Proceedings of The Physiological Society
University College London December 2005 (2006) Proc Physiol Soc 1, PC2
Repeated transcranial direct current stimulation (tDCS) of the motor cortex: a new form of inducing 'homeostatic-like' timing-dependent plasticity in the human
Fricke, Kristina; Seeber, Antje; Nitsche, Michael A.; Rothwell, John C.;
1. Sobell Department, Institute of Neurology, London, United Kingdom. 2. Department of Clinical Neurophysiology, Georg-August University, Goettingen, Germany.
Plasticity in the excitability of neural circuits is necessary for learning and memory. However, it is also necessary to regulate how easy it is to make such changes in order to prevent one or a small number of inputs from dominating activity in a pathway. Several rules have been put forward to control the amount of plasticity in a circuit (metaplasticity). One of these, homeostatic plasticity, suggests that the ease with which a connection can be facilitated/suppressed depends on the previous amount of activity in the system. Here we describe a 'homeostatic-like' interaction between two protocols that are currently used in the human motor cortex to produce lasting changes in corticospinal excitability. We used transcranial direct current stimulation (tDCS; 1 mA) of motor cortex in 9 healthy subjects using the method described by Nitsche & Paulus (2000). As reported previously, 5 min of anodal tDCS facilitated motor-evoked potentials (MEPs) evoked in relaxed contralateral hand muscles for the next 5 min; 10 min anodal stimulation facilitated MEPs for up to 1 h. Cathodal tDCS suppressed MEPs for similar periods. We then investigated the effect of splitting 10 min of tDCS into two 5 min periods separated by a pause. If 5 min tDCS was followed by a rest period of 30 min, application of a second period of 5 min tDCS had the same effect as the first period of tDCS. We refer to this protocol as 5/30/5. However, the effects were quite different if the pause between two successive periods of tDCS was only 3 min (i.e. 5/3/5). In this case, the second 5 min tDCS was applied while the cortex was still facilitated/depressed following the first period of tDCS. For anodal stimulation, the second period of tDCS initially led to facilitation, but this only lasted 5 min; from 10-30 min MEPs were suppressed. With cathodal stimulation, the second 5 min tDCS no longer produced any suppression, but was followed by facilitation that was particularly evident from 10-30 min. There was no effect of the 3 min pause on the after-effects of 5 min tDCS on measures of intracortical inhibition/facilitation. We conclude that the after-effects of tDCS depend on the excitability of the cortex at the time the stimulation is applied; a period of increased excitability reverses the effect of anodal tDCS on MEPs from facilitation to suppression. A period of decreased excitability reverses the effect of cathodal stimulation from suppression to facilitation. This is compatible with a 'homeostatic-like' rule governing the response of the human motor cortex to plasticity probing protocols. Furthermore, this rule appears to take into account the delay between previous activity and application of the protocol. K. Fricke and A. Seeber contributed equally to this work.
Where applicable, experiments conform with Society ethical requirements