TransPhoM-DS Study Grant Report: Rationale and Protocol for Investigating the Efficacy of Low-Power Transcranial Photobiomodulation on Language, Executive Function, Attention, and Memory in Down Syndrome

Abstract

Down syndrome (DS) is the leading genetic cause of intellectual disability globally, affecting about 1 in every 800 births. Individuals with DS often face various neuropsychiatric conditions alongside intellectual disabilities due to altered brain development. Despite the diverse phenotypic expressions of DS, typical physical characteristics frequently influence language development and acquisition. EEG studies have identified abnormal oscillatory patterns in individuals with DS. Emerging interventions targeting the enhancement of gamma (40 Hz) neuronal oscillations show potential for improving brain electrical activity and cognitive functions in this population. However, effective cognitive interventions for DS remain scarce. Extensive research indicates that transcranial photobiomodulation (t-PBM) with near-infrared (NIR) light can penetrate deeply into the cerebral cortex, modulate cortical excitability, and enhance cerebral perfusion and oxygenation. Furthermore, t-PBM has been shown to improve cognitive functions such as language, attention, inhibition, learning, and memory, including working memory. This study presents the rationale and design of an ongoing randomized, sham-controlled clinical trial aimed at assessing the effectiveness of t-PBM using NIR light in enhancing the language abilities of individuals with DS.

1. Introduction

1.1. Down Syndrome, Language, Executive Function, Attention and Memory, Brain Oscillations, and Mitochondrial Dysfunction

Down syndrome (DS) is the most prevalent genetic cause of intellectual disability worldwide, occurring in approximately 1 in 800 births [1]. DS is caused by the presence of an extra partial or full copy of chromosome 21 in humans (HSA21). The DS phenotype is characterized by a variety of clinical manifestations associated with many organ systems, especially the musculoskeletal, neurological, and cardiovascular systems [2]. Common physical and cognitive characteristics of individuals with DS include short stature, muscle hypotonia, atlantoaxial instability, congenital heart defects, reduced neuronal density, cerebellar hypoplasia, and intellectual disability [2]. Moreover, this population also has higher chances of developing some co-occurring neuropsychiatric conditions [3]. Although there is a wide range of phenotypes among people with DS, the typical physical and cognitive features of DS often lead to delays in language development and acquisition [4,5]. Although research in this area is still limited, it is evident that deficits in expressive language abilities persist into adulthood for individuals with DS [6,7]. In addition to impairments in language, difficulties with executive function are commonly observed in people with DS [8]. Executive function refers to a set of cognitive processes involved in goal-oriented behavior, which include working memory for temporary storage and manipulation of information, attentional focus for directing and shifting attention, cognitive flexibility for task-switching, and inhibition for overriding dominant or automatic responses [9].

To date, there is a considerable shortage of effective interventions for boosting cognition and its related aspects such as memory and attention, in people with DS [10,11]. Almost all individuals with DS have an intellectual disability (IQ < 70), which appears to be related to altered brain development [12]. Additionally, adults with DS are at an extremely high risk of developing Alzheimer’s disease (AD), with a lifetime prevalence of 50%; virtually all adults with DS show the neuropathological changes of AD by the age of 40 years [13,14]. Both the intellectual disability and the risk of AD are at least partially attributed to the triplication of the gene for the amyloid precursor protein (APP), located on chromosome 21, as amyloid-β typically accumulates in the brain across the lifespan of people with DS [15]. In animal models, normalization of APP expression restored neurogenesis and neurite length and decreased astrogliogenesis. In a case report, an elderly man with phenotypic DS and partial trisomy of chromosome 21 lacked triplication of APP. His intellectual disability was mild in severity and no dementia was detected on his neurological examinations; his neuropathological findings showed only normal aging but no signs of AD [16].

In addition, a decrease in neuronal firing rate and deficits in gamma oscillations (20–50 Hz) have been identified in the prefrontal cortices (PFCs) of transgenic DS mice due to decreased inhibition by the parvalbumin-positive interneurons [17]. In animal models of AD, a reduction of neuronal gamma oscillations preceded the onset of plaque formation (amyloid-β accumulation) and cognitive decline [18]. Optogenetically driving parvalbumin-positive interneurons at gamma (40 Hz) reduced levels of amyloid-β. Similarly, a non-invasive 40 Hz light-flickering regimen reduced levels of amyloid-β in the visual cortex [18]. These findings uncover a previously unappreciated function of gamma rhythms in recruiting both neuronal and glial responses to attenuate AD-associated neuropathology [18]. Altered gamma has been observed in multiple brain regions in several neurological and psychiatric disorders, including a reduction in spontaneous gamma synchronization in AD patients and reduced gamma power in multiple AD mouse models [19,20,21,22]. Alterations in oscillatory patterns have also been detected in electroencephalography (EEG) recordings in individuals with DS [23,24,25]. Several authors have detected shifts toward lower frequencies in mean EEG signals up to the beta band, as well as decreased low beta (13–18 Hz) oscillatory activity in adults with DS [25,26]. Babiloni et al. (2009) [24] found decreased beta and gamma waves in adolescents with DS that were correlated with cognitive psychometric measurements. Fast cortical oscillations in the beta-gamma range are involved in several cognitive processes involving the prefrontal cortex (PFC), such as attention, decision making, learning, and working memory [27,28,29].

