Pinel basics ch04

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Pinel basics ch04

  1. 1. Chapter 4 The Visual System How We See <ul><li>This multimedia product and its contents are protected under copyright law. The following are prohibited by law: </li></ul><ul><li>any public performance or display, including transmission of any image over a network; </li></ul><ul><li>preparation of any derivative work, including the extraction, in whole or in part, of any images; </li></ul><ul><li>any rental, lease, or lending of the program. </li></ul>
  2. 2. What you see is not necessarily what you get <ul><li>Somehow a distorted and upside-down 2D retinal image is transformed into the 3D world we perceive </li></ul><ul><li>2 types of research needed to study vision </li></ul><ul><ul><li>Research probing the components of the visual system </li></ul></ul><ul><ul><li>Research assessing what we see </li></ul></ul>
  3. 3. Light Enters the Eye and Reaches the Retina <ul><li>No species can see in the dark, but some are capable of seeing when there is little light </li></ul><ul><li>Light </li></ul><ul><ul><li>Photons of energy </li></ul></ul><ul><ul><li>Waves of electromagnetic radiation </li></ul></ul><ul><li>Humans see light between 380-760 nanometers in length </li></ul>
  4. 5. Light enters the eye <ul><li>Wavelength – perception of color </li></ul><ul><li>Intensity – perception of brightness </li></ul><ul><li>Light enters the eye through the pupil whose size is regulated by the irises </li></ul><ul><li>Sensitivity – the ability to see when light is dim </li></ul><ul><li>Acuity – the ability to see details </li></ul>
  5. 6. Light enters the eye <ul><li>Lens – focuses light on the retina </li></ul><ul><li>Ciliary muscles adjust the shape of the lens as needed </li></ul><ul><li>Accommodation – the process of adjusting the lens to bring images into focus </li></ul>
  6. 7. Binocular cues <ul><li>Convergence – eyes must turn slightly inward when objects are close </li></ul><ul><li>Binocular disparity – difference between the images on the 2 retinas </li></ul><ul><li>Both are greater when objects are close – provides brain with distance information </li></ul>
  7. 8. The Retina <ul><li>Built inside-out </li></ul><ul><ul><li>Light passes through several cell layers before reaching its receptors </li></ul></ul><ul><li>Vertical pathway – receptors > bipolar cells > retinal ganglion cells </li></ul><ul><li>Lateral communication </li></ul><ul><ul><li>Horizontal cells </li></ul></ul><ul><ul><li>Amacrine cells </li></ul></ul>
  8. 10. The Retina <ul><li>No receptors where information exits the eye </li></ul><ul><ul><li>Creates the blind spot </li></ul></ul><ul><li>Fovea </li></ul><ul><ul><li>At the center of the retina, high acuity </li></ul></ul><ul><ul><li>Reduced light distortion </li></ul></ul>
  9. 11. Cone and Rod Vision <ul><li>Duplexity theory of vision – cones and rod mediate different kinds of vision </li></ul><ul><li>Cones – photopic vision </li></ul><ul><ul><li>High-acuity in good lighting </li></ul></ul><ul><li>Rods – scotopic vision </li></ul><ul><ul><li>High-sensitivity, allowing for low-acuity vision in dim light </li></ul></ul>
  10. 13. Cone and Rod Vision <ul><li>More convergence in rod system, increasing sensitivity while decreasing acuity </li></ul><ul><li>Which receptors are found at the fovea? </li></ul><ul><li>Cones </li></ul>
  11. 14. Eye Movement <ul><li>We continually scan the world with small and quick eye movements – saccades </li></ul><ul><li>These bits of information are then integrated </li></ul><ul><li>Stabilize retinal image – see nothing </li></ul><ul><li>Visual system responds to change </li></ul>
  12. 15. The Conversion of Light to Neural Signals <ul><li>Transduction –conversion of one form of energy to another </li></ul><ul><li>Visual transduction – light energy to neural signals by visual receptors </li></ul><ul><li>Pigments absorb light </li></ul><ul><li>Absorption spectrum determines spectral sensitivity </li></ul>
  13. 17. Rhodopsin <ul><li>The pigment found in rods, a G-protein-linked receptor that responds to light </li></ul><ul><li>In the dark, sodium channels are partially open </li></ul><ul><ul><li>Rods depolarized </li></ul></ul><ul><li>When light strikes, sodium channels close </li></ul><ul><ul><li>Rods hyperpolarize in response to light </li></ul></ul>
  14. 18. Retina-geniculate-striate pathway <ul><li>~90% of axons of retinal ganglion cells </li></ul><ul><li>Information from the left visual field of each eye projects to the right lateral geniculate nucleus (LGN) and vice versa </li></ul><ul><li>Most LGN neurons that project to primary visual cortex (V1, striate cortex) terminate in the lower part of cortical layer IV </li></ul>
  15. 20. Retinotopic organization <ul><li>Information received at adjacent portions of the retina remains adjacent </li></ul><ul><li>More cortex is devoted to areas of high acuity – like the disproportionate representation of sensitive body parts in somatosensory cortex </li></ul><ul><li>About 25% of primary visual cortex is dedicated to input from the fovea </li></ul>
