Abstract
The past decades have witnessed a huge interest in uncovering the neural bases of intelligence (e.g., Stelmack, & Houlihan, 1995; Stelmack, Knott, & Beauchamp, 2003). This study investigated the influence of transcranial alternating current stimulation (tACS) on fluid intelligence performance and corresponding brain activation. Previous findings showed that left parietal theta tACS leads to a transient increase in fluid reasoning performance. In an attempt to extend and replicate these findings, we combined theta tACS with fMRI. In a double-blind sham-controlled experiment, N=20 participants worked on two intelligence tasks (matrices and paper folding) after theta tACS was applied to the left parietal cortex. Stimulation-induced brain activation changes were recorded during task processing using fMRI. Results showed that theta tACS significantly increased fluid intelligence performance when working on difficult items in the matrices test; no effect was observed for the visuo-spatial paper folding test. Whole-brain analyses showed that left parietal brain stimulation was accompanied by lower activation in task-irrelevant brain areas. Complemental ROI analyses revealed a tendency towards lower activation in the left inferior parietal cortex. These findings corroborate the functional role of left parietal theta activity in fluid reasoning and are in line with the neural efficiency hypothesis.
Highlights
Intelligence is associated with diverse relevant real-life outcomes such as educational accomplishment, occupational performance, and even health (Deary, 2012)
In a double-blind sham-controlled experiment, N = 20 participants worked on two intelligence tasks after theta transcranial alternating current stimulation (tACS) was applied to the left parietal cortex
Post-hoc paired-sample t-tests showed that participants solved significantly more difficult Raven's progressive matrices (RPM) items (28.44%, M (SD) = 9.10 (1.87)) when verum tACS was applied than after sham stimulation (24.69%, M (SD) = 7.90 (2.53); t(19) = 2.40, p = 0.03, d = 0.53), but there was no stimulation effect for easy items (45.83%, M (SD) = 8.25 (1.07), and 46.39%, M (SD) = 8.35 (0.88) for verum and sham stimulation, respectively; t(19) = −0.42, p = 0.68; Fig. 1)
Summary
Intelligence is associated with diverse relevant real-life outcomes such as educational accomplishment, occupational performance, and even health (Deary, 2012). The parieto-frontal integration theory of intelligence (P-FIT; Jung & Haier, 2007) postulates a four-phase information processing model which highlights the importance of frontal and parietal brain areas and the associated communication patterns between those. Another important theory is the neural efficiency hypothesis. It posits that more intelligent people use their brain resources more efficiently as compared to less intelligent individuals in terms of lower brain activation (Haier et al, 1988) or faster neural transmission time (Stelmack, Knott, & Beauchamp, 2003). There has been an increasing interest to use and extend our understanding of the brain in attempts to improve intelligence via brain stimulation (Enriquez-Geppert, Huster, & Herrmann, 2013)
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