August 2009
Volume 9, Issue 8
Free
Vision Sciences Society Annual Meeting Abstract  |   August 2009
Stimulus magnification compensates for eccentricity dependent sensitivity loss for first and second order biological motion stimuli
Author Affiliations
  • Rick Gurnsey
    Department of Psychology, Concordia University, Montreal, QC, Canada
  • Nikolaus Troje
    Department of Psychology, Queen's University, Kingston, ON, Canada
Journal of Vision August 2009, Vol.9, 610. doi:10.1167/9.8.610
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      Rick Gurnsey, Nikolaus Troje; Stimulus magnification compensates for eccentricity dependent sensitivity loss for first and second order biological motion stimuli. Journal of Vision 2009;9(8):610. doi: 10.1167/9.8.610.

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      © ARVO (1962-2015); The Authors (2016-present)

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Abstract

Purpose: Size scaling compensates for eccentricity-dependent sensitivity loss in a point-light-walker (PLW) direction discrimination task (Gurnsey et al., Vision Research, 2008) and PLW direction discrimination thresholds reach similar asymptotically low levels at large sizes for eccentricities of 0 to 16° (Gurnsey et al., submitted). Here we ask how PLW direction discrimination thresholds change as a function of stimulus size and eccentricity for first and second order stimuli.

Methods: On each trial a PLW was shown moving left or right at an angle (±α°) from straight ahead. An adaptive threshold procedure was used to determine threshold α at a range of stimulus sizes (uniform magnifications) at eccentricities from 0 to 16° in the right visual field. Second order walkers comprised uniform luminance dots embedded in dynamic noise (SO1) or vice versa (SO2). First order walkers were structurally identical to the second order walkers but had a higher mean luminance in the uniform luminance region; FO1 and FO2, respectively.

Results: Within each condition dividing stimulus size at each eccentricity (E) by an appropriate F = 1 + E/E2 (where E2 is the eccentricity at which stimulus size must double to achieve equivalent-to-foveal performance) collapsed all thresholds onto a single psychometric function. The average E2 values were: E2(S01) = 2.85, E2(S02) = 2.03, E2(F01) = 1.50 and E2(FO2) = 0.80; asymptotic thresholds averaged ±3.91°, ±3.83°, ±3.90° and ±4.17° respectively. However, SO1 stimuli could not be discriminated at 8 and 16° and had to be much larger at fixation in order for thresholds to be measured.

Conclusions: Second order signals can elicit PLW direction discrimination thresholds similar to first order signals. For second order stimuli, noise dots in a uniform background convey information about walker direction at much smaller sizes than do uniform dots in a noise background.

Gurnsey, R. Troje, N. (2009). Stimulus magnification compensates for eccentricity dependent sensitivity loss for first and second order biological motion stimuli [Abstract]. Journal of Vision, 9(8):610, 610a, http://journalofvision.org/9/8/610/, doi:10.1167/9.8.610. [CrossRef]
Footnotes
 This research was supported by NSERC grants to Rick Gurnsey.
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