Can cannabis impair your ability to drive safely, and if so, how can we objectively measure this impairment? As cannabis use becomes increasingly accepted and legal, these questions are more critical than ever.
Traditional methods for assessing impairment, like breathalyzers, fall short when it comes to cannabis. So, can advanced brain devices, such as EEGs, fill this gap and accurately identify cannabis-impaired drivers?
Can We Trust Tests to Measure Driving Impairment from Cannabis?
There’s an urgent need to measure cannabis impairment accurately with the ongoing rise of cannabis legalization. Unlike alcohol, where blood alcohol concentration directly correlates with impairment, cannabis presents a more complex challenge.1
THC, the primary psychoactive component in cannabis, can be detected in blood, saliva, urine, and hair. However, these measurements often show weak correlations with actual impairment, making them unreliable for determining whether someone is fit to drive.1
For instance, chronic cannabis users can have detectable THC levels even after days of abstinence, leading to potential false positives in standard tests. The “high” of cannabis is influenced by factors such as the user’s history and the method of cannabis consumption. This emphasizes the need for instruments that can measure the psychoactive effects of cannabis objectively in order to decrease dependence on subjective evaluations.1,2
How long until cannabis effects wear off?
Subjective effects like “feeling high” significantly decrease within 30 minutes post-inhalation. For most users, perceived intoxication reduces to about 40% after 4 hours, with noticeable declines at 1.5, 3, and 4 hours. The inhalation method (e.g., smoking, vaping, bong use) influences these timelines, with smoking showing more consistent effects.2
Reliable measurements of cannabis intoxication are crucial not just for law enforcement and employers but also for users themselves. With a dependable tool to gauge their own impairment, users could make informed decisions about whether they are fit to drive, helping to prevent accidents and enhancing overall safety.
How Does Cannabis Impair Driving?
Cannabis impairs driving by affecting essential skills like lane maintenance and reaction time.1,2
A study led by Brown et al. provides detailed insights into how this impairment manifests. They recruited licensed drivers aged 18 to 40, administering a single dose of cannabis containing 6.7% THC or a placebo with 0.09% THC. The cannabis was inhaled via vaporization, and the driving performance of the participants was subsequently tested using a driving simulator.1
Results showed a strong connection between cannabis impairment and driving, using the standard deviation of lane position (SDLP) as a measure of lane-keeping ability. On average, cannabis-impaired drivers exhibited a 6.29 cm increase in SDLP, highlighting the negative impact of cannabis on driving ability (Fig. 1). These findings emphasize the importance of developing reliable methods to assess cannabis impairment.1
Exploring EEG as a Tool for Measuring Cannabis Intoxication
Electroencephalography (EEG) has emerged as a promising tool for assessing cannabis-induced impairment. EEG sensors measure the brain’s electrical activity, capturing the fluctuating electrical signals generated by large groups of neurons. This makes EEG devices a portable and objective option for evaluating impairments.3
Unlike traditional breathalyzers, which have limitations in detecting cannabis impairment, EEG biomarkers offer a more nuanced approach. These biomarkers, derived from a brief, easily administered neurocognitive test, provide highly sensitive and specific results.1,2
Research supports the use of EEG devices in measuring cannabis impairment, showing strong correlations between EEG biomarkers and cannabis ingestion. This technology could replace or complement existing methods, offering a more accurate assessment of whether someone can drive.1,3
How Does Cannabis Affect Your Electrical Brain Activity?
Cannabis changes your brain’s electrical activity, with effects that can be observed shortly after use. The effects manifest quickly with an early onset of psychoactive effects, a rapid escalation to peak potency, and a sustained duration lasting several hours.2
In Bosnyak et al.’s study, researchers explored how cannabis affects the brain’s electrical activity using the Cognalyzer®, an EEG-based device designed to detect and quantify THC-induced psychoactive effects on a scale from 0 to 100%. The study included participants aged 19 to 65, all of whom were regular cannabis users with a history of consumption at least a few times per month but no more than two to three times per week.2
The study followed a carefully designed protocol where participants abstained from cannabis for three days before the experiment. After this period, they inhaled cannabis until they reported a subjective feeling of being “high” at a level of 7 out of 10. Immediately after cannabis consumption, the researchers administered a Drug Effects Questionnaire (DEQ-5) and conducted EEG recordings to capture the brain’s response to THC.2
The findings revealed that the Cognalyzer® could effectively quantify the onset, maximum potency, and duration of the psychoactive effects induced by cannabis. Notably, the EEG data showed significant increases in psychoactive effect (PE) levels, which persisted for up to 3.5 hours post-inhalation. The study also demonstrated a strong correlation between the Cognalyzer® PE levels and participants’ self-reported drug effects, reinforcing the device’s potential as an objective measure of cannabis impairment (Fig. 2).2
Furthermore, the study found that the effects of cannabis on brain activity were influenced by several factors, including the participant’s cannabis use history, the type of cannabis product inhaled, and the method of consumption. For example, more experienced cannabis users exhibited different EEG patterns compared to less frequent users, indicating that individual sensitivity to THC can vary widely.2
One of the key findings from this study was the reduction in slow theta power (3–5 Hz) in the parietal and occipital regions of the brain during a resting state with eyes closed. The observed reduction in theta power suggests that cannabis intoxication disrupts the brain’s normal resting state, potentially altering consciousness and reducing relaxation. This disruption may contribute to the impaired driving performance observed in the study, as the brain’s ability to maintain a calm and focused state is compromised.1
What are theta brainwaves?
