It’s Now Possible to Quantify a Cannabis “High”

The behavioral consequences of intoxication with THC are well documented and include impaired cognitive function, including temporary amnesia and altered psychomotor performance, such as impaired driving.

Scientists do not currently understand how THC produces these behavioral changes. What is well known is that THC prevents the release, and thus function, of many different neurotransmitter systems scattered throughout the brain.

Specifically, which neurotransmitters are primarily disturbed, and in which brain areas that account for the behavioral changes remain a mystery. If scientists have any hope of gaining insights into the underlying neurobiology of a subjective high, they will need to discover a way to quantify it.

In the past, scientists would measure the concentration of THC metabolites in the breath and body fluids. Unfortunately, this approach is vulnerable to false-positive results. The problem is that THC metabolites remain in the blood and tissues for weeks after the intoxication is over.

Blood and tissue levels of THC never correlate with the level of subjective intoxication or behavioral impairment. Without an accurate, objective, and reliable biometric test, it will not be possible to study the neurobiology of cannabis-induced intoxication.

Recent studies have assessed cannabis intoxication using functional near-infrared spectroscopy (fNIRS). This scanning method is non-invasive, portable, and inexpensive. How does it work? fNIRS measures changes in oxygenated hemoglobin in the blood as it passes through the brain’s blood vessels. The initial studies discovered that THC intoxication induces regional changes in the concentration of oxygenated hemoglobin in the brain.

The current study examined individual differences in fNIRS scans to determine impairment at the level of a single brain region. The 169 subjects were 18 to 55 yrs old and regular cannabis users with no serious medical illness, no lifetime history of schizophrenia spectrum or bipolar disorder.

Each subject was given a single dose of THC designed to produce intoxication. The best dose for each participant was determined by taking a history of participants’ usual use pattern and estimated dose when used recreationally.

Other factors were considered, including sex and BMI, to maximize the likelihood of intoxication for each individual participant while minimizing adverse effects such as nausea, anxiety, and hemodynamic change.

The study found increased fNIRS activation within the prefrontal cortex that was specific to acute THC intoxication and cognitive impairment. More importantly, when the changes in hemoglobin oxygenation determined using fNIRS are combined with performance during a simple memory task, it became possible with high accuracy to distinguish individual participants who were impaired due to THC intoxication from those who are not clearly impaired or only mildly intoxicated.

What does increased fNIRS activation within the prefrontal cortex during intoxication mean? The authors concluded that THC-induced intoxication leads to reduced brain efficiency. Essentially, this means that in the presence of intoxicating levels of THC, individual neurons require greater effort to complete a simple task. Interestingly, a very similar pattern was observed in patients with schizophrenia.

This study also showed that there was no difference in THC dose between those who became impaired from those who did not. This finding suggests that blood and oral levels of THC are not sensitive or reliable measures and should not be used to distinguish THC impairment from simple THC exposure.

In the future, a portable fNIRS system may be used by law enforcement to determine whether someone is intoxicated and cognitively impaired with THC.