Visual System

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Visual System
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Sensory systems

• All sensory systems carry information from the
  environment to the brain, where it is processed.
• Sensory receptors in the periphery detect
  physical events
• Stimuli act on these receptors and alter their
  membrane potential
• Sensory transduction is the process of
  transferring stimuli into changes in the membrane
  potential
• Receptor potential change in membrane potential
  results in change in NT released
• Receptor cells lack an axon no action potential

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Visual system anatomy - Eyeball

• Sclera
• Conjunctiva
• Cornea
• Iris
• Pupil
• Lens
• Accomodation

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Visual system anatomy - Retina

• Retina the lining at the back of the eye
  contains photoreceptors
• Rods
• Cones
• Fovea mediates most acute vision
• Optic disk
• Blind spot no receptors

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Visual system anatomy - Retina

• Retinal layers
• Photoreceptor layer
• Bipolar layer contains
• Bipolar cells
• Horizontal and amacrine cells

• Ganglion cell layer axons optic nerve

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Photoreceptors

• Contain molecules of photopigments
• Each molecule consists of an opsin and a retinal
• Change in the membrane potential
• NT released
• Photoreceptors secrete NT constantly rate
  determines the message

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Primary visual pathway

• Each eye receives stimulation from both halves of
  the visual field
• Receptor cells in the left half of each retina
  respond to stimuli in the right visual field
• Retinal ganglion cell axons combine in the optic
  nerve
• At the optic chiasm, axons from the medial halves
  of the retina cross axons from the lateral
  halves continue to the ipsilateral hemisphere

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Primary visual pathway

• Axons carrying information from the left visual
  field proceed to the right lateral geniculate
  nucleus where they synapse.
• Neurons in the dorsal lateral geniculate nucleus
  project to the primary visual cortex

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Other visual pathways

• In addition to the retino-geniculo-cortical
  pathway, axons from the retina carry visual
  sensations to the
• Hypothalamus
• Superior colliculus (Optic tectum)

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Visual processing Striate cortex

• The primary visual (striate) cortex is the first
  stop for incoming visual information
• Striate cortex contains a map of the
  contralateral visual field

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Visual processing Striate cortex

• Neurons in the striate cortex respond to
  information from multiple ganglion cells
• Each striate neuron responds to its own receptive
  field

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Visual processing in the striate cortex
Orientation and movement

• Orientation-sensitive neurons only respond to
  objects in a particular spatial placement
• Simple cells
• Complex cells
• Hypercomplex cells

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Visual processing in the striate cortex Depth

• Binocular vision provides the most acute
  depth-perception
• Stereopsis
• Many neurons in the visual cortex respond to
  visual stimulation from both eyes
• Retinal disparity
• Binocular cells respond strongly to retinal
  disparity

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Visual processing in the striate cortex - Color

• Information from color-sensitive ganglion cells
  is transmitted to cytochrome oxidase blobs in the
  striate cortex
• Wavelength-sensitive neurons
• Central to color vision processing

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Visual processing in the striate cortex -
organization

• Striate cortex is organized into modules
• Each module contains the neurons that analyze
  information from a single region of the visual
  field
• Input from both eyes
• Share the same occular dominance (amount of input
  from each eye)
• Highly connected and mostly binocular
• Centered around a CO blob, surrounded by neurons
  sensitive to orientation, movement, frequency,
  texture and depth.

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Visual association cortex

• Striate cortex (V1) begins processing visual
  information
• The visual association cortex combines
  information from individual modules in V1 in
  order to produce visual perception.

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Visual association cortex

• The extrastriate cortex (V2) is adjacent to V1
• Two streams of visual information project from V1
  to V2
• Dorsal stream
• Ventral stream

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Visual association cortex Color

• Color perception is mediated by extrastriate
  neurons in area V4 of the ventral stream
• Color constancy
• Achromatopsia

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Visual association cortex - Form

• Orientation-sensitive neurons in V1 send
  information to V2 along the ventral stream
• Damage to the visual association cortex can
  result in a visual agnosia

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Visual agnosias

• Apperceptive visual agnosia inability to
  perceive and identify common objects by sight
• Prosopagnosia inability to identify a familiar
  face
• Associative visual agnosia inability to name an
  object that is visually perceived, despite the
  ability to define and draw it.

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Visual association cortex - movement

• Perception of movement is mediated by
  movement-sensitive neurons
• Area V5 (or medial temporal area) of the
  extrastriate cortex
• Damage to V5 can produce akinetopsia

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Visual association cortex - location

• Perception of spatial location is mediated by
  neurons in the dorsal stream
• Spatial perception takes place in the posterior
  parietal cortex