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Pinel basics ch04

Pinel basics ch04






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    Pinel basics ch04 Pinel basics ch04 Presentation Transcript

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