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CHAPTER 10. Vision and visual perception Form Vision. Review: eye to Brain pathway. Rods or cones  bipolar cells  ganglion cells Ganglion cell  optic nerve; Optic nerves  optic chiasm Optic chiasm  optic tracts (L and R)

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chapter 10


Vision and

visual perception

Form Vision

review eye to brain pathway
Review: eye to Brain pathway
  • Rods or cones bipolar cells  ganglion cells
  • Ganglion cell  optic nerve;
  • Optic nerves  optic chiasm
  • Optic chiasm  optic tracts (L and R)
  • Optic tracts  lateral geniculate nuclei of thalamus
  • Optic radiations leave LGN  occipital cortex
  • Several areas of occipital cortex
    • Areas 17, 18 and 19
two kinds of pathways
Two kinds of pathways
  • Ambient pathway:
    • Spatial vision pathway
    • Where things are, not what they are
    • Magnocelluar pathway
  • Focal vision
    • Object vision pathway
    • What things are, but not where they are
    • Parvocellular pathway
  • Cortical blindness
    • Damage to brain, not eyes or even optic tract
    • Typically lose form vision first, then all of it
    • Very rare to just lose ambient pathway
form vision
Form Vision
  • Incoming information organized into a retinotopic map
    • exists in the visual cortex
    • adjacent retinal receptors activate adjacent cells in the visual cortex.
  • Form vision:
    • detection of an object’s boundaries and features (such as texture)
    • Differs from spatial location (that is spatial vision!).
    • Primarily using cones
  • Processed using lateral inhibition
    • On/off cell activity
    • each neuron’s activity inhibits the activity of its neighbors
    • in turn, its activity is inhibited by them.
  • the inhibition is delivered by horizontal cells to nearby synapses of receptors with bipolar cells.
form vision8
Form Vision
  • In on center cells
    • light in the center increases firing
    • light in the off surround reduces firing below resting levels.
    • Only responds if center of receptive field is stimulated
  • Other ganglion cells have an off center
    • Thus an on surround.
    • Work the opposite
form vision11
Form Vision
  • Simple cells
    • respond to a line or an edge that is at a specific orientation
    • May also respond to specific place on the retina.
  • Complex cells:
    • continue to respond when a line or edge moves to a different location.
  • According to spatial frequency theory
    • visual cortical cells function like a Fourier frequency analysis of the luminosity variations in a scene.
    • Analyze the shape of the sine wave patterns
    • visual cortical cells can detect not just edges but gradations of change.
do we process input individually globally or hierarchically
Do we process input individually, globally or hierarchically?
  • Modular processing
    • segregation of the various components of processing into separate locations.
    • Process each unit or module separately; then build into meaningful unit
  • Hierarchical processing:
    • lower levels of the nervous system analyze their information
    • pass the results on to the next higher level for further analysis.
    • Layered analysis
  • Distributed vs. localized processing:
    • Some neuroscientists reject modular notion,
    • visual functions distributed not separate locations
    • E.g., occurs across a relatively wide area of the brain.

Two routes and

two types of vision

  • Remember:
    • Visual information follows two routes from the retina through the brain
    • Again, are the focal vs. spatial pathways
  • Parvocellular system: form pathway
    • Focal vision
    • Object vision pathway
    • What things are, but not where they are
  • Magnocellular system.: spatial pathway
    • Ambient pathway
    • Spatial vision pathway
    • Where things are, not what they are

Parvocellular pathway

  • Parvocellular ganglion cells:
    • located mostly in the fovea.
    • Thus mostly involves cones
    • They have circular receptive fields
  • Receptive fields:
    • small and color opponent
    • These smaller receptive fields best for discrimination of fine detail and color.
    • Thus this is the form pathway
    • Form pathway = figures, specific shapes and critical information
    • Tells us the WHAT

Magnocellular pathway

  • Magnocellular ganglion cells
    • larger circular receptive fields
    • Mainly rods
    • brightness opponent
    • respond only briefly to stimulation.
    • specialized for brightness contrast and for movement
    • This is the spatial pathway.
  • Parvocellular and magnocellular pathways travel to
    • the lateral geniculate nucleus (LGN of thalamus)
    • then to the primary visual cortex, also known as V1.
    • One tract lies on top of other: magnocellular is on top (dorsal)!!

