Participants

Thirty undergraduate students of the Leiden University (29 females, 1 male, mean age = 19.5 years, range 18–22) participated in the experiment. Participants were recruited via an on-line recruiting system and offered course credits for participating in a behavioural pharmacological study. Participants were screened individually via a phone interview by the same lab-assistant using the Mini International Neuropsychiatric Interview (M.I.N.I.). The M.I.N.I. is a short, structured interview of about 15 minutes that screens for several psychiatric disorders and drug use. The M.I.N.I. is often used in clinical and pharmacological research43,44,45. Participants without cardiac, hepatic, renal, neurological or psychiatric disorders, personal or family history of depression, migraine and medication or drug use were considered suitable to participate in this study. Written informed consent was obtained from all participants, all experimental protocols and remuneration arrangements of course credits were approved by the local ethical committee (Leiden University, Institute for Psychological Research). The methods were carried out in accordance with the approved guidelines.

A double-blind, randomised, between-group design was used. After signing the informed consent, participants were administered an oral dose (powder) of 800 mg of synthetic GABA in the GABA group or 800 mg of microcrystalline cellulose in the placebo group. An independent person not further involved in this study prepared a list that coded for participants to receive either placebo or GABA and the matching treatment tubes containing either placebo or GABA. Hence, participants were randomly assigned to one of the two experimental groups: placebo (N = 15; mean age = 19.3, SD = 1.1; mean Body Mass Index = 21.6, SD = 1.9), or GABA (N = 15; 1 male; mean age = 19.8, SD = 1.2; mean Body Mass Index = 20.9, SD = 1.3). Both synthetic GABA and placebo were dissolved in 200 ml of orange juice. Following Markus and colleagues46 and Colzato et al.47,48, only women currently using contraception were tested. Participants arrived at the laboratory at 9:30 a.m. and had been instructed to fast overnight; only water or tea without sugar was permitted. In addition, subjects were not allowed to use any kind of drugs before and during the experiment or to drink alcohol the day before their participation and arrival at the laboratory. Thirty minutes after the administration of either synthetic GABA or the neutral placebo participants were allowed to eat an apple.

Apparatus and procedure

All participants were tested individually. Upon arrival, participants were asked to rate their mood on a 9 × 9 Pleasure × Arousal grid41 with values ranging from –4 to 4. Heart rate (HR) and systolic and diastolic blood pressure (SBP and DBP) were collected from the non-dominant arm with an OSZ 3 Automatic Digital Electronic Wrist Blood Pressure Monitor (Spiedel & Keller). Thirty minutes following the administration of synthetic GABA (corresponding to the peak of the plasma concentration, which remains stable until 60 minutes after administration49) or placebo, participants again rated their mood before having HR, SBP and DBP measured for the second time. Immediately after, participants started with the practice procedure of the stop-change paradigm, which took about 20 minutes. After completing the practice, participants performed the task, which took about 25 minutes. Upon completion, participants again rated their mood before having their HR, SBP and DBP measured for the third time.

Stop-Change paradigm

The experiment was controlled by an Asus laptop running on an Intel Core i3-3217U processor, attached to an LG Flatron 776FM 16 inch monitor (refresh rate of 60 Hz). Stimulus presentation and data collection were controlled using Presentation software system (Neurobehavioral Systems, Inc., Berkeley, CA). The stop-change (SC) paradigm was adapted from Yildiz et al.50, see Fig. 1. Responses were given using the index and middle fingers of the right hand during the GO trials and the same fingers of the left hand for the SC trials.

Throughout each trial, a white rectangle of 55 × 16 mm was displayed on a black background in the centre of the screen. Within this rectangle, three horizontal reference lines (line thickness 1 mm, width 13 mm) separated four vertically aligned circles (diameter 7 mm). At 250 ms after trial onset, one of the circles was filled white, thus becoming the GO target stimulus. In the GO condition (67% of all trials), the participant’s response was expected to indicate whether this target was located above or below the middle reference line. Responses were given by pressing the outer right key with the right middle finger (“above” judgment) or by pressing the inner right key with the right index finger (“below” judgment). All stimuli remained visible until the participant responded. When RTs were longer than 1000 ms, the word “Quicker” was presented above the box until the participant responded.

