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Katharina Anton-Erxleben, Jared Abrams, Marisa Carrasco; Attention does alter apparent contrast: Evaluating comparative and equality judgments. Journal of Vision 2010;10(7):273. doi: 10.1167/10.7.273.
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Introduction: Covert attention not only improves performance in many visual tasks but also modulates the appearance of several low-level visual features (e.g. Carrasco, Ling & Read, 2004). Studies on attention and appearance have assessed subjective appearance using a task contingent upon a comparative judgment between two stimuli. Recently, Schneider and Komlos (2008) questioned the validity of those results because they did not find a significant effect of attention on contrast appearance using a same-different task. They claim that such equality judgments are bias free whereas comparative judgments are bias prone and propose an alternative interpretation of the previous findings based on a decision bias. However, there is no empirical support for the superiority of the equality procedure. Here, we compare the sensitivity of both paradigms to shifts in perceived contrast.
Methods: In four experiments, we measured contrast appearance using either a comparative or an equality judgment. With both paradigms, the same observers judged the contrasts of two simultaneously presented stimuli, while either the contrast of one stimulus was physically incremented (Experiments 1&2) or exogenous attention was drawn to it (Experiments 3&4). Observers' points of subjective equality (PSEs) were derived from cumulative- or scaled-Gaussian model fits for the comparative and equality judgments, respectively.
Results & Conclusions: We demonstrate several methodological limitations of the equality paradigm. For instance, changes in the frequency of ‘same’ responses make an additional scaling parameter necessary to explain the data. PSE estimates are less accurate: unlike in the comparative judgment, asymmetric criteria for low and high contrasts lead to a consistent underestimation of the PSE relative to veridical contrast. Furthermore, variability across observers is higher in the equality judgment. Nevertheless, both paradigms capture shifts in PSE due to physical (Experiments 1&2) and perceived changes (Experiments 3&4) in contrast. Regardless of the paradigm used, attention significantly increases apparent contrast.
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