Left lateralizing transcranial direct current stimulation improves reading efficiency

Abstract

Background: Poor reading efficiency is the most persistent problem for adults with developmental dyslexia. Previous research has demonstrated a relationship between left posterior temporal cortex (pTC) function and reading ability, regardless of dyslexia status.

Objective/hypothesis: In this study, we tested whether enhancing left lateralization of pTC using transcranial direct current stimulation (tDCS) improves reading efficiency in adults without dyslexia.

Method: Twenty-five right-handed adults with no history of learning disorder participated. Real and sham "Left lateralizing" tDCS were applied to the pTC in separate sessions. Standardized word and nonword reading tests were given immediately after stimulation.

Results: Modeling of the induced electrical field confirmed that tDCS was likely to increase left pTC excitability and reduce right pTC excitability as intended. Relative to sham, real tDCS induced improvements in word reading efficiency in below average readers.

Conclusions: Enhancing left lateralization of the pTC using tDCS improves word reading efficiency in below-average readers. This demonstrates that left lateralization of the pTC plays a role in reading ability, and provides stimulation parameters that could be used for a trial of tDCS in adults with developmental dyslexia. Such short-term gains could amplify the effect of appropriate reading interventions when performed in conjunction with them.

Copyright © 2012 Elsevier Inc. All rights reserved.

Figures

Figure 1

Modeling of electrical fields induced by tDCS. Placement of the electrode pads are shown in the first three columns. The anode is shown in red, and the cathode in black. The fourth column shows the magnitude of the induced electrical field, which is maximal directly under the electrodes. The rightmost column shows the directional fields normal to the cortical surface, demonstrating mainly inward current in left pTC, and outward in right pTC. This pattern of directional current is expected to increase left pTC excitability, and decrease right pTC excitability, thus enhancing left lateralization of the pTC. Individual variation in gyral anatomy may result in differences in the precise location of the effect across subjects, but the model serves to confirm the expected lateralizing effect of this montage on the pTC in general.

Figure 2

Effect of tDCS on standardized reading scores. Standardized scores have a population mean of 100 and SD of 15. Word Efficiency = TOWRE Sight Efficiency Test. Nonword Efficiency = TOWRE Phonetic Decoding Efficiency Test. Word Accuracy = WJRMT Word ID. Nonword Accuracy = WJRMT Word Attack. *P = 0.034.

Figure 3

Word reading efficiency results by reading ability. Standardized scores have a population mean of 100 and SD of 15.