Theta brainwaves oscillate between 4–8 Hz and play a crucial role in cognitive processes such as memory, attention, and information transfer. Theta power, particularly in the slow theta band (3–5 Hz), reflects neuronal excitation and inhibition phases that facilitate information processing. EEG research often studies them for their role in linking low- and high-frequency brain activity.4-6
The relationship between EEG biomarkers and cannabis impairment was further explored in Brown et al.’s study, where researchers used the STAT® X-24 EEG wireless sensor headset to assess the impact of cannabis on driving performance. This study involved a similar participant group, who were administered either a 6.7% THC dose or a placebo, followed by a series of neuropsychological tests.1
Additionally, the study found a negative correlation between the decrease in theta power and the increase in SDLP during the driving task. This indicates that as the brain’s theta power diminishes, driving performance worsens, reinforcing the potential of EEG as a tool for assessing cannabis impairment (Fig. 3).1
The study also observed a reduction in the Late Positive Potential (LPP) amplitude—an indicator of heightened attention to emotional stimuli—during tasks demanding sustained attention and verbal memory. This outcome suggests that cannabis might lower the brain’s responsiveness to particular stimuli, impacting both memory and attention (Fig. 4).1
The Role of EEG in Shaping Cannabis Intoxication Policies and Public Safety
The findings from these studies suggest that EEG technology has significant potential for improving public safety by providing an objective, sensitive, and specific measure of cannabis impairment. The identified EEG biomarkers, such as reduced theta power and decreased LPP amplitude, correlate strongly with impaired driving performance, offering a reliable metric for assessing whether an individual is too impaired to drive.
By understanding how different cannabis products, dosages, and methods of consumption affect their ability to drive, users can make more informed decisions, enhancing road safety. Devices like the Cognalyzer® could be used in real-world settings, allowing drivers to self-assess their driving eligibility and potentially preventing accidents caused by impaired driving.
From a policy perspective, the adoption of EEG-based impairment assessments could provide law enforcement and employers with a more accurate tool for evaluating cannabis intoxication. This would not only reduce the risk of unjust penalties due to residual THC levels in chronic users but also ensure that genuinely impaired individuals are identified and prevented from engaging in potentially dangerous activities.
References
- Brown T, McConnell M, Rupp G, Meghdadi A, Richard C, Schmitt R, Gaffney G, Milavetz G, Berka C. Correlation of EEG biomarkers of cannabis with measured driving impairment. Traffic injury prevention. 2019 Nov 25;20(sup2):S148-51.
- Bosnyak D, McDonald AC, Gasperin Haaz I, Qi W, Crowley DC, Guthrie N, Evans M. Use of a novel EEG-based objective test, the Cognalyzer®, in quantifying the strength and determining the action time of cannabis psychoactive effects and factors that may influence them within an observational study framework. Neurology and Therapy. 2022 Mar 1:1-22.
- Soufineyestani M, Dowling D, Khan A. Electroencephalography (EEG) technology applications and available devices. Applied Sciences. 2020 Oct 23;10(21):7453.
- Aktürk T, de Graaf TA, Güntekin B, Hanoğlu L, Sack AT. Enhancing memory capacity by experimentally slowing theta frequency oscillations using combined EEG-tACS. Scientific Reports. 2022 Aug 20;12(1):14199.
- Tan E, Troller-Renfree SV, Morales S, Buzzell GA, McSweeney M, Antúnez M, Fox NA. Theta activity and cognitive functioning: Integrating evidence from resting-state and task-related developmental electroencephalography (EEG) research. Developmental Cognitive Neuroscience. 2024 Jun 1;67:101404.
- Gordon PC, Belardinelli P, Stenroos M, Ziemann U, Zrenner C. Prefrontal theta phase-dependent rTMS-induced plasticity of cortical and behavioral responses in human cortex. Brain stimulation. 2022 Mar 1;15(2):391-402.
