Comparing the benefits of caffeine, naps and placebo on verbal, motor and perceptual memory

Abstract

Caffeine, the world's most common psychoactive substance, is used by approximately 90% of North Americans everyday. Little is known, however, about its benefits for memory. Napping has been shown to increase alertness and promote learning on some memory tasks. We directly compared caffeine (200mg) with napping (60-90min) and placebo on three distinct memory processes: declarative verbal memory, procedural motor skills, and perceptual learning. In the verbal task, recall and recognition for unassociated words were tested after a 7h retention period (with a between-session nap or drug intervention). A second, different, word list was administered post-intervention and memory was tested after a 20min retention period. The non-declarative tasks (finger tapping task (FTT) and texture discrimination task (TDT)) were trained before the intervention and then retested afterwards. Naps enhanced recall of words after a 7h and 20min retention interval relative to both caffeine and placebo. Caffeine significantly impaired motor learning compared to placebo and naps. Napping produced robust perceptual learning compared with placebo; however, naps and caffeine were not significantly different. These findings provide evidence of the limited benefits of caffeine for memory improvement compared with napping. We hypothesize that impairment from caffeine may be restricted to tasks that contain explicit information; whereas strictly implicit learning is less compromised.

Figures

Figure 1. Experimental timeline

All subjects tested on Word List 1 in the morning. At 1pm, nappers slept with PSG monitoring. At 3pm non-nappers received an unmarked pill (200mg of caffeine or placebo). All subjects retested on Word List 1 after 7hr retention interval. All subjects were then trained and tested on Word List 2 with a 20min retention interval.

Figure 2. Declarative Verbal Memory Task

Verbal memory performance in Placebo (black bar), Nap (striped bar), and Caffeine groups (grey bar) (means and standard errors), with p-values of significant group differences. 7hr retention of Morning Words in Recall (2a) and Recognition in d’ (2b), 20min retention of Evening Words in Recall (2c) and Recognition in d’ (2d).

Figure 3. Motor Skill Learning

Differences Scores on Finger Tapping task represent increase in number of correct sequences completed in session two compared with session one in placebo (black bar), Nap (striped bar) and caffeine (grey bar) groups.

Figure 4. Perceptual Learning Task

Threshold Difference Scores for Texture Discrimination task represents change in threshold from session one to session two in placebo (black bar), Nap (striped bar) and caffeine (grey bar) groups.

Figure 5

Subjective sleepiness ratings across the day for each group (averages and standard errors). Caffeine group showed a short period of increased alertness immediately after a dose of caffeine (*).