State-dependent Transcranial Magnetic Stimulation (TMS) protocols

In this chapter we describe a novel approach which enhances the functional resolution of transcranial magnetic stimulation (TMS) to a level that allows for differential stimulation of functionally distinct neuronal populations within a cortical area. It is based on the well-known principle of state - dependency : a phenomenon whereby the response of a system to an external stimulus is affected not only by the properties of that stimulus but also by the internal state of the system. With regard to TMS, the neural impact of an applied pulse is determined not only by the stimulation parameters but also by the initial activation state of the affected neurons; therefore, neurons within a cortical area will be differentially affected by TMS if their initial activation states at the time of stimulation are dissimilar. The basic idea in state-dependent TMS is to control this initial state/TMS interaction. By selectively increasing the susceptibility of a specific neuronal population via adaptation and priming, one can differentially stimulate this population from other neurons in the area. The main benefit of state-dependent TMS is that it allows TMS research to move beyond questions of “Is region X necessary for task Y” and investigate the functional neuronal properties within a targeted area. So far, this approach has been successfully used to investigate neuronal representations associated with a wide range of cognitive functions such as numerical cognition, action observation, and conceptual knowledge, and it may hold much promise for future research.