Participants

31 participants (14 males) were recruited for this study through advertisements placed at the University of East London (UEL) and on psychology websites, via emails to UEL students, and through friends. A pre-participation health questionnaire was sent to interested individuals, to exclude persons for whom overnight fasting might have been a potential health risk (e.g., pregnant women and people suffering from diabetes or a heart condition). The ages of participants ranged from 21 to 46 years (mean 31 years; SD 7.24).

Tasks and questionnaires

Task were presented to participants in the following order. Parallel forms of all tasks were administered counterbalanced across water condition and task version.

International positive and negative affect schedule short form (I-PANAS-SF)

The I-PANAS-SF scale is a shorter version of the original PANAS consisting of 10 items instead of 20 (Thompson, 2007) used to measure general affect. Half of the emotion words presented reflect negative affect states (ashamed, afraid, hostile, nervous, upset) and the other half reflect positive affect states (active, alert, attentive, determined, inspired). Participants rated their positive and negative affect on a 5-point scale that ranged from “very slightly or not at all” (1) to “extremely” (5).

Thirst scale

Participants were asked to indicate their level of thirst by marking an X on a continuous horizontal line (17.8 cm) with anchors indicating “not at all” to “very thirsty” (Edmonds et al., 2013). This was converted to a percentage where a higher percentage indicated a higher level of thirst.

Cambridge neuropsychological test automated battery (CANTAB)

The CANTAB eclipse software (Sahakian & Owen, 1992) contains an array of tests used to assess cognitive performance. We administered six tests from this platform: the motor screening test (MOT), the simple reaction time (SRT), the choice reaction time (ChoiceRT), the big/little circle (BLC), the intra-extra dimensional set shifting (IED), and the rapid visual information processing (RVP). The IED, RVP and ChoiceRT assess executive functions including visual attention, which is the focus of this report.

ChoiceRT is a 2-choice reaction time test with stimulus and response uncertainty introduced by having two possible stimuli and two possible responses. Participants were instructed to press the left-hand button if the stimulus (an arrow) was displayed on the left-hand side of the screen, and the right-hand button if the stimulus was displayed on the right-hand side of the screen. A practice stage (24 trials) was followed by two assessment stages (50 trials each). The dependent variable was reaction time.

IED is a test of rule acquisition and reversal. Two patterns are displayed on the screen, first simple (colour-filled shapes) and then compound (white lines overlying colour-filled shapes). The participant must learn which of the two stimuli was correct by trial and error learning. When six consecutive correct responses were recorded, the contingencies were reversed and this pattern of stimulus addition and reversal continued for nine blocks. If the participant failed to reach six consecutive responses after 50 trials, the test was terminated. The dependent variable for this task was the total errors committed.

RVP is a sensitive measure of general performance and in particular of visual sustained attention. Numbers appear one at a time in a box in the centre of the screen at the rate of 100 digits per minute. Participants were instructed to press the button on the press pad whenever they spotted a target sequence of three consecutive numbers. A practice stage (lasting 2 min) in which participants were prompted as to when a sequence had begun and when to press the button was followed by a test stage (lasting 4 min) in which no cues were displayed and the participant had to spot three different sequences on their own. Target sequences occurred at the rate of 16 every 2 min. The measured dependent variable was total error rate.

Measuring cognitive reflection performance

To assess judgement and decision-making performance, we employed tasks that are typically used to assess the use of heuristic (automatic) processing that can be overcome by reflective (controlled and analytic) thinking (following largely Toplak et al., 2011). This consisted of nine vignettes or puzzles in total per session. Six of these were heuristics-and-biases vignettes from widely cited publications that reflect important aspects of rational thought such as probabilistic reasoning, hypothetical thought, theory justification, scientific reasoning, and the tendency to think statistically. Each answer to a heuristic vignette task was scored as correct or incorrect (1 or 0 score), resulting in a total maximum score of 6 (per session). The battery was comprised of the following:

1. Causal base rate (Fong, Krantz, & Nisbett, 1986). 2. Sample size (Tversky & Kahneman, 1974). 3. Gambler’s fallacy (Toplak et al., 2011). 4. Conjunction fallacy (Tversky & Kahneman, 1983). 5. Bayesian reasoning (Doherty & Mynatt, 1990). 6. Sunk cost (Arkes & Blumer, 1985).

