The Structure of the Eye

The Macula and Photoreceptors

• The macula has a pit called the fovea, specialized for detailed vision
• There are two types of photoreceptors: rods and cones
• Rods:
  • 100 million in the human eye
  • Responsible for scotopic vision (seeing in dim light)
  • Contain the photopigment rhodopsin, responding to 502nm (bluish-green)
• Cones:
  • 3 million in the human eye
  • Responsible for photopic vision (seeing in bright light)
  • Three classes of cones:
    • Blue or short-wave cones (respond to 419nm, violet)
    • Green or middle-wave cones (respond to 531nm, green)
    • Red or long-wave cones (respond to 502nm, yellow)

Transduction by Photoreceptors

• Rhodopsin has two parts: opsin (protein chain) and retinal (made from Vitamin A)
• When light enters the eye, rhodopsin changes structure, breaking apart and hyperpolarizing the photoreceptor
• In the dark, rods have a resting potential of -30mV due to open sodium channels maintained by cGMP
• When rhodopsin breaks apart, cGMP is broken down, and sodium channels close, hyperpolarizing the photoreceptor

Processing by Retinal Interneurons

• Horizontal cells:
  • Receive input from photoreceptors, integrating information from nearby photoreceptors
• Bipolar cells:
  • Receive input from photoreceptors and horizontal cells, communicating to amacrine cells
  • Two types: diffuse and midget cells
  • Identify contrast by responding to light falling on photoreceptors in their receptive fields
• Amacrine cells:
  • Form connections with bipolar cells, ganglion cells, and other amacrine cells
  • Typically release GABA and glycine, integrating visual messages and processing changes in light over time
• Ganglion cells:
  • Receive input from bipolar and amacrine cells, producing action potentials
  • Axons form the optic nerve, integrating communication from 103 million photoreceptors through their receptive fields

Colour Contrast and Constancy

• Colour contrast: colours appear different depending on their context
• Colour constancy: an object's colour appears the same despite changes in lighting
• Edwin Land suggests that the brain normalizes the responses of all cones exposed to a scene before making local comparisons, maintaining colour constancy

Life-Span Development of the Visual System

• Development of contrast sensitivity: unclear
• Vision as we age:
  • Presbyopia (old sight): lens accommodates more slowly to changes in focal distance
  • Decreased response to changes in lighting
  • Perceptions of blue and green change due to lens growing and taking on a yellow hue
  • Pupil size decreases, affecting vision quality
  • Ageing affects cortical processing of visual information negatively

Disorders of the Visual System

• Amblyopia (lazy eye): one eye cannot focus on objects
• Cataracts: unclear