Jared Cooney Horvath

Quantitative Review Finds No Evidence of Cognitive Effects in Healthy Populations From Single-session Transcranial Direct Current Stimulation (tDCS)

BACKGROUND Over the last 15-years, transcranial direct current stimulation (tDCS), a relatively novel form of neuromodulation, has seen a surge of popularity in both clinical and academic settings. Despite numerous claims suggesting that a single session of tDCS can modulate cognition in healthy adult populations (especially working memory and language production), the paradigms utilized and results reported in the literature are extremely variable. To address this, we conduct the largest quantitative review of the cognitive data to date. METHODS Single-session tDCS data in healthy adults (18-50) from every cognitive outcome measure reported by at least two different research groups in the literature was collected. Outcome measures were divided into 4 broad categories: executive function, language, memory, and miscellaneous. To account for the paradigmatic variability in the literature, we undertook a three-tier analysis system; each with less-stringent inclusion criteria than the prior. Standard mean difference values with 95% CIs were generated for included studies and pooled for each analysis. RESULTS Of the 59 analyses conducted, tDCS was found to not have a significant effect on any - regardless of inclusion laxity. This includes no effect on any working memory outcome or language production task. CONCLUSION Our quantitative review does not support the idea that tDCS generates a reliable effect on cognition in healthy adults. Reasons for and limitations of this finding are discussed. This work raises important questions regarding the efficacy of tDCS, state-dependency effects, and future directions for this tool in cognitive research.

Evidence that transcranial direct current stimulation (tDCS) generates little-to-no reliable neurophysiologic effect beyond MEP amplitude modulation in healthy human subjects: A systematic review

BACKGROUND Transcranial direct current stimulation (tDCS) is a form of neuromodulation that is increasingly being utilized to examine and modify a number of cognitive and behavioral measures. The theoretical mechanisms by which tDCS generates these changes are predicated upon a rather large neurophysiological literature. However, a robust systematic review of this neurophysiological data has not yet been undertaken. METHODS tDCS data in healthy adults (18-50) from every neurophysiological outcome measure reported by at least two different research groups in the literature was collected. When possible, data was pooled and quantitatively analyzed to assess significance. When pooling was not possible, data was qualitatively compared to assess reliability. RESULTS Of the 30 neurophysiological outcome measures reported by at least two different research groups, tDCS was found to have a reliable effect on only one: MEP amplitude. Interestingly, the magnitude of this effect has been significantly decreasing over the last 14 years. CONCLUSION Our systematic review does not support the idea that tDCS has a reliable neurophysiological effect beyond MEP amplitude modulation - though important limitations of this review (and conclusion) are discussed. This work raises questions concerning the mechanistic foundations and general efficacy of this device - the implications of which extend to the steadily increasing tDCS psychological literature.

Transcranial direct current stimulation: five important issues we aren't discussing (but probably should be)

Transcranial Direct Current Stimulation (tDCS) is a neuromodulatory device often publicized for its ability to enhance cognitive and behavioral performance. These enhancement claims, however, are predicated upon electrophysiological evidence and descriptions which are far from conclusive. In fact, a review of the literature reveals a number of important experimental and technical issues inherent with this device that are simply not being discussed in any meaningful manner. In this paper, we will consider five of these topics. The first, inter-subject variability, explores the extensive between- and within-group differences found within the tDCS literature and highlights the need to properly examine stimulatory response at the individual level. The second, intra-subject reliability, reviews the lack of data concerning tDCS response reliability over time and emphasizes the importance of this knowledge for appropriate stimulatory application. The third, sham stimulation and blinding, draws attention to the importance (yet relative lack) of proper control and blinding practices in the tDCS literature. The fourth, motor and cognitive interference, highlights the often overlooked body of research that suggests typical behaviors and cognitions undertaken during or following tDCS can impair or abolish the effects of stimulation. Finally, the fifth, electric current influences, underscores several largely ignored variables (such as hair thickness and electrode attachments methods) influential to tDCS electric current density and flow. Through this paper, we hope to increase awareness and start an ongoing dialog of these important issues which speak to the efficacy, reliability, and mechanistic foundations of tDCS.

