NEUROPSYCHOLOGY

• Sensation


  Registering of sensory information by the brain
• Perception


  Assignment of meaning to that sensory information
• How is sensory information implemented neurally?

  1. Sensory organs absorb energy

  2. Energy is transduced into a neural signal

  3. The neural signal is sent throughout the brain where further processing takes place
• Light and the electromagnetic spectrum

  • Humans' sensory system is only responsive to a narrow wavelength called the visible spectrum
  • Wavelength = colour and amplitude = brightness
• Cornea


  Transparent, involved in focusing an image onto the retina

  
• Choroid (vascular tunic)


  Eyes' blood supply providing nutrients to keep tissue alive and discard waste
• Iris


  Muscle that gives eye its colour
• Pupil


  Opening between the iris allowing more or less light
• Lens


  Focusing near and far - cataracts = cloudy lens

  
• Retina


  Contains photoreceptors (rods and cones) that convert electromagnetic energy into a neural signal

  
• Rods and cones


  • 120 million rods and 7 million cones
  • Perform transduction - converting electromagnetic energy into a neural signal
  • Cones detect colour and function in the day with high resolution
  • Rods don't detect colour, function at night and have low resolution

  
• Bipolar cells and retinal ganglion cells

  Optic nerve, blind spot
• Sensation


  Decreases
• Perception


  Increases
• Optic axis
• Brain organisation


  • 4 lobes - frontal, parietal, occipital and temporal
  • Cortex - all the convulsions
  • Subcortex - everything below

  
• Visual pathways


  1. Eyes → subcortex

  2. Eyes → lateral geniculate nucleus (LGN) - half of the fibres from each eye remain on the same side and half cross over

  3. Subcortex → cortex - LGN → primary visual cortex (V1) in the occipital lobe

  
• Mishkin and Ungerleider (1982)


  • Monkeys were trained on both an object discrimination task and a landmark discrimination task
  • Object discrimination involved telling shapes apart and landmark involved discriminating the position of a cylinder
• Ventral stream


  Involved in pattern vision, also known as the 'what' pathway
• Dorsal stream


  Involved in spatial vision, also known as the 'where' pathway
• Electrophysiology studies


  • Drop wires into brain and listen to action potentials of neurons
  • All neurons (cells) fire at a baseline rate
  • If a cell is interested in something it will either increase or decrease its firing rate relative to baseline
• From eyes to V1

  1. Rods and cones - changes in illumination

  2. Retinal ganglion (RG) cells - spots of light

  3. LGN cells - spots of light

  4. V1 cells - lines in different orientations

  
• Grandmother cells


  Cells that respond to discrete features, also known as feature detectors
• Retinotopic mapping


  • Point-to-point mapping of external world onto a brain area
  • V1 and before: there is retinotopic mapping
  • After V1: no retinotopic mapping

  
• As you go higher into the visual system (away from eyes) the information becomes more complex
• The features that drive a cell change from basic illumination levels - rods and cones to spots of light - RG cells, LGN cells to lines -V1 to complex features- IT cortex cells
• Lateral inhibition


  • RGN likes dots - centre-surrounded architecture
  • Enhances contrast
  • Brightens contrast
• Herman grid illusion


  Arises out of centre-surrounded architecture of RG cells

  
• If bathed in diffuse light or darkness, no information is sent to the brain
• If presented with a light or dark dot, maximum signal
• At the fovea, cones are more tightly packed so RG cell assembly is smaller and falls completely within the white lines
• When you foveate the intersection
• The right side of the brain looks at the left visual word-left side of brain looks at right visual field

  
• Blindsight


  • Patient DB had surgical removal of a tumor in right occipital lobe
  • He suffers from left homonymous hemianopia
  • He can't identify a static visual image but can localise it in space
  • He can't identify a moving visual object but can localise it in space
• Localising an object he can't see is a result of information reaching V5
• Sensing a moving object is a result of information reaching V5
• Not being able to identify a static or moving object is info bypassing V1
• Achromatopsia


  • Absence of colour vision
  • Damage to V4 or missing cone photoreceptors
  • People with achromatopsia are colour blind
  • Only see black, white and shades of grey

  
• Akinetopsia


  • Absence of motion vision
  • Damage to V5 (MT)
  • Difficulty perceiving objects in motion
  • Rare

  
• Appreciative agnosia


  • Failure of object recognition due to failure of visual perception
  • Preserved elementary visual function such as colour and motion perception
  • Poor matching and copying
  • Neuropathology: bilateral damage to V1