Cannabis has always had a complicated relationship with memory and cognition. One of the signature traits of the stereotypical stoner is forgetting the punchline midway through the setup of a joke that he/she has randomly remembered. These effects are often believed to stick around for a while, especially for heavy cannabis users.
However, the matter isn’t so straightforward; THC-induced memory and cognitive impairment can often be minimal, and THC may produce cognitive improvement in certain ways.  Encouraged by this, three scientists conducted a study published in Psychopharmacology which “aimed to detail the acute and residual cognitive effects of THC in infrequent cannabis users,” using time marks within a 48-hour period following cannabis use and nuanced assessment tasks.  Infrequent use was defined as less than once a week, and 15 male participants formed the sample population.
The researchers employed a balanced, double-blind cross-over design, juxtaposing the effects of 7.5 mg and 15 mg of delta-9-tetrahydrocannabinol (Δ9-THC) and placebo 1, 2, 4, 6, 8, 24, and 48 hours after oral consumption. The amounts of THC “were chosen as representative of social use, as a typical “joint” will deliver a dose of 5–25 mg Δ9-THC.”
The assessments included selective reminding, visual processing, reasoning, subtraction, choice reaction time, single and double digit target cancellation, and simple reaction time tasks, as well as classics like prose recall and verbal fluency, Gibson spiral maze (a perceptual motor task), a perceptual priming task, and mood and subjective effects rating scales.
As expected, the strongest cognitive impairment was observed 2 hours after 15 mg of THC intake, with the 2-hour mark considered as the peak plasma concentration. That impairment surfaced in explicit memory tasks like prose recall, “which are more predicative of everyday memory function,” while “performance on an implicit memory task was preserved intact.”
The same tendency, but even more definitive, was again observed 2 hours after 15 mg of THC intake on the selective reminding task. It had participants remember 16 unrelated words that were presented to them for 2 seconds each with 1-second intervals between them, followed by reminders of any words that weren’t recalled the first time around. Participants completed the task a second time, took a 30-minute break, and then gave their final (third) try. “The higher dose of Δ9-THC resulted in no learning whatsoever occurring over a three-trial selective reminding task at 2 h.”
Another interesting pattern that emerged across several tasks was a correlation between THC intake and an increase in speed and error rates, which is a testament to “riskier speed-accuracy trade-offs.”
So, what conclusions did the researchers draw from these findings? “[O]ral Δ9-THC impairs episodic memory and learning in a dose-dependent manner whilst sparing perceptual priming and working memory.” However, none of the effects persisted to the 24 and 48 hour-marker assessments.
This idea more or less aligns with general mixed findings, as researchers demonstrated that cannabis impairs explicit memory, so to speak, like recalling learned word lists, yet it’s largely neutral or even beneficial to more implicit forms of memory and cognition, like concept formation. 
- Curran HV, et al. “Cognitive and Subjective Dose-Response Effects of Acute Oral Δ9-Tetrahydrocannabinol (THC) in Infrequent Cannabis Users.” Psychopharmacology, vol.164, no.1, 2002, pp. 61-70. Journal Impact Factor = 3.875; Times Cited = 259
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