Abstract
Abstract: :
Purpose: To define the spatiotemporal limits of flicker defined form (FDF) contrast thresholds (also known as the phantom contour illusion). Methods:FDF phase contrast thresholds were determined for a variety of spatial stimulus characteristics using a temporal frequency of 30Hz, a background mean luminance of 50 cdm–2 and circular stimuli generated from a random dot background with tapered dots of 0.25o diameter. The FDF stimulus characteristics investigated (n = 3) included stimulus size (2, 3, 4, 5, 6, 7 and 8o diameter), eccentricity (0, 3, 9, 15 and 21o in the inferior temporal visual field), and number of random dots (1000 – 6500). The proportion of the stimulus area occupied by random dots was termed the spatial content percentage (SC%). In total, one eye of each subject was tested using 280 stimulus combinations divided into 28 sessions. Results: Increasing either target size or random dot number significantly decreased FDF contrast thresholds at all eccentricities (repeated measures ANOVA; F (df 6, 12) = 122.18, p < 0.001 and F (7, 14) = 1159.87, p < 0.001 respectively). However, random dot number had a greater influence on threshold than stimulus size. Although eccentricity had an overall significant effect on thresholds (F(df 4,8) = 14.83. p = 0.012), this effect was dependant on the random dot number / target size combination. Regression analysis of the relationship between FDF phase contrast thresholds and the product of SC% and stimulus size revealed a significant (p < 0.001) co–efficient of determination (r2) at all eccentricities (0.903, 0.973, 0.709, 0.959 and 0.882 for 0, 3, 9, 15 and 21o respectively). Less SC% was required within the stimulus area to maintain a constant FDF contrast threshold with increasing eccentricity. Conclusions: We have determined the spatiotemporal characteristics and perceptual limits of the FDF illusion. Equations will be presented that enable prediction of the combination of stimulus variables that would give an equivalent phase contrast threshold. The FDF stimulus is a high temporal frequency, contrast based detection task with the potential for preferentially stimulating the magnocellular pathway.
Keywords: shape, form, contour, object perception • temporal vision • perception