Transcranial direct current stimulation (tDCS) of frontal cortex decreases performance on the WAIS-IV intelligence test

Abstract

Transcranial direct current stimulation (tDCS) modulates excitability of motor cortex. However, there is conflicting evidence about the efficacy of this non-invasive brain stimulation modality to modulate performance on cognitive tasks. Previous work has tested the effect of tDCS on specific facets of cognition and executive processing. However, no randomized, double-blind, sham-controlled study has looked at the effects of tDCS on a comprehensive battery of cognitive processes. The objective of this study was to test if tDCS had an effect on performance on a comprehensive assay of cognitive processes, a standardized intelligence quotient (IQ) test. The study consisted of two substudies and followed a double-blind, between-subjects, sham-controlled design. In total, 41 healthy adult participants were included in the final analysis. These participants completed the Wechsler Adult Intelligence Scale, Fourth Edition (WAIS-IV) as a baseline measure. At least one week later, participants in substudy 1 received either bilateral tDCS (anodes over both F4 and F3, cathode over Cz, 2 mA at each anode for 20 min) or active sham tDCS (2 mA for 40 s), and participants in substudy 2 received either right or left tDCS (anode over either F4 or F3, cathode over Cz, 2 mA for 20 min). In both studies, the WAIS-IV was immediately administered following stimulation to assess for performance differences induced by bilateral and unilateral tDCS. Compared to sham stimulation, right, left, and bilateral tDCS reduced improvement between sessions on Full Scale IQ and the Perceptual Reasoning Index. This demonstration that frontal tDCS selectively degraded improvement on specific metrics of the WAIS-IV raises important questions about the often proposed role of tDCS in cognitive enhancement.

Keywords: Brain stimulation; IQ; Intelligence; Perceptual reasoning; WAIS-IV; tDCS.

Figure 1

tDCS administered over DLPFC. Two stimulators were used to deliver double-blinded unilateral or bilateral tDCS over DLPFC. In all stimulation conditions, three electrodes were placed, one each at F3, F4 (anodes, red electrodes), and Cz (cathode/return, blue electrode) (A). For bilateral tDCS, both stimulators delivered 20 minutes of 2mA stimulation with a ramp up and ramp down of current (B). For sham tDCS, both stimulators administered 20 seconds of stimulation with a ramp up and ramp down of current, in order to mimic the sensations of stimulation (C). For right tDCS, 20 minutes of stimulation was delivered through the stimulator attached to electrodes F4 and Cz, while only the 20 seconds of sham stimulation were delivered through the stimulator attached to F3 and Cz (D). For left tDCS, 20 minutes of stimulation was delivered through the stimulator attached to F3 and Cz, while the stimulator attached to F4 and Cz administered 20 seconds of stimulation (E).

Figure 2

Electric field modeling of tDCS. Anodal tDCS was administered bilaterally over frontal cortex (AD), unilaterally on the left (E–G), or unilaterally on the right (H–J). Red electrodes represent anodes, blue electrodes represent cathodes, and purple electrodes represent stimulation electrodes which were attached, but only received sham stimulation (A, E, H). For bilateral stimulation, the greatest magnitude of the electric field was mostly localized to areas underneath and between the electrodes (B, left panels: gray matter; right panels: white matter). For unilateral stimulation, electric field was mostly localized to the hemisphere targeted for stimulation (F and I, left panels: gray matter; right panels: white matter). The middle frontal gyri (green) and superior frontal gyri (blue) were the areas of frontal cortex with the highest amplitude electric field (D). Axial, coronal, and sagittal sections showing electric field induced by tDCS (D, G, J).

Figure 3

Double-blind, randomized, placebo-controlled study design with a repeated-measure of IQ. During Session 1, each participant was administered the Wechsler Adult Intelligence Scale, Fourth Edition (WAIS-IV). During Session 2, each participant received either sham stimulation or bilateral tDCS (A, substudy 1) or either right or left tDCS (B, substudy 2). The WAIS-IV was immediately administered following stimulation in order to assess stimulation-induced modulation in performance.

Figure 4

Unilateral and bilateral tDCS significantly decreased practice gains in the Full Scale IQ (FSIQ) compared to sham stimulation. The difference in FSIQ between sessions (Session 2 – Session 1) was calculated for each participant, and then averaged across participants. Group means of individual differences are plotted. Error bars show 1 SEM. * indicates significant at p < 0.05.

Figure 5

The WAIS-IV provides four index scores which represent the major components of intelligence: (A) Verbal Comprehension Index (VCI), (B) Perceptual Reasoning Index (PRI), (C) Working Memory Index (WMI), and (D) Processing Speed Index (PSI). Right tDCS significantly decreased practice gains on the PRI of the WAIS-IV, while left tDCS and bilateral tDCS decreased practice gains on the PRI at trend level. Bars represent the group means of the individual differences of scores between Session 1 and Session 2. Error bars show 1 SEM. * indicates significant at p < 0.05.

Figure 6

Perceptual reasoning abilities are assessed by 3 subtests: Block Design (A), Matrix Reasoning (B), and Visual Puzzles (C). Differences between stimulation conditions were found in Matrix Reasoning, Visual Puzzles, and Picture Completion. For Matrix Reasoning, right and bilateral tDCS decreased practice gains, at trend level, compared to sham stimulation. Interestingly, practice gains in Matrix Reasoning were unchanged by left tDCS compared to sham stimulation. For Visual Puzzles, right and left stimulation tDCS significantly reduced practice gains compared to sham stimulation. Bars represent the group means of the differences (by participant) of scores between Session 1 and Session 2. Error bars show 1 SEM. * indicates significant at p < 0.05.

Figure 7

Changes in performance on the subtests were not influenced by test order for substudy 1 (A, sham or bilateral tDCS) or substudy 2 (B, right or left tDCS). Group means of the individual differences of each subtest score between Session 1 and Session 2 are plotted in order of test administration. Error bars show 1 SEM.