells. Wavelength insensitive. Involved in the analysis of form
- 83 parallel chains specialised to analyse a particular feature of the visual sceneMagnocellular pathway (motion)
Layer IVB. Binocular RF. Simple and complex cells. Orientation selective. Direction selective. Wavelength insensitive Analysis of object motion and guidance of motor actions - 9Blob pathway (colour) Layer II and III. Centre surround RF. Double colour opponent. Monocular RF. Orientation insensitive. Analysis of object colourParvo-interblob pathway (shape)
Layer II and III. Wavelength insensitive. Binocular RF. Simple and complex cells. Smallest orientation selective RF. Analysis of fine object shape - 10It is now known that orientation columns, ocular dominance columns, blobs and interblobs are arranged into cortical modules.Each single hypercolumn covers 1mm of chunk of striate cortex.
- 11Evidence that visual perception occurs in V1 – Lawrence Weiskrantz 1970s (Oxford) – blindsightPatient TN (a 50-year-old man) suffered two strokes resulting in a complete bilateral loss of the striate cortex. Registered as blind by all normal physiological tests. Imaging studies demonstrated a completely inactive V1. Was able to walk around a room and avoid walking into objects. Did crude sight remain which could not be perceived? There are redundant pathways going to other brain regions – V1 is not everything.
- 12Information is processed in two parallel streams: Ventral stream Fed by parvo interblob and blob neurons in V1 Perception of the visual world Object/form recognition ‘What’ pathwayDorsal stream
Fed by magnocellular neurons in V1
Motion recognition
Visual motion
Control of actions
‘Where’ pathway - 13V5 bilateral lesionsNot only is perception affected, but also impairment of visio-motor co-ordination
Patient LM - 14Area V4 - involvement in shape and colout recognitionElectrophysiology – shows V4 neurons are both orientation and colour selective. In monkeys, damage to V4 causes perceptual deficits involving shape and colour. Achromatopsia – loss of colour vision despite normal functioning cones. Also associated with deficits in form perception. Damage to temporal and occipital lobes, but normal LGN, V1 – suggestive disorder of processing in ventral stream ‘WHAT’ pathway.
Información
- Descripción
- Brain Fichas sobre Visual Cortex and Perception , creado por Eleanor Smith el 30/12/2018.
- Creado por
- Eleanor Smith
- Visitas
- 3
- Valoración