Experimental design

A mixed model design for three-factors (between-between-within design) was adopted. The first factor (between) was the pop-out times of green ring (Times): 4, 6, and 8. The second factor (between) was the contrast levels of the Gabor gratings at the beginning in the incidental memory task (Levels): 0.3 and 0.9. The contrast level was defined by the visibility of the stimulus relative to the white background, ranging from 0.0 (transparent) to 1.0 (operant). Table 1 shows the corresponding relation between the Weber Contrast and the contrast defined in this study. The third factor (within) was the report/no-report condition (R/NR). A total of 48 participants were randomly assigned to one of the six groups (Times × Level = 6; Table 2).

Corresponding relation between Weber contrast and contrast in this study

Table of the experimental design

A mixed model design for three-factors. Times: pop-out times of green ring; Levels: the contrast levels of the Gabor gratings at the beginning in the incidental memory task; N: numbers of the participants. A total of 48 participants were divided into six groups, which is the combination of the three levels in Times and the two levels in Levels. All participants completed both no-report condition and report condition.

The procedure of the experiment is shown in Figure 1. Each participant completed all four phases of the experimental procedure in a soundproof room with controlled illumination. All of them completed the experiment in a fixed order: (1) the no-report condition with 64 trials; (2) an incidental memory task on the critical stimuli; (3) the report condition with 64 trials; (4) an incidental memory task on the critical stimuli.

Design of the experiment. A, Each participant completed this experiment in the same sequence. B, In the no-report condition, the green ring was the target. Participants had to report how many times they saw the green ring. In the report condition, participants answered questions with regard to the Gabor grating they saw trial by trial. C, Each trial started with 300 ms of fixation followed by 33 ms of the stimulus, and then 1300 ms of ITI. ITI, Intertrial interval.

The no-report condition consisted of 32 critical trials and 32 blank trials with randomized sequence. Each critical trial began with a red dot at the center of the screen for 300 ms, followed by a Gabor grating with 45° incline or 135° incline for 33 ms, and then a white background for 1300 ms as the intertrial interval (ITI). The Gabor gratings were 300 × 300 pixels (px) in size with a Gaussian mask: spatial frequency = 0.035 (10.5 cycles across 300 px; opacity = 0.6; 0 as transparent and 1 as opaque). In blank trials, the 33-ms Gabor grating was replaced with a blank screen. A green ring appeared in some of the trials and served as the target in the no-report condition. The green (RGB: 196, 255, 199) ring was a circle dugout with a smaller circle from the center (longer diameter = 750 px; shorter diameter = 400 px). The onset of the green ring within a trial was selected randomly from 0 to 1266 ms, and the duration was always 300 ms. The pop-out times of green ring (the number of trials containing a green ring) depended on which group the participants were assigned (4, 6, 8 times, respectively). Participants in this condition were instructed to fixate at the red dot at the center of the screen and count how many times they see a green ring. After the participants passively viewed these 64 trials (∼106.6 s in total), they were unexpectedly asked to report the pop-out times of the green ring.

In the incidental memory task, participants was presented with the same stimuli as in the no-report condition except that the contrasts of the Gabor gratings were different. The contrast of Gabor gratings in both the report and the no-report conditions were always 0.6. However, in the incidental memory task, the Gabor gratings had an initial contrast of either 0.3 or 0.9. Participants were instructed to adjust the contrast of the Gabor gratings to match what they just saw in the previous phase (i.e., in the no-report or the report condition).

In the report condition, the stimuli and procedure were identical to those in the no-report condition (Fig. 1C) except the participants were instructed to respond to the Gabor gratings rather than the green rings. Participants were asked to provide a 3-AFC button-press response which indicated whether they saw a “Gabor grating inclined to 45 degrees,” a “Gabor grating inclined to 135 degrees,” or “nothing” in each trial. The green rings were still randomly presented in some trials, and the pop-out times depended on which group the participants were assigned. However, participants were not asked to report anything about the green rings.

All the experimental stimuli were controlled using Psychopy version 3.0, a psychology software in Python. All of the stimuli were displayed on a white background and located at the center of the screen. Participants perceived the stimuli with their chin resting on a chin rest to keep visual angles the same. Distance between the participant’s eyes and the monitor was 78.5 cm, making the critical stimuli 6.27°, and the green ring 15.88°. Also, participants were asked to maintain fixation on a red dot with a visual angle of 0.31° in the center of the screen throughout the whole procedure.