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John M. Foley, Srinivassa L. Varadharajan, Chin C. Koh, Mylene C. Q. Farias; Detection of gabor patterns. Journal of Vision 2005;5(8):181. doi: 10.1167/5.8.181.
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© ARVO (1962-2015); The Authors (2016-present)
We measured contrast thresholds of vertical Gabor patterns as a function of their eccentricity, size (standard deviation, SD), shape (ratio of collinear to orthogonal SD's), and phase using a 2AFC method with threshold corresponding to 82% correct. The patterns were 4 c/deg and they were presented for 90 or 240 msec. Log thresholds increase linearly with eccentricity at a mean rate of 0.47 dB/wavelength. Thresholds decrease as the SD of the pattern increases (TvS function). The TvS functions are concave up on log-log coordinates. Thresholds continue to decrease with SD over the entire range of 0.07 to 12 wavelengths. The threshold decrease is less than proportional to the area increase over the entire size range. For small patterns only, threshold depends on shape, and there is an interaction between shape and phase such that, for patterns with the same area, patterns in cosine phase have the lowest thresholds when they are narrow; patterns in sine phase have the lowest thresholds when they are short. Threshold energy is a U-shaped function of SD with a minimum in the vicinity of 0.4 wavelength indicating detection by small receptive fields (RF). A supplementary experiment showed that observers can discriminate among patterns of different sizes when the patterns are at threshold indicating that more than one mechanism is involved. For small patterns, TvS functions for all sizes and shapes are well fitted by a one Gabor linear receptive field model. The best RF is either circular or slightly longer in the collinear direction. Larger patterns require at least several additional RF's. Thresholds for all sizes and shapes are described by a model in which peripheral RF's are in phase with the center mechanism and falloff in sensitivity at 0.47 dB/wavelength. Their excitations are combined nonlinearly according to Quick's rule to determine the threshold.
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