Participants were 13 Tel-Aviv University students (eight female) who volunteered to participate in the experiment. All were right-handed and reported normal or corrected-to-normal visual acuity and normal color vision.
Displays were generated by an Intel Pentium IV computer using software by E-prime. The stimuli were presented on a 17-inch color monitor, using a 1024 × 768 resolution graphics mode with an 85 Hz refresh rate. Responses were collected via the computer keyboard. A chin rest was used to set the viewing distance to approximately 50 cm from the monitor.
The fixation display was a gray 0.2° × 0.2° plus sign (+) in the center of a black background. Each search display consisted of six uppercase letters (1° in length and 0.8° in width) with one unique letter, the target, among five homogenous distractors. The target and distractor letters were randomly selected from a set of 14 letters (A, E, H, I, K, L, M, N, T, V, W, X, Y, and Z). Each target display included exactly three blue letters (RGB 130, 39, 152) and three yellow letters (RGB 149, 54, 56). The letters appeared within an imaginary 3 × 3 matrix with each cell subtending a 2.4° visual angle in side. Each letter was centered inside its cell with a random jitter of up to 0.15°. The masking display was similar to the search display except that both the target and distractor letters that had appeared in the search display were superimposed in each of the filled locations and were of the same color as the single letter that had appeared at that location.
In each trial, the participants had to make a forced-choice response as to whether the unique letter (the target) was blue or yellow by pressing the appropriate keys as quickly as possible with their right hands while maintaining high accuracy. Key-to-response mapping was counterbalanced between participants. Trials were organized in pairs: a prime trial in which the search display was not followed by a mask and a probe trial in which the search display was followed by a mask. In each pair, the two letters were the same for both trials, and their assignment to the roles of target or distractor either remained the same or switched.
Each trial began with the fixation display, which appeared for 1000 ms. In unmasked-display (prime) trials, the fixation display was followed by the search display, which remained on the screen for 200 ms and after which a blank screen appeared until the participant's response to the target's color. In masked-display (probe) trials, the search display remained visible for an individual exposure time, the duration of which was determined during the calibration block, described below. Then, the masking display was presented until the participant's response to the target's color. It was followed by a question mark, which prompted the participants to report their awareness of the target shape. Using their left hands, they had to report whether (a) they had clearly seen the target letter in the masked display, (b) they had seen fragments of it or were unsure about whether they had seen it, or (c) they had not seen it or any part of it at all by pressing designated keys. Thus, there were three possible awareness responses (henceforth, “fully aware,” “partially aware,” and “fully unaware,” respectively).
Note that presenting the unmasked displays for a fixed amount of time rather than until response ensured that the exposure duration of the prime displays was not confounded with reaction time (RT) on the prime trial. Also note that the masks had the same colors as the letters they replaced. Thus, the difficulty in this task resided in finding the target, not in discriminating its color.
The experiment began with a 40-trial calibration block. The calibration trials were similar to the experimental trials except that the search display duration in the masked-display trials was varied according to the participant's awareness report. Initial duration was set to 200 ms. Using a staircase procedure (Levitt,
1971), the display duration in the masked trials was decremented by 13 ms if the participant had made a “fully aware” response on the previous trial and incremented by 13 ms if the participant had made either a “fully unaware” or a “partially aware” response. Using this procedure, we aimed at obtaining roughly 50% of “fully aware” trials. The mean duration across the last 10 trials of the calibration phase was saved as the individual exposure time to be used in the experimental phase.
All possible letter pairs were equiprobable, and each of the letters had an equal probability (p = 0.07) of appearing in a given trial pair. In the probe trial, the target and distractor letters were equally likely to repeat or switch relative to the prime trial. In order to prevent priming effects between unmasked-display trials separated by a masked-display trial, a minimum of four trial pairs separated repetition of the same letter between trial pairs. All letters, including the target, were equally likely to be blue or yellow.
The calibration phase was followed by 20 practice trials. The experimental trials that followed consisted of 10 blocks of 30 trials each. Participants were allowed a short rest after each block.