A pilot study examining the effects of transcranial direct current stimulation on cognitive and visual impairments

Abstract

Objective:

The study aimed to assess the impact of a single session of transcranial direct current stimulation (tDCS) on people with both cognitive and visual impairments.

Methods:

Sixteen individuals with visual and cognitive impairments were recruited and randomized to either the experimental or sham group. While the sham group only received 30 seconds of initial stimulation, participants in the experimental group received an active tDCS intervention for 20 minutes. The Cantonese Mini-Mental State Examination (CMMSE), the Digit Span Forward Test, the Chinese Verbal Learning Test (CVVLT), and the memory subtest from the Neurobehavioral Cognitive Status Examination (NCSE) were the neuropsychological assessments. To ascertain whether there are statistical differences between groups, repeated measures ANOVA was used to analyze data gathered prior to and following the intervention.

Results:

Age, educational attainment, and baseline CMMSE scores did not significantly differ between the groups. The type of stimulation and CVVLT total scores showed a significant interaction effect, with improvements in scores seen after active tDCS treatment. Other measured variables did not show any additional effects that were statistically significant.

Conclusion:

These findings provided preliminary support for the potential efficacy of tDCS in improving immediate memory functions among individuals with both cognitive and visual deficits.

Keywords

transcranial direct current stimulation cognitive impairment visual impairment memory attention

1 Introduction

As people age, they typically exhibit quantifiable declines in cognitive abilities, including areas like processing speed, executive function, memory, and attention [1]. Dementia and other pathological conditions worsen cognitive impairments brought on by progressive neurodegeneration. Visual impairment is one of the most prevalent conditions affecting older adults, along with dementia [2]. Research has indicated a link between visual impairment and a higher chance of cognitive decline and dementia [2, 3]. People who have both visual and cognitive impairments face more difficulties; they frequently have higher rates of disability and less independence in day-to-day activities [4].

Behavioral strategies, cognitive training, physical exercise, and pharmaceutical treatments are all used to try to stop or postpone cognitive decline [5]. The efficacy of these traditional approaches is still being questioned. Pharmacological treatments frequently exhibit limited impact across multiple randomized controlled trials, according to a review by Petersen [6]. The effectiveness of aerobic exercise may be limited to people with relatively good physical health, and while it has been proposed as a potential strategy to slow the progression toward dementia [6], the evidence for this claim has been conflicting [7]. Likewise, behavioral or lifestyle interventions necessitate extended training durations, rendering them unfeasible in certain situations [8].

An alternate method for improving cognition in individuals with cognitive difficulties is non- invasive brain stimulation (NIBS) [9]. Its use has become increasingly popular over the last 20 years, as research indicates that it can improve cognitive function and can be a time-saving technique [9, 10]. Transcranial magnetic stimulation (TMS) and transcranial direct current stimulation (tDCS) are two of the most widely used NIBS techniques [10]. Both strategies promote neuroplasticity and alter cortical excitability [9]. Of the two, tDCS is known for being portable and cost-effective. By raising the membrane potential in the cortical regions beneath the electrode, anodal tDCS specifically promotes neuronal activity and promote neuronal depolarization [11]. This neural activation mechanism has been linked to tDCS’s ability to enhance cognitive performance immediately after stimulation [12].

The impact of tDCS on cognitive processes has been the subject of an increasing amount of research. Anodal tDCS temporarily improves cognitive function and reverses age-related changes in brain activity and connectivity, according to studies conducted on older adults [13]. According to a review of thirteen studies on healthy older populations, tDCS has shown encouraging results in reducing the effects of cognitive aging [14]. The left dorsolateral prefrontal cortex (dlPFC), a part of the brain frequently linked to cognitive functions, was the focus of the majority of studies. Studies involving people with cognitive impairments have also been carried out. For example, Meinzer and Lindenberg [8] identified improved semantic-word-retrieval abilities and normalized brain activity in participants with mild cognitive impairment after tDCS treatments. However, in some cases, these benefits were short-term. Findings from sham-controlled, multi-session studies suggest that tDCS enhances subjective perceptions of memory ability [15] and leads to improvements in executive functions, episodic verbal memory, figure naming tasks, verbal fluency, and working memory [16 –18 ].

For those with cognitive impairments, the use of tDCS continues to produce encouraging results. However, no research has looked into how it affects people who have both visual and cognitive impairments. This report describes a pilot study that investigated how a single tDCS session affected participants who had both visual and cognitive impairments. Because of its established involvement in a number of cognitive processes, such as attention [19, 20], working memory [21], and episodic memory [22], the left dlPFC was chosen as the stimulation site.

2 Methods

2.1 Participants

From residential facilities serving senior citizens with visual impairments, sixteen participants were recruited. Participants had to meet three requirements in order to be considered: (1) be 65 years of age or older; (2) report cognitive complaints; and (3) have a visual acuity of 20/200 or worse in the better eye, which indicates moderate to severe visual impairment or blindness. Those who (1) were unable to follow directions, (2) were taking drugs that could affect cognitive function or cognitive-enhancing medications, (3) had a history of intellectual disability, central nervous system infections, brain tumors, traumatic brain injuries, serious psychiatric conditions (like major depression or schizophrenia), or substance abuse, and (4) had communication barriers, such as deafness or other severe language-related problems, were excluded. Individuals with a history of cerebrovascular surgery or the presence of a metal plate in their skull were also excluded. Participants were recruited through a convenience sampling approach, and the study received ethical approval from the Tung Wah College (Reference number: REC2018017, approved on 19 November 2018). Written informed consent was obtained from participants or their family members before the study began.

