This paper reports the findings of a quasi-experimental biological investigation examining how acute caffeine ingestion affects visual-motor response times in undergraduate students. Seven participants completed a computer-based color-change reaction task before and after consuming water, Red Bull (approximately 80 mg caffeine), or coffee (approximately 122 mg caffeine). Results showed a dose-dependent improvement: coffee improved response times by roughly 13 ms and Red Bull by approximately 6 ms compared to water. Despite high between-subjects variability attributable to genetic, habitual, and weight-related factors, findings align with prior published research. The paper reviews caffeine's pharmacology, mechanism of action via adenosine receptor blockade, and implications for future study design.
Caffeine represents the most widely consumed psychoactive substance in the world, so understanding how this chemical affects an individual's physiology is essential to providing the best healthcare advice for the general public. Towards this goal, the response times of college students were studied before and after ingestion of water, Red Bull, or coffee. The task involved clicking a mouse button as fast as possible in response to a computer monitor screen changing color. Compared to water, response times improved by almost 6 and 13 milliseconds for Red Bull and coffee, respectively. Based on published information suggesting that Red Bull and coffee ingestion would provide approximately 80 and 122 mg of caffeine, respectively, these results indicate a dose-dependent improvement in task performance as caffeine dosage increased. Although between-subjects variability was high, these results are remarkably consistent with previous findings. Basing dosage on subject weight may reduce between-subjects variability and is recommended for future studies.
Low doses of caffeine in non-caffeine users produce a sensation of euphoria, increased alertness, and improved cognition, but at high doses nausea, anxiety, and trembling are not uncommon (Yang, Palmer, & de Wit, 2010). For example, low to moderate doses of caffeine injected into rats exposed to stress-inducing noise were protective against increased production of stress hormones, but high doses were not (Patz, Day, Burow, & Campeau, 2006). Long-term, chronic consumption can lead to dependence, withdrawal, and an increased risk of cardiovascular disease, but one upside of chronic consumption is protection against neurodegenerative disease (Yang et al., 2010). Another advantage of caffeine consumption was revealed when 40 college-aged women — habitually consuming less than 90 mg or more than 750 mg per day, with a mean weight of 58.2 kg — were given 2.5 or 5.0 mg per kg of caffeine (Jacobson & Thurman-Lacey, 1992). Only the low-caffeine users suffered significantly in terms of hand dexterity and steadiness after acute caffeine ingestion. Despite the many benefits and hazards of consuming caffeine, it is the immediate sensation of alertness and improved cognitive performance that makes caffeine the most widely used psychoactive substance on the planet.
Ingested caffeine is rapidly absorbed through the gastrointestinal tract in a dosage-dependent manner and then metabolized by cytochrome P-450 in a rate-limiting manner (Yang et al., 2010). The immediate downstream demethylation metabolites of caffeine are paraxanthine, theobromine, and theophylline. Peak blood concentrations of caffeine are obtained within 1 to 2 hours after ingestion of 250 or 500 mg of anhydrous caffeine (Bruce, Scott, Lader, & Marks, 1986). The half-life for plasma caffeine is approximately 5 hours; however, peak plasma levels can occur in as little as 15 minutes (Brunye, Mahoney, Leiberman, & Taylor, 2010). Due to genetic variation in the CYP1A2 gene, which produces P-450, the metabolic clearance of caffeine from the blood can vary up to 40-fold between individuals (Yang et al., 2010). Within-individual variation depends on habitual caffeine use, smoking status, and drug use.
The pharmacologic activity of caffeine depends on competitive binding to the A1 and A2A adenosine receptors. The dopaminergic system is the primary neurotransmitter system affected during acute ingestion, while chronic ingestion of caffeine will induce changes in the density of A1, muscarinic, nicotinic, and GABA receptors in the brain. Caffeine acts to block A1 and A2A receptor activity, thereby reducing dopaminergic inhibition of the motor system and improving psychomotor performance.
Empirical support for enhanced cognitive and motor activity has come from visual response tasks. For example, Kenemans and Lorist (1995) tested the visual response time for accepting or rejecting visual cues by pressing a button with either the left or right hand and discovered that caffeine decreased reaction times from 404.6 to 382.9 ms (p < .0001). Electroencephalography (EEG) recordings attributed the improved performance to better selectivity of visual information processing, discrimination of visual stimuli, and motor processing. Similar findings were obtained when subjects were asked to respond to specific colors while EEG recordings were made (Ruijter, De Ruiter, & Snel, 2000). The authors of that study concluded that subjects who ingested caffeine experienced higher arousal levels, better processing of attended and unattended information, and improved motor responses. When low-caffeine users (< 42.5 mg/day) were given a placebo or 100, 200, or 400 mg of caffeine, caffeine significantly improved visual alerting and executive control networks, but slightly diminished orienting network performance (Brunye et al., 2010). The most improvement was observed for the first two traits at the 200 mg caffeine dose.
Based on the research findings reviewed above, the quickness of a motor response to visual stimuli should be enhanced by acute caffeine consumption. Accordingly, the null hypothesis for this study is that acute caffeine ingestion will have no effect on task performance. To test this hypothesis, a simple experiment was conducted measuring visual-motor response times to a computer screen changing color.
A quasi-experimental study design, without randomization of subjects or treatment group, was used for this study. The independent variable was the ingestion of a beverage between the pretest and posttest performance trials. The beverage choices were 7.57 oz. of water, 6.74 oz. of coffee, or 8.4 oz. of Red Bull. The predicted dosage of caffeine was 0 mg for water, between 80 and 170 mg for coffee (Mayo Clinic Staff, 2011), and 80 mg for Red Bull (Red Bull GmbH, n.d.); drip coffee averages approximately 122 mg for a 6.74 oz. serving (Caffeineinformer, 2014). The dependent variable was how quickly a subject could click a mouse button in response to a visual stimulus.
Seven undergraduate students were recruited to participate in the study. All participants were required to provide informed consent before being allowed to take part. The study design was approved by the Institutional Review Board for human studies before the study commenced.
Subjects were asked to sign on to the website www.humanbenchmark.com three times per week for three weeks. Once seated and attending to the screen, subjects would click the mouse button when the screen turned green. After ten repetitions of this task, the website generated a mean response time in milliseconds. Subjects then drank water, coffee, or a can of Red Bull, waited 15 minutes, and then performed 10 more repetitions of the task. The total time to complete the task was less than 30 minutes, so task fatigue should not have been a factor.
The seven subjects were divided into three groups, with one group containing three subjects. Each week of the trial, the three groups consumed water, coffee, or Red Bull once per week on separate days. Within a single day of the trial, each member of a group consumed the same beverage; however, no group ever drank the same beverage as the other two groups on the same day. This strategy resulted in each group ingesting all three beverages each week across three different days.
The mean response times for pretest and posttest were recorded and compared to determine the difference. Data were reported in terms of the response time difference (ms) and the standard error.
"Dose-dependent response time improvements across beverage groups"
"Comparison to prior work, limitations, and future recommendations"
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