Visual integration, notes to the presentations

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Visual integration, notes to the presentations

• December 5, 2007

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Williams syndrome

• A rare genetic disorder, caused by the deletion
  of a circumscribed region of chromosome 7.
• Symptoms
• -- cardiovascular
• -- kidney, heart, and joint problems (unusually
  high Ca levels in their blood)
• -- locomotor disease
• -- CNS and cognitive functions are also affected

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Neurological and cognitive symptoms

• Reduced size of the cerebral (forebrain) tissue,
  especially that of the occipital and parietal
  cortices
• General IQ deficit (IQ 40-90)
• Williams syndrome subjects are very sociable
  (nice, friendly) and have good language skills
  (their vocabulary is close to normal)
• Spatial orientation (finding their way), memory
  for visual stimuli, and other visual functions
  are affected
• On closer scrutiny, there is language deficit as
  well, for instance for instance, the
  understanding of spatial prepositions (in, on
  under, between, among) studies with Hungarian
  WMS children

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Oblique effect, inverse oblique effect

• Oblique effect contrast detection thresholds,
  and orientation discrimination thresholdsare
  found to be smallest for vertical or horizontal
  stimuli and significantly higher for stimuli near
  the 45 obliques. (Wilson et al., 2001, Vis
  Res.)
• Inverse oblique effect thresholds for detecting
  structure in Glass patterns are lowest for
  oblique orientations, and higher for horizontal
  and vertical structure.
• Psychophysical experiments suggest that this
  results from larger pooling areas for oblique
  orientations in these patterns, and certain
  filtering properties of complex cells in the
  visual cortex.

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The staircase method

• A psychophysical procedure to measure absolute or
  differential thresholds.
• The method of limits
• -- Start with a far-from-threshold stimulus
• -- the experimenter adjusts stimulus value
  changes come in equal steps until the subjects
  response changes
• Adaptive techniques
• -- the subject, not the experimenter adjusts
  stimulus intensity
• -- Steps can vary. For instance, in auditory
  threshold measurement, the goal is to keep the
  sound just audible

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Short-range vs. long-range connectivity
underlying orientation discrimination

• The orientation-discrimination of individual
  Gabor patches is underlain by individual V1
  receptive fields (which are, arise, we saw
  earlier, from combinations of retinal ganglion
  cells).
• This ability does not presuppose long-range
  connections that connect different V1
  orientation-selective cells
• Thus if the orientation-discrimination of
  individual stimuli is affected already, then that
  presumably signals a deficit in more local V1
  connectivity (that underlying orientation-selectiv
  e individual receptive fields).

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