Perception of Objects, Color, and Movement

  • Two pathways = highly interconnected,
    • parvocellular system dominates the ventral stream that flows from the visual cortex into the temporal lobes,
    • magnocellular system dominates the dorsal stream from the visual cortex to the parietal lobes.
  • Ventral stream = what of visual processing
    • parvocellular
  • Dorsal stream = where of visual processing
    • magnocellular

The Perception of Objects, Color, and Movement

  • Beyond cortical V1
    • the ventral stream passes through V2 and into V4,
    • mostly concerned with color perception.
  • Then projects to the inferior temporal cortex
    • lower boundary of the temporal lobe.
    • This area shows remarkable specialization for object recognition
    • Damage to this area = loss of much of form vision, even if pathway is intact.

The Perception of Objects, Color, and Movement

  • The dorsal stream proceeds to
    • V2 through to V5,
    • also known as Medial temporal or MT
      • Why? It is on the middle temporal gyrus in the monkey.
    • Neurons there have strong directional sensitivity
    • Contributes to the perception of movement.
  • The dorsal stream then travels to posterior parietal cortex
    • located just behind the somatosensory cortex.
    • Primary role: locate objects in space
    • Significant behavioral implications of this function VERY important
    • Loss of this area = loss of spatial vision even if pathway intact

Visual disorders

  • Object agnosia:
    • impaired ability to recognize objects
    • Often due to damage of inferior temporal cortex
    • Also if parvocellular pathway damaged
  • Prosopagnosia :
    • Special type of object agnosia
    • inability to visually recognize familiar faces.
    • Can no longer recognize individual people or sometimes recognize person vs object
  • Both object agnosia and prosopagnosia are caused by damage to the inferior temporal cortex (part of the ventral stream).

Visual disorders

  • Fusiform face area
    • Specialized “area”, although somewhat disperse
    • face-responsive neurons are often intermingled with object-responsive neurons,
    • But, part of the fusiform gyrus on the underside of the temporal lobe is critical for face recognition
    • Hence, the name!
    • Damage to this produces the prosopagnosia

Other Vision disorders

  • Blindsight: Patients blinded by damage to V1
    • can locate and track the movement of objects; discriminate colors,
    • But say they are guessing, unaware of abilities.
    • Occassionally also due to damage to magnocellular pathway
  • Color agnosia: loss of the ability to perceive colors
    • Relies on Color constancy: critical behavior
    • The ability to recognize the “natural color “ of object despite of illuminating wavelength
    • If could not, objects would seem to change colors as the sun shifted position through the day or as we went indoors into artificial light
  • Movement agnosia: inability to perceive movement.
    • Often involves damage to medial temporal and posterior parietal
    • Unable to integrate incoming information

Sensory neglect

  • Function of posterior parietal cortex:
    • combines input from visual, auditory, and somatosensory areas
    • helps the individual locate objects in space
    • Helps orient the body in the environment.
  • Damage here impairs several abilities:
    • reaching for objects
    • often produces sensory neglect
    • patient ignores visual, touch, and auditory stimulation on the side opposite the injury.
  • Neglect not due to any defect in visual processing
    • Due to deficit in attention.

The Perception of Objects, Color, and Movement

  • Many researchers have wondered: where is all the information about a visual object brought back together?.
  • Suggestions:
    • ultimate understanding of an object occurs in superior temporal gyrus
    • Area receives input from both dorsal and ventral neural streams
    • Alternatively: in part of the parietal cortex where damage causes neglect.
    • Other investigators suspect frontal areas where both streams converge.
  • Bottom line: meaning and understanding require careful integration of information by several brain areas working actively together
    • Loss of any one disrupts this process
    • Delicate balance