Describe the structure and function of the visual pathway from stimulus through to visual representation

From Eye to Cortex

  • Reception – absorption of physical energy
  • Transduction – Physical energy is converted into an electrochemical pattern in the neurons
  • Coding – one-to-one correspondence between aspects of the physical and aspects of the resultant nervous system activity

Transduction of Light

  • Light travels through the retina to photoreceptors at the back of the eye
  • Cones are in centre of retina (fovea) and are sensitive to fine detail and colour
  • Rods are in periphery of the retina and are sensitive to movement but not fine detail
  • Light bleaches a pigment contained within the photoreceptors which leads to a graded receptor potential that eventually produces an action potential in the ganglion cell

From the Eye to the Brain

 Route of visual signals – signals reaching the left visual cortex come from the left side of the two retinas, and signals reaching the right visual cortex come from the right side of the two retinas

The Retina-Geniculate-Striate System

  • Parvocellular Pathway – Ventral (inferno-temporal cortex) “what” stream – sensitive to colour, form and fine detail – more input comes from cones
  • Magnocellular Pathway –  Dorsal (parietal cortex) “how” stream – most sensitive to motion – most input comes from rods

What and Where Pathways  Primary visual cortex (V1)

–     2 processing streams:

  • “what” pathway involved in determining what an object is (perception for perception)
  • “where” pathway involves in locating objects in space, following movements of objects and guiding movements and actions (perception for action)

Describe how visual illusions and case studies of acquired brain injury support the idea that there are two distinct visual processing pathways: one specialised for recognising objects and one for dealing with the spatial location and movement of objects

The Muller-Lyer Illusion

 Both lines are the same

  • When pointing (using vision for action system) the illusion was 5.5 %
  • When verbalizing a response (using vision for perception system) the illusion size was

22.4%

Illusions and Reality

  • Illusions can be useful in showing the ways in which information is interpreted in normal vision
  • Many illusions rely on lack of context so that it is difficult to dis-ambiguate information
  • Illusions draw attention to how much of our perceptual world we ‘fill in’ based on what makes sense

List monocular, binocular and oculomotor cues for depth, and explain how these types of information might be used to judge distance and depth in real and virtual environments

Depth or Distance Perception

  • Monocular cues – visual input from each eye separately
  • Binocular cues – visual input integrated from the 2 eyes (stereopsis, eye convergence, disparity and yielding depth)
  • Oculomotor cues – accommodation and convergence – depth/distance perception

Monocular Depth Perception Cues

  • Interposition – one object blocks another
  • Linear perspective – lines converge
  • Texture gradient – distant objects finer
  • Shading – 3D objects cast shadows
  • Aerial perspective – far objects are fuzzy
  • Familiar size – familiar objects that appear small are inferred to be distant
  • Relative size – the smaller of 2 objects is seen as further away

Virtual Grasping

  • Perception uses relative (allocentric) frames of reference
  • Motor acts directed towards objects are computed within an egocentric frame of reference