title: Exploration of the psychophysics of a motion displacement hyperacuity stimulus.
abstract: OBJECTIVE
  To explore the summation properties of a motion-displacement hyperacuity stimulus with respect to stimulus area and luminance, with the goal of applying the results to the development of a motion-displacement test (MDT) for the detection of early glaucoma.
  
  
  METHODS
  A computer-generated line stimulus was presented with displacements randomized between 0 and 40 minutes of arc (min arc). Displacement thresholds (50% seen) were compared for stimuli of equal area but different edge length (orthogonal to the direction of motion) at four retinal locations. Also, MDT thresholds were recorded at five values of Michelson contrast (25%-84%) for each of five line lengths (11-128 min arc) at a single nasal location (-27,3). Frequency-of-seeing (FOS) curves were generated and displacement thresholds and interquartile ranges (IQR, 25%-75% seen) determined by probit analysis.
  
  
  RESULTS
  Equivalent displacement thresholds were found for stimuli of equal area but half the edge length. Elevations of thresholds and IQR were demonstrated as line length and contrast were reduced. Equivalent displacement thresholds were also found for stimuli of equivalent energy (stimulus area x [stimulus luminance - background luminance]), in accordance with Ricco's law. There was a linear relationship (slope -0.5) between log MDT threshold and log stimulus energy.
  
  
  CONCLUSIONS
  Stimulus area, rather than edge length, determined displacement thresholds within the experimental conditions tested. MDT thresholds are linearly related to the square root of the total energy of the stimulus. A new law, the threshold energy-displacement (TED) law, is proposed to apply to MDT summation properties, giving the relationship T = K logE where, T is the MDT threshold, Kis the constant, and E is the stimulus energy.
authors.affiliation: Department of Visual Science, Institute of Ophthalmology, London, United Kingdom. g.m.verdon-roe@ucl.ac.uk
authors.forename: Gay Mary
authors.initials: GM
authors.lastname: Verdon-Roe
authors.forename: Mark C
authors.initials: MC
authors.lastname: Westcott
authors.forename: Ananth C
authors.initials: AC
authors.lastname: Viswanathan
authors.forename: Frederick W
authors.initials: FW
authors.lastname: Fitzke
authors.forename: David F
authors.initials: DF
authors.lastname: Garway-Heath
mesh_terms: D000328:Adult; D003941:Diagnostic Techniques, Ophthalmological; D005901:Glaucoma / Q000175:diagnosis*; D006801:Humans; D008875:Middle Aged; D009039:Motion Perception / Q000502:physiology*; D011601:Psychophysics; D014792:Visual Acuity / Q000502:physiology*