Lecture 10 Visual Agnosias

Agnosias and Higher-Level Vision

Goals

  • Understand functional dissociations and their significance in cognitive neuropsychology.
  • Examine different types of visual agnosias, including apperceptive and associative forms.
  • Explore the neural basis and behavioral phenomena associated with disorders like prosopagnosia and simultanagnosia.
  • Introduce key neuroimaging findings related to visual pathways.

Functional Dissociations in Neuropsychology

  • Single Dissociation:
    • Damage to brain area X disrupts function A but not function B.
    • Weak evidence due to possible differences in task difficulty.
  • Double Dissociation:
    • Damage to brain area X impairs function A but spares function B, while damage to brain area Y does the opposite.
    • Provides strong evidence for functional specialization.

Application to Visual Pathways

  • Haxby et al. (1994):
    • PET study dissociating dorsal and ventral pathways:
      • Dorsal Pathway: Activated during spatial location tasks.
      • Ventral Pathway: Activated during object recognition tasks.

Types of Visual Agnosias

Apperceptive Agnosia

  • Definition: Impaired perceptual integration of visual stimuli.
  • Symptoms:
    • Difficulty identifying shapes, copying objects, and matching forms.
    • Disrupted perception by low-level noise or unusual views (e.g., shadows, rotated objects).
    • “Exploded” drawings where parts of objects are not integrated into a whole.
  • Neuroanatomy:
    • Often associated with damage to occipital or parietal lobes.
  • Simultanagnosia:
    • Subtype of apperceptive agnosia.
    • Dorsal Simultanagnosia:
      • Patients perceive only one object at a time.
      • Drawings show an “exploded” appearance.
      • Impairments in spatial localization.
    • Ventral Simultanagnosia:
      • Difficulty processing multiple letters or symbols simultaneously.
      • Associated with reading deficits (alexia).

Associative Agnosia

  • Definition: Failure to assign meaning to intact percepts.
  • Symptoms:
    • Patients can copy or match objects but cannot recognize them.
    • Impaired ability to link visual forms with semantic knowledge.
  • Subtypes:
    • Category-Specific Agnosias:
      • Loss of knowledge about specific domains (e.g., living vs. non-living things).
      • Represent a semantic double dissociation.

Prosopagnosia (Face Blindness)

  • Definition: Inability to recognize familiar faces, despite intact visual perception.
  • Symptoms:
    • Difficulty identifying individuals but can recognize facial expressions or gaze direction.
    • Sometimes associated with skin conductance responses indicating “partial knowledge.”
  • Neuroanatomy:
    • Fusiform gyrus in the ventral temporal lobe (face-processing module).
  • Double Dissociation:
    • Prosopagnosia can be dissociated from object agnosia and facial expression deficits.
  • Notable Cases:
    • Patient CK: Retained face recognition but had severe object agnosia.
    • Developmental and acquired forms of prosopagnosia.
  • Capgras Delusion:
    • Familiar individuals are perceived as imposters, possibly due to dissociation between recognition and emotional processing.
  • Fregoli Delusion:
    • Belief that strangers are familiar people in disguise.

Neural Basis of Visual Processing

Dorsal and Ventral Pathways

  • Dorsal Pathway (“Where/How”):
    • Processes motion and spatial relationships.
    • Associated with area MT/V5.
  • Ventral Pathway (“What”):
    • Processes object identity and form.
    • Includes V4 and fusiform gyrus.
  • Blindsight:
    • Patients with V1 lesions can respond to visual stimuli without conscious awareness.
    • Involves subcortical pathways like the superior colliculus and pulvinar.

Physiological Studies

  • Face Cells:
    • Discovered in monkeys (Bruce, Desimone, and Gross, 1981).
    • Fusiform face area (FFA) in humans shows face-specific activity in fMRI and ERP studies.
  • Motion Perception:
    • MT/V5 neurons remain responsive even without input from V1.
    • TMS studies confirm area MT’s role in motion detection.

Other Visual Disorders

Topographic Agnosia

  • Definition: Impairment in recognizing landmarks or navigating familiar environments.
  • Symptoms:
    • Patients rely on verbal cues (e.g., street names) to navigate.
  • Neuroanatomy:
    • Right posterior lesions often implicated.
  • Overlap with Prosopagnosia:
    • Shared deficits in ventral visual processing.

Charles Bonnet Syndrome

  • Definition: Visual hallucinations in blind or visually impaired individuals.
  • Mechanism:
    • Spontaneous activity in deprived visual areas creates vivid imagery.

Anton Syndrome

  • Definition: Denial of blindness despite clear evidence.
  • Neuroanatomy:
    • Bilateral occipital damage leads to confabulated visual experiences.

Key Concepts

  • Dissociations: Critical for understanding functional specialization.
  • Blindsight: Highlights non-conscious pathways in visual processing.
  • Category-Specific Deficits: Reveal organization of semantic knowledge in the brain.

This lecture explored the neural and behavioral underpinnings of visual agnosias, emphasizing dissociations between perception and recognition. Future discussions will delve deeper into attentional mechanisms and their role in higher-order vision.

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