Foveal contrast thresholds exhibit spatial-frequency- and polarity-specific contour interactions.
Traditionally, contour interaction has been investigated at the visual acuity limit using a Landolt C and flanking bars, performance being quantified in terms of a percent correct measure. More recently, it has been shown that the properties of the contour interaction are different when larger stimuli are used: Contour interaction is not polarity specific, and spatial frequency tuning for an unflanked C is broader. Here we quantify contour interaction for stimuli 5x larger than the resolution limit in terms of contrast thresholds. We show that polarity of bars has little effect on unfiltered stimuli but does show very different effects on the spatial-frequency-tuning curves for discrimination of the Landolt C. This explains the polarity dependence of crowding at the visual acuity limit and its independence for larger unfiltered targets. Thus the underlying filtering function is composed of more than one mechanism, affected differently depending on the relative polarity of the test and flank contours.
Ehrt
Oliver
O
McGill Vision Research, Department of Ophthalmology, McGill University, Montreal, Quebec H3A 1A1, Canada. oliver.ehrt@ak-i.med.uni-muenchen.de
Hess
Robert E
RE
Williams
Cristyn B
CB
Sher
Khurram
K
D015350:Contrast Sensitivity / Q000502:physiology; D004192:Discrimination, Psychological / Q000502:physiology; D005584:Fovea Centralis / Q000502:physiology*; D006801:Humans; D008954:Models, Biological; D012684:Sensory Thresholds / Q000502:physiology*; D014787:Vision Tests; D014785:Vision, Ocular / Q000502:physiology*; D014792:Visual Acuity