Years of Cannabis Use Impaired Attention Filtering, While Frequency of Use Slowed Processing Speed | Research

The takeaway

Brain recordings showed two distinct cognitive impairments in cannabis users: the ability to filter irrelevant information worsened with more years of use, while information processing speed slowed with more frequent use.

Read this if you are a long-term or frequent cannabis user and want to understand how different patterns of use affect different aspects of cognition.

Two distinct cognitive deficits linked to different dimensions of cannabis use

What the researchers found

Building on their earlier ERP work, researchers studied long-term cannabis users while sober and identified two dissociable cognitive deficits that were affected by different patterns of use.

The ability to focus attention and filter out irrelevant information, measured by frontal brain processing negativity, worsened progressively with the number of years of cannabis use but was unrelated to how often someone used. This suggested a cumulative, long-term effect that builds over time.

Information processing speed, measured by the latency of the parietal P300 brain wave, was delayed with increasing frequency of use but was unaffected by total duration. This suggested a more acute, dose-related effect.

The researchers interpreted these findings as evidence that cannabinoids produce both short-term impairments (related to how much is in the system) and long-term impairments (related to cumulative exposure), through different mechanisms.

Why it matters

This was one of the first studies to separate different dimensions of cannabis use (duration vs. frequency) and link them to distinct cognitive deficits. This nuance is important because it suggests that reducing frequency might improve processing speed while not addressing the cumulative attentional filtering deficit.

The numbers in context

Two distinct ERP measures affected: frontal processing negativity worsened with years of use; P300 latency increased with frequency of use. Users were tested while sober.

How the study worked

Cross-sectional ERP study of long-term cannabis users in the unintoxicated state. Two ERP measures of distinct attentional components were analyzed: frontal processing negativity (attentional filtering) and parietal P300 latency (processing speed). Both were correlated with years and frequency of use.

What this study cannot tell us

Cross-sectional design cannot establish causation. Pre-existing cognitive differences might predispose both to longer cannabis use and poorer attentional filtering. Sample size was not specified in the abstract. Users were sober but "sober" timing was not detailed.

How to read the evidence

A cross-sectional ERP study with objective brain measurements and sophisticated analysis of use pattern variables. Stronger than behavioral testing alone but still observational.

When this study was published

Published in 1995. Subsequent research has confirmed the distinction between frequency and duration effects on cannabis-related cognitive changes.

The bigger picture

This study pioneered the idea that cannabis produces different types of cognitive change through different mechanisms. The distinction between cumulative and frequency-dependent effects has been confirmed by subsequent research and has implications for how we think about recovery after quitting.

Questions still open

Does the cumulative attentional filtering deficit reverse after long-term abstinence? Are the frequency-dependent processing speed changes immediately reversible? Do these two deficits interact to produce the concentration difficulties cannabis users report?

Common questions

Does how long or how often you use matter more?

Both matter, but they affect different things. Years of use predicted worse attentional filtering, while frequency of use predicted slower processing speed.

Were users high during testing?

No. Users were tested in the unintoxicated state, meaning these effects persisted beyond the acute intoxication period.

Read the original research

Differential impairments of selective attention due to frequency and duration of cannabis use.

Biological psychiatry, 37(10), 731-9

View on PubMed ↗

Citation

Solowij, N; Michie, P T; Fox, A M. (1995). Differential impairments of selective attention due to frequency and duration of cannabis use.. Biological psychiatry, 37(10), 731-9.