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Journal of the Optical Society of America A

Journal of the Optical Society of America A

| OPTICS, IMAGE SCIENCE, AND VISION

  • Vol. 15, Iss. 8 — Aug. 1, 1998
  • pp: 2023–2035

Simultaneous discrimination of velocity and contrast of drifting gratings

Rolf Müller and Mark W. Greenlee  »View Author Affiliations


JOSA A, Vol. 15, Issue 8, pp. 2023-2035 (1998)
http://dx.doi.org/10.1364/JOSAA.15.002023


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Abstract

Discrimination thresholds for velocity and contrast were measured as a function of (1) the stimulus duration, (2) the reference contrast of the stimuli, (3) the stimulus velocity, and (4) whether the observer knew on which dimension, velocity or contrast, the gratings would differ. Two vertically oriented grating patches were presented centered 2 deg left and right of the fixation point. The stimuli drifted under a stationary envelope to the right at a speed of either 1.25 deg/s (Experiment 1) or 5 deg/s (Experiment 2). The reference contrast was varied over five interleaved staircases from 0.02 to 0.32 in equal logarithmic steps. The results of two different tasks were compared. In the single-judgment task, the subject knew along which dimension the stimuli would change and was asked to judge which stimulus had the higher value along that dimension. In the dual-judgment task, the stimuli could differ in either velocity or contrast but not both. In this task the subject first indicated which dimension differed and second which stimulus had the higher value along that dimension. The dual-/single-judgment threshold ratios remained constant over a wide range of stimulus conditions. The mean value of these ratios, however, significantly exceeds that expected by the vector model presented by Greenlee and Thomas [J. Opt. Soc. Am. A 10, 395 (1993)]. A modification of the model, which assumes that velocity and contrast are not independently coded, appears to be sufficient to account for the observed differences. The results are in line with the known dependency of perceived speed on stimulus contrast.

© 1998 Optical Society of America

OCIS Codes
(050.2770) Diffraction and gratings : Gratings
(330.1800) Vision, color, and visual optics : Vision - contrast sensitivity

History
Original Manuscript: October 28, 1997
Revised Manuscript: March 4, 1998
Manuscript Accepted: March 25, 1998
Published: August 1, 1998

Citation
Rolf Müller and Mark W. Greenlee, "Simultaneous discrimination of velocity and contrast of drifting gratings," J. Opt. Soc. Am. A 15, 2023-2035 (1998)
http://www.opticsinfobase.org/josaa/abstract.cfm?URI=josaa-15-8-2023


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  25. These distributions are significantly different from that expected by chance. Observer RM showed error distributions that significantly differ from that of the guessing model (speed: chi2=8.5,df=2,p<0.02; contrast: chi2=40.6,df=2,p<0.001) but also differ from that expected by the vector model (speed: chi2=35.7,df=2,p<0.001; contrast: chi2=4.7,df=2,p<0.1). Likewise, observer ES exhibited error distributions that deviate from those expected by the guessing model (speed: chi2 =327.3,df=2,p<0.001; contrast: chi2=116.9,df =2,p<0.001) and the vector model (speed: chi2=116.0,df=2,p<0.001; contrast: chi2=210.5,df=2,p<0.001).
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