  16. 21. The M and P Channels <ul><li>Magnocellular layers (M layers) </li></ul><ul><ul><li>Big cell bodies, bottom 2 layers of LGN </li></ul></ul><ul><ul><li>Particularly responsive to movement </li></ul></ul><ul><ul><li>Input primarily from rods </li></ul></ul><ul><li>Parvocellular layers (P layers) </li></ul><ul><ul><li>Small cell bodies, top 4 layers of LGN </li></ul></ul><ul><ul><li>Color, detail, and still or slow objects </li></ul></ul><ul><ul><li>Input primarily from cones </li></ul></ul>
  17. 22. The M and P Channels <ul><li>M layers – movement, rods </li></ul><ul><li>P layers – color and detail, cones </li></ul><ul><li>Project to slightly different areas in lower layer IV in striate cortex, M neurons just above the P neurons </li></ul><ul><li>Project to different parts of visual cortex beyond V1 </li></ul>
  18. 23. Lateral Inhibition and Contrast Enhancement <ul><li>Visual system detects change </li></ul><ul><li>Mach bands – nonexistent stripes that visual system creates to enhance the contrast and make edges easier to see – an example of contrast enhancement </li></ul><ul><li>A consequence of lateral inhibition </li></ul>
  19. 24. Receptive Fields of Visual Neurons <ul><li>The area of the visual field within which it is possible for a visual stimulus to influence the firing of a given neuron </li></ul><ul><li>Hubel and Wiesel looked at receptive fields in cat retinal ganglion, LGN, and lower layer IV of striate cortex </li></ul>
  20. 25. Receptive Fields of Visual Neurons <ul><li>Similarities seen at all 3 levels: </li></ul><ul><ul><li>Receptive fields of foveal areas smaller than those in the periphery </li></ul></ul><ul><ul><li>Circular receptive fields </li></ul></ul><ul><ul><li>Monocular </li></ul></ul><ul><ul><li>Many had an excitatory area and an inhibitory area separated by a circular boundary </li></ul></ul>
  21. 28. Receptive Fields in Striate Cortex <ul><li>Neurons of lower layer IV are an exception – circular receptive fields (as in retinal ganglion cells and LGN) </li></ul><ul><li>Most neurons in V1 are either </li></ul><ul><ul><li>Simple – receptive fields are rectangular with “on” and “off” regions </li></ul></ul><ul><ul><li>Complex – also rectangular, larger receptive fields, respond best to a particular stimulus anywhere in its receptive field </li></ul></ul>
  22. 29. Receptive Fields in Striate Cortex <ul><li>SIMPLE </li></ul><ul><li>Rectangular </li></ul><ul><li>“ on” and “off” regions, like cells in layer IV </li></ul><ul><li>Orientation and location sensitive </li></ul><ul><li>All are monocular </li></ul><ul><li>COMPLEX </li></ul><ul><li>Rectangular </li></ul><ul><li>Larger receptive fields </li></ul><ul><li>Do not have static “on” and “off” regions </li></ul><ul><li>Not location sensitive </li></ul><ul><li>Motion sensitive </li></ul><ul><li>Many are binocular </li></ul>
  23. 30. Columnar Organization of Primary Visual Cortex <ul><li>Cells with simpler receptive fields send information on to cells with more complex receptive fields </li></ul><ul><li>Functional vertical columns exist such that all cells in a column have the same receptive field and ocular dominance </li></ul><ul><li>Ocular dominance columns – as you move horizontally, the dominance of the columns changes </li></ul><ul><li>Retinotopic organization is maintained </li></ul>
  24. 31. Seeing Color – 2 Theories <ul><li>Trichromatic theory (component theory) </li></ul><ul><li>Proposed by Young, refined by Helmholtz </li></ul><ul><li>3 types of receptors, each with a different spectral sensitivity </li></ul>
  25. 32. Seeing Color – 2 Theories <ul><li>Opponent-process theory </li></ul><ul><li>Hering </li></ul><ul><li>2 different classes of cells encoding color, and another class encoding brightness </li></ul><ul><li>Each encodes two complementary color perceptions </li></ul>
  26. 33. Seeing Color – 2 Theories <ul><li>Both are correct – coding of color by cones seems to operate on a purely component basis, opponent processing of color is seen at all subsequent levels </li></ul>
  27. 35. Color Constancy and the Retinex Theory <ul><li>Color constancy – color perception is not altered by varying reflected wavelenths </li></ul><ul><li>Retinex theory – color is determined by the proportion of light of different wavelengths that a surface reflects </li></ul><ul><li>Relative wavelengths are constant, so perception is constant </li></ul>
  28. 36. Visual Cortex <ul><li>Primary – receives most of its input from the LGN </li></ul><ul><li>Secondary – receives most of its input from primary visual cortex </li></ul><ul><li>Visual association cortex – receives input from secondary visual cortex and other secondary sensory systems </li></ul>
  29. 38. Scotomas: Completion <ul><li>Damage to an area of primary visual cortex produces a scotoma, an area of blindness </li></ul><ul><li>Completion prevents many patients of from being aware of their deficit – the mind fills in the blanks </li></ul>
  30. 41. Prosopagnosia <ul><li>Visual agnosia for faces </li></ul><ul><li>Agnosia – a failure of recognition </li></ul><ul><li>How is it that you can have an agnosia for a particular type of information? What does this say about how information is organized in the brain? </li></ul>

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