Announcement

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Announcement

• MIDTERM
• When 2/23 8-10 PM
• Where 182 Dennison

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Last Lecture

• Examining the what pathway
• The Visual Agnosias
• Neurocognitive architecture of visual recognition

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This Lecture

• Two-processor model of visual recognition
• What vs. How
• The Where Pathway
• Balints syndrome
• Unilateral Neglect

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Farah theory Two specialized (lateralized?)
processors

• I. PART DECOMPOSITION
• used heavily for word identification but also
  objects
• LH dominant
• II. WHOLISTIC PROCESSING
• Used for complex objects without part
  decomposition--gt faces
• RH dominant
• Object recognition relies on both
• to varying degrees--gt
• one may compensate for the other.

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Evidence for two processors

• Object agnosia co-occurs with pure alexia.
• Object agnosia co-occurs with prosopagnosia
• Rarely, if ever, does object agnosia occur
  without either pure alexia or prosopagnosia.

Words
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Case D.F. (Milner Goodale, 1995)

• Classic Apperceptive Agnosic
• Severly impaired FORM perception
• Damage to V2, V3, V4-- Ventral Stream
• Intact abilities should reflect operation of
  dorsal stream

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Dissociating What from How

• Orientation reports IMPAIRED
• verbal
• matching
• Posting behavior SPARED
• Implication Orientation shape representations
  available for guiding action

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Damaging the Where Pathway...

• Balint's Syndrome (1909) A disorder of space
  exploration and space cognition...
• A cluster co-occuring visuomotor visuospatial
  disturbances
• Results from Bilateral Posterior Parietal damage

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A note about Syndromes

• Collection of deficits or symptoms that co-occur
  
• Are all symptoms due to one underlying
  impairment? (common mechanism?)
• OR
• Do they co-occur because underlying mechanisms
  are located in adjacent brain structures?
  (co-localization?)
• If latter, then dissociations should eventually
  be found.

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Normal scanpaths
Gaze apraxic scanpaths
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Gaze apraxia
Back and forth between two points
normal
Outlining the Perimeter of a rectangle
normal
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Elements of Balint's

• gaze apraxia inability to move the eyes
  voluntarily to points in the visual field.
• spontaneous, reflexive movements spared
• optic ataxia (disorder of reaching) impairment
  in visually guided movements of the limbs.
• some improvement with eyes closed.

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Posting performance Balints
Normal Optic Ataxia Optic Ataxia
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Elements of Balint's

• gaze apraxia inability to move the eyes
  voluntarily to points in the visual field.
• spontaneous, reflexive movements spared
• optic ataxia (disorder of reaching) impairment
  in visually guided movements of the limbs.
• some improvement with eyes closed.
• spatial disorientation inability to appreciate
  the "spatial properties" of objects.
• distance and size estimates impaired.
• impaired whole body movement in space.

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Spatial Disorientation
Place a point in the center
Divide the circle in two
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Spatial Disorientation
Draw the outline of a face
Draw the facial features
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Elements of Balint's

• gaze apraxia inability to move the eyes
  voluntarily to points in the visual field.
• spontaneous, reflexive movements spared
• optic ataxia (disorder of reaching) impairment
  in visually guided movements of the limbs.
• some improvement with eyes closed.
• spatial disorientation inability to appreciate
  the "spatial properties" of objects.
• distance and size estimates impaired.
• impaired whole body movement in space.
• simultanagnosia the inability to see more than
  one object at a time.

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Simultanagnosia
Drawing a circle
Tracing a circle
It is difficult I dont see at the same time
both --- the pencil and the circle. The hand
does not movewhen I want it to move. (from Luria
et al., 1954)
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Simultanagnosia
The drawing of a circle in between two already
drawn
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Lesion site in Balints
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Implications

• Parietal lobes are important for
• representations that guide spatial behaviors
• the how and where pathway
• allocating attention (shifting overtly and
  covertly)

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Covert Attention as a Spotlight

• Attention selects information.
• Fate of unattended input?

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Filter theory of Attention
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Locus of selection?
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Brain Regions Activated During Attention Shifting
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What is wrong with this patient?

• Symptoms observed
• left hemiparetic arm
• anosagnosia- unawareness / denial of illness.
• rightward gaze deviation
• no obvious hemianopia
• Visual extinction to double simultaneous
  stimulation
• Tactile extinction to DSS
• Constructional apraxia deficit in constructional
  and drawing tasks
• apraxia disorder of skilled movement
• allesthesia (gross) mislocalization of
  stimulation

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Unilateral (LEFT) neglect

• A deficit in perceiving responding to
  stimulation on the side opposite to the damaged
  hemisphere.
• Typical lesion site unilateral
  parietal-occipital junction (Brodmann's area 7,
  40)

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Examples of Neglect Performance