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Visual System
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Visual System






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Visual System Visual System Presentation Transcript

  • The Visual System
  • What Do We See?• Somehow a distorted and upside-down 2-D retinal image is transformed into the 3-D world we perceive• Two types of research needed to study vision – Research probing the components of the visual system – Research assessing what we see
  • Light Enters the Eye• No species can see in the dark, but some are capable of seeing when there is little light• Light can be thought of as – Particles of energy (photons) – Waves of electromagnetic radiation• Humans see light between 380-760 nanometers
  • The electromagnetic spectrum: colors and wavelengths visible to humans
  • Light Enters the Eye (continued)• Wavelength – perception of color• Intensity – perception of brightness• Light enters the eye through the pupil, whose size changes in response to changes in illumination• Sensitivity – the ability to see when light is dim• Acuity – the ability to see details
  • Light Enters the Eye (continued)• Lens – focuses light on the retina• Ciliary muscles alter the shape of the lens as needed• Accommodation – the process of adjusting the lens to bring images into focus
  • A diagram of the human eye
  • Eye Position and Binocular Disparity• Convergence – eyes must turn slightly inward when objects are close• Binocular disparity – difference between the images on the two retinas• Both are greater when objects are close – provides brain with a 3-D image and distance information
  • The Retina and Translation of Light into Neural Signals• The retina is in a sense “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• Blind spot: no receptors where information exits the eye – The visual system uses information from cells around the blind spot for “completion,” filling in the blind spot• Fovea: high acuity area at center of retina – Thinning of the ganglion cell layer reduces distortion due to cells between the pupil and the retina
  • Cone and Rod Vision• Duplexity theory of vision – cones and rod mediate different kinds of vision – Cones – photopic (daytime) vision • High-acuity color information in good lighting – Rods – scotopic (nighttime) vision • High-sensitivity, allowing for low-acuity vision in dim light, but lacks detail and color information
  • Cone and Rod Vision (continued) Distribution of rods and cones• More convergence in rod system, increasing sensitivity while decreasing acuity• Only cones are found at the fovea
  • Spectral Sensitivity• Lights of the same intensity but different wavelengths may not all look as bright• A spectral sensitivity curve shows the relationship between wavelength and brightness• There are different spectral sensitivity curves for photopic (cone) vision and scotopic (rod) vision
  • Spectral Sensitivity (continued) Human photopic and scotopic spectralsensitivity curves
  • 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
  • Visual Transduction: Conversion of Light to Neural Signals• Transduction – conversion of one form of energy to another• Visual transduction – conversion of light to neural signals by visual receptors• Pigments absorb light• Absorption spectrum describes spectral sensitivity
  • VisualTransduction (continued) The absorption spectrum of rhodopsincompared with the human scotopic spectral sensitivity curve
  • Rhodopsin• The pigment found in rods• A G protein-linked receptor that responds to light rather than to neurotransmitters• In the dark – Na+ channels remain partially open (partial depolarization), releasing glutamate• When light strikes – Na+ channels close – Rods hyperpolarize, inhibiting glutamate release
  • From Retina to Primary Visual Cortex The Retinal-Geniculate-Striate Pathways• ~90% of axons of retinal ganglion cells• The left hemiretina of each eye (right visual field) connects to the right lateral geniculate nucleus (LGN); the right hemiretina (left visual field) connects to the left LGN• Most LGN neurons that project to primary visual cortex (V1, striate cortex) terminate in the lower part of cortical layer IV
  • From Retina toPrimary Visual Cortex The retina- geniculate- striate system
  • Retinotopic Organization• Information received at adjacent portions of the retina remains adjacent in the striate cortex• 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 two layers of LGN – Particularly responsive to movement – Input primarily from rods• Parvocellular layers (P layers) – Small cell bodies, top four layers of LGN – Color, detail, and still or slow objects – Input primarily from cones
  • The M and P Channels (continued)• 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
  • Seeing Edges: Lateral Inhibition and Contrast EnhancementVisual system detects change• Mach bands – nonexistent stripes that visual system creates for contrast enhancement – Makes edges easier to see – 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: Neurons of the Retina- Geniculate-Striate System• Similarities seen at all three levels: – Receptive fields of foveal areas are smaller than those in the periphery – Neurons’ receptive fields are circular in shape – Neurons are monocular – Many neurons at each level had receptive fields with excitatory and inhibitory area
  • Receptive Fields• Many cells have receptive fields with a center-surround organization: excitatory and inhibitory regions separated by a circular boundary• Some cells are “on- center” and some are “off-center”
  • Receptive Fields in Striate Cortex• In lower layer IV of the striate cortex, neurons with circular receptive fields (as in retinal ganglion cells and LGN) are rare• Most neurons in V1 are either – Simple – receptive fields are rectangular with “on” and “off” regions, or – Complex – also rectangular, larger receptive fields, respond best to a particular stimulus anywhere in its receptive field
  • Receptive Fields in Striate Cortex (continued)SIMPLE COMPLEX• Rectangular • Rectangular• “On” and “off” • Larger receptive fields regions, like cells in • Do not have static “on” layer IV and “off” regions• Orientation and • Not location sensitive location sensitive • Motion sensitive• All are monocular • Many are binocular
  • Columnar Organization of V1• 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: Component and Opponent Processing (continued)• Component theory (trichromatic theory) – Proposed by Young, refined by Helmholtz – Three types of receptors, each with a different spectral sensitivity
  • Seeing Color: Component and Opponent Processing (continued)Opponent-process theory• Proposed by Hering• Two different classes of cells encoding color, and another class encoding brightness• Each encodes two complementary color perceptions• Accounts for color afterimages and colors that cannot appear together (reddish green or bluish yellow)
  • Seeing Color: Component and Opponent Processing (continued)• 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
  • Absorption spectra of the three classes of cones
  • Color Constancy and the Retinex Theory• Color constancy – color perception is not altered by varying reflected wavelengths• 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
  • Cortical Mechanisms of Vision and Conscious AwarenessVisual areas of the human cerebral cortex • Flow of visual information: – Thalamic relay neurons, to – 1˚ visual cortex (striate), to – 2˚ visual cortex (prestriate), to – Visual association cortex • As visual information flows through hierarchy, receptive fields – become larger – respond to more complex and specific stimuli
  • Damage to Primary Visual Cortex• Scotomas – Areas of blindness in contralateral visual field due to damage to primary visual cortex – Detected by perimetry test• Completion – Patients may be unaware of scotoma – missing details supplied by “completion”
  • Damage to Primary Visual Cortex (continued)• Blindsight – Response to visual stimuli without conscious awareness of “seeing” – Possible explanations of blindsight • Islands of functional cells within scotoma • Direct connections between subcortical structures and secondary visual cortex, not available to conscious awareness
  • Functional Areas of Second and Association Visual Cortex• Neurons in each area respond to different visual cues, such as color, movement, or shape• Lesions of each area results in specific deficits• Anatomically distinct• Retinotopically organized
  • Dorsal and Ventral Streams• Dorsal stream: pathway from primary visual cortex to dorsal prestriate cortex to posterior parietal cortex – The “where” pathway (location and movement), or – Pathway for control of behavior (e.g. reaching)• Ventral stream: pathway from primary visual cortex to ventral prestriate cortex to inferotemporal cortex – The “what” pathway (color and shape), or – Pathway for conscious perception of objects
  • Prosopagnosia• Inability to distinguish among faces• Most prosopagnosic’s recognition deficits are not limited to faces• Often associated with damage to the ventral stream• Prosopagnosics have different skin conductance responses to familiar faces compared to unfamiliar faces, even though they reported not recognizing any of the faces