Alcohol and cannabis co-use altered task performance and brain activity

Increasingly potent concentrated cannabis products containing high levels of Δ9-tetrahydrocannabinol (THC) are being consumed with alcohol. This combination is known to acutely impair cognitive and psychomotor abilities, although few studies have evaluated alcohol and cannabis co-use using neural measures. This naturalistic feasibility study explored the acute effects of co-intoxication on behavioural performance and brain activity, measured via electroencephalography (EEG), during a performance monitoring task administered three times over a 4.5-h period. Participants (N = 17, 65% male) were heavy drinking community members (age M = 31.7 years; SD = 10.27) who regularly use cannabis. EEG was recorded while participants completed a speeded visual-flanker task at baseline (sober) before consuming a standardized dose of alcohol and a cannabis concentrate product, followed by EEG recordings 1-h (intoxicated) and 4-h post co-consumption (recovery). Recordings were processed to obtain six event-related potential components: the stimulus-locked N1, P2, N2 and P3 and response-locked error-related negativity (ERN) and error positivity (Pe). Repeated measures ANOVAs revealed that behavioural measures at the intoxicated timepoint showed an increase in errors (p < 0.005; η2 p = 0.35) and significant decrease in response time (RT) (p < 0.001; η2 p = 0.50) compared to sober and recovery periods. Measures of brain activity also demonstrated significant change across time with largest amplitudes when sober, smaller amplitudes when intoxicated, then an increase in amplitude to near sober levels at recovery for stimulus-locked (P2, N2 and P3) and response-locked components (ERN and Pe). Thus, compared to sober and recovery periods, the behaviour at the intoxicated timepoint revealed significantly greater impulsivity (RT) leading to significantly higher error rates, while measures of cognitive processing (i.e., decision-making, error detection, performance monitoring/adaptation) showed significantly diminished levels of brain activity. Performance and neurophysiology measures remained diminished at recovery compared to baseline. Understanding neurocognitive functioning during co-intoxication may provide insights into specific cognitive processes that may impact function in everyday activities such as driving. Trial Registration: NCT06259916.