Cannabis Users See Depth Illusions Differently, Similar to People with Schizophrenia | Research

The takeaway

Regular cannabis users showed reduced binocular depth inversion illusion scores, a pattern also seen in schizophrenia patients, suggesting subtle visual processing changes that standard cognitive tests could not detect.

Read this if you are interested in how cannabis affects visual perception and its possible relationship to psychosis.

Cannabis users showed BDII pattern similar to schizophrenia, missed by standard tests

What the researchers found

Ten regular cannabis users showed significantly higher scores on the binocular depth inversion illusion (BDII) for inverted images compared to 10 matched controls. This was not due to basic visual processing problems, as there was no difference for normal image depth perception. The BDII impairment did not correlate with time since last dose, suggesting it reflected chronic rather than acute effects.

Standard neuropsychological tests of memory and executive function showed no significant differences between groups. The BDII appeared to be a more sensitive tool for detecting subtle visual information processing changes. The pattern of reduced BDII is also seen in schizophrenia patients with positive symptoms and in other altered states including alcohol withdrawal and sleep deprivation.

Why it matters

The overlap between the BDII pattern seen in cannabis users and schizophrenia patients was intriguing, as it raised questions about whether cannabis-related visual processing changes shared underlying mechanisms with psychotic states. The finding that standard tests missed what the BDII detected supported the use of more specialized assessments in cannabis research.

The numbers in context

Ten cannabis users and 10 controls. BDII scores significantly higher for inverted images in users. No significant differences on standard memory or executive function tests. Cannabis users consumed significantly more alcohol.

How the study worked

This was a cross-sectional study comparing 10 regular cannabis users with 10 healthy controls matched for age, sex, and premorbid IQ. The binocular depth inversion illusion task, standard neuropsychological tests, and personality measures (EPQ-R) were administered.

What this study cannot tell us

The sample was very small (10 per group) and cross-sectional, preventing causal conclusions. Cannabis users also used more alcohol, which could confound results. Pre-existing perceptual differences could explain the findings rather than cannabis effects. The BDII, while sensitive, is a specialized measure with limited clinical applicability.

How to read the evidence

This is a small cross-sectional study with 10 participants per group, providing preliminary evidence of a perceptual finding.

When this study was published

Published in 2003. The relationship between cannabis and psychosis-like perceptual changes continues to be investigated.

The bigger picture

The relationship between cannabis use and schizophrenia-like perceptual changes continues to be studied. The BDII and similar perceptual tasks have been used to explore the continuum between normal perception and psychotic-like experiences, with cannabis use appearing to shift perception in the direction of psychosis-like processing.

Questions still open

Does the shared BDII pattern between cannabis users and schizophrenia patients reflect a common neural mechanism? Would BDII normalize after sustained cannabis abstinence? Could BDII serve as a marker for individuals at risk of cannabis-related psychotic experiences?

Common questions

Does cannabis make you see things like someone with schizophrenia?

Not exactly. This study found that cannabis users processed a specific visual illusion in a way similar to schizophrenia patients. This does not mean cannabis users have schizophrenia, but it suggests some overlap in how the brain processes visual information.

Why can't standard cognitive tests detect these changes?

The depth inversion illusion tests a very specific aspect of how the brain integrates visual information. Standard memory and executive function tests measure broader cognitive abilities that may be preserved even when subtle perceptual processing is altered.

Read the original research

Reduced binocular depth inversion in regular cannabis users.

Pharmacology, biochemistry, and behavior, 75(4), 789-93

View on PubMed ↗

Citation

Semple, David M; Ramsden, Fiona; McIntosh, Andrew M. (2003). Reduced binocular depth inversion in regular cannabis users.. Pharmacology, biochemistry, and behavior, 75(4), 789-93.