2.2 Assessments

Among the neuropsychological tests were the Cantonese version of the Mini-Mental State Examination (CMMSE) [23], the digit span forward test [24], the Chinese version of the Verbal Learning Test (CVVLT) [25], and the memory subtest of the Neurobehavioral Cognitive Status Examination (NCSE) [26]. Overall cognitive function and participant eligibility were determined using the CMMSE, which has a maximum score of 30. Selective attention was assessed using the digit span forward test, which has a maximum score of 9. Nine two-character Chinese nouns were included in the CVVLT, which also included a 10-minute delayed recall trial with a maximum score of nine and four learning trials with a maximum score of nine each, for a total maximum score of 36. The memory subtest of the NCSE requires the memorization of 4 two- character Chinese nouns, which participants must later recall (with a maximum score of 12).

2.3 Procedure

The experimental group and the sham group were assigned to participants at random. The participants in this study were not informed of their group assignments because it was a single-blind trial. Assessments were conducted both before and after the one tDCS session. The proposed studyʹs tDCS protocol complied with accepted safety standards and recommendations for human research [27]. To ensure the participants’ comfort and safety, the stimulation was hardware-restricted.

With electrodes placed over the left dorsolateral prefrontal cortex (dlPFC) (which corresponds to F3 in the international 10-20 system as an anodal/sham electrode) and the right dlPFC (F4, acting as the reference electrode), tDCS was administered using a Starstim 8 system (Neuroelectrics, Spain) (see Figure 1). Participants received a constant 1.5 mA current directed at the designated area for 20 minutes during anodal stimulation. This comprised a 30-second ramp-up period at the beginning and a ramp-down phase at the conclusion of the session. On the other hand, current was only used for the first 30 seconds of the 20-minute process for the sham stimulation condition. Im et al. [28], Yun et al. [15] and Bowie et al.’s [29] research methods served as a guide for the dosage and electrode locations that were selected.

Figure 1

A flowchart of study procedure

2.4 Statistical Analysis

SPSS Statistics 26.0 (IBM, USA) was used for statistical analyses. To investigate differences in neurocognitive test scores between two time points (pre and post), a repeated measures analysis of variance was performed, using the stimulation group (sham or anodal) as the between-subjects factor. Post-hoc analyses were performed on significant interaction effects. The Greenhouse– Geisser correction was used when the results did not meet the sphericity assumption. P < 0.05 was chosen as the significance level.

3 Results

All sixteen participants were female (refer to Table 1). Each group consisted of eight participants.

Table 1

Demographic information from the participants

Demographic information	Experimental (n=8)	Sham (n=8)	p-value
Age (years)	79.9±10.5	86.5±10.9	0.24^a
Gender	8 females	8 females	1.0^b
Education (years)	6.6±4.2	5.3±4.9	0.55^a
CMMSE	19.9±6.9 (range: 11–26)	18.3±5.3 (range: 13–26)	0.69^a

: by independent samples t-test

: by Chi-square test

Abbreviations: CMMSE, Cantonese Mini-Mental State Examination.

The independent samples t-test revealed that there were no significant differences in age, years of education, and CMMSE scores between the two groups. The CMMSE score ranges suggested that these participants might exhibit mild cognitive impairment, as well as mild or moderate dementia.

Neuropsychological assessment scores are shown in Table 2. There were no significant baseline (pre-assessment) differences between the stimulation and sham groups across all neuropsychological assessments (p’s > 0.06). Results indicated that there was an interaction effect between stimulation group and CVVLT total score (F(1,14)=5.034, p=0.042). Post hoc test indicated that CVVLT total score increased after real tDCS stimulation while scores in sham group remained the same (p<0.01). Main effect of stimulation group for NCSE–memory was found significant (F(1,14)=6.725, p=0.021). However, post hoc test did not confirm the effect. No other significant main or interaction effects were observed.

Table 2

Assessment results

Assessment	Session	Sham	Experimental	p-value
Digit span forward test	Pre	5.8±0.5	6.3±1.3
Post	6.5±1.4	6.1±1.4
CVVLT first trial	Pre	2.1±1.4	2.4±2.0
	Post	2.9±2.0	3.5±2.6
CVVLT total	Pre	12.8±5.2	17.5±6.3	0.042^a
	Post	12.5±6.5	20.5±7.3
CVVLT delayed recall	Pre	1.4±1.7	3.5±2.4
Post	1.5±2.0	3.9±2.9
NCSE—memory	Pre	8.3±3.1	6.5±3.4	0.021^b
Post	8.8±3.5	7.8±3.0

: group x time interaction effect

: group main effect

Abbreviations: CVVLT, the Chinese version of the Verbal Learning Test; NCSE, the Neurobehavioral Cognitive Status Examination.

