Michal Lavidor

tDCS polarity effects in motor and cognitive domains: a meta-analytical review.

In vivo effects of transcranial direct current stimulation (tDCS) have attracted much attention nowadays as this area of research spreads to both the motor and cognitive domains. The common assumption is that the anode electrode causes an enhancement of cortical excitability during stimulation, which then lasts for a few minutes thereafter, while the cathode electrode generates the opposite effect, i.e., anodal-excitation and cathodal-inhibition effects (AeCi). Yet, this dual-polarity effect has not been observed in all tDCS studies. Here, we conducted a meta-analytical review aimed to investigate the homogeneity/heterogeneity of the effect sizes of the AeCi dichotomy in both motor and cognitive functions. The AeCi effect was found to occur quite commonly with motor investigations and rarely in cognitive studies. When the anode electrode is applied over a non-motor area, in most cases, it will cause an excitation as measured by a relevant cognitive or perceptual task; however, the cathode electrode rarely causes an inhibition. We found homogeneity in motor studies and heterogeneity in cognitive studies with the electrode’s polarity serving as a moderator that can explain the source of heterogeneity in cognitive studies. The lack of inhibitory cathodal effects might reflect compensation processes as cognitive functions are typically supported by rich brain networks. Further insights as to the polarity and domain interaction are offered, including subdivision to different classes of cognitive functions according to their likelihood of being affected by stimulation.

Activation of inhibition: Diminishing impulsive behavior by direct current stimulation over the inferior frontal gyrus

A common feature of human existence is the ability to reverse decisions after they are made but before they are implemented. This cognitive control process, termed response inhibition, refers to the ability to inhibit an action once initiated and has been localized to the right inferior frontal gyrus (rIFG) based on functional imaging and brain lesion studies. Transcranial direct current stimulation (tDCS) is a brain stimulation technique that can facilitate as well as impair cortical function. To explore whether response inhibition can be improved through rIFG electrical stimulation, we administered focal tDCS before subjects performed the stop signal task (SST), which measures response inhibition. Notably, activation of the rIFG by unilateral anodal stimulation significantly improved response inhibition, relative to a sham condition, whereas the same tDCS protocol did not affect response time in the go trials of the SST and in a control task. Furthermore, the SST was not affected by tDCS at a control site, the right angular gyrus. Our results are the first demonstration of response inhibition improvement with brain stimulation over rIFG and further confirm the rIFG involvement in this task. Although this study was conducted in healthy subjects, present findings with anodal rIFG stimulation support the use of similar paradigms for the treatment of cognitive control impairments in pathological conditions.

Semantically convergent and semantically divergent priming in the cerebral hemispheres: lexical decision and semantic judgment.

The effects of semantically divergent and semantically convergent priming on the processing of lexical ambiguity by the two cerebral hemispheres were examined in two visual hemifield experiments. The experiments investigated the ability of the right hemisphere (RH) and the left hemisphere (LH) to summate activation from two single word primes followed by a laterally presented ambiguous target word. In a lexical decision task (experiment 1), the two priming words were either both related to the dominant meaning of the target (new, fresh-NOVEL), or to the subordinate meaning (story, book-NOVEL), or to one dominant and one subordinate meaning (new, story-NOVEL). Results indicated that the LH benefited most from semantically convergent primes that converged onto the dominant meaning of the ambiguous target word, whereas the RH benefited most from semantically mixed (divergent) primes, that diverge on alternate meanings of the ambiguous target word. We used the same stimuli in a semantic relatedness judgment task (experiment 2), and found that the facilitation in the RH was significantly larger when the primes were mixed than when both primes converged on a single (i.e. either dominant or subordinate) meaning of the to-be-presented target word. In contrast, the only facilitation found in the LH was when the two primes were associated with a single meaning (either dominant or subordinate) of the to-be-presented target word. When the primes were mixed, there was no facilitation in the LH. These results support previous findings indicating that during word recognition, the RH activates a broader range of related meanings than the LH, including alternate meanings of ambiguous words. Thus, by summating activation for seemingly incongruous elements, the RH may be critically involved in at least one important component of verbal creativity.

The nature of foveal representation.

A fundamental question in visual perception is whether the representation of the fovea is split at the midline between the two hemispheres, or bilaterally represented by overlapping projections of the fovea in each hemisphere. Here we examine psychophysical, anatomical, neuropsychological and brain stimulation experiments that have addressed this question, and argue for a shift from the current default view of bilateral representation to that of a split representation, to provide a greater understanding of higher visual processes.

Word Length and Orthographic Neighborhood Size Effects in the Left and Right Cerebral Hemispheres

Previous studies have reported an interaction between visual field (VF) and word length such that word recognition is affected more by length in the left VF (LVF) than in the right VF (RVF). A reanalysis showed that the previously reported effects of length were confounded with orthographic neighborhood size (N). In three experiments we manipulated length and N in lateralized lexical decision tasks. Results showed that length and VF interacted even with N controlled (Experiment 1); that N affected responses to words in the LVF but not the RVF (Experiment 2); and that when length and N were combined, length only affected performance in the LVF for words with few neighbors.

Evaluating a split processing model of visual word recognition: effects of word length

A new theory of visual word recognition is based on the fact that the fovea is split in humans. When a reader fixates the center of a written word, the initial letters of the word that are to the left of fixation are projected first to the right cerebral hemisphere (RH) while the final letters are projected to the left cerebral hemisphere (LH). This paper explores the possibility that this has consequences for the early processing of the beginning and ends of centrally fixated words: specifically that lexical decision RTs are affected by the number of letters to the left of fixation but not by the number of letters to the right of fixation. For centrally presented five- and eight-letter words, we manipulated number of letters presented to the right or to the left of a fixation point (Experiment 1). We found that longer latencies to longer letter strings characterised the processing of the initial letters of words while LH word recognition features characterised the ends of words. Experiment 2 was a lateralized version of Experiment 1, and revealed the well established visual field and word length interaction. The results supported the split fovea theory.

