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Borota D, Murray E, Keceli G, Chang A, Watabe JM, Ly M, Toscano JP, Yassa MA. 
“Post-study caffeine administration enhances memory consolidation in humans”. 
Nature Neuroscience. 2014 Jan 112.
Brief Communication [not fully peer reviewed]

It is currently not known whether caffeine has an enhancing effect on long-term memory in humans. We used post-study caffeine administration to test its effect on memory consolidation using a behavioral discrimination task. Caffeine enhanced performance 24 h after administration according to an inverted U-shaped dose-response curve; this effect was specific to consolidation and not retrieval. We conclude that caffeine enhanced consolidation of long-term memories in humans.

We conducted a randomized, double-blind, placebo-controlled trial in caffeine-naive participants. On day 1, participants incidentally studied images of objects, then received either 200 mg of caffeine or placebo. We collected salivary samples at baseline and 1 h, 3 h and 24 h after administration of caffeine or placebo to quantify caffeine metabolites (Fig. 1a and Supplementary Fig. 1). Twenty-four hours after the study session, we evaluated participants’ recognition performance using some items they saw the previous day (targets), some new items (foils) and some items that were similar but not identical to ones they saw before (lures; Fig. 1a). Correctly identifying these lures as ‘similar’ has been previously shown to be associated with hippocampal activity3.

We found that participants in the group that received caffeine had a significant increase in caffeine metabolites (Supplementary Fig. 2) at the 1 h and 3 h time points, which then returned to baseline amounts over a 24-h washout period. Participants who received caffeine were more likely to call lure items ‘similar’ rather than ‘old’ compared to participants who received the placebo (Fig. 1b), whereas we found no group differences in rates of target hits (Fig. 1c) or foil rejection (Fig. 1d). D′ (z(target hits) – z(false alarms)) was not significantly different among groups (t42 = 0.60, two-tailed P = 0.55), hence basic recognition memory was unaltered. We calculated a lure discrimination index (LDI) as P(‘similar’|lure) minus P(‘similar’|foil) to correct for response bias and found a significant difference between groups (t42 = 2.0, two-tailed P = 0.05; Fig. 2a). This suggests that caffeine enhanced consolidation of the initial study session such that discrimination during retrieval was improved. Both groups had similar variance as assessed by Levene’s test for (D′) (P = 0.44) and LDI (P = 0.96).
Comments and Responses to this Article
Status: display
Jan 13, 2014 16:00
Differences/Improvement Not that Large #

The news reporting didn't point out very clearly that the differences between caffeine and placebo groups is pretty darned small.

News here:

The most interesting thing about this paper is that they administered the caffeine -after- the memory training portion of the experiment. The had the participants look at and try to memorize a series of iconic images. Then the participants were either given caffeine or placebo. Then 24 hours later, they were shown a set of images again that the researchers had grouped into old (ones shown the previous day), lure/similar for images that looked like the old images in some way, and new images that were not considered similar to the old images.

The folks who got caffeine scored better than placebo group when looking at 'similar lure' images, though the difference between the groups was quite small.

Also, it's worth noting that Nature Neuroscience has this article tagged as a brief communication meaning that they are not putting their full weight behind it and this paper is preliminary and not as strictly peer reviewed as some of the material Nature publishes.
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