Sensory systems: From eyes to cortex

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Light enters the eye...

  • Pupil detects wavelengths and intensities of light.
  • The human visual spectrum is between 380 - 780 nm.
  • This travels at 186,000 mps. 
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The structure of the eye

1) The eye detects change in wavelength and intensity of light.

2) The cornea gathers light.

3) Light passes through the cornea

4) Onto the pupil, the hole in the iris.

5) The iris regulates the amount of light

6) Then onto the lens which focuses light onto the retina.

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The structure of the eye

  • The iris is controlled by the ANS (Autonomic Nervous System).
  • When light enters the eye, the iris dilates when dark and aroused, and becomes constricted when there is high illumination and increases acuity.
  • The lens brings image into focus on the retina.
  • It is controlled by muscles - bending more for close images and flatenning more for far images.
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Rods and cones

There are two types of light sensitive receptors on the retina:

  • Rods - 120 million
  • Cones  - 6 million

Three types of cones (they respond to specific portions of the visible light spectrum)

  • Cones that absorb long wavelength (600nm): Red
  • Cones that absorb middle wavelength (500nm) : Green
  • Cones that absorb short wavelength (400nm): Blue

Dual opponent cells:

  • Red/green opponent cells- when excited, signal red, and when inhibited, signal green.
  • Yellow/blue opponent cells - when excited by red/green input, signal yellow and when inhibited signal blue.
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The structure of the retina

  • Ganglion cell axons (800,000) bunch together to form the optic nerve.
  • These are the pathway receptor cells to te brain, via (splices) trains of action potentials (EEG).

From the retina to V1

  • Optic Chiasm - cross-over point. Nasal side of retina cross to opposite side of the brain.
  • Then about 80% of lateral geniculate nucleus to the primary visual cortex or striate cortex in occipital lobe.

Cortical Visual Areas

  • Organised into two major streams:
  • A ventral stream that serves in the recognition of faces and objects.
  • A dorsal (where/how) stream that serves in location and visuomotor skills.

Part of the dorsal visual stream includes mirror neurons that respond to the sight of another individuals actions, information and for gripping objects.

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The M system

The M system

  • Parallel visual pathways
  • Small P and large M cells.
  • Main evidence: Behavioural experiments with monkeys (Ungerleider and Mishkin, 1982), using lesions or single cell recordings.
  • Milner and Gooddale (1995): Ventral stream > conscious pathway. Dorsal stream > unconscious pathway. Project to distinct layer of LGN. Cells have distinct physiological properties.

Distinct physiological properties

  • Parvocelluar > High spatial, low temporal frequency, colour sensitive.
  • Magnocelluar > Low spatial, high temporal frequencies, insensitive to changes in colour.

Impaired M- cell development has been found in prematurity, foetal alcohol syndrome, developmental dyslexia, dyspraxia, dysphasia, ADHD, ASD, Williams, SZ, depression and violent personalities.

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From the primary visual cortex

  • V1 - visual information passes through V2, V3, V4 and V5
  • Colour, motion and orientation largely independent.
  • Channeled through where/what pathways.

Visual Agnosia

  • Visual agnosia is seeing without recognizing.
  • Supports the distinction between ventral and dorsal visual pathways, vision must rely on two processes:
  • 1) Perceptual integration 
  • 2) Sensory data organized into meaningful wholes
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