Transcranial direct current stimulation as a non-medication modality for attention enhancement: A review of the literature

Ali Yadollahpour¹, Halime Mansoury Asl^2*, Samaneh Rashidi²

¹Department of Medical Physics, School of Medicine, Ahvaz Jundishapur University of Medical Sciences, Ahvaz, Iran

²Student Research Committee, Ahvaz Jundishapur University of Medical Sciences, Ahvaz, Iran

*Corresponding Author E-mail: halime_mansoury@yahoo.com

ABSTRACT:

Objective: Attention is a higher cognitive function and its controlled manipulation has been always a research interest. Transcranial direct current stimulation (tDCS) is a non-invasive neuromodulation technique with promising therapeutic outcomes in different neuropsychiatric disorders. Initial studies have shown potential efficacy of tDCS for enhancing attention in neuropsychiatric and healthy individuals. The present study aims to review the tDCS studies on modulating attention among healthy individuals. The potentials of this technique for controlled modulation of attention related performance are discussed. Methods: The databases of PubMed (1980-2016), Web of Sciences (1985-2016), PsychInfo (1985-2016), and Google Scholar (1980-2016) were explored using the search terms "transcranial direct current stimulation" or 'tDCS", "attention", " healthy individuals”, and "modulation". The obtained results were screened for the title and abstract and the relevant papers were selected for a comprehensive review. Results: Ten studies were included in this review. The protocols applied for improving attention in healthy subjects to modulate attention under different attention specific tasks are discussed. Although studies on attention modulation in healthy individuals are in initial step, the outcomes are relatively promising. The most frequent used regions of brain were dorsolateral prefrontal cortex (DLPFC) and right inferior frontal gyrus. The current density ranging 30-35 µA/cm²and anodal over left DLPFC can enhance attention in healthy individuals. Conclusion: Current evidence shows relative efficiency of tDCS for controlled modulation of attention in healthy individuals. For clinical application of this technique as an alternative or adjunctive modality of attention modulation further controlled studies are needed.

KEYWORDS: Transcranial Direct Current Stimulation, Healthy Individual, Attention, Modulation

1. INTRODUCTION:

Attention is among the main higher cognitive functions in humans. It is defined as selective collection and processing of information from the environment (1, 2). Attention plays a pivotal role in occupational and individual performance, especially in the occupations that require high level of attention and concentration including arts, professional sports, and armed force tasks. Improving or controlled modulation of attention has been always of significant interest for physicians and researchers in different fields.

The first line of approaches for improving attention included pharmaceutical agents. Systematic research on development of pharmacological agents for attention related cognitive functions dated back to 1917 following the report of learning facilitating effects of strychnine in rats (3). A main class of attention enhancement medications are stimulant drugs (4, 5). However, medications are associated with different side effects. In this regard, a new line of studies have been devoted to develop non-medication modalities with no serious side effects to alter or enhance mental faculties (2). On the other hand, the abuse of drugs such as methylphenidate , with the commercial name of Ritalin, as an attention improvement agent especially among college students has been dramatically increased during the recent decade (6). Developing non-medication modality with no or tolerable side effects for controlled modulating cognitive functions including attention, memory, and learning has gain a plenty of research interest (2, 7). Electromagnetic fields (EMFs) due to their unique characteristics including deep penetrating capability, non-invasiveness, and controllable parameters have been widely used in therapeutic medicine for different disorders (8-17). In addition, the underpinning phenomena of all living creatures, particularly biological cells are based on electromagnetic interactions. Therefore, using external EMFs we can alter or modulate the basic functions of cells and also the functions of different organisms in human body. In this regard, different EMFs based techniques have been developed for modulation and treatment of different cognitive functions in patients and also in healthy individuals (18, 19) (20).

Transcranial direct current stimulation (tDCS) is a noninvasive method for neuro modulation for the human brain. The method involves attaching two electrodes to the scalp and conducting a weak electrical current from the positively charged cathode to the negatively charged anode. The effects of the different electrodes on brain activity are not fully understood and are still under research. Currently, it is assumed that anodal stimulation enhances the neural firing rate by depolarizing the stimulated area, whereas cathodal stimulation hyperpolarizes cortical neurons in the stimulated area (21). The advent of transcranial brain stimulation influences neurocognitive enhancement in healthy volunteers (22, 23). There is growing interest in developing methods of neurocognitive enhancement for healthy adults, for example to accelerate learning and skill acquisition in complex tasks that would otherwise take very long to master (24). TDCS involves the passage of a small current (typically in the range of 0.5–4.0 mA) through the scalp and skull to modulate brain activity. Evidence for the use of electrical stimulation on the nervous system, employing animals such as electric fish and eels that produce electricity, dates back more than 2000 years. Later, other sources of electricity were used and electrical stimulation of the brain was sporadically studied in the middle of the 20th century under the term “brain polarization” (25).

