B.2.5.1. Method

Subjects. Twenty healthy volunteers (mean age 21.0, S.E.: 0.7, 3 men) participated in the first experiment. All subjects had normal or corrected to normal self-reported vision. One subject was discarded of the analysis, because of a number of missing errors in the second task that exceeded the mean by more than two standard errors.

Stimuli and apparatus. The stimuli were grey squares (37,37 cd/m²) appearing inside a circle of equal luminance, presented on a black background (0,034 cd/m²). At a viewing distance of 30 cm, the angular size of each square was 0.5° x 0.5°, and the circle’s radius was 1°. During the pre-mask, the squares were complete (i.e. not gaped). After the pre-mask, the square gaps were revealed. Then, each square had a small gap on one side, either on top or bottom (distractors) or on right or left side (target). The stimuli were displayed centrally on a colour screen of a Dell Latitude portable computer with a Pentium-II 200 MHz processor. The experiments took place in a dimly lit room. The auditory list was presented via two loudspeakers, one at each side of the screen.

Figure 4: Synopsis of the visual search task in Experiments 2.2 and 2.3, in the no-distractor condition (top right) and onset distractor condition (bottom right) Only the low load (6 items) condition is shown.

Procedure. A trial began with the appearance of a central fixation dot for 1000 ms (see Figure 4). A pre-mask was then added, which consisted of three or six equally spaced circles containing a square in their centre. At the same time, an arrow was added, pointing from the centre to the location of one of the three or six presented items. These items appeared on the circumference of an imaginary circle centred at fixation. The distance between the fixation point and each item was 4.8° of visual angle. The six circles formed a hexagon; the three circles formed either an upward or a downward pointing equilateral triangle. The pre-mask remained visible for 200 ms, and the arrow only for 100 ms, so this latter disappeared 100 ms before the end of the pre-mask. Finally, the search display was revealed: the central dot went off and a small gap appeared on one side of each square. The target was the only item with a gap on the right or left side. Subjects were asked to indicate the location of the target’s gap (right or left) by pressing one of two predefined response buttons (the 'J' key for right and the 'F' key for left). The display remained present until the response was given. The following conditions were tested: (a) in the no-distractor condition, the three or six items were presented, with no abrupt onset; (b) in the distractor onset condition, a new item, which contained a non-target item (i.e. open vertically), was abruptly added to the display, when the search display was revealed. It was added in an empty place of the circle, approximately opposite to the target, in order to enhance the distracting effects. Each subject completed a 12-trial training session followed by an experimental session of 96 trials (24 trials per display size per condition). Display size and conditions were randomly intermixed.

Such an experimental block was completed by the subject under two different conditions of WM load. In the single task condition, participants only had to achieve the visual search task, with no additional element. In the dual task condition, they were asked to complete the same task, while listening to an auditory list in order to detect the occurrence of two possible targets. The list consisted of random digits (from 0 to 9), read at the speed of one per second. The targets were the digits 4 and 6, which were each presented on one tenth of the stimuli, on average. The order of the blocks was counterbalanced between subjects. No feedback was given.