title: On the perception of the left thoracic paraspinal line.
abstract: OBJECTIVE
  We investigated whether the left thoracic paraspinal line represents an actual change in optical density corresponding to the posterior pleural reflections or whether it represents a Mach band effect.
  
  
  METHODS
  Using photodensitometry, we obtained averaged light intensity tracings across the left thoracic paraspinal line in 14 normal digitized anteroposterior (AP) thoracic spine radiographs. We also performed a mathematical simulation of relative brightness perception by using the contrast sensitivity function of the human visual system (assumed to represent the modulation transfer function) to filter the light intensity profiles by fast Fourier transformation techniques.
  
  
  RESULTS
  Results from 14 normal AP thoracic spine radiographs showed no significant increase in light transmitted over the perceived location of the paraspinal line. A simple linear systems analysis of the behavior of the visual system predicted a Mach band effect corresponding in location to the observed paraspinal line.
  
  
  CONCLUSIONS
  The spatial pattern of optical densities in the region of the left thoracic paraspinal line rather than the tissue composition is the crucial factor responsible for the perception of the paraspinal line.
authors.affiliation: Department of Diagnostic Radiology, McGill University, Montreal General Hospital, Quebec, Canada.
authors.forename: J
authors.initials: J
authors.lastname: Casullo
authors.forename: B G
authors.initials: BG
authors.lastname: Fallone
mesh_terms: D015502:Absorptiometry, Photon; D000328:Adult; D000368:Aged; D000369:Aged, 80 and over; D015350:Contrast Sensitivity; D005260:Female; D005583:Fourier Analysis; D006801:Humans; D008027:Light; D008297:Male; D008875:Middle Aged; D019047:Phantoms, Imaging; D012189:Retrospective Studies; D013904:Thoracic Vertebrae / Q000000981:diagnostic imaging*; D014796:Visual Perception