Visual Agnosias

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Visual Agnosias
by Aubrey Siebert Neuro 405 Neuropsychology 8
December 2003
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Basics of Visual Processing

• Light enters eye and is defracted by
• the lens so it falls on the retina
• Information leaves the eyes via
• optic nerves, which cross at the
• optic chiasm
• From here information is relayed to
• the lateral geniculate nucleus (LGN)
• via the optic tract
• LGN axons fan out through deep
• white matter as optic radiations that
• ultimately synapse in the 1o visual
• cortex

www.thalamus.wustl.edu/course/basvis.html

• From V1, info is subject to higher cortical
  processing to determine
• global aspects of object, such as color,
  shape or motion(Carlson,1999)

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Basics of Visual Processing

• From the primary visual cortex, information is
• then processed by two somewhat separate and
• parallel pathways
• - Ventral stream Object recognition
• - Dorsal stream Object location
• Information is exchanged between these two
• pathways, each supporting the function of the
• other
• Ventral stream travels from the extrastriate
• cortex and terminates in the inferior temporal
  
• cortex (Carlson, 1999)
• Damage to this pathway leads to various visual
• agnosias

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Primary Visual Cortex
Picture of brain.
Inferior Temporal Cortex
http//www.icomm.ca/geneinfo/agnos.htm
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What Are Visual Agnosias?

• A group of deficits characterized by the
  inability
• to recognize a stimulus visually
• Have relatively normal visual acuity and
• intellectual capacity
• Often result from cerebrovascular accidents or
• traumatic brain injury, such as stroke, CO
• poisoning, car accidents (Zolton, 1996)
• Two main types of visual agnosia
• Apperceptive
• Associative

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Apperceptive Visual Agnosia

• Failure to perceive objects even though visual
• acuity is relatively normal
• Cannot identify objects visually, but can often
• describe it or its use, or identify it through
  other
• senses (e.g. touch, sound)
• Ex. When shown a pencil, patient can describe
  its color, shape and use, but will not be able to
  label object as a pencil just from sight.
• Likely due to damage of early perceptual
• processes failure of high levels of
  perception
• (Vecera Gilds, 1998)

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Associative Visual Agnosia

• Inability to identify objects that are perceived
• visually, even though the form of the perceived
  
• object can be drawn or matched with similar
• objects
• Patients unable to verbally label objects they
• see or comply with verbal requests (e.g. draw
  an
• anchor)
• Involves a deficit in the ability to transfer
• information between the visual cortex and brain
  
• mechanisms involved in language (Carlson,
• 1999)

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A
B
C
D
Carlson, 1999
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Sub-Types of Visual Agnosias

• Achromatopsia inability to see color
• world in shades of gray
• Akinetopsia inability to perceive
• objects in motion objects jump from
• place to place
• Prosopagnosia failure to recognize
• people by the sight of their face
• Simultagnosia inability to perceive
• more than one object at a time sees
• objects in parts

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• Group Deficit Theory (Vecera Gilds, 1990)
• Deficits in pre-attentive perceptual grouping
• difficulty organizing segments of visual
• information into a comprehensive whole
• Peppery Mask Theory (Farah, et al., 1991)
• Random visual noise due to air bubbles or blood
• clots in cerebrovascular system
• This produces random areas of diminished vision
• throughout visual field, leading to low levels
  of
• visual processing
• Current research seeks to determine which
• theory best explains visual agnosias

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Dr. P The Man Who Mistook His Wife for a Hat

• Unable to recognize faces unless they had a
• distinguishing feature such as a strong jaw
  line
• or a large nose.
• Unable to visually recognize objects (e.g. feet
• and slippers)
• He recognized people based on their body
• music
• Often used touch to identify objects he could
  not
• recognize visually
• Used songs to keep himself on track,
• performing daily task such as eating or
  dressing
• himself

(Sacks, 1985)