Does Augmenting Virtual Reality-Based Cognitive Training with Longitudinal,Anodal Transcranial Direct Current Stimulation Improve Visual Search Performance? A Neurobehavioral Evaluation

Abstract

This study investigated the effectiveness of combining Virtual Reality (VR)-based cognitive training with transcranial direct current stimulation (tDCS) in enhancing visual processing. Eighty participants undertook a 20-day, single-blind, placebo-controlled study across pre-intervention (Day 1), intervention (Day 2–Day 9), and post-intervention phases (Day 10 and Day 20). Participants were randomly assigned to four groups: tDCS + VR, VR + placebo tDCS, tDCS + placebo VR, and placebo tDCS + placebo VR. After undertaking three visual processing tasks on Day 1, participants underwent intervention from Day 2 to Day 9 and followed up with post-intervention testing on Days 10 and 20. Results revealed that the tDCS + VR condition showed significantly improved sensitivity in visual search on Day 10. EEG results indicated that anodal tDCS coupled with VR training enhanced cortical activation in the tDCS + VR condition. This study underscores the potential of cognitive training programs integrating VR and tDCS.

Objectives

Visual search is a complex cognitive process that consists of extracting information from visual stimuli, distinguishing the target stimuli from a multitude of distractions, and constructing a coherent perceptual representation (Duncan & Humphreys, 1989). This process involves several sub-cognitive activities, such as visuospatial processing, attention, and object recognition (Duncan & Humphreys, 1989). In recent years, synthetic training environments in Virtual Reality (VR) have emerged as a dependable platform for training individuals in cognitive processes like executive functioning, motor coordination, multitasking, and dynamic decision-making (Newman et al., 2022). VR environments offer a high degree of immersion and presence and facilitate efficient interaction, making them a valuable tool for enhancing cognitive abilities (Rao et al., 2022). VR allows users to engage their proprioceptive senses in a virtual environment, promoting a more immersive and engaging experience (Rao et al., 2019). Previous studies have successfully demonstrated the advantages of training in “immersive” VR (through a head-mounted display [HMD]) compared to traditional desktop-based virtual environments in cognitive tasks ranging from spatial learning and motor coordination to executive functioning (Srivastava et al., 2019). Some studies have highlighted the propensity of immersive VR to efficiently localize spatial and temporal information in the virtual environment, which led to higher accuracy and reaction times (Rao et al., 2020). However, there is limited research on how VR can effectively improve visual search and its consequential cognitive processes.

The use of non-invasive brain stimulation (NIBS) techniques, specifically transcranial direct current stimulation (tDCS), for cognitive enhancement has experienced a significant rise in the past decade. tDCS influences cortical excitability by adjusting neural activity through electrodes placed on the scalp, resulting in polarity-specific shifts in the resting membrane potential (Shah et al., 2023). Extensive research supports anodal tDCS, mainly when administered longitudinally, as a promising intervention for enhancing cognitive and sub-cognitive processes. For instance, Gan et al. (2022) investigated the effectiveness of anodal tDCS on the visual cortex for improving visual perception. The study revealed that participants in the experimental group (who received 1 mA anodal tDCS on the visual cortex over three sessions) exhibited a considerable increase in contrast sensitivity compared to the control group. Similarly, Sung and Gordon (2018) demonstrated that anodal stimulation of the dorsolateral prefrontal cortex (dlPFC) effectively facilitated longer cumulative visual fixations on key attributes in a standard visual search task.

The concept of combining VR-based training with transcranial direct current stimulation (tDCS) to enhance cognitive function has primarily been explored in the context of cognitive rehabilitation for motor impairments, stroke, and post-traumatic stress disorders. For example, Lee and Chun’s (2014) study demonstrated that the combination of tDCS and VR exposure therapy administered longitudinally led to significant improvements in clinical measures related to stroke. However, there is a dearth of research examining the effectiveness of VR and tDCS interventions in enhancing visual search and associated cognitive processes. This study addresses this research gap by investigating the efficacy of longitudinal VR-based training and anodal tDCS on visual search, visual processing, and visuospatial memory.

