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Detection and Discrimination Thresholds are Differentially Affected in Contour Interaction
Ehrt O., Williams C. B., Hess R. F.,
Augenklinik der LMU München (Ehrt O.) und
McGill Vision Research, Department of Ophthalmology, McGill University (Montréal)
Purpose: The detrimental effect of flanking structures on the discrimination of optotypes known as contour interaction has usually been studied close to the visual acuity limit. We are interested to see whether these results apply also to larger targets and whether detection and discrimination of the central target are differentially affected by flanking structures.
Methods: A Landolt Cs subtending a visual angle of 0,25° to 1° were generated by a PC system (VSG, Cambridge Research, GB) and presented on a calibrated monitor with a stimulus duration of 500 ms. Polarity and separation of the flanking bars ere varied and the stimulus presented at eccentricities of 0° to 20°. Contrast threshold and confidence intervals for detecting the presence of the central target and discrimination of its orientation were calculated using a bootstrapping.
Results: Discrimination thresholds for unflanked targets were 30-60% above detection thresholds irrespective of eccentricity. Bringing in the bars does affect discrimination more than detection. The threshold ratio (discrimination/detection) increases to 1.8 to 4 fold especially for opposite polarity flanks and small targets.
Conclusions: Contour interaction could be seen as a combination of masking of the central stimulus (dominant with same polarity bars and large targets) and shape discrimination (most prominent with opposite polarity flanks and small optotypes). How these mechanisms are affected in amblyopia is open to further studies.