Evaluating comparative and equality judgments in contrast perception: attention alters appearance

Abstract

Covert attention not only improves performance in many visual tasks but also modulates the appearance of several visual features. Studies on attention and appearance have assessed subjective appearance using a task contingent upon a comparative judgment (e.g., M. Carrasco, S. Ling, & S. Read, 2004). Recently, K. A. Schneider and M. Komlos (2008) questioned the validity of those results because they did not find a significant effect of attention on contrast appearance using an equality 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, to date there is no empirical support for the superiority of the equality procedure. Here, we compare biases and sensitivity to shifts in perceived contrast of both paradigms. We measured contrast appearance using both a comparative and an equality judgment. Observers judged the contrasts of two simultaneously presented stimuli, while either the contrast of one stimulus was physically incremented (Experiments 1 and 2) or exogenous attention was drawn to it (Experiments 3 and 4). We demonstrate several methodological limitations of the equality paradigm. Nevertheless, both paradigms capture shifts in PSE due to physical and perceived changes in contrast and show that attention enhances apparent contrast.

Figure 1

Experimental design. (A) In both increment Experiments (1 and 2), each trial started with a fixation period of 510 ms; then two Gabors were presented for 50 ms at 4 deg eccentricity left and right of fixation. In the equality judgment (Experiment 1), observers were asked to indicate if the contrasts of the two stimuli were the same or different, while in the comparative judgment (Experiment 2), they had to report which stimulus had higher contrast. (B) Illustration of the increment conditions in Experiments 1 and 2. For each, the numbers on the left show the increment magnitude (log contrast), the axes on the right illustrate the resulting standard (thick tick mark) and test contrast (thin tick marks). No increment (baseline, black), test incremented by 0.05 (dark blue), 0.1 (medium blue), 0.15 (light blue) log contrast steps, or standard incremented by 0.05 (dark red), 0.1 (medium red), or 0.15 (light red) log contrast steps. (C) In the attention Experiments (3 and 4), the fixation period was followed by a cue which was flashed for 70 ms at fixation (neutral condition) or at 1.5 deg above the center of one of the Gabors (test cued and standard cued conditions). After an interstimulus interval of 50 ms, the two Gabors were presented for 50 ms, and observers reported if the contrasts were the same or different or which stimulus had higher contrast in the equality (Experiment 3) and comparative (Experiment 4) judgments, respectively. Drawings are not to scale.

Figure 2

Results Experiments 1 and 2 (Physical increments). (A) Experiment 1: Frequency of reporting same contrast as a function of no-increment test contrast (baseline) for each increment condition averaged across observers. Increment conditions: test incremented by 0.15 (light blue, *), 0.1 (medium blue, ×), or 0.05 (dark blue, +) log contrast steps, no increment (black, ○), standard incremented by 0.05 (dark red, □), 0.1 (medium red, ◆), or 0.15 (light red, △) log contrast steps. Error bars are standard error of the mean. Solid lines are fits to the average data. (B) Experiment 2: Frequency of reporting higher contrast as a function of baseline test contrast for each increment condition averaged across observers. Same format as A. (C and D) PSE in the test-incremented and standard-incremented conditions as a function of the PSE in the no increment condition for each observer in the equality (C) and comparative (D) experiment. Same symbols and color code as in A and B.

Figure 3

PSE estimates Experiments 1 and 2 (Physical increments). Average PSE as a function of POE for each increment condition from the equality (large symbols) and comparative (small symbols) task. Lines mark the linear regression to the average data (dotted: equality, dashed: comparative experiment). Same symbols and color code as in Figure 2. Error bars are standard error of the mean.

Figure 4

Results Experiments 3 and 4 (Attention). (A) Experiment 3: Frequency of reporting same contrast as a function of test contrast for test-cued (blue, *), neutral (black, ○), and standard-cued (red, △) conditions, averaged across observers. Error bars are standard error of the mean. Solid lines are fits to the average data. (B) Experiment 4: Frequency of reporting higher contrast as a function of test contrast for test cued (blue, *), neutral (black, ○), and standard-cued (red, △) conditions, averaged across observers. Same format as A. (C–D) PSE in the test-cued and standard-cued conditions as a function of the PSE in the neutral condition for each observer in the equality (C) and comparative (D) experiment. Same symbols and color code as in A and B.

Figure A1

Simulation results. Top row: Equality experiment with ideal observer; distribution of PSE (A), standard deviation (B), and amplitude (C) estimates from 10,000 simulations with 20 trials per contrast level and ideal observer criterion. Medium row: Equality experiment with adjusted criterion; distribution of PSE (D), standard deviation (E), and amplitude (F). Bottom row: Comparative experiment; distribution of PSE (G), and standard deviation (H). Solid lines mark the mean, dashed lines one standard deviation. See text for details.