Why Normal Finger Tapping Did Not Exclude Motor Effects After Cannabis Concentrates

Finger tapping remained unchanged in a concentrate-use study even when limb speed and balance worsened. A normal result on one brief motor task therefore did not establish normal performance across the battery.

Experienced users and naturalistic exposure

Hitchcock and colleagues assessed 65 experienced concentrate users before use, immediately afterward, and one hour later. A mobile laboratory supported plasma sampling and smart-device measurements of arm speed, leg speed, finger tapping, and balance.

Participants used concentrates ad libitum. This reflects ordinary variation in intake but does not provide a uniform-dose placebo-controlled comparison.

Different functions changed at different times

Arm speed slowed immediately and remained slower at one hour. Leg speed was reduced at one hour. Balance worsened immediately, but the same significant difference was not present at the later assessment. Finger tapping did not significantly change.

The observed exposure-related performance changes did not significantly differ between women and men. That null interaction should not be confused with a claim that all baseline performance values were identical between sexes.

Plasma THC did not summarize the task results

Changes in plasma THC showed little relationship to the motor changes. The study did not establish a concentration that could substitute for measuring the particular function.

There was no matched nonuse session to fully separate fatigue, practice, and elapsed time from cannabis effects. Isolated limb acceleration also is only part of complex driving or workplace movement. The findings support selecting and interpreting tasks individually, without treating either a normal finger-tapping score or an elevated THC value as a complete assessment of a person’s ability.

The timing also changes how a negative result should be described. Balance at one hour did not show the earlier significant change, but arm and leg measures still did. Calling that later visit a complete recovery would merge different endpoints. A report should retain the task name, comparison interval, and direction of the observed change.

Related reading: why a normal result on one task does not settle every function.

Primary research

Leah N. Hitchcock; Brian L. Tracy; Angela D. Bryan; Kent E. Hutchison; L. Cinnamon Bidwell. Acute Effects of Cannabis Concentrate on Motor Control and Speed: Smartphone-Based Mobile Assessment. Frontiers in Psychiatry (2021). DOI: 10.3389/fpsyt.2020.623672.