Current study
Our first aim was to replicate the pilot study in a new experiment. We
refer to this new experiment as the Oddball task . We
conservatively assumed that the true effect size of the late
alcohol-induced SPN difference is half that found in the pilot
(dz = 0.58) and collected a larger sample accordingly (N
= 26, power = 0.8, alpha = 0.05, two-tailed).
As well as increasing sample size, we increased the number of trials to
100 per condition and increased the number of oddball trials to 60 (from
80 and 32 in the pilot study). This improved signal quality, and the
additional oddball trials allowed us to measure a more reliable P300
component. Furthermore, to reduce variability between participants, the
absorption period of alcohol was kept to 10 minutes.
Given the pilot study results, we predicted that alcohol would have no
effect on positive P1 peak, reduce the negative N1 dip, and enhance the
SPN in the 400-1000 ms window (https://aspredicted.org/hj9eh.pdf). We
also predicted that alcohol would reduce the P300 response to oddballs.
Analysis of the Oddball task confirmed our predictions regarding P1, N1
and P300. However, the late SPN was slightly reduced, rather than
enhanced, in the alcohol session (the opposite of the pilot results).
Following those results, we completed another new experiment, with
another group of 26 participants. These participants discriminated
symmetry from random trials and ignored colour. We refer to this as theRegularity task . This is an important comparison. It might be
that automatic symmetry responses are more vulnerable to alcohol induced
changes. Here we predicted alcohol would have no effect on P1, reduce
N1, and have no effect on the SPN. These predictions were confirmed. We
also predicted that the SPN would be larger overall in the Regularity
task than the Oddball task, although this was not confirmed
(https://aspredicted.org/yn3vq.pdf).
Results of the Oddball and Regularity tasks are presented together and
as separate groups in a mixed design.