1 / 39

Chapter 5

Chapter 5. Visual System. Anatomy of the eye. The ‘ purpose ’ of the eye is to focus light to create an image on the retina. Retina. Retinal pigment epithelium , which provides critical metabolic and supportive functions to the photoreceptors

wildat
Download Presentation

Chapter 5

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Chapter 5 Visual System

  2. Anatomy of the eye The ‘purpose’ of the eye is to focus light to create an image on the retina

  3. Retina • Retinal pigment epithelium, which provides critical metabolic and supportive functions to the photoreceptors • Receptor layer, which contains the light sensitive outer segments of the photoreceptors • Outer nuclear layer, which contains the photoreceptor cell bodies • Outer plexiform layer, where the photoreceptor, horizontal and bipolar cells synapse

  4. Retina continued • Inner nuclear layer, which contains the horizontal, bipolar and amacrine cell bodies • Inner plexiform layer, where the bipolar, amacrine and retinal ganglion cells synapse • Retinal ganglion cell layer, which contains the retinal ganglion cell bodies • Optic nerve layer, which contains the ganglion cell axons travelling to the optic disc

  5. Photoreceptors: two sorts Rods • Relatively sensitive to light • Located mostly at the periphery of the retina • Operate at low levels of light • High convergence of information • Sensitive to light • Low visual acuity • Around 120 million in each retina Cones • Relatively insensitive to light • Located mostly in the centre of the retina • Operate under high levels of light • Low convergence of information • Relatively insensitive to light • High visual acuity • Around 6 million in each retina

  6. Major types of retinal ganglion cells • The P ganglion cell(s) – colour sensitive • greater in number than the M-ganglion • synapses with one to a few cone bipolars cells • colour sensitive • has a small concentric receptive field • generates a sustained, slowly adapting reaction • produces weak responses to stimuli • The M ganglion cell(s) – motion sensitive • visibly larger than P ganglion cells • synapses with many bipolar cells • not sensitive to colour • has a big concentric receptive region • more sensitive to small centre-surround variations • has the strongest possible response

  7. Behaviour of retinal ganglion cells • Three different types of retinal ganglion cell have been identified: 1. On ganglion cells that are energized by bipolar cells reacting to light conditions 2. Off ganglion cells that become excited with the removal of the light stimulus – they are inhibited by amacrine cells when light is present 3. On-off ganglion cells that become excited by bipolar cells in the presence of a light stimulus and are not inhibited by amacrine cells upon the removal of the light stimulus

  8. Pathways from eye to brain Cortical route 1. Retina  LGN  Visual cortex (V1) Sub-cortical routes 2. Retina  hypothalamus (circadian rhythm: biological clock) 3. Retina  superior colliculus (eye movement control)

  9. Pathways from eye to brain • Input from the left visual field is projected on the right hemi-retina of each eye • Information from the right hemi-retina of each eye (left visual fields) travels to the lateral geniculate nucleusof the thalamus on the right side •  half of the optic fibres cross at the optic chiasm

  10. Pathways from eye to brain • Information from the two eyes goes to different vertical layers: ipsilateral eye (right) to 2, 3, and 5, contralateral eye (left) to 1, 4, 6 • LGN layers 1, 2, 3, 4 are parvocellular layers, input from cones (colour, details) • LGN layers 5, 6 are magnocellular layers, input from rods (light/dark contrasts, movement)

  11. The visual cortex • Six layers • 4 parvocellular layers • Ganglion cells – concentrated around the fovea • Blobs – sensitive to specific colours • 2 magnocellular layers • Ganglion cells – equally distributed in the retina • Respond to movement

  12. Visual cortex • Topographic relationships between the retina and the visual cortex

  13. Dorsal & ventral streams • Visual information travels from the striate cortex in the occipital lobe to: • The parietal lobe (the dorsal stream) and • The temporal lobe (the ventral stream)

  14. Lateral inhibition • Visual edge: a place where two different areas of a visual image meet  perception of an edge = perception of a contrast • Contrast enhancement: contrasts are being highlighted, making us very good at perceiving edges

  15. Seeing edges – Mach Bands At each edge, the brighter stripe looks brighter than it is, the darker stripe looks darker than it is  edges stand out

