Long-term enhancement of brain function and cognition using cognitive training and brain stimulation

Abstract

Noninvasive brain stimulation has shown considerable promise for enhancing cognitive functions by the long-term manipulation of neuroplasticity. However, the observation of such improvements has been focused at the behavioral level, and enhancements largely restricted to the performance of basic tasks. Here, we investigate whether transcranial random noise stimulation (TRNS) can improve learning and subsequent performance on complex arithmetic tasks. TRNS of the bilateral dorsolateral prefrontal cortex (DLPFC), a key area in arithmetic, was uniquely coupled with near-infrared spectroscopy (NIRS) to measure online hemodynamic responses within the prefrontal cortex. Five consecutive days of TRNS-accompanied cognitive training enhanced the speed of both calculation- and memory-recall-based arithmetic learning. These behavioral improvements were associated with defined hemodynamic responses consistent with more efficient neurovascular coupling within the left DLPFC. Testing 6 months after training revealed long-lasting behavioral and physiological modifications in the stimulated group relative to sham controls for trained and nontrained calculation material. These results demonstrate that, depending on the learning regime, TRNS can induce long-term enhancement of cognitive and brain functions. Such findings have significant implications for basic and translational neuroscience, highlighting TRNS as a viable approach to enhancing learning and high-level cognition by the long-term modulation of neuroplasticity.

Copyright © 2013 Elsevier Ltd. All rights reserved.

Figures

Figure 1

Schematic Representation of the Arithmetic Learning Regimes (A) The calculation task. Answers to each calculation problem were obtained by manipulation of two numerical operands according to a particular algorithm (either [(right number – left number) + 1] + right number or [(right number + left number) − 10] + right number). Participants were instructed to enter their two-digit solution on the number pad of a standard QWERTY keyboard. Positive and negative feedback was provided for each answer (500 ms duration), and participants were only allowed to progress to subsequent problems once they had obtained the correct solution. (B) The drill task. Each trial began with the presentation of two numerical operands accompanied by the problem’s answer. After the initial presentation, the problem would disappear from the screen and reappear without the answer, at which point subjects were required to enter their two-digit solution. Positive and negative feedback was provided (500 ms duration), and if participants answered incorrectly, the whole presentation cycle would repeat. Calculation and drill problems were presented in alternating groups of 18, referred to as “blocks.” In line with previous studies [7], the total number of blocks varied according to the day of training: ten blocks on the first day, 12 on the second, 14 on the third, 16 on the fourth, and 14 on the fifth. The ratio of calculation to drill blocks was the same on each day, at 1:1. For both tasks, the round-edged boxes represent the individual presentation screens, and the square-edged boxes the time delays between each presentation screen.

Figure 2

The Effect of TRNS on Arithmetic Performance (A) Calculation learning rates during training were significantly higher in the TRNS group relative to sham controls. (B) Drill learning rates during training were significantly higher in the TRNS group relative to sham controls. (C) Calculation RTs during testing were significantly faster in the TRNS group relative to sham controls for both old and new problems. (D) Drill RTs during testing did not differ between TRNS and sham groups for either old or new problems. Error bars indicate one SEM. Significant differences are marked with asterisks. See also Figure S2.

Figure 3

NIRS: The Effect of TRNS on Hemodynamic Response Amplitudes and Latencies within the Left LPFC during Training (A) Combined TRNS-NIRS setup. The NIRS plate (orange) is embedded with two transmitting and six receiving optodes and secured to the forehead: transmitting optodes are capped with blue labels, and receiving optodes are unmarked. Recordings were taken during the training phase on the first day and the fifth (last) day, as well as during the testing phase 6 months later. The fiber optic cables connecting the optodes to the NIRS device can be seen emanating from the top of the image. TRNS electrodes are positioned over the bilateral DLPFC and encased in blue and red saline-soaked sponges, as shown. This innovative TES-NIRS combination allowed us to quantify the hemodynamic response to functional activation and to assess how it varied as a function of brain stimulation and arithmetic training. (B) TRNS reduced the amplitude of HbO2 and HbT responses by the end of training. A significant three-way interaction between hemodynamic measure, day, and group in the left LPFC indicates a significant decrease in peak amplitude for HbO2 and HbT at the end of the training (fifth day) in the TRNS group relative to sham controls. (C) Reduced peak latencies emerged in the TRNS group compared to sham controls, for HbO2, HHb, and HbT responses, independent of day. Both these effects were restricted to the left LPFC. Error bars indicate one SEM. Significant differences are marked with asterisks. See also Figure S3.

Figure 4

The Effect of TRNS on Hemodynamic Response Latencies during Testing: Relationship with Behavioral Performance (A) A significant two-way interaction existed between learning regime and group for peak latency in the left LPFC, 6 months after the end of training. While the TRNS and sham groups did not differ for drill problems, the TRNS group showed a significant decrease in peak latency compared to sham controls for calculation problems. Error bars indicate one SEM. Significant differences are marked with asterisks. (B) Significant correlations existed between calculation RTs and the peak time of changes in HbO2, HHb, and HbT concentrations 6 months after the completion of training. See also Figure S4.