In addition to the aforementioned changes observed in DS, there is also evidence that dysregulated mitochondrial-dependent activation of intracellular stress cascades plays a pivotal role in various neuropathologies seen in DS [30,31]. This dysregulation involves an interaction between overexpressed genes from HSA21 and dysregulated genes elsewhere in the genome, leading to disrupted molecular signaling pathways and the emergence of DS phenotypes [31]. Many of these HSA21 genes are associated with oxidative phosphorylation (OXPHOS) and mitochondrial functions, impacting multiple organs or tissues in DS, including blood [32], muscle [33], and the brain [34]. The malfunctioning of the mitochondrial OXPHOS apparatus in DS brains is characterized by a notable increase in reactive oxygen species (ROS), leading to heightened oxidative stress. This dysfunction also involves the compromised performance of electron transport chain subunits, specifically complexes I to IV, and a transition towards less efficient bioenergy profiles [31]. Additionally, DS fetal astrocytes exhibit impaired mitochondrial energy metabolism, indicated by significantly reduced mitochondrial membrane potential (MMP) and diminished mitochondrial redox activity [35]. Considering this, the changes in brain oscillatory patterns observed in EEG recordings among individuals with DS could potentially be linked to mitochondrial dysfunctions, as maintaining fast neuronal network oscillations within the gamma range requires high energy demand and augmented mitochondrial oxidative metabolism [36].

In light of the significant influence that neuromodulation can have on brain oscillations, language abilities, and executive functions, we have initiated a pilot study to investigate the effectiveness of neuromodulation in individuals with DS, focusing specifically on language enhancement. Our primary objective is to explore potential clinical improvements in neuropsychiatric function by examining the synchronization of brain oscillations. Our study employs transcranial photobiomodulation (t-PBM), a technique involving the delivery of near-infrared (NIR) light to the brain. This method is closely linked to mitochondrial metabolism and has demonstrated promise in modulating brain oscillations. Notably, t-PBM has also shown the ability to increase intrinsic brain activity within irradiated areas in early AD [37], which further reinforces our decision to utilize this approach.

1.2. Transcranial Photobiomodulation (t-PBM)

t-PBM, utilizing NIR light, is an innovative intervention that has recently surfaced as a promising therapeutic approach for various neuropsychiatric conditions (for review, see Salehpour et al., 2018) [38]. This technique involves the application of low-level lasers or light-emitting diodes (LEDs) and has displayed promising therapeutic effects in individuals with stroke [39], traumatic brain injury [40], neurodegenerative disorders [41], and major depressive disorder (for review, see Vieira et al., 2023a, 2023b) [42,43]. Moreover, t-PBM has been found to offer cognitive benefits to healthy populations, improving attention, memory, working memory, and learning [40,44,45,46,47,48,49]. A substantial body of literature proves that t-PBM with NIR light penetrates deeply into the cerebral cortex [50,51,52], modulates cortical excitability [53,54], and improves cerebral perfusion [55,56,57] and oxygenation [58]. Its safety was demonstrated in 1,410 acute stroke patients [59,60,61].

A compelling hypothesis suggests that t-PBM has the potential to enhance brain energy metabolism by promoting optimal mitochondrial function [62]. Adenosine triphosphate (ATP) serves as the primary intracellular source of energy, crucial for sustaining neural activity. Mitochondria play a key role in ATP production through OXPHOS. This process involves a respiratory chain consisting of five enzyme complexes, and any alterations to these complexes can impact ATP synthesis. It is proposed that t-PBM, by delivering photons (energy particles) to the tissue, can stimulate one of these complexes, specifically the cytochrome c oxidase (CCO) or respiratory chain complex IV. Numerous studies have documented the upregulation of CCO following LED and laser light therapy, particularly in the NIR range. This upregulation of CCO has been associated with an increase in neuronal activity [62].

As previously mentioned, the presence of mitochondrial dysfunction has been observed in various prevalent neuropsychiatric disorders [63], and the utilization of t-PBM could potentially compensate for such dysfunction, thereby restoring cognitive capacity. Research findings have shown that increased mitochondrial activity might have an impact on brain activity, particularly in high-frequency oscillations. Extensive studies conducted on hippocampal slice cultures have provided evidence of a connection between mitochondrial function and fast neural oscillations within the gamma band range (approximately 30–90 Hz) [64,65].

