Visual Cortex and Beyond

Goals

• Explore the organization of the primary visual cortex (V1) and its columnar structure.
• Understand the hierarchical feedforward processing model, including the roles of simple, complex, and hypercomplex cells.
• Discuss the dorsal (“where”) and ventral (“what”) pathways.
• Examine clinical findings related to visual system damage, such as Anton’s syndrome, blindsight, akinetopsia, and achromatopsia.

Organization of the Visual Cortex

Columnar Structure of V1

• Primary Visual Cortex (V1):
  • Also known as Brodmann area 17.
  • Organized retinotopically, with a hypercolumn representing a specific point in visual space.
  • Hypercolumns contain:
    • Ocular dominance columns: Alternating inputs from the left and right eyes.
    • Orientation columns: Neurons sensitive to different line orientations.
    • Blobs: Regions for color processing.
    • Inter-blobs: Regions for form processing.

Hubel and Wiesel’s Model

• Simple Cells:
  • Respond to bars/edges with specific orientation.
  • Have a center-surround receptive field structure.
• Complex Cells:
  • Respond to oriented edges or bars, regardless of position in the receptive field.
  • Some are motion-sensitive.
• Hypercomplex Cells (End-Stopped):
  • Respond to edges of a specific length or corners.
  • Involved in detecting complex shapes.

Hierarchical Processing

• Feedforward model: Information is processed from simple to complex structures.
• Homunculus Problem:
  • Potential issue with hierarchical processing, suggesting the need for a single “grandmother cell” for specific recognition.

Pathways from V1

Dorsal Pathway (“Where”)

• Originates from magnocellular input in V1.
• Projects to:
  • Thick stripes in V2.
  • Area MT/V5 in the parietal lobe.
• Specializes in:
  • Motion detection.
  • Spatial awareness and navigation.
• Clinical Relevance: Damage can result in akinetopsia (motion blindness).

Ventral Pathway (“What”)

• Originates from parvocellular input and blobs/inter-blobs in V1.
• Projects to:
  • Thin and pale stripes in V2.
  • Area V4 in the temporal lobe.
• Specializes in:
  • Object recognition.
  • Color and form processing.
• Clinical Relevance: Damage can result in achromatopsia (color blindness).

Clinical Syndromes

Anton’s Syndrome

• Blind individuals confabulate visual experiences, believing they can see.
• Often associated with damage to V1 or its pathways.

Blindsight

• Visual abilities without conscious awareness, due to V1 damage.
• Example: Patient G.Y.
  • Exhibits preserved motion detection and facial expression recognition in the blind visual field.
  • Uses dorsal pathway for unconscious processing.

Akinetopsia

• Motion blindness caused by damage to MT/V5 in the dorsal pathway.
• Patients report seeing a “strobe-like” motion.

Achromatopsia

• Loss of color perception due to damage in V4 of the ventral pathway.
• Patients can perceive luminance and motion but not color.

Experimental Insights

• Neuroimaging Studies:
  • PET and fMRI reveal activity in MT/V5 for motion and V4 for color.
  • Example: Stimulation of V4 evokes colored phosphenes, demonstrating its role in color perception.
• Blindsight Studies:
  • TMS applied to V1 in healthy individuals creates transient scotomas.
  • Participants perform above chance on forced-choice tasks despite denying perception.

Summary

This lecture examined the visual system’s intricate processing from the retina to cortical areas. It highlighted the specialized roles of dorsal and ventral pathways and their contributions to motion, form, and color perception. The discussion of clinical syndromes underscored the functional significance of these pathways and the implications of their disruption.