Transient attention does increase perceived contrast of suprathreshold stimuli: a reply to Prinzmetal, Long, and Leonhardt (2008)

Abstract

Carrasco, Ling, and Read (2004) showed that transient attention increases perceived contrast. However, Prinzmetal, Long, and Leonhardt (2008) suggest that for targets of low visibility, observers may bias their response toward the cued location, and they propose a cue-bias explanation for our previous results. Our response is threefold. First, we outline several key methodological differences between the studies that could account for the different results. We conclude that the cue-bias hypothesis is a plausible explanation for Prinzmetal et al.'s (2008) results, given the characteristics of their stimuli, but not for the studies by Carrasco and colleagues, in which the stimuli were suprathreshold (Carrasco, Ling, & Read, 2004; Fuller, Rodriguez, & Carrasco, 2008; Ling & Carrasco, 2007). Second, we conduct a study to show that the stimuli used in our previous studies are not near-threshold, but suprathreshold (Experiment 1, Phase 1). Furthermore, we found an increase in apparent contrast for a high-contrast stimulus when it was precued, but not when it was postcued, providing more evidence against a cue-bias hypothesis (Experiment 1, Phase 2). We also show that the visibility of the stimuli in Prinzmetal et al. (2008) was much lower than that of Carrasco, Ling, and Read, rendering their stimuli susceptible to their cue-bias explanation (Experiment 2). Third, we present a comprehensive summary of all the control conditions used in different labs that have ruled out a cue bias explanation of the appearance studies. We conclude that a cue-bias explanation may operate with near-threshold and low-visibility stimuli, as was the case in Prinzmetal et al. (2008), but that such an explanation has no bearing on studies with suprathreshold stimuli. Consistent with our previous studies, the present data support the claim that attention does alter the contrast appearance of suprathreshold stimuli.

Figure 1

Trial sequence as used by Carrasco, Ling, and Read (2004). The observers' task is to report the position (relative to fixation) and orientation of the higher (or lower, depending on instructions) Gabor stimulus in a simultaneously presented pair. The peripheral cue is completely uninformative about the relative contrasts, positions, and orientations of the stimuli.

Figure 2

Trial sequences for appearance experiment (Experiment 1, Phase 2). (A) Precue trials: Uninformative cue precedes presentation of stimuli. Cue color, cue type and position, and stimulus contrasts, positions, and orientations were fully randomized. (B) Postcue trials: The trial follows stimulus presentation, with reversed temporal spacing relative to the precue trials. Precue and postcue trials were in separate runs of 1,000 trials, with order counterbalanced across observers.

Figure 3

Localization performance (Phase 1). Probability of correct localization, for 10 observers participating in an appearance task (Phase 1), of a single stimulus on the right or left of fixation (vertical axis) versus a stimulus's Michelson contrast. Contrasts and all other stimulus parameters in the localization phase were the same as in the appearance task. Error bars are ±1 SEM.

Figure 4

Appearance results (Experiment 1, Phase 2). We show results pooled across 10 observers. (A) Precue trials: The psychometric function shifts toward lower test contrast when the test is cued, and toward higher test contrast when the standard is cued, indicating that transient attention increases the apparent contrast of the cued stimulus. (B) Postcue trials: The psychometric functions for test-cued and standard-cued conditions are not statistically different from the neutral cue condition. Error bars are ±1 SEM.

Figure 5

Localization performance with the Prinzmetal et al. (2008) stimuli: Probability of correct localization of a single stimulus on the right or left of fixation (vertical axis) versus a stimulus's Michelson contrast. (A) Triangles indicate performance for 10 observers with 11-cpd, 1.05° visual angle Gabors, as reported by Prinzmetal et al. (2008, accepted version). (B) Circles indicate performance for 9 observers with 7-cpd, 1.05° visual angle Gabors, as reported by Prinzmetal et al. (2008, published version). For comparison, we include the localization performance for 10 observers with 4-cpd, 2° visual angle Gabors at 4° eccentricity, as used by Carrasco, Ling, and Read (2004; squares).