Example of sample size:

A certain town is served by two hospitals. In the larger hospital about 45 babies are born each day, and in the smaller hospital about 15 babies are born each day. As you know, about 50% of all babies are boys. However, the exact percentage varies from day to day. Sometimes it may be higher than 50%, sometimes lower. For a period of 1 year, each hospital recorded the days on which more than 60% of the babies born were boys. Which hospital do you think recorded more such days?

(a) The larger hospital. (b) The smaller hospital. (c) About the same (that is, within 5% of each other).

In addition to the vignettes inducing heuristic thinking, the Cognitive Reflection Test (CRT; Frederick, 2005) was used. The CRT is designed to measure participants’ tendency to override an intuitive first response and to engage in reflective thinking to arrive at the correct answer (similar to the mechanism proposed to work in solving heuristic vignettes, Kahneman, 2011). The dependent variable was the total number of correct responses (maximum of 3 per session). The original CRT comprised of only three questions. We used the extended version by Toplak et al. (2014) resulting in different three questions in each of the two sessions. The answers to the six heuristic vignettes and the three CRT puzzles formed the cognitive reflection score (a maximum of nine correct answers per session). An example of the CRT is the following: in a lake, there is a patch of lily pads. Every day, the patch doubles in size. If it takes 48 days for the patch to cover the entire lake, how long would it take for the patch to cover half of the lake? (Intuitive answer: 24; correct answer: 47).

Measurement of hydration status

A Vitech Advanced Multi Sample Micro freezing point osmometre from Advanced Instruments Inc. was used to determine urine osmolality (mOsm/kg) to assess participants’ hydration status. A higher value indicates a greater degree of dehydration. According to the US National Institutes of Health, a concentration of 500–800 mOsm/kg is considered normal, whereas a 12–14 h fluid restriction should yield a value in excess of 850 mOsm/kg (Chernecky & Berger, 2012). A higher value indicates a greater degree of dehydration.

Procedure

A pre-participation health questionnaire was sent to interested individuals, to exclude persons for whom overnight fasting might have been a potential health risk (e.g., pregnant women and people suffering from diabetes or a heart condition). They were also provided with an empty sample container in which they supplied their waking urine sample, which they also brought with them to each of their sessions.

Participants visited UEL’s Psychology Research Suite on two occasions, 1 week apart, after having fasted (no food or drink) from 9 p.m. the night before. Participants were asked to collect a urine sample upon waking (in sterile sample pots already provided), which they brought with them. Testing took place in the mornings (8 a.m.–11 a.m.). To standardise the water content of breakfast, before each testing session, participants received a choice of cereal bar (113 kcal or 119 kcal). On one occasion (counterbalanced across participants), they were also given a 500 ml bottle of water (at room temperature). Participants were explicitly and clearly instructed to drink as much as they wanted before beginning the tasks. There was no time pressure, but all participants stopped drinking after 2 min. They were not allowed to continue drinking during testing.

Participants then completed the tasks in the order they have been described above. At the end of testing, they were asked to provide another urine sample. The second session followed the same procedure and at the end of the second session they were debriefed and compensated for their time and participation. Tasks in both sessions were completed in approximately 1 h.

The order of water supplementation and tasks administered was counterbalanced so that 15 participants had water in their first session and 14 in their second session, and 15 had version A of decision-making tasks in their first session and 14 had version B of decision-making tasks in their second session.

Data analysis

The main aim of this study was to investigate the effect of water supplementation on cognitive performance. To test hypothesis 1 and 2, the data was subjected to a series of mixed analyses of variance (ANOVA) in which water supplementation (water/no water given) was a within-participants factor, and order (water first/no water first), thirst (thirsty/not thirsty), and urine osmolality (high/low) were between-participants factors. The same analyses were also performed for the combined cognitive reflection scores.

For thirst and hydration, median splits were performed grouping participants as either thirsty/not thirsty and hydrated/not hydrated based on the respective medians of 63% and 827.5 mOsm/kg on the ‘no water day’. The post-test osmolality data was used in the present analyses. The pre-test data was used to confirm fasting (see “Results”).

To investigate hypothesis 3 and 4, correlation analyses were also performed in an attempt to tease apart a possible relationship between performance on the judgment and decision-making tasks and performance on the ChoiceRT, IED, and RVP.