When less is more: Evidence for a facilitative cathodal tdcs effect in attentional abilities

Many previous studies reported that the hyperpolarization of cortical neurons following cathodal stimulation (in transcranial direct current stimulation) has resulted in cognitive performance degradation. Here, we challenge this assumption by showing that cathodal stimulation will not always degrade cognitive performance. We used an attentional load paradigm in which irrelevant stimuli are processed only under low but not under high attentional load. Thirty healthy participants were randomly allocated into three interventional groups with different brain stimulation parameters (active anodal posterior parietal cortex [PPC], active cathodal PPC, and sham). Cathodal but not anodal stimulation enabled flanker processing even in high-loaded scenes. A second experiment was carried out to assert whether the improved flanker processing under cathodal stimulation is because of altered attention allocation between center and surround or, alternatively, enhanced attentional resources. In this experiment, the flanker was presented centrally. The results of Experiment 2 replicated Experiment 1s finding of improved flanker processing. We interpret the results from these two experiments as evidence for the ability of cathodal stimulation to enhance attentional resources rather than simply change attention allocation between center and periphery. Cathodal stimulation in high-loaded scenes can act like a noise filter and may in fact enhance cognitive performance. This study contributes to understanding the way the PPC is engaged with attentional functions and explains the cathodal effects, which thus might lead to more efficient brain stimulation protocols.

Cerebral lateralization of frontal lobe language processes and lateralization of the posterior visual word processing system

The brain areas involved in visual word processing rapidly become lateralized to the left cerebral hemisphere. It is often assumed this is because, in the vast majority of people, cortical structures underlying language production are lateralized to the left hemisphere. An alternative hypothesis, however, might be that the early stages of visual word processing are lateralized to the left hemisphere because of intrinsic hemispheric differences in processing low-level visual information as required for distinguishing fine-grained visual forms such as letters. If the alternative hypothesis was correct, we would expect posterior occipito-temporal processing stages still to be lateralized to the left hemisphere for participants with right hemisphere dominance for the frontal lobe processes involved in language production. By analyzing event-related potentials of native readers of French with either left hemisphere or right hemisphere dominance for language production (determined using a verb generation task), we were able to show that the posterior occipito-temporal areas involved in visual word processing are lateralized to the same hemisphere as language production. This finding could suggest top-down influences in the development of posterior visual word processing areas.

Mechanisms of Magnetic Stimulation of Central Nervous System Neurons

Transcranial magnetic stimulation (TMS) is a stimulation method in which a magnetic coil generates a magnetic field in an area of interest in the brain. This magnetic field induces an electric field that modulates neuronal activity. The spatial distribution of the induced electric field is determined by the geometry and location of the coil relative to the brain. Although TMS has been used for several decades, the biophysical basis underlying the stimulation of neurons in the central nervous system (CNS) is still unknown. To address this problem we developed a numerical scheme enabling us to combine realistic magnetic stimulation (MS) with compartmental modeling of neurons with arbitrary morphology. The induced electric field for each location in space was combined with standard compartmental modeling software to calculate the membrane current generated by the electromagnetic field for each segment of the neuron. In agreement with previous studies, the simulations suggested that peripheral axons were excited by the spatial gradients of the induced electric field. In both peripheral and central neurons, MS amplitude required for action potential generation was inversely proportional to the square of the diameter of the stimulated compartment. Due to the importance of the fibers diameter, magnetic stimulation of CNS neurons depolarized the soma followed by initiation of an action potential in the initial segment of the axon. Passive dendrites affect this process primarily as current sinks, not sources. The simulations predict that neurons with low current threshold are more susceptible to magnetic stimulation. Moreover, they suggest that MS does not directly trigger dendritic regenerative mechanisms. These insights into the mechanism of MS may be relevant for the design of multi-intensity TMS protocols, may facilitate the construction of magnetic stimulators, and may aid the interpretation of results of TMS of the CNS.

Modulating oscillatory brain activity correlates of behavioral inhibition using transcranial direct current stimulation

OBJECTIVE Studies have mainly documented behavioral changes induced by transcranial direct current stimulation (tDCS), but recently cortical modulations of tDCS have also been investigated. Our previous work revealed behavioral inhibition modulation by anodal tDCS over the right inferior frontal gyrus (rIFG); however, the electrophysiological correlates underlying this stimulation montage have yet to be established. The current work aimed to evaluate the distribution of neuronal oscillations changes following anodal tDCS over rIFG coupled with cathodal tDCS over left orbitofrontal cortex (lOFC) using spectral power analysis. METHODS Healthy subjects underwent sham and real tDCS (15 min, 1.5 mA, anodal rIFG; cathodal lOFC) stimulation conditions in a single-blind, placebo-controlled cross-over trial. Following tDCS session, resting EEG recordings were collected during 15 min. RESULTS Analysis showed a significant and selective diminution of the power of theta band. The theta diminution was observed in the rIFG area (represented the anode electrode), and was not found in the lOFC area (represented the cathode electrode). A significant effect was observed only in the theta but not in other bands. CONCLUSIONS These results are the first demonstration of modulating oscillatory activity as measured by EEG with tDCS over rIFG in general, and documenting theta band reduction with this montage in particular. SIGNIFICANCE Our results may explain the improvement in behavioral inhibition reported in our previous work, and although this study was conducted with healthy subjects, the findings suggest that tDCS may also modulate electrophysiological changes among ADHD patients, where decreasing theta activity is the target of neuro-feedback methods aimed to improve cognitive control.