Stimulating language: insights from TMS

Abstract

Fifteen years ago, Pascual-Leone and colleagues used transcranial magnetic stimulation (TMS) to investigate speech production in pre-surgical epilepsy patients and in doing so, introduced a novel tool into language research. TMS can be used to non-invasively stimulate a specific cortical region and transiently disrupt information processing. These 'virtual lesion' studies offer not only the ability to explore causal relations between brain regions and language functions absent in functional neuroimaging, but also spatial and temporal precision not typically available in patient studies. For instance, TMS has been used to demonstrate functionally distinct sub-regions of the left inferior frontal gyrus; to clarify the relationship between pre-morbid language organization and susceptibility to unilateral lesions and to investigate the contribution of both left and right hemisphere language areas in recovery from aphasia. When TMS is used as a measure of functional connectivity, it demonstrates a close link between action words and motor programmes; it suggests a potential evolutionary link between hand gestures and language and it suggests a role in speech perception for the motor system underlying speech production. In combination with functional neuroimaging, it can elucidate the circuits responsible for this involvement. Finally, TMS may even be useful for enhancing recovery in aphasic patients. In other words, TMS has already become an important tool for studying language at both the cognitive and neural levels, and it is clear that further developments in TMS methodology are likely to result in even greater opportunities for language research.

Figures

Figure 1

Motor excitability during speech perception. A) Data from stimulation of the left primary motor face area in a single subject when listening to speech, listening to non-verbal sounds, viewing speech, and viewing eye movements. EMG recordings from individual trials are superimposed and the dotted line indicates the time of stimulation. The horizontal bar represents 10msec and the vertical bar 0.5mV. B) Average MEP sizes for the same four stimulus conditions with stimulation to the left face area, right face area, and hand area of motor cortex. The x-axis through the 0% level represents the mean MEP size in the control condition and error bars represent standard error of the mean. Panel A) & B) are modified and reprinted from Neuropsychologia, 41, Watkins, K. W., Strafella, A. P. and Paus, T. “Seeing and hearing speech excites the motor system involved in speech production”, pp. 990–992, Copyright (2003), with permission from Elsevier. C) The relation between regional cerebral blood flow in Broca’s area and the size of the MEP evoked by single pulse TMS over the mouth region of primary motor cortex (left panel). On the right, an activation map showing the anatomical location of the significant positive relationship illustrated in the graph. Reprinted from Journal of Cognitive Neuroscience, 16(6), Watkins, K. W. and Paus, T. “Modulation of motor excitability during speech perception: The role of Broca’s area”, pp. 990–992, Copyright (2004), with permission from MIT Press.

Figure 2

Effects of stimulation on rostral and caudal LIFG. A) The bar plots show the mean normalized TMS effects as percent change in reaction times from the non-TMS baseline during synonym judgments (left), homophone judgments (middle), and visual matching (right). Error bars indicate the standard error of the mean and significant differences are indicated with an * (p

Figure 3

Effects of rTMS treatment on aphasic patients. A) Picture naming accuracies and response times for four patients (P1-4) shown before (white bars) and after (black bars) 10 sessions of rTMS to RIFG. Reprinted from Brain and Language, 93, Naeser et al., “Improved picture naming in chronic aphasia after TMS to part of right Broca's area: an open-protocol study,” p. 101, Copyright (2005), with permission from Elsevier. B) The white triangle indicates the site of RIFG stimulation in a single patient who was followed up two and eight months post-TMS treatment. The bar plots show her picture naming accuracy improving after treatment. From Naeser et al. (2005a), permission pending.