Differential effects of motor cortical excitability and plasticity in young and old individuals: a Transcranial Magnetic Stimulation (TMS) study

Aging is associated with changes in the motor system that, over time, can lead to functional impairments and contribute negatively to the ability to recover after brain damage. Unfortunately, there are still many questions surrounding the physiological mechanisms underlying these impairments. We examined cortico-spinal excitability and plasticity in a young cohort (age range: 19–31) and an elderly cohort (age range: 47–73) of healthy right-handed individuals using navigated transcranial magnetic stimulation (nTMS). Subjects were evaluated with a combination of physiological [motor evoked potentials (MEPs), motor threshold (MT), intracortical inhibition (ICI), intracortical facilitation (ICF), and silent period (SP)] and behavioral [reaction time (RT), pinch force, 9 hole peg task (HPT)] measures at baseline and following one session of low-frequency (1 Hz) navigated repetitive TMS (rTMS) to the right (non-dominant) hemisphere. In the young cohort, the inhibitory effect of 1 Hz rTMS was significantly in the right hemisphere and a significant facilitatory effect was noted in the unstimulated hemisphere. Conversely, in the elderly cohort, we report only a trend toward a facilitatory effect in the unstimulated hemisphere, suggesting reduced cortical plasticity and interhemispheric communication. To this effect, we show that significant differences in hemispheric cortico-spinal excitability were present in the elderly cohort at baseline, with significantly reduced cortico-spinal excitability in the right hemisphere as compared to the left hemisphere. A correlation analysis revealed no significant relationship between cortical thickness of the selected region of interest (ROI) and MEPs in either young or old subjects prior to and following rTMS. When combined with our preliminary results, further research into this topic could lead to the development of neurophysiological markers pertinent to the diagnosis, prognosis, and treatment of neurological diseases characterized by monohemispheric damage and lateralized motor deficits.

No significant effect of transcranial direct current stimulation (tDCS) found on simple motor reaction time comparing 15 different simulation protocols.

BACKGROUND Research exploring the behavioral impact of transcranial direct current stimulation (tDCS) over M1 has produced homogenous results. The most common explanations to address this homogeneity concerns the differential impact of varied tDCS parameters (such as stimulation intensity or electrode montage). To explore this, we systematically examined the effects of 15 different tDCS protocols on a well-elucidated neurobehavioral system: simple visual motor reaction time (smRT). METHODS For the initial phase of this study, 150 healthy participants were randomly assigned to one of 5 experimental groups (2mA anodal, 2mA cathodal, 1mA anodal, 1mA cathodal, or sham) across 3 different conditions (orbitofrontal, bilateral, or extracephalic reference electrode location). The active electrode was always placed over M1 and tDCS lasted for 20min. Starting ~5min prior to stimulation and running continuously for ~30min, participants were repeatedly presented with a visual cue centered on a computer monitor and asked to press a response button as quickly as possible at stimulus onset (stimuli number: 100 pre-, 400 during-, and 100-post stimulation - interstimulus interval: 1-3s). Ex-gaussian distribution curves, miss, and error rates were determined for each normalized batch of 100 RTs and compared using a two-way ANOVA. As the largest group differences were seen with 2mA anodal (compared to sham) stimulation using an orbitofrontal montage, an additional 60 healthy participants were recruited to further test for significance in this condition. RESULTS No significant impact of tDCS was seen on any parameter of smRT distribution, error rate, or miss rate, regardless of polarity, stimulation intensity, electrode montage, or stimulation-to-task relationship. CONCLUSION Our results suggest that tDCS over M1 might not have a predictable or reliable effect on short duration smRT. Our results raise interesting questions regarding the mechanisms by which tDCS might modulate more complex motor behaviors. Additional research utilizing multiple tDCS protocols as undertaken here will help address and clarify these concerns.

