Pathway of Visual Information Through the Brain

Overview

• Focus on the pathway of visual information from the eye to the visual cortex.
• General introduction to how visual information travels through the brain.

Starting Point: The Eye

• Vision begins with the eye.
• Retina: The neural structure of the eye where visual processing starts.
  • Outlined in blue in diagrams.
• Other components of the eye:
  • Cornea: Transparent layer that refracts light onto the retina.
  • Lens: Modifiable shape for focusing on objects at varying distances.
    • Becomes less flexible with age, leading to the need for reading glasses.
  • Pupil: Adjustable opening in the iris to regulate light intake.
    • Dilation in low-light and constriction in bright light.

The Retina

• Function: Detects light and generates electrical and chemical signals.
• Structure:
  • Contains 5 basic types of neurons situated in distinct layers.
  • Photoreceptors (at the back of the retina) convert light into signals (phototransduction).
    • Photoreceptors:
      • Rods: Sensitive to light, low spatial resolution, no color perception.
      • Cones: Less sensitive to light, high spatial resolution, responsible for color vision.
  • Fovea: Area with a high density of cones providing the highest visual acuity.

Visual Processing in the Retina

• Bipolar Cells: Relay signals from photoreceptors to ganglion cells.
• Ganglion Cells: Carry visual information to the brain.
• Horizontal Cells: Modulate photoreceptor function, enhance contrast, adapt to lighting changes.
• Amacrine Cells: Modify functions of bipolar and ganglion cells, refine visual signals.

Blind Spot

• Optic Disc: Area where ganglion cells exit the eye, creating a blind spot with no photoreceptors.
• The brain fills in this blind spot using information from other areas.

The Optic Nerve and Chiasm

• Optic Nerve: Formed by axons of ganglion cells, extends to the optic chiasm.
• Optic Chiasm:
  • 60% of axons cross to the opposite side of the brain.
  • Nasal retina fibers cross over; temporal retina fibers do not.
  • Right visual field info goes to the left brain and vice versa.

Visual Pathways After the Optic Chiasm

• Optic Tract: Continues to various brain regions.
  • Pretectum: Involved in pupillary light reflex.
  • Suprachiasmatic Nucleus: Regulates circadian rhythms based on light.
  • Superior Colliculus: Coordinates head and eye movements.
• Lateral Geniculate Nucleus (LGN): Main destination of optic tracts; synapses with neurons that extend to the primary visual cortex.

Primary Visual Cortex (V1)

• Located around the calcarine sulcus in the occipital lobe.
• Function: Creates visual images from signals received from the retina.
• Neurons in V1 activate based on various visual characteristics (orientation, movement, contrast, depth).

Further Visual Processing

• Visual Areas: V2, V3, V4, V5, and V6 communicate with V1 for higher-level processing.
  • V5 specializes in motion detection.
• Higher-level processing enables complex tasks like object recognition and meaning attribution based on experience.

Conclusion

• Overview provided insight into the major pathway of visual information in the brain.