MCAT Practice Test 6

Caffeine, Signal Detection, and Arousal

¹Adenosine accumulates in the brain during prolonged wakefulness. By binding A1 and A2A receptors, it inhibits arousal-promoting neurons and increases sleepiness. Caffeine is an adenosine receptor antagonist. Researchers asked whether caffeine improves the detection of faint sounds after sleep restriction and whether any improvement reflects greater sensitivity or only a change in willingness to report a sound.

²Sixty adults slept 5 hours on the night before each of two laboratory sessions. In counterbalanced order, they received 200 mg of caffeine in one session and a placebo in the other, under double-blind conditions. On each trial of the detection task (Figure 1), white noise played and a faint tone was embedded on half of the trials; participants answered "yes" (tone) or "no" (no tone). Blocks used either a neutral payoff (equal points for hits and correct rejections) or a liberal payoff (3 points per hit, 1 point lost per false alarm). Sensitivity (d′) was computed from hit and false-alarm rates; a higher d′ indicates better discrimination of tone from noise regardless of response tendency.

³Results are shown in Table 1. In a separate loudness-rating task, participants rated the loudness of the white noise on a 0–10 scale at the start of a block and again after 10 minutes of continuous exposure. Mean ratings fell from 6.0 to 4.2 in both sessions, although the physical intensity of the noise never changed, and they returned to 5.9 after a 2-minute silent break.

⁴Participants were also genotyped for a variant of ADORA2A, the gene encoding the A2A receptor. Caffeine raised d′ by 0.52 on average in carriers of the variant but by only 0.10 in noncarriers; under placebo, the two genotype groups did not differ. Finally, caffeine raised mean heart rate by 9 beats per minute. After each session, participants rated their irritability. In the caffeine session, those who attributed their pounding heart to the harsh noise reported irritability ratings nearly twice as high as those who attributed it to "maybe getting the caffeine."