Approach

Eighty participants (34 females, 46 males; mean age = 24.1 years; all right-handed) participated in the study. None of the participants reported any history of psychiatric disorders, and all had normal vision. The study complied with the ethical standards laid down by the Declaration of Helsinki, and the Institute Ethical Committee at the institute approved it. The participants were initially briefed about the experimental methodology and were screened for potential risks during tDCS administration via a screening questionnaire.

The study was a 20-day, randomized, single-blind, sham-controlled trial with four phases: pre-intervention, intervention, post-intervention-1, and post-intervention-2. On the first day, participants undertook a visual search task, a change detection task (testing visuospatial memory), and a Corsi-block tapping task (testing visual working memory) in VR with synchronous 32-channel EEG data recorded. Participants were randomly assigned to four groups: tDCS + VR, VR only, tDCS only, and sham. The tDCS + VR group received VR-based visual search task-based training and 2 mA tDCS on the dlPFC for 20 min daily from Day 2 to Day 9. The VR group underwent only VR-based visual search training with sham tDCS on the dlPFC for 20 min daily. While training in an unrelated VR task, the tDCS group received anodal tDCS administration on the dlPFC. The sham group underwent sham tDCS on the dlPFC while training in an unrelated VR task. Participants’ performance in all three VR-based tasks was evaluated on days 10 (post-intervention-1) and 20 (post-intervention-2) after the intervention.

Findings

For statistical analysis, we carried out mixed factorial ANOVAs to evaluate the main and interaction effects across different intervention conditions (tDCS + VR, only VR, only tDCS, and sham) and different phases of training (Day 1, Day 10, and Day 20). Results revealed a significant interaction effect between the four intervention conditions and the different training phases on the sensitivity parameter in the visual search task (F[1.77, 140.23] = 8.5, p < .05, η_p² = .25), with Bonferroni adjustment revealing that the sensitivity was significantly different in the tDCS + VR condition compared to other conditions on day 10 (tDCS + VR: µ = .87 > VR only: µ = .68 > tDCS only: µ = .59 > Sham: µ = 0.49). Similar results were observed across the total score and reaction time in the Corsi-block tapping task. Based on the results obtained in the study, we inferred that combining VR-based cognitive training with tDCS can enhance neuroplasticity and improve visual search performance. Specifically, stimulating the dlPFC increased activation and efficiency in working memory and decision-making, which are critical for visual search. Longitudinal VR-based cognitive training also improved information processing by providing an immersive, realistic environment where participants could see and manipulate objects in space, facilitating a sense of “presence.” This study suggests that integrating VR with tDCS may provide a more flexible and rigorous method to enhance visual search and visuospatial memory while modulating the neural circuits responsible for these processes, resulting in a more substantial beneficial effect.

Takeaways

Our study indicates that combining VR-based cognitive training with tDCS may be a promising approach for enhancing cognitive and sub-cognitive processes such as visual search and visuospatial memory. Our findings support the theory proposed by Yao et al. (2020) that constant upregulation of the dorsolateral prefrontal cortex (dlPFC) through VR-based cognitive training and tDCS leads to improved spatial learning, processing speed, and cognitive flexibility. Our results emphasize the critical role of anodal tDCS on the dlPFC in enhancing neural activation and optimizing cognitive functions essential for visual search, such as working memory and decision-making. The longitudinal nature of both interventions highlights the importance of sustained engagement with VR-based cognitive tasks in promoting neuroplastic changes that improve visual search performance. Our study highlights the potential of combining VR-based cognitive training with tDCS to achieve cognitive enhancement by augmenting neural activation and optimizing cognitive functions.

Proficient visual search/visuospatial memory is widely recognized in numerous domains and programs, such as aviation, screening, and medical imaging interpretation. This study sheds light on the relationship between immersive VR-based cognitive training and non-invasive neuromodulation techniques like tDCS, highlighting the need for further investigation into developing personalized cognitive training programs that cater to individual cognitive requirements. In the future, we plan to explore the spatiotemporal effects of this combination of interventions using functional near-infrared spectroscopy (fNIRs). In addition, we also intend to measure the paradoxical homeostatic effects of tDCS and transfer effects to more complex cognitive processes.