Comparing the electric fields of transcranial electric and magnetic perturbation.

Significance.Noninvasive brain stimulation (NIBS) by quasistatic electromagnetic means is presently comprised of two methods: magnetic induction methods (transcranial magnetic perturbation or TMP) and electrical contact methods (transcranial electric perturbation or TEP). Both methods couple to neuronal systems by means of the electric fields they produce. Both methods are necessarily accompanied by a scalp electric field which is of greater magnitude than anywhere within the brain. A scalp electric field of sufficient magnitude may produce deleterious effects including peripheral nerve stimulation and heating which consequently limit the spatial and temporal characteristics of the brain electric field. Presently the electromagnetic NIBS literature has produced an accurate but non-generalized understanding of the differences between the TEP and TMP methods.Objective.The aim of this work is to contribute a generalized understanding of the differences between the two methods which may open doors to novel TEP or TMP methods and translating advances, when possible, between the two methods.Approach.This article employs a three shell spherical conductor head model to calculate general analytical results showing the relationship between the spatial scale of the brain electric fields and: (1) the scalp-to-brain mean-squared electric field ratio for the two methods and (2) TEP-to-TMP scalp mean-squared electric field ratio for similar electric fields at depth.Main results.The most general result given is an asymptotic limit to the TEP-to-TMP ratio of scalp mean-squared electric fields for similar electric fields at depth. Specific example calculations for these ratios are also given for typical TEP electrode and TMP coil configurations. While TMP has favorable mean-squared electric field ratios compared to TEP this advantage comes at an energetic cost which is briefly elucidated in this work.

Individual differences in TMS sensitivity influence the efficacy of tDCS in facilitating sensorimotor adaptation.

BACKGROUND: Transcranial direct current stimulation (tDCS) can enhance cognitive function in healthy individuals, with promising applications as a therapeutic intervention. Despite this potential, variability in the efficacy of tDCS has been a considerable concern. OBJECTIVE: /Hypothesis: Given that tDCS is always applied at a set intensity, we examined whether individual differences in sensitivity to brain stimulation might be one variable that modulates the efficacy of tDCS in a motor learning task. METHODS: In the first part of the experiment, single-pulse transcranial magnetic stimulation (TMS) over primary motor cortex (M1) was used to determine each participant's resting motor threshold (rMT). This measure was used as a proxy of individual sensitivity to brain stimulation. In an experimental group of 28 participants, 2 mA tDCS was then applied during a motor learning task with the anodal electrode positioned over left M1. Another 14 participants received sham stimulation. RESULTS: M1-Anodal tDCS facilitated learning relative to participants who received sham stimulation. Of primary interest was a within-group analysis of the experimental group, showing that the rate of learning was positively correlated with rMT: Participants who were more sensitive to brain stimulation as operationalized by our TMS proxy (low rMT), showed faster adaptation. CONCLUSIONS: Methodologically, the results indicate that TMS sensitivity can predict tDCS efficacy in a behavioral task, providing insight into one source of variability that may contribute to replication problems with tDCS. Theoretically, the results provide further evidence of a role of sensorimotor cortex in adaptation, with the boost from tDCS observed during acquisition.

Recovery from moderate aphasia in the first year poststroke: effect of type of therapy.

OBJECTIVES: (1) To determine whether 2 model-based remediation programs affect writing performance in unselected subjects with moderate aphasia and whether there is consequent improvement in everyday life, and (2) to interpret the potential changes observed by recourse to a theoretical model. DESIGN: Consecutive sample, multiple baseline, within subject crossover study. SETTING: Ambulatory care units. PARTICIPANTS: Eight subjects with moderate aphasia from 6 to 12 months postonset. INTERVENTION: A standardized test for reading and writing skills was given at the beginning and the end of each therapy program and 1 month after therapy stopped. MAIN OUTCOME MEASURES: Functional outcome measures were the Communicative Abilities in Daily Living (CADL) test and subtests from standardized aphasia assessment. RESULTS: After the 2 programs, there was improved writing performance, which was maintained after therapy stopped. Patterns of improvement corresponded to each of the 2 programs. Learning transfer was observed on the CADL test and functional writing, but gains on oral language were limited. Only 1 program was effective for 6 of the 8 patients. CONCLUSION: Specific rehabilitation programs aid recovery from aphasic symptoms from 6 to 12 months postonset. Individual response is linked to type of treatment. The interpretation is linked to a model-based description of aphasic symptoms and mechanisms of functional recovery.