The remaining 33% of trials were SC trials. The SC condition began with the presentation of a white GO stimulus. After a variable stop signal delay (SSD), which was adjusted using a staircase procedure, a STOP signal (a red rectangle replacing the previous white frame) was presented. This STOP signal remained on the screen until the end of the trial and requested the participant to try to inhibit the response to the GO stimulus. The SSD was initially set to 250 ms and was adapted to each participant’s performance by means of a staircase procedure to yield a 50% probability of successfully inhibited GO responses. In the case of a completely correct SC trial (no response to GO stimulus, no response prior to the CHANGE stimulus in the SCD300 condition (explained below) and a correct left hand response to the CHANGE stimulus), the SSD of the following SC trial was adjusted by adding 50 ms to the SSD of the evaluated trial. In the case of an erroneous SC trial (if any of the above criteria were not met), the SSD was adjusted by subtracting 50 ms from the SSD of the evaluated trial. Limiting this procedure, the SSD values were set to not fall below a value of 50 ms and not to exceed a value of 1000 ms. Stop-signal reaction times (SSRTs), which index the duration of the stop process, were calculated by subtracting the mean SSD from the mean RT on GO trials2,33.

Irrespective of the stopping performance/inhibition, every stop signal was combined with one of three possible CHANGE stimuli. The CHANGE stimulus was a 100 ms sine tone presented via headphones at 75 dB SPL and could be high (1300 Hz), medium (900 Hz) or low (500 Hz) in pitch. The tone assigned a new reference line in relation to which the CHANGE stimulus (the previous white GO target circle on the screen) had to be judged. The high tone represented the highest of the three lines as the new reference, the medium tone represented the middle line and the low tone represented the lowest line (see Fig. 1). All three reference lines were used with equal frequency. The required CHANGE response had to be performed with the left hand. RTs for the stop-change trials were measured from the onset of the CHANGE stimulus. If the target was located above the newly assigned reference line, an outer left key press (left middle finger) was required; if the target circle was located below the newly assigned reference line, a left inner key press (left index finger) was required. In half of the SC trials, there was a stop change delay (SCD) with a stimulus onset asynchrony (SOA) of 300 ms between the STOP and the CHANGE signals (SCD300 condition); in the other half of the SC trials, the two stimuli were presented simultaneously (SOA of 0 ms, SCD0 condition). In the case of a RT-SCD longer than 2000 ms, the English word “Quicker” was presented above the box until the participant responded. During the inter-trial interval (ITI; fixed duration of 900 ms), a fixation cross was presented in the center of the screen. In total, 864 trials were administered in the task (576 GO, 144 SCD0 and 144 SCD300), which took the participants approximately 25 minutes to finish.

Statistical Analyses

Mood (pleasure and arousal), HR, DBP and SBP were analysed separately by means of repeated-measures analyses of variance (ANOVAs) with treatment group (GABA vs. placebo) as between-subjects factor and effect of time (first vs. second vs. third measurement) as within-subjects factor. To assess the effect of GABA on action cascading, correct reaction times (RTs) were submitted to separate repeated-measures ANOVAs with condition (GO, SCD0, SCD300) as within-subject factor and treatment group (GABA vs. placebo) as between-subject factor. Greenhouse—Geisser correction was applied when the sphericity assumption was violated. The corrected degrees of freedom are reported along with the corrected test values. All post-hoc tests were Bonferroni-corrected. Kolmogorov–Smirnov tests indicated that all variables subsequently tested with t-tests were normally distributed (i.e. BMI, SSRTs and the error percentage for the GO trials), all z < 0.22; p > 0.06. A significance level of p < 0.05 was adopted for all statistical tests.