This study is the first to examine the relationship between alcohol intoxication and task difficulty on primary task performance and rates of IB in a dynamic, real-world IB paradigm (Simons and Chabris 1999). The key findings are summarised as follows. Increasing task difficulty had a significant adverse effect on task performance that was independent of the influence of alcohol intoxication. Contrary to our hypothesis, however, the task difficulty manipulation had no effect on our measure of IB. Moreover, although alcohol intoxication was associated with a decrease in primary task accuracy and an increase in IB, these expected effects were shown only for the easy version of our primary task. This finding nevertheless aligns with the work of Clifasefi et al. (2006), who observed a similar effect of alcohol intoxication on gorilla spotting using the same “easy” Simons and Chabris’ (1999) ball-pass counting task, thus lending further support to the attention-narrowing account of AMT. According to this view, alcohol depletes attentional resources with those leftover being prioritised for processing only the most important, task-relevant features of the visual field. This channelling of attention effectively narrows the scope of peripheral monitoring making it harder for the intoxicated viewer to detect an unexpected task irrelevant stimulus—even one as salient as a fake gorilla amidst a basketball game.

Why alcohol did not impair counting performance or increase IB among our hard task group is less obvious. We suggest that the mean level of intoxication in our sample—relatively low for a field study of this nature (BAC ≈ .05%) and possibly associated with our early evening test slots—was not high enough to significantly impair primary task performance. Yet, it is quite possible that the additional demands of maintaining two separate pass counts left hard task participants with no spare attentional resources for alcohol to deplete. If so, we should expect intoxication to have a smaller impact on IB, and peripheral attention generally, in studies where the attentional demands of the primary task are high.

We have recently reported an example of such a study in Bayless and Harvey (2017), where an increase in difficulty of a dual (central/peripheral) attentional task impaired the performance of a sober group but not their intoxicated counterparts. Participants of this study were told to fix their gaze on a centrally positioned fixation cross and count its flashes as an array of coloured circles was displayed around this focal point in the screen’s periphery. On each trial, one of the peripheral circles was cued by a small arrow, also positioned in the screen’s centre and, following a blank screen interval, participants had to decide if the single circle presented next was either the same or different in colour to the one just previously cued. For trials showing four circles in the peripheral array, sober performance was superior to the alcohol group, and particularly so for peripheral recognition, which is obviously consistent with AMT. However, the sober group’s peripheral recognition advantage was significantly reduced for six-circle trials while the alcohol group’s performance was unaffected by the increase in array size. Although the Bayless and Harvey (2017) task is somewhat different to that of the present study, this earlier result of ours is at least consistent with the idea that alcohol will probably not worsen peripheral awareness when the drinker’s cognitive resources are already fully deployed with the demands of an ongoing task.

Another surprising feature of the present study is the absence of a significant effect of task difficulty on IB, which is inconsistent with previous studies (e.g. Cartwright-Finch and Lavie 2007; Lavie 1995; Simons and Chabris 1999; Simons and Jensen 2009). Our only explanation for this is that the lower accuracy scores of the hard group reflect a waning focus on this more challenging primary task, which produced attentional drifts away from it and thus a noticing rate for task irrelevant events comparable to that of the easy group. This view is supported by the fact that attentional drift or mind wandering, while more commonly experienced during mundane tasks, has been shown to cause greater impairment to the performance of difficult tasks (Feng et al. 2013). Mind wandering is also known to be associated with low attentional (or working memory) capacity (Kane et al. 2007) and, interestingly, is more likely to occur when participants are alcohol intoxicated than when sober (Sayette et al. 2009). We nevertheless acknowledge that this attentional drift account is speculative and we are currently examining the effects of alcohol on IB in a laboratory-based study with eye-tracking and a more tightly controlled computer-based IB task to shed further light on this issue.

So what does the present study teach us about the attentional focus of the drunken driver or the intoxicated witness? On the basis of our easy task data, we draw the same conclusion as Clifasefi et al. (2006), namely, that intoxicated viewers cognitively engaged with one aspect of their visual environment—such as traversing a chaotic road junction or anticipating the actions of an armed assailant—are less likely than sober counterparts to notice other unexpected scene developments, even if these events are novel and occur in plain sight. But our hard task data suggest that the extent to which alcohol increases IB is also determined by the attentional demands of the primary task. Should this be so burdensome as to consume all of the viewer’s mental resources—such as driving on ice perhaps, or convincing an armed assailant not to shoot—then we expect any additional attentional narrowing effects of intoxication to be negligible, as alcohol cannot drain resources from an empty cognitive reserve.

Unfortunately, though, as our participants were required to focus on the primary task continuously and prior to presentation of the unexpected stimulus, the present study reveals little about the attentional orientation of drinkers suddenly and simultaneously presented with objects equally matched in importance or goal relevance. Anticipating which of multiple external stimuli drinkers will prioritise remains a considerable challenge, not least because a fluid range of internal cues are also likely to be competing for their awareness. Previous basic research on perception and memory has emphasised the importance of arousal as a mechanism for biasing the viewer’s attention, facilitating swift selection of one of a number of compelling visual stimuli (see Mather and Sutherland 2011, for a review). But the matter is further complicated by the possibility that our viewer may attend not to one specific spatial location but to a stimulus distributed across the visual field, such as an object superimposed by other less important items, or perhaps to multiple objects grouped on the basis of some gestalt principle (Duncan 1984; Emmanouil and Magen 2014; O’Craven et al. 1999). Visual attention and memory researchers should therefore consider carefully the extent to which critical stimuli elicit participant arousal in future alcohol studies and ensure that these target objects are not always confounded with their spatial location.

On the basis of the present findings, we conclude that visual awareness of a salient but task-irrelevant stimulus is significantly less likely following alcohol intoxication if the drinker is engaged in an ongoing primary task, but only when his attentional capacity is not already exhausted by that task’s perceptual and cognitive demands.