Contrast Energy and Contour Interaction

SIGNIFICANCE
Contour interaction describes an impairment of visual acuity produced by nearby flanking features, which exerts a significant impact in many clinical tests of visual acuity. Our results indicate that the magnitude of interaction depends either on the flanker contrast energy (i.e., the product of flanker contrast and width) or the flanker contrast alone, depending on the contrast energy of the flankers.
PURPOSE
The discrimination of acuity targets is impaired by the presence of nearby flanking contours, a phenomenon known as contour interaction.
METHODS
In this study, we measured percent correct identification for threshold size, high-contrast Sloan letters at the fovea and at 5° in the inferior visual field for different combinations of flanking-bar width, and Weber contrast corresponding to specific fixed values of contrast energy (width × contrast, in %-min arc).
RESULTS
For flanking bars with low-contrast energy, contour interaction exhibited no systematic dependence on the flanking-bar width. However, when the flanking bars had higher contrast energy, narrower high-contrast bars produced significantly greater contour interaction than wider bars of lower contrast.
CONCLUSIONS
The results are consistent with the interpretation that contour interaction depends primarily on the contrast energy of flanking contours when their contrast energy is low. As the contrast energy of the flanking contours increases, the magnitude of contour interaction depends on the flanker contrast. For high-contrast flanking contours, the magnitude of contour interaction saturates when the width of the flanking contours is approximately 20% of letter size.