title: Awareness and filling-in of the human blind spot: linking psychophysics with retinal topography.
abstract: OBJECTIVE
  To link psychophysical thresholds for blind spot awareness and filling-in with early neural components that underpin these perceptions.
  
  
  METHODS
  Blind spot dimensions were quantified, after which an intrinsic stimulus (i.e., a rectangular bar of varying length centered within the blind spot) was used to determine blind spot awareness and filling-in for five subjects. Histologic examination of six human retinas at 20-μm intervals from the temporal and nasal neural rims of the optic nerve head out to 1040 μm allowed the quantification of outer nuclear layer thickness, a direct correlate of photoreceptor density.
  
  
  RESULTS
  Blind spot awareness was reported for bar extensions beyond 0.4° to 0.8° from the edge of the blind spot. Partial and total blind spot filling-in were reported between 1.1° and 1.3° and beyond 1.5°, respectively. Histologic measures of ONL thickness were correlated with previously published data of photoreceptor spatial density to determine the percentage of photoreceptor density required to trigger a 75% probability response. Blind spot awareness was achieved by stimulating 43% to 70% of the maximum photoreceptor density. Partial and total filling-in of the blind spot required between 78% and 83% and more than 85% photoreceptor spatial densities, respectively.
  
  
  CONCLUSIONS
  A novel intrinsic stimulus has been used to concurrently investigate blind spot awareness and blind spot filling-in. Retinal neural correlates of each visual experience have been quantified. Future computational models will have to integrate bottom-up constraints with long-range cortical receptive field activity and higher order cognitive factors.
authors.affiliation: Faculty of Life Sciences, University of Manchester, Manchester, United Kingdom. r.abadi@manchester.ac.uk
authors.forename: Richard V
authors.initials: RV
authors.lastname: Abadi
authors.forename: Glen
authors.initials: G
authors.lastname: Jeffery
authors.forename: Jonathan S
authors.initials: JS
authors.lastname: Murphy
mesh_terms: D000328:Adult; D000368:Aged; D001364:Awareness / Q000502:physiology*; D005403:Fixation, Ocular / Q000502:physiology; D006801:Humans; D008297:Male; D008875:Middle Aged; D009898:Optic Disk / Q000502:physiology*; D020419:Photoreceptor Cells, Vertebrate / Q000166:cytology*; D011601:Psychophysics; D055351:Retinal Neurons / Q000502:physiology*; D012684:Sensory Thresholds / Q000502:physiology; D014019:Tissue Donors; D014796:Visual Perception / Q000502:physiology*; D055815:Young Adult