Procedure

The procedure is described in figure 10.1. Each trial started with the presentation of the fixation point. Participants were instructed to look at this

Figure 10.1

Time course of a single trial. Stimuli could appear either above or below the fixation point. Prime and mask color were independent; both stimuli could be either red or green.

Figure 10.1

Time course of a single trial. Stimuli could appear either above or below the fixation point. Prime and mask color were independent; both stimuli could be either red or green.

point and not move their eyes during the presentation of stimuli. After presentation of a fixation point, the prime appeared for 17 msec either above or below the fixation point. After a prime-mask SOA of 17, 34, 51, 68, or 85 msec, the mask appeared at the same position as the prime and remained on the screen until the participant pressed one of two response keys on the computer keyboard, using the left or right index finger, respectively. The interval between the onset of the fixation point and the onset of the mask was constant at 500 msec. After a constant intertrial interval (ITI) of 500 msec, the next trial started. Prime color, mask color, stimulus position, and prime-mask SOA were varied randomly and independently. Stimulus conditions were identical for the mask and prime identification tasks.

Two Tasks: Mask and Prime Identification

In the mask identification task, participants had to indicate the color of the mask by pressing the appropriate key (left or right). They were instructed to respond as quickly and accurately as possible. After an incorrect response, a tone (100 Hz, 500 msec) was sounded and the following ITI was prolonged to 1000 msec. At the end of the block, participants received visual feedback about their average response time and number of errors.

The prime identification task was similar, but participants had to identify the prime rather than the mask. However, to diminish possible non-conscious influences from the prime, responses were slightly delayed. A warning signal (1000 Hz, 200 msec) was sounded when the response time was shorter than 600 msec, and the following ITI was prolonged to 1000 msec. There was no feedback about the accuracy of the response. At the end of the block, however, participants received visual feedback about the number of errors.

In both tasks, the same keys on the computer keyboard were used (either "y" or "-" on a European-style QWERTZ keyboard). The mapping of colors to responses was constant across tasks for each participant and counterbalanced across participants.

Sessions

Each participant took part in 4 sessions. Each session started with 6 blocks of mask identification, followed by 6 blocks of prime identification, another 6 blocks of mask identification, and another 6 blocks of prime identification, with 40 trials per block. The first block in each new task was excluded from data analysis. The computer program announced every change in task and repeated the most important parts of the instructions. Participants could take a short break after each block.

At the beginning of the first session, participants were carefully instructed and practiced both tasks in a demonstration program. In the first block of this program, participants performed the mask identification task, with stimuli in the same temporal sequence as in the main experiment. In the second and third blocks, they performed the prime identification task with prolonged prime-mask SOAs so that the primes were clearly visible. All prime-mask combinations were used, so that participants had full information about stimulus combinations except the variation in SOAs.

At the beginning of each subsequent session, the most important parts of the instructions were repeated. After the final session, participants were informed about the purpose of the experiment and could examine their data patterns.

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