Discussion
Previous research has shown that the SPN is robust to experimental
variations of task, spatial attention, and visual memory load. Here we
investigated whether the SPN is also robust to alcohol induced changes
in mental state for the first time. A pilot Oddball task found a
surprising alcohol induced SPN enhancement. In contrast, the new Oddball
task showed a small alcohol induced SPN reduction. There was no effect
of alcohol on SPN amplitude in the new Regularity task. It could be that
task-relevant symmetry processing in the Regularity task is more robust
and can thus survive moderate doses of alcohol.
However, exploratory analysis indicated that task differences are partly
explained by individual differences in drinking behaviour. Participants
who drink more show an alcohol induced SPN enhancement, participants who
drink less show an alcohol induced SPN suppression. By chance, the
participants in the Oddball task were lighter drinkers. This aspect of
the results requires replication. However, it means we cannot
overinterpret task differences.
Despite the remaining questions, we can confidently state that there was
a large SPN in all tasks and conditions. The SPN is probably altered by
alcohol in a complicated way, which depends partly on the task and
partly on individual differences in drinking behaviour, but it was never
substantially reduced, let alone abolished.
In contrast, N1 was consistently reduced by alcohol in the Pilot study,
the Oddball task, and the Regularity task. We conclude that alcohol
induced changes in N1 amplitude are more consistent and reliable than
alcohol induced changes in SPN amplitude. The N1 is also generated by
mid-level visual processes involved in perceptual grouping and gestalt
formation, and these initial visual operations may be more directly
sensitive to alcohol that the subsequent SPN.
Interestingly, stimulus regularity and alcohol consumption both affected
N1 amplitude, but these effects did not interact. This suggests that
early symmetry processing during the N1 time window is not strongly
altered by alcohol. Moreover, there was some weak evidence that the P1
was enhanced by alcohol, and that the P300 in the Oddball task was
reduced by alcohol. These two effects were much smaller than the alcohol
induced N1 reduction.
Our reliable N1 results are broadly consistent with the
attention-allocation model (Sayette, 1999). Alcohol may narrow
attention, so potential distractions go unnoticed. This lets drinkers
live in the moment, untroubled by stressful memories and duties.
Distracting visual stimuli are also more likely to remain unconscious if
activations that generate the N1 are dampened.
This project highlights the importance of replication in cognitive
neuroscience. It would probably have been possible to publish the pilot
results by rhetorically blurring the distinction between confirmatory
and exploratory research. The narrative ‘alcohol enhances symmetry
sensitivity’ is readily understandable and leads to interesting
discussions about aesthetic experience. For instance, it has been
suggested that alcohol makes faces look more beautiful because it makes
them look more symmetrical (Halsey et al., 2010; Souto et al., 2008).
Our pilot results were consistent with this. However, hasty publication
of unexpected results from small sample experiments has a net negative
effect on reproducibility (Bishop, 2019; Button et al., 2013; Munafò et
al., 2017). We were keen to avoid this, and a strength of the current
work is that we replicated the pilot experiment and expanded it with a
new regularity task. While we are now less confident that the SPN is
systematically altered by alcohol consumption, we have not polluted the
literature with an eye-catching fluke!
The current work adds to previous studies that have found an SPN in
oddball tasks (Höfel & Jacobsen, 2007, Makin et al., 2013). Here we
went one step further and found an SPN in an Oddball task when
participants were intoxicated. However, this automaticity may be
qualified by known boundary conditions: Weaker regularities, such as
symmetry embedded in noise, might not produce an SPN under these
conditions (Makin et al., 2020). Moreover, SPNs generated by other kinds
of visual symmetry, such as rotation, translation, and Glass patterns,
might be more vulnerable to task and alcohol manipulations. Most
importantly, these results only generalize to symmetry in the retinal
image. This is an important limitation: Many objects are symmetrical,
but they only project a symmetrical image onto the retina when viewed in
the frontoparallel plane (Sambul et al., 2013; Sawada & Pizlo, 2008).
Symmetrical objects do not project a symmetrical image when viewed in
perspective. Perspective symmetry can be detected when it is task
relevant (Bertamini et al., 2022) but may not be processed automatically
when it is not relevant (Keefe et al., 2018; Makin et al., 2015; Rampone
et al., 2019).