A Bridge Too Far – Revisited: Reframing Bruer’s Neuroeducation Argument for Modern Science of Learning Practitioners

In Education and the Brain: A Bridge Too Far, John Bruer argues that, although current neuroscientific findings must filter through cognitive psychology in order to be applicable to the classroom, with increased knowledge the neuroscience/education bridge can someday be built. Here, we suggest that translation cannot be understood as a single process: rather, we demonstrate that at least four different ‘bridges’ can conceivably be built between these two fields. Following this, we demonstrate that, far from being a matter of information lack, a prescriptive neuroscience/education bridge (the one most relevant to Bruer’s argument) is a practical and philosophical impossibility due to incommensurability between non-adjacent compositional levels-of-organization: a limitation inherent in all sciences. After defining this concept in the context of biology, we apply this concept to the learning sciences and demonstrate why all brain research must be behaviorally translated before prescriptive educational applicability can be elucidated. We conclude by exploring examples of how explicating different forms of translation and adopting a levels-of-organization framework can be used to contextualize and beneficially guide research and practice across all learning sciences.

Are current blinding methods for transcranial direct current stimulation (tDCS) effective in healthy populations

In their paper What Do You Feel if I Apply Transcranial Electrical Stimulation?, Fertonanai et al. (2015) present the largest quantitative analysis to date of the sensorial sensations and side-effects generated by varied forms of transcranial electrical stimulation (tES) within healthy populations. This group concludes that, by and large, transcranial direct current, alternating current, and random noise stimulation are safe and painless techniques. Interestingly, this group reports that anodal transcranial direct current stimulation (tDCS) generates an average discomfort rating 25% higher than sham stimulation. Though this difference was only nearly significant (p = 0.056), it leads to interesting questions regarding the effectiveness of current blinding techniques during tDCS experimentation within healthy individuals: an issue that has been raised several times previously in the literature.

The hard problem of ‘educational neuroscience’

Differing worldviews give interdisciplinary work value. However, these same differences are the primary hurdle to productive communication between disciplines. Here, we argue that philosophical issues of metaphysics and epistemology subserve many of the differences in language, methods and motivation that plague interdisciplinary fields like educational neuroscience. Researchers attempting interdisciplinary work may be unaware that issues of philosophy are intimately tied to the way research is performed and evaluated in different fields. As such, a lack of explicit discussion about these assumptions leads to many conflicts in interdisciplinary work that masquerade as more superficial issues. To illustrate, we investigate how philosophical assumptions about the mind (specifically the hard problem of consciousness and mind-body problem) may influence researchers in educational neuroscience. The methods employed by researchers in this field are shaped by their metaphysical beliefs, and arguments around these issues can threaten accepted disciplinary ontologies. Additionally, how a researcher understands reduction in the special sciences and how they place their colleagues in this ontology constrains the scope of interdisciplinary projects. In encouraging researchers to explicitly discuss the philosophical assumptions underlying their research we hope to alleviate some of the conflict and establish realistic expectations for collaborative projects.

Translating neuroscience, psychology and education: An abstracted conceptual framework for the learning sciences

Abstract Educators strive to understand and apply knowledge gained through scientific endeavours. Yet, within the various sciences of learning, particularly within educational neuroscience, there have been instances of seemingly contradictory or incompatible research findings and theories. We argue that this situation arises through confusion between levels of analysis applicable to various disciplines. In this article, we propose a conceptual framework for the science of learning which integrates sociological, psychological, biological and neurological perspectives of learning. This framework seeks to recognise the distinction between learning—essentially a complex neurological phenomenon—and education, an even more complex sociocultural phenomenon. As such, the framework allows a coherent perspective to emerge that can help resolve a number of key issues. Specifically, we argue that its adoption will (a) provide the science of learning with a foundation to assist in the development of a translational paradigm for neuroscience, psychology and education professionals, (b) enable neuromyths to be more easily identified and (c) help prevent unhelpful debates in the future.

Transcranial Magnetic Stimulation (TMS) Safety Considerations and Recommendations

Ensuring patient and participant safety during transcranial magnetic stimulation (TMS) is of paramount importance. In this chapter, we begin by exploring a number of general safety concerns and the prevalence of reported side-effects in the TMS literature. Next, we outline contraindications and the recommended safety parameters for each of the major stimulation paradigms (including single and repetitive pulse patterns). Finally, we offer several practical tips to ensure TMS is delivered in the safest and most ethical manner.