Lecture Notes: Vision and Visual Processing (Part 4)

Overview

• Focus on neural processing in the retina and lateral geniculate nucleus (LGN).
• Understanding early stages of visual processing.
• Introduction to the retina geniculate striate pathway.

Retina Geniculate Striate Pathway

• Pathway: Retina → LGN → Striate Cortex (Primary Visual Cortex, Occipital Lobe).
• Retinotopic Organization: Information organized as a map of the retina.
  • Fovea is overrepresented due to high focal vision and one-to-one spatial summation.
  • Concept of cortical magnification.

Lateral Inhibition

• Definition: Ability of an excited neuron to reduce the activity of its neighbors.
• Example: Study with horseshoe crabs (ommatidia and lateral plexus).
• Contrast Enhancement: Neural basis explained by lateral inhibition.
  • Highlighting of borders and perception of intensity differences.
  • Examples: Mach bands, Hermann grid, and atmospheric effects in landscapes.

Lateral Geniculate Nucleus (LGN)

• Layers and Systems:
  • Parvocellular System (Layers 3-6): Responsive to color, fine patterns, stationary/slower moving objects.
    • Input primarily from photopic system (cones).
  • Magnocellular System (Layers 1-2): Responsive to movement, low illumination, lacks fine detail.
    • Input primarily from scotopic system.
• Monocular layers: Each layer corresponds to one eye (either ipsilateral or contralateral).

Receptive Fields and Center-Surround Antagonism

• Receptive Fields: Discovered by Hubel and Wiesel.
  • Neurons respond to light within their receptive field.
  • Smaller in the fovea, larger in the periphery.
  • Circular and monocular with inhibitory/excitatory areas in a center-surround arrangement.
  • On-center and off-center cells:
    • On-center: Excitation in the center, inhibition in the surround.
    • Off-center: Inhibition in the center, excitation in the surround.
• Importance: Critical for detecting contrast and discerning where stimuli start and stop.

Conclusion

• Center-surround antagonism helps in understanding visual information processing by modulating neuron firing rates.
• Next lecture: Exploration of visual cortex processing and perceptual features.