VISUAL PATHWAY

• The 17th-century philosopher ReneDescartes believed that the nerves from theeye would send the brain a pattern ofimpulses arranged like a picture of the perceived object, right side up. In
• Johannes Müller held that whatever excites a particular nerve establishes a special kind of energy unique to that nerve.
• Light is a particle (photon) but, it is also an electromagnetic

  wave Visible portion of the electromagnetic spectrum
• Amplitude is related to the intensity or brightness of a stimulus.
• Higher amplitude = brighter perception
• Brightness is also determined by the amount of photons released.
• CORNEA

  ➢ Exterior surface of the eye

  ➢ Highly innervated with pain receptors.

  ➢ Fastest growing tissue in the body.

  ➢ Collects oxygen from the environment.
• Refraction

  • the major function of the cornea
  • Bends light on focus images on the

  retina
• The air/cornea difference allows for

  proper refraction but the Water/cornea does not
• Amount of refraction is related to the

  air/cornea difference
• IRIS

  ➢ Colored ring of muscle tissue

  ➢ Controls the size of the pupil
• PUPIL

  ➢ Adjustable opening at the center of

  the eye

  ➢ Controls the amount of light entering

  the eye

  ➢ Consensual pupillary response

  ➢ Important social cue
• Accommodation

  • The process by which light is focus on

  the retina
• Acuity

  ○ The sharpness of vision
• Nearsightedness

  • myopia, light is focus in front of the

  retina
• Farsightedness

  • hyperopia, light is focus behind the

  retina
• Presbyopia

  “Old eyes”

  Lens become thick, and less elastic

  Older adults unable to accommodate

  • Also changes color perception and

  sensitivity.
• Rods

  ○ Only detect the presence or absence

  of light

  ○ Function only under conditions of low

  illumination (e.g. night)

  ○ Involved in perception of motion

  ○ Do not discriminate color, detail

  ○ Occur only in the periphery
• Cones

  ○ Three types of cones - S, M, and L

  ○ Each absorbs light at specific

  wavelengths

  ○ Color vision, detailed vision

  ○ Not as sensitive to light, function in

  higher illumination only

  ○ Located primarily in the fovea
• FOVEA

  ➢ Indentation in central retina

  ➢ Densely packed with cones

  ➢ Light is focused here

  ➢ No rods
• OPTIC NERVE

  ➢ Bundle of cells which exit the eye as

  the optic tract

  ➢ Transmit signals from the retina to

  the thalamus
• BLIND SPOT

  ➢ Optic disc

  ➢ Area where the optic nerve exits the

  eye (nasal side)

  ➢ No photoreceptors here
• Inner Nuclear Layer - bipolar,

  horizontal, and amacrine cells.
• Ganglion Cell Layer - All information

  leaves the retina via the ganglion cells. These

  cells exit via the optic nerve and synapse at

  the lateral geniculate nucleus of the

  thalamus.
• NEURAL PROCESSING IN THE RETINA

  FROM RETINA TO THE LGN

  a. Overview of the retina-geniculate-striate pathway

  b. Retinotopic Organization

  c. Lateral Inhibition

  d. The M and P layers of the LGN

  e. Receptive Fields and Center-Surround
• Nasal fibers are contralateral,

  temporal fibers are ipsilateral
• Synapse at the LGN:

  ○ Ipsilateral - layers 2,3, and 5

  ○ Contralateral - layers 1, 4, and 6
• Retinotopic Organization

  ➢ At each level of the pathway,

  information is organized like a map of the

  retina.

  ➢ The fovea is overrepresented in this

  map (cortical magnification
• Lateral Inhibition

  ➢ Is the capacity of an excited neuron to

  reduce the activity of its neighbors.
• The top four layers of the LGN

  • comprise the parvocellular system (layers 3-6)
  • Responsive to color, fine patterns or

  details, stationary, or slowly moving objects

  • Primary input from the photopic

  system (cones)
• The lower 2 layers of the LGN

  comprise the magnocellular system (layers 1

  & 2)

  ➢ Responsive to movement

  ➢ Primary input from the scotopic

  system (rods)
• Parvocellular neurons

  • with small cell bodies and small receptive fields, are mostly in or near the fovea.
  • They also respond to color, each neuron being excited by some wavelengths and inhibited by others.
• Parvocellular means "small celled”,

  from the Latin root parv, meaning "small".
• Magnocellular neurons

  • with larger cell bodies and receptive fields, are distributed evenly throughout the retina
  • with their larger receptive fields, respond strongly tomovement and large overall patterns, but they do not respond to color or fine details.
• Magnocellular means “large celled”, from the Latin root magn, meaning "large".
• Koniocellular neurons

  • have small cell bodies, similar to the parvocellular neurons, but they occur throughout the retina.
  • have several functions, and their axons terminate in several locations.
• Hubel & Wiesel won the Nobel prize for

  their discovery of receptive fields
• Some people with damage to area VI

  show a surprising phenomenon called

  blindsight, the ability to respond in limited

  ways to visual information without

  perceiving it consciously.
• Types of Cells in Visual Cortex

  1. Simple Cells

  2. Complex Cells

  3. End-stopped Cells
• Simple Cells

  ○ Side-by-side excitatory and inhibitory receptive fields

  ○ Result of center-surround antagonism