4 Discussion

To our knowledge, this is the first tDCS trial to include a group of older individuals with both visual and cognitive impairments. Except for the CVVLT total score, the scores in the experimental and sham groups were largely comparable, according to the results acquired before and after the intervention. These results implied that improving attention with a single tDCS session was insufficient. Nonetheless, some results suggested that for people with comparable profiles, a single tDCS session might improve memory (immediate recall). Eight sessions of cognitive training may improve attention and processing speed, according to a previous case study involving visually impaired older adults with dementia [30]. This aligns with the perspective that suitable external interventions could still facilitate neuroplastic and behavioral modifications in individuals with visual and cognitive impairments [31, 33]. Nonetheless, the effectiveness of such interventions may be contingent upon their intensity.

Single session design might be partly to blame for the modest improvement that was seen. This could represent a constraint of tDCS, as the neuroplastic changes elicited by a single tDCS session may be minimal or transient [32]. The fact that these changes were temporary may help to explain why assessments of delayed recall did not show any improvements. The treatment effect may have been identified because the CVVLT total sub-test has been shown to be a more sensitive measure than the CVVLT first trial sub-test [25].

The NCSE-memory and CVVLT delayed recall tests did not reveal any noteworthy variations. The purpose of both tests is to assess similar cognitive domains. However, there were differences in how these tests were administered, scored, and how complex they were. Participants in the CVVLT were given the same list of nine two-character nouns four times, with memory registration tests administered after each reading. On the other hand, there were no limitations on how many times participants could receive the NCSE-memory nouns. Until they were able to successfully repeat all four nouns twice, they were free to practice as often as needed. Furthermore, if a participant failed to recall a noun in the NCSE-memory subtest, categorical cues and options were provided, allowing them to earn partial score for items recalled with assistance. Despite the differences in administration methods, the outcomes were consistent, revealing that single tDCS did not enhance delayed recall in the experimental group. This may be attributed to the transient effects of tDCS. Future research could explore similar tests with varying levels of support to determine if tDCS might yield benefits when additional assistance is provided during the retrieval phase.

Enhancements in delayed memory have previously been observed in individuals with cognitive impairment. Rasmussen and Boayue [33] conducted tDCS over a span of 2 days, comprising 6 sessions of 20 minutes each, which resulted in improved delayed memory among patients diagnosed with Alzheimer’s disease. A multi- session approach is frequently necessary to achieve more significant and sustained improvements in cognitive function [32]. Gu et al.[34] assessed the impact of tDCS on episodic memory in people with mild cognitive impairment (MCI) in a different study. According to their research, tDCS can improve episodic memory in MCI patients for up to four weeks when given for 20 minutes each day for five days. This improvement has been connected to tDCS-induced improvements in synaptic remodeling and cortical excitability modulation. The Montreal Cognitive Assessment (MoCA), which measures general cognition, did not, however, reveal any changes after the intervention. Possible learning effects and the insensitivity of the MoCA items have been suggested as reasons for this result.

To enhance the effects of tDCS, previous research has suggested that intensified tDCS protocols, such as performing multiple stimulation sessions in a single day and prolonging the duration of each session, should be taken into consideration in addition to using a multi-session design [35, 36]. Additionally, combining tDCS stimulation with a concurrent cognitive training program offers yet another potential tactic to improve effectiveness; however, more research is needed to fully understand how the various modalities interact [37, 38].

There were various limitations on this study. For example, only one session of tDCS stimulation was used, the sample size was small, and the participants were heterogeneous (the baseline cognitive performance varied). Despite the limitations, this study provided preliminary evidence that tDCS may be beneficial for people with both visual and cognitive impairments. From one session per day to several sessions daily, multi-session tDCS protocols can vary significantly across studies and last anywhere from six to eight weeks or longer [39]. Each training session has a more consistent duration, usually lasting 20 to 30 minutes [40]. Future multi-session studies should involve protocols that are practical and promising.

5 Conclusions

These findings provided preliminary support for the potential efficacy of tDCS in improving immediate memory functions among individuals with both cognitive and visual deficits. The use of more intensive protocols or a multi-session design should be investigated in future studies.

Footnotes

Acknowledgements

The authors thank the Hong Kong Society for the Blind for the support provided to carry out this research.

Funding information

This study was supported by a College Research Fund awarded to Dr. Michael Kuo (ref no.: 2017-00-51-CRG170202).

Authors Contributions

MCCK: Conceptualization, Formal analysis, Validation, Visualization, Writing - original draft, Writing - review & editing, Investigation, Resources, Supervision; ATSC: Data curation, Validation, Writing - review & editing, Investigation, Project administration, Resources.

Declaration of Conflicting interests

The authors declare no conflict of interest.

Data Availability Statement

The data that support the findings of this study are available from the corresponding author upon reasonable request.

Ethics Statement

The study was approved by the Ethics Committee of the Tung Wah College, Hong Kong, China (approval number: REC2018017), and it was compliant with the Helsinki Declaration of 1975, as revised in 2008.

Informed consent

Written informed consent was acquired from each participant or their family member before the start of the experiment.

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