The current study set out to examine whether retrograde facilitation of memory following alcohol is observed in a naturalistic setting. As hypothesised, people in the alcohol condition exhibited improved memory on the retrograde task the following day after consumption compared to before they had drunk, but no improvement was observed in the sober group. The improvement in the alcohol group was related to the dose of alcohol consumed. The study also found poorer recognition memory performance on the anterograde task on the second day of testing in the sober group compared to the first, but no difference in the alcohol group across the two sessions.

The results of this study support the notion that alcohol can facilitate memory for previously learned information. By replicating retrograde facilitation in people drinking in their own homes, this finding extends past laboratory findings2,3,4,5 to suggest that these controlled studies have ecological validity. In addition to this, a correlation suggested that within the alcohol condition, the total amount of alcohol consumed during session one was positively related to performance on the retrograde task the following day. Thus, individuals in the alcohol condition who consumed more alcohol made more correct responses during cued recall the subsequent morning. This is similar to previous findings in the laboratory administering three doses of alcohol2, where the two highest doses of alcohol caused significantly greater enhancement of memory for information learnt prior to drinking compared with the lowest dose. The current study thus suggests that alcohol dose can have a gradient effect on consolidation, implying that as greater quantities of alcohol are consumed, there is less retroactive interference. This may be explained by the creation of a neural state which better facilitates cellular and systems consolidation as dose increases. It is also interesting to note that performance on the retrograde task was better on the day following alcohol use in the alcohol group. Recent work has suggested that sleep may facilitate access to memory traces that are initially too weak to recover on a recall task such as this19, and these data may suggest that alcohol might be interacting with this process, rather than simply reducing forgetting.

The idea that alcohol prevents subsequent encoding and has an impact on memory performance the following day would be further supported by observing a relationship between extent of anterograde amnesia in session one and subsequent memory performance, yet this was not found. In the absence of this additional correlation, it is difficult to definitively conclude that reduced retrograde interference is solely responsible for the memory improvements found. Our anterograde task that measured alcohol intoxication was not sensitive enough to the memory impairing effects of alcohol, as performance in the alcohol condition did not change between sessions on this measure, despite the amnestic effects of alcohol. Nonetheless, alcohol dose was significantly related to improved memory performance the subsequent day, suggesting the gradient effect of alcohol intoxication on consolidation.

In light of the previous point, future work should make attempts to empirically rule-out alternative neurobiological explanations for retrograde facilitation by controlling for sleep. A large proportion of the literature has ignored the influence of sleep on consolidation, as many studies that report retrograde facilitation have comprised a period of sleep between sessions2, 4, 5. Since alcohol can increase the proportion of slow wave sleep (SWS) the night following drinking20, and SWS is crucial for declarative memory consolidation15, it is possible that retrograde facilitation may partially be a consequence of increased SWS, as opposed to a reduction in interference alone. It is important to rule out this explanation by controlling for SWS between sessions using polysomnography, and taking measures of sleep quality and sleep architecture.

The ability to reproduce this subtle effect on memory consolidation in an environment with less experimental control is not only important for enhancing the ecological validity of laboratory findings, it is also informative on how alcohol interacts with memory processes in the environments where alcohol consumption generally occurs. In essence, this enables researchers to make more generalisable claims about the cognitive consequences of alcohol consumption in environments where alcohol is frequently consumed, which is important for advising public health interventions. Rather than the wholly negative effect that alcohol is presumed to have on memory, this study suggests that there may be some subtle enhancing effects, that may be informative in future for developing novel pharmacotherapies and tailored cognitive enhancing interventions. Clearly, from a public health perspective, the communication of these subtle and highly constrained positive effects of alcohol on memory should be carefully considered in the context of the wide-ranging longer term cognitive impairments associated with frequent heavy consumption of this drug.

Performance on the memory anterograde task revealed a decline in performance of the sober group from session one to session two. There was not a significant increase in performance in the alcohol group across the two sessions, which might have been expected considering the acute memory impairing effects of alcohol. It may be that the task was not sensitive enough to pick up the alcohol-induced impairment on session one, although there was a tendency towards this difference. Alcohol has been suggested to have relatively confined effects on memory at the dose participants were tested at in the current study, which affects memory for peripheral rather than central material21, 22, and with memory for pictures less impaired by alcohol than memory for words.

The poorer performance on the anterograde task in session two in the sober group might be explained by considering the impact of interference on memory. It is possible that during the second session the sober condition experienced proactive interference from completing the task the prior evening, meaning that the pictures they had encoded then were making the task more difficult the following day. The alcohol group would not be subject to this interference. Although the current study used two stimulus sets between sessions to reduce practice effects, the within subjects design could have elicited proactive interference so that the temporally close sessions could have caused interference within the sober condition. This possible explanation would also align with the assumptions made by theories explaining retrograde facilitation: memory for previously learned information is enhanced via a reduction in encoding new information following alcohol consumption, which could explain why the alcohol condition did not experience proactive interference from the memory anterograde task.

In addition to the above, analysis of the behavioural pattern separation score taken from the memory anterograde task did not reveal any significant differences in performance between conditions or sessions. This score is believed to give a more sensitive measure of participants’ pattern separation performance, since it corrects for any response biases that may be present in the traditional measure of recognition memory14. Including an additional phase of the memory anterograde task before intoxication would have provided a baseline to compare performance with, both immediately after intoxication and the following morning, but was not possible in this study due to time constraints.

There are inevitable limitations of a naturalistic design. We allowed participants the freedom to consume the alcoholic beverage and quantity of their choice, but then were largely reliant on the participants’ estimates for amount of alcohol consumed, which can be inaccurate particularly as alcohol is a memory impairing drug23. One method to improve this would be to include objective and remote measurements of blood alcohol concentrations (BAC) throughout the duration of the study, such as fitting participants with a continuous transdermal alcohol monitor that continually assesses BAC through perspiration. The inclusion of this objective measure would enable researchers to assess the trajectory of BAC over a session of drinking and compare this with cognitive performance, and allow for comparisons between participants; in relation to dose, peak intoxication, and length of time until BAC readings decline back to zero. Such a measure would enable researchers to control for these differences in intoxication between participants in studies with naturalistic designs, which could be included in future research.

Related to the point above, participants were tested in their home environment, which arguably is not the most prototypical environment in which drinking takes place. Efforts to test in more naturalistic settings, such as a bar or club, were met with heavily resistance from these businesses, and hence the most available option was to test participants in their homes during a drinking session that preceded a night out.

Overall, the current research has provided compelling evidence that retrograde facilitation can occur outside a laboratory setting, in a natural environment in which alcohol is frequently consumed. Furthermore, correlative evidence suggests that amount of alcohol is related to rate of consolidation; where larger doses improve memory more than smaller doses, putatively due to reduced interference at higher doses. Initially, these findings would indicate that alcohol could cause disruptions to hippocampal encoding, as the current study suggests alcohol interferes with the ability to store information encountered following intoxication. However, without the confirmatory evidence from the anterograde task, this claim should be made with caution. Nonetheless, this finding in naturalistic settings has greater generalisability than laboratory studies to real-life situations where alcohol is frequently consumed; and potential clinical implications, for example reducing peritraumatic alcohol use.