Primary Visual Pathway

• What is the primary visual pathway?

  retina --> leteral geniculate body --> visual cortex
• What makes up the retina?

  photoreceptors, bipolar cells, retinal ganglion cells
• Central items in vision stimulate...

  the fovea, other items in field stimulate the peripheral retina
• What does the optic nerve connect?

  the retina and the lateral geniculate body
• rod cells are ____ abundant, escpecially in ____

  more, peripheral retina
• Other key features of rod cells...

  • no colour discrimination
  • sensitive in low light
  • can track high-rate changes
• cone cells are ____ abundant, but highly concentrated in the ____

  less, fovea
• Other key features of cone cells...

  • 3 types dicriminate wavelengths (colours)
  • less sensitive in low light
  • cannot track high-rate changes
• How are photoreceptors and bipolar cells analogue signals?

  they vary their voltage as they a stimulated
• How are all other visual cells all-or-nothing digital signals?

  they vary their spike rate and only fire at a certain voltage
• What is photoreceptor detection of light translated into?

  excitation or inhibition of RGCs via bipolar cells
• What are receptive fields?

  portions of the retina/visual field where visual stimulation evokes a change in the firing rate of the visual neuron
• What is a substructure of a receptive field?

  A description of how visual stimuli need to be presented in the receptive field of a visual neuron in order to evoke firing-rate changes
• What do retinal ganglion cells do?

  recieve input from multiple photoreceptors via bipolar cells
• RGCs have ON-OFF centre-surround receptive fields...

  • Light presented in ‘ON’ regions excites cell (increasing firing rate), and light in ‘OFF’ regions inhibits cell (decreasing firing rate)
  • ON and OFF regions are organised in ‘centre-surround’ fashion
• The response rate of RGCs depends on...

  the sum of stimulation in ON regions minus stimulation in OFF region
• ON-OFF cenre-surround receptive fields in RGCs lead to...

  enhancement of contrast and boundaries
• What is the functional significance of centre-surround fields?

  • allows us to only respond to changes and boundaries (edges) which is more efficient than reacting to everything that is constant
  • helps to preserve appearance of objects in different light levels
• Colour sensitivity of RGCs and LGN neurons...

  Both recieve input from cones and are sensitive to colour
• What is centre-surround colour oppency?

  colour sensitive RGCs and LGNN have receptive fields where positive and negative parts of fields are opposite colours
• What is the functional significance of colour oppency?

  not clear, but can help to explain negative afterimages
• What is the primary visual cortex also known as?

  V1 or striate cortex
• most V1 neurons respond to...

  elongated stimuli with specific orientation, 2 mains types (simple and complex)
• What are simple V1 neurons?

  fields have inhibitory and excitatory regions, can be thought of as combining inputs from ON and OFF cells
• What are complex V1 neurons?

  fields have no discrete ON and OFF regions, can be thought of as combining inputs from simple cells
• Why are complex V1 neurons better?

  provides best response to moving stimuli (reflecting response adaptation)
• What is a retino-topic map?

  • orderly mapping of retina/visual field onto visual cortex
  • orderly because neighbouring regions in visual field excite neighbouring regions in retina, exciting neighbouring regions in visual cortex
• What are modules?

  V1 is divided into small columnar modules that combine neurons sensitive to different aspects of stimuli presented in a small part of the visual field
• Visual information from the modules in V1 are combined and processed in the...

  visual association cortices (V2-V5, inferior temporal cortex, posterior parietal cortex)
• What is blindsight?

  people with lesions to V1 and apparent ‘blindness’ can show appropriate responses to visual stimuli of which they are not ‘conscious’
• What does blindsight highlight?

  apart from the primary visual pathway that is critical for conscious vision, there are additional visual pathways responsible for unconcious vision