Alcohol Disrupted Eye Tracking in Ways That Cannabis Did Not | Research

The takeaway

Alcohol impaired smooth eye tracking movements at multiple doses, while cannabis had no measurable effect on the same visual tracking tasks.

Read this if you want to understand how cannabis and alcohol differ in their effects on basic visual-motor coordination.

Cannabis produced no measurable impairment on smooth or saccadic eye tracking

What the researchers found

Researchers tested experienced substance users on a visual tracking task where participants followed a small moving dot with their eyes. The dot oscillated faster and faster until smooth tracking broke down.

Alcohol consistently degraded smooth eye tracking, reducing the frequency at which participants could maintain smooth pursuit. It also increased the time the brain needed to initiate saccadic (jumping) eye movements. These effects appeared dose-dependent.

Cannabis, by contrast, produced no measurable impairment on either smooth or saccadic tracking compared to placebo. The researchers attributed alcohol's effects to disruption of central processing in brainstem and cerebellar regions that coordinate eye movements.

Why it matters

Eye tracking is used as a proxy for neurological impairment because it reflects how quickly the brain processes and responds to visual information. This study suggested cannabis and alcohol affect different neural pathways, with alcohol targeting brainstem and cerebellar structures involved in eye movement coordination while cannabis left those systems intact.

The numbers in context

The tracking test ranged from 0.5 to 3.0 Hz over 40 seconds. The visual field span was 7.5 degrees. Alcohol reduced both smooth and saccadic cutoff frequencies; cannabis did not differ from placebo on either measure.

How the study worked

Experienced alcohol and cannabis users tracked a horizontally oscillating dot while eye movements were recorded. The dot's frequency increased from 0.5 to 3.0 Hz over 40 seconds. Researchers measured the frequency at which smooth tracking and saccadic tracking broke down under alcohol, cannabis, and placebo conditions.

What this study cannot tell us

The study used experienced users who may have developed tolerance. The sample size was small. Only one type of visual tracking was tested, which does not capture all aspects of visual-motor coordination. The cannabis dose and route of administration were not detailed in the abstract.

How to read the evidence

A controlled laboratory experiment with experienced users. Small sample and limited dosing detail reduce generalizability.

When this study was published

Conducted in 1976. Cannabis potency and consumption methods have changed substantially since this era.

The bigger picture

This is one of several early studies attempting to identify which specific cognitive and motor functions cannabis impairs versus those it leaves intact. The finding that cannabis spared eye tracking while alcohol disrupted it helped researchers understand that these substances act on fundamentally different neural circuits.

Questions still open

Do higher cannabis doses eventually impair eye tracking? Would cannabis-naive participants show different results? How do these eye tracking findings relate to real-world driving performance?

Common questions

Did cannabis impair any aspect of eye tracking?

No. In this study, cannabis did not significantly affect either smooth pursuit or saccadic eye movements compared to placebo.

How did alcohol impair eye tracking?

Alcohol reduced the frequency at which smooth tracking could be maintained, increased the time needed to initiate saccadic eye movements, and slightly decreased saccadic velocity.

Read the original research

Alcohol and marijuana effects on ocular tracking.

American journal of optometry and physiological optics, 53(12), 764-73

View on PubMed ↗

Citation

Flom, M C; Brown, B; Adams, A J; Jones, R T. (1976). Alcohol and marijuana effects on ocular tracking.. American journal of optometry and physiological optics, 53(12), 764-73.