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Bruno Richard, Bruce Hansen, Dave Ellemberg, Aaron Johnson; Size matters: Increasing stimulus size reduces thresholds in an amplitude spectrum discrimination task.. Journal of Vision 2012;12(9):845. doi: https://doi.org/10.1167/12.9.845.
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The relation of spatial content to spatial scales falls at a 1/fα in natural scenes (Field, 1987). Previous studies have demonstrated that the primate visual system is sensitive to this regularity by investigating peak alpha discrimination sensitivity. The findings surrounding human sensitivity have shown to vary considerably, ranging from peaks in sensitivity of alphas (i.e., amplitude spectrum slope) between 1.0-1.3, or peaks across the entire range of alphas – excluding 0.8 (Knill et al., 1990; Tadmore & Tolhurst, 1994; Hansen & Hess, 2006), while still, some have found no peak sensitivity across all alphas (Thompson & Foster, 1997; Johnson et al., 2011). One possible account of these differences may lie in the size of stimuli previously presented to participants, which has ranged widely from 0.75° to 10° of visual angle. Although the 1/fα relation has long been considered scale invariant (Field, 1987), it is likely that alpha discrimination may be benefited at coarser scales, therefore reducing thresholds and affecting peak sensitivity. We presented 7 different stimulus sizes (ranging from 1° to 8° in steps of 1.84) at the fovea and measured alpha discrimination sensitivity of 1/f noise stimuli. Results indicate that sensitivity to select alphas disappeared when stimulus size increased and surpasses 2° of visual angle, while smaller sizes exhibited similar peak sensitivity (1.0 – 1.3) as was found in more recent studies (e.g. Hansen & Hess, 2006). In addition, overall alpha discrimination thresholds decreased as a function of stimulus size and demonstrated a size tuning to the amplitude spectrum slope, which explains part of the variability between measured thresholds of studies that presented small stimuli (Knill et al., 1991) as opposed to larger stimuli (Johnson et al., 2011; Thompson & Foster, 1997).
Meeting abstract presented at VSS 2012
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