Effects of Diazepam on Reaction Times to Stop and Go

Transcranial direct current stimulation over the right intraparietal sulcus improves response inhibition

Various situations in our everyday life call for response inhibition, mechanisms deputed to outright stop an ongoing course of action. This function reportedly involves the activity of the right intraparietal sulcus (rIPS). This study aimed to determine whether transcranial direct current stimulation (tDCS) intervention to the rIPS alters response inhibition. We investigated 15 healthy adults performing a stop signal task before and after tDCS intervention. We applied tDCS with 1.5 mA to the rIPS directly above (P4) and the left supraorbital area for 20 min. The stimulation conditions involved Anodal, cathodal, and pseudo-stimulation. Each participant performed a stop signal task under all stimulation conditions. The changes in response inhibition function were evaluated by comparing the stop signal reaction times (SSRT) before and after the tDCS intervention. Under the Anodal condition, SSRT was significantly shorter after than before the intervention (p = 0.014). Under the Anodal and Cathodal conditions, we could observe a significantly positive correlation between the SSRT before the tDCS intervention and the difference in SSRT before and after tDCS intervention (Anodal condition: r = 0.823, p < 0.001; Cathodal condition: r = 0.831, p < 0.001). No such correlation could be found under the Sham condition. In summary, this study demonstrated that Anodal-tDCS intervention for rIPS improves response-inhibitory function and the stimulus effect depends on the response-inhibitory function of the participant prior to stimulation.

GABA Supplementation Negatively Affects Cognitive Flexibility Independent of Tyrosine

Increasing evidence, particularly from animal studies, suggests that dopamine and GABA are important modulators of cognitive flexibility. In humans, increasing dopamine synthesis through its precursor tyrosine has been shown to result in performance improvements, but few studies have reported the effects of GABA supplementation in healthy participants. We conducted a double-blind, placebo-controlled, randomized experiment to test the interactive effects of tyrosine and GABA administration on two measures of cognitive flexibility, response inhibition and task switching. A total of 48 healthy volunteers were split into four groups (placebo, tyrosine alone, GABA alone, and tyrosine and GABA combined). They completed cognitive flexibility tasks at baseline and after drug administration. We found that tyrosine alone had no impact on the measures of cognitive flexibility, whereas GABA alone and in combination with tyrosine worsened task switching. Our results provide preliminary evidence that putative increases in GABA and dopamine synthesis do not interact to affect cognitive flexibility performance.