  16. Simple cells in V1: orientation detectors Hubel and Wiesel, 1959

  17. Complex cells in V1: motion detectors • Importance of orientation • Direction sensitivity

  18. Hypercomplex cells • End stop cells • Been found to be subtype of simple and complex cells

  19. Regions: V1, V2, V3, V4, V5 (MT)

  20. Extrastraite cortex • Visual Area V2 • The first area in the secondary visual cortex • Strong connections from V1 (both forwards and backwards) • Transmits strong forward connections to upstream areas V3, V4, and V5. Illusory contours and figure ground segmentation (Qiu & von der Heydt, 2005) • Visual Area V3 • Found directly in front of V2 • Debate as to what section of the cortex this area resides • Dorsal and Ventral V3? • Might be involved in the processing of global movement (Braddick et al., 2001)

  21. Extrastraite cortex • Visual Area V4 • Visual area V4 obtains information from V2 and relays information to the posterior inferotemporal cortex • Processing of colour information (S.M. Zeki, 1977) • Attentional modulation (Moran & Desimone, 1985) • Lesions of V4 result in achromotopsia (S. Zeki & Bartels, 1999) • Visual Area V5 (area MT) • Motion perception • V5 connects to many brain areas, inputs include the visual areas V1, V2 and dorsal V3 • It transmits information to neighbouring cortex and the frontal eye regions • Lesion of V5 may result in akinotopsia, a condition in which the individual is unable to perceive movement (Zihl, 1983)

  22. Colour vision • S-cones: short wavelength= ‘blue’ cones • M-cones: medium wavelength= ‘green’ cones • L-cones: large wavelength= ‘red’ cones

  23. Colour: trichromatic theory • Young (1802) & Helmholz (1852) • Perception of colour depends on the wavelengths of light that an object reflects • Any colour in the visible spectrum can be reproduced by mixing three different wavelengths of light

  24. Colour: trichromatic theory • Three types of cones: • Small wavelength  blue • Medium wavelength  green • Large wavelength  red • Every colour produces a unique set of responses in the cones

  25. Colour blindness

  26. Colour opponency • Cones  bipolar cells  retinal ganglion cells • Ganglion cells respond to pairs of primary colours: • red / green • blue / yellow

  27. Colour constancy – retinex theory • Different light sources differ in wavelengths they contain  wavelengths reflected by the same object differ with the use of different light sources • Thus, we should perceive the colour of the same object as being different, when light sources are changed • However, perceived colour of objects remains constant under varying illumination conditions

  28. Colour constancy What happens? • Your brain compares the wavelengths of light coming from different parts of the retina • Based on this comparison, your brain calculates what the wavelength of the light source is, and takes this into account when it determines a perception of colour for each object

  29. Motion perception – area V5 • Pathway from area V1 to area V5 and up to the posterior parietal lobe have a selective responsibility for perceiving motion • V5 in the human body is analogous to area MT in primates • Disrupted – akinotopsia

  30. Faces special? • ‘Face Area’ in the Brain • Kanwisher et al., 1997

  31. Faces or expertise? • Diamond & Carey 1986 (dog experts) • Greebles • Gauthier et al., 1998

  32. Expertise • Gauthier et al., 1998

  33. The binding problem • How is fragmented visual processing of shape, colour, motion, etc. unified into complete objects? • The hippocampus and the prefrontal cortex appear to be important to binding • Gamma-band oscillations of neural activity

  34. Disorders of object perception • Visual Agnosia • Inability to recognize objects • Associative agnosia – unable to assign meaning to an item • Prosopagnosia – inability to recognize a face • Blindsight • An individual can sustain damage to area V1 and be unable to experience conscious vision from part of the visual field, yet still be able to experience a response and localize visual objects • transmitted to the superior colliculus in the midbrain and on to the motor control output systems • Hemispatial Neglect • Condition in which, after one hemisphere of the brain has been damaged, a deficit in attention to the opposite side of space is observed • Right hemisphere damage is a more frequent and more severe cause of unilateral neglect than left hemisphere damage

  35. Rods

  36. Cones

  37. Visual system

  38. Readings • Barnes, J. (2011). Essential Biological Psychology (Chapter 5). London: Sage. • The Essentials Haxby, J.V., Grady, C.L., Horwitz, B., Ungerleider, L.G., Mishkin, M., Carson, R.E., et al. (1991). Dissociation of object and spatial visual processing pathways in human extrastriate cortex. Proceedings of the National Academy of Science USA, 88(5), 1621-1625. • Next Steps Zeki, S. (1980). The representation of colours in the cerebral cortex. Nature, 284(5755), 412-418. • Delving Deeper Gauthier, I., & Tarr, M.J. (1997). Becoming a ‘Greeble’ expert: exploring mechanisms for face recognition. Vision Research, 37(12), 1673-1682.

More Related