Stimuli
The stimuli were the same as the pilot experiment. Exemplars are shown in Figure 4. They were comprised on 64 small Gaussian-masked dot elements with either 4 axes of reflectional symmetry, or randomly arranged. The dot elements were on a grey disk with a diameter of approximately 3.5 degrees of visual angle.
The patterns were generated by an element positioning algorithm during the experiment, so the same pattern was never repeated. The number of individual dot elements around the axes was either 0% (random) or 100% (symmetrical). Elements were constrained from overlapping or falling at the centre of the pattern and were either green (50 Cd/m2) or black (0.15 Cd/m2). Elements were positioned on a grey background disk (40 Cd/m2) and a black background screen (0.15 Cd/m2). The individual dot element diameter was 0.43°.
For half the participants normal trials were black and rare oddball trials were green, for half it was the other way around (black oddball, green normal). Participants who had green (black) oddballs in the alcohol condition also had green(black) oddball in the placebo condition (with one exception). Due to complications with sampling and replacing participants, this balancing was not achieved in the Oddball task group (14/26 had green oddballs in the alcohol condition, and 15/26 participants had green oddballs in the placebo condition). Correct balancing was achieved in the Regularity task group.