In this paper, we review the methodological history and currently known physiological effects of tDCS, followed by a discussion of peer-reviewed research examining the effects of tDCS on attention in healthy adults (26, 27). Furthermore, individual differences in executive attention are known to mediate inter-individual variation in complex decision making (28). Research into the use of tDCS for enhancement of attention may therefore inform efforts to improve the efficiency of higher-order cognitive processes. Such research may also lead to the development of new clinical treatments for.

2. METHOD:

The databases of PubMed, Web of Science, and Google Scholar were searched from the first data available to 2016. The following key words were used "transcranial direct current stimulation" OR "tDCS" AND "healthy" AND "attention" AND "cognitive". The obtained records were reviewed for the title and abstract by two authors independently. Then, a consensus decision was made whether the studies are relevant for the review topic. Human studies that evaluate the effects of tDCS in healthy individuals of attention were included for further review. Limited number of studies in this field and heterogeneity in the design and methodology of the studies, we aimed to provide a comprehensive and descriptive overview of all aspect of applications of tDCS for treatment of attention deficit whether in children, or adults.

2.1. Search Strategy:

The scientific records were retrieved by a systematic search of multiple bibliographic databases and the last update of the search was performed on to Sep 30th 2016 including PubMed, Web of Science, and Google Scholar. The language of search was limited to English. The search key words based on the MeSH heading included "transcranial direct current stimulation" OR "tDCS" AND "healthy" AND "attention" AND "cognitive ". The titles and abstracts of all the records retrieved by the search strategy were reviewed by two authors (AY and HM) and the relevant papers with full texts available were used for further assessments. Moreover, the reference lists of the relevant papers were checked manually to identify additional eligible studies. These papers also were included for the full review.

2.2. Inclusion and Exclusion Criteria:

The identification and screening of the titles for inclusion or exclusion were performed independently by the two reviewers (AY and HM) and disagreements were resolved by discussion. Only original articles were eligible if they provided the following characteristics: human studies evaluating the effects of tDCS in healthy individuals on attention function. Studies were excluded if: (a) abstract only, (b) review or meta-analysis, (c) books, (d) letters, (e) conference documents, (f) case reports, (g) editorial (h) guideline (i) pilot study and (j) animal models. The flowchart of the study process is depicted in figure 1.

Table 1. Characteristics of the reviewed studies

Study

Electrodes placement

Simulation types

Parameters

Sample Size

Outcomes

Pope PA(2012) UK (29)

One electrode on the right cerebellar cortex and other on the right deltoid muscle

Anodal

/cathodal/sham

2mA , 20 min

66 healthy right hand

participants’ verbal responses were facilitated by cathodal stimulation

Clarke P (2014) Australia (30)

anodal electrode on LDLPFC /cathode on left superior region of the trapezius muscle

Anodal/

sham

1mA , 17 min

77 volunteers

receiving active stimulation showed greater evidence of attentional bias

Jacobson L (2012) Israel (31)

anodal on left intraparietal sulcus/superior parietal cortex (IPS/SPL), cathodal on right inferior parietal

cortex (IPL)

Anodal/

cathodal

1mA , 10 min

12 healthy young adult

significant effect of

higher accuracy in the Left Dorsal Anodal

Christopher S.Y. Benwell (2015) UK (32)

one left and the other over the right parietal

Anodal/

cathodal/sham

Half of participant 1mA and half 2mA ,20 min

40 right-handed

The opposite polarity (left cathodal/right anodal) resulted in no change in subjective

midpoint estimation

Loftus A.M. (2012) Australia (33)

left PPC P3, right PPC P4

Anodal/cathodal/ sham

1mA, 20 min

30 right hand

Significantly reduced by anodal tDCS

Pecchinenda A (2015) Italy (34)

Anodal on left DLPFC in anodal group, cathodal on left DLPFC in cathodal group

Anodal/cathodal/ sham

1.5 mA, 15 min

43 healthy young

Altering the DLPFC activity increase selective attention efficiency

Roe J M (2015) Norway (35)

Anodal on left DLPFC in anodal group, cathodal on left DLPFC in cathodal group

Anodal/cathodal/sham

1mA

34 right hand healthy

High attentional loads was significantly reduced in both stimulation relative to sham

Tommaso M (2014) Italy

(36)

Left parietal cortex (P3), reference cathode electrode on supraorbital

Anodal/ sham

2mA, 20 min

20 right hand

Attention task was not influenced by the menstrual cycle for women, men: significant increase in errors toward the left side.

