title: Is the middle between both halves?: midpoint location and segment size estimation in neglect.
abstract: BACKGROUND
  Line bisection errors in neglect are attributed to perceptual size distortions. In order to compare the two segments of the line to determine if they are equal, one might first estimate the location of a midpoint that defines the two line segments to be compared.
  
  
  OBJECTIVE
  The authors attempted to determine whether estimating a line's midpoint can be dissociated from comparing the two segments of this line, and if so, what the relative contribution of each of these tasks is to the perceptual bias in neglect.
  
  
  METHODS
  The authors studied two patients with hemispatial neglect from right hemisphere lesions by asking them where bisection marks were placed on prebisected lines and whether the two adjacent line segments were equal.
  
  
  RESULTS
  There was a stronger bias judging the position of the bisecting marks ("where" determination) than comparing the size of two adjacent line segments.
  
  
  CONCLUSIONS
  These results suggest that perceptual size distortion of line segments alone cannot explain the subjects' line bisection bias, but postperceptual deficits in "where" computations may better account for their errors locating the midpoint. "Where" determinations might require more attentional capacity, depend more heavily on viewer-centered allocation of attention, and be mediated by the right hemisphere's "where" dorsal stream. In contrast, comparing the length of two segments might be mediated by the left hemisphere's "what" ventral stream.
authors.affiliation: National Center for Environmental Health of the National Institute of Public Health, Mexico City, Mexico. alonso_riestra@intramedios.com.mx
authors.forename: Alonso R
authors.initials: AR
authors.lastname: Riestra
authors.forename: Kyle B
authors.initials: KB
authors.lastname: Womack
authors.forename: Gregory P
authors.initials: GP
authors.lastname: Crucian
authors.forename: Kenneth M
authors.initials: KM
authors.lastname: Heilman
mesh_terms: D000368:Aged; D002544:Cerebral Infarction / Q000150:complications / Q000523:psychology; D005556:Form Perception / Q000502:physiology*; D006801:Humans; D008279:Magnetic Resonance Imaging; D008297:Male; D008875:Middle Aged; D009483:Neuropsychological Tests; D010468:Perceptual Disorders / Q000209:etiology / Q000523:psychology*; D012858:Size Perception / Q000502:physiology*