Coffman A (2012) USA

(37)

Anodal on F10.Cathodal on left arm

Anodal/ sham

9 participants received 2.0 mA, 10 received 0.1 mA. 20 min

20 healthy

Significantly higher for participants receiving 2.0 mA compared with 0.1 mA tDCS

Scheldrup M (2014) USA (38)

Anode placed at (a) C3, (b) C4, (c) F9, or (d) F10, (cathode on contralateral arm).

Anodal/ sham

2mA.30min

100 healthy, right-handed

No effects were seen with anodes over sites that stimulated only dorsal (C3) or only ventral (F10) attention networks

4. DISCUSSION:

Research examining the modulation of cognition using tDCS is one of the most rapidly advancing fields in cognitive neuroscience today. Recent studies have demonstrated significant, often strong, effects of tDCS on cognitive processes that, in many cases, are relevant to both clinical and non-clinical populations. Attention is all affected by tDCS, and some of these effects are profound.

The literature reviewed in this article has primarily demonstrated tDCS is particularly well-suited for the manipulation of cognitive processing for healthy subjects. Importantly, the effects of tDCS occur on a time scale that may provide a potential performance enhancer in healthy individuals. In many of the studies reviewed in this article, tDCS has shown lasting effects that persist after stimulation has ended. In a few of these studies, this lasting effect of tDCS has persisted for hours or days after stimulation was ceased. This, along with the portability, relative ease of application, and profound acute effects, makes tDCS a powerful candidate for performance enhancement in healthy adults, or for intervention in attention networks.

Intriguing results have been demonstrated using tDCS for cognitive enhancement in healthy volunteers, and some studies have additionally found positive effects of tDCS in clinical populations. Though further studies are needed, cognitive enhancement with tDCS may even eventually be accepted as an alternative form of treatment for clinical populations, and neuro enhancement for healthy populations. Compared to other techniques, tDCS offers many advantages due to its relative safety, noninvasiveness, low-cost, and portability. Research into the effects of tDCS on cognition will undoubtedly continue and, along with other brain stimulation methods, may spark a new age in the way we think about treating neurocognitive dysfunction. The immediate impact of tDCS on cognitive networks, as compared to drug treatment; might make it a promising approach to improve cognitional function like attention. Future studies should explore various aspects likely relevant for the efficacy of tDCS. For example, it is not clear which brain areas should be directly targeted with tDCS to achieve optimal improvement of attentional networks.

According to the existing research with appropriate training, tDCS will become a common clinical approach to neurotherapy. Thus, it might be assumed that the combination of this stimulation with other modulating neural activity like rTMS and specially neurofeedback causes the comparatively large effects of tDCS. Despite this, as this field moves forward, it will be important future studies include measures which directly replicate prior work, explore potential state dependent effects within and between studies, and report quantitative data for all explored outcome measures to depict a big picture on the state of the field.

5. CONCLUSION:

The current literature showed that the target stimulation site to improve attention and also cognitive functions is frontal cortex. However, because of the limited number of the conducted studies and small sample size concluding decision on the efficacy of tDCS as well as the optimum parameters of tDCS is not possible (36). Although previous studies have claimed the efficacy of tDCS in modulating cortical excitability and in improving cognitive performance in healthy subjects (36, 38), further controlled studies with large sample size are necessary to reach a conclusive answer. The tDCS studies on modulating attention are in their initial step and the first line studies have indicated promising outcomes.

6. ACKNOWLEDGMENT:

The present study was financially supported by the student research committee of Ahvaz Jundishapur University of Medical Sciences, Ahvaz, Iran (Grant No.: 94s70).

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Received on 02.12.2016 Modified on 19.12.2016

Research J. Pharm. and Tech. 2017; 10(1): 311-316.

DOI: 10.5958/0974-360X.2017.00064.6