1.3. Innovation

The treatment for neuropsychological symptoms in DS consists of non-biological interventions, such as behavioral training, education, vocational training, early stimulation therapy, and creating a supportive home environment [66]. Pharmacotherapy is also available for symptomatic treatment of co-occurring conditions, such as attention-deficit/hyperactivity disorder (ADHD), obsessive-compulsive disorder (OCD), depression, and anxiety [11]. In response to this gap in biological therapeutics for DS, our main goal is to test neuromodulation as a biological intervention targeted to specific brain areas, aiming at improving language and cognitive functions. We are interested in associating precise t-PBM dose with target engagement—such as change in brain spectral power and connectivity—and with clinical response. Thus, the current project is innovative because of the following: 1. It tests t-PBM with NIR, an innovative technology with a well-established safety profile, broadly to the left brain in DS; 2. It tests t-PBM to improve comprehension and production in language; 3. Preliminary data suggest potential effects of t-PBM for the improvement in cognition (memory and attention) associated with gamma and beta EEG power; 4. t-PBM represents a new intervention for children, i.e., acting during neuroplasticity and brain development; 5. We are looking at the durability of the presumed therapeutic effects after treatment discontinuation; we chose a follow-up assessment after 4 weeks from the end of t-PBM sessions; and 6. We offer open-label treatments to participants who did not respond to t-PBM/sham, which consists of an alternative paradigm of t-PBM with high-power application to the PFC.

The effects of t-PBM on language and cognition in individuals with DS are yet unknown. Based on preliminary research, we hypothesize that subjects may experience improvements in language, executive function, memory, and attention. This study will improve our understanding of the pathology underlying DS, which may lead to the development of novel therapeutics. If our hypotheses are confirmed, the present study will both support the clinical development of an innovative neuromodulation strategy and offer ideal methods to test its application in other neuropsychiatric conditions.

1.4. Preliminary Data

We conducted a single-blind, sham-controlled pilot study to test the efficacy of continuous t-PBM (830 nm on Fp1, Fp2, F3, F4) and sham on EEG oscillations in a sample of ten subjects without DS. EEG recordings were performed before and after the t-PBM session. t-PBM significantly boosted beta (t = 2.91, df = 7, p < 0.03) and gamma (t = 3.02, df = 7, p < 0.02) EEG spectral powers in eyes-open recordings and gamma power (t = 3.61, df = 6, p < 0.015) in eyes-closed recordings, with the largest effects in the posterior regions [67].

A collaboration between MGH and Piero Mannu’s private practice in Cagliari, Italy, piloted NIR t-PBM in three children with DS. The patients were treated with either one or two series of NIR t-PBM—each consisting of two applications per week for four weeks—and were evaluated every two weeks. t-PBM was started with the Omnilux New-U device (PhotoMedex Inc.) and applied to the forehead (targeting PFC). t-PBM parameters were 830 nm; 33.2 mW/cm²; 40 J/cm²; 28.7 cm² and 20 min per two sites (F3/F4); 2.3 kJ per session. Preliminary results displayed an increased ability in motor skills, with greater control in fine movements, greater capacity in drawings (spiral drawing test), and ability to draw portraits. Improved speech, attention, and interpersonal skills suggested a positive effect of t-PBM on cognition. Aggressive behaviors, agitation, hyperactivity, and emotional lability decreased. No serious adverse effects occurred [68].

2. Aims

The current study aims to test the t-PBM effect on resting-state EEG gamma power, language, attention, and memory in individuals with DS. Moreover, it aims to investigate the correlation between the t-PBM effect on EEG gamma power and language and cognition.

Specific Aims

• Aim 1: To test the effect of t-PBM on resting-state EEG gamma power in individuals with DS. Hypothesis: t-PBM will increase EEG gamma power (40 Hz) significantly more than sham, as tested after completion of all 18 t-PBM sessions (after 6 weeks from baseline, e.g., 2 or 3 days after the final t-PBM session) and at long-term follow-up (after 10 weeks from baseline, e.g., 4 weeks after the final t-PBM session).
• Aim 2: To test the effect of t-PBM on language in individuals with DS. Hypothesis: t-PBM will improve language (intelligibility, vocabulary, syntax, and grammar) significantly more than sham, as tested after completion of all 18 t-PBM sessions (after 6 weeks from baseline, e.g., 2 or 3 days after the final t-PBM session) and at long-term follow-up (after 10 weeks from baseline, e.g., 4 weeks after the final t-PBM session).
• Aim 3: To test the effect of t-PBM on attention and memory in individuals with DS. Hypothesis: t-PBM will improve attention (decrease in reaction times) and visual memory (increase in correct matches of images) significantly more than sham, as tested after completion of all 18 t-PBM sessions (after 6 weeks from baseline, e.g., 2 or 3 days after the final t-PBM session) and at long-term follow-up (after 10 weeks from baseline, e.g., 4 weeks after the final t-PBM session). Executive function will be used as an additional, exploratory outcome measure; however, its metrics were not defined a priori.
• Aim 4: To test the correlation between the effect of t-PBM on EEG gamma power and language and cognition in individuals with DS. Hypothesis: The increase in EEG gamma power will directly correlate with improvements in the language (intelligibility, vocabulary, syntax, and grammar) and cognition (decrease in reaction times and increase in correct matches of images).
• Exploratory aim: To test the effect of t-PBM on resting state functional connectivity in the brains of individuals with DS. Hypothesis: t-PBM will improve functional connectivity within the Default Mode Network (DMN), Salience Network (SN), Central Executive Network (CEN), and Language Network after completion of all 18 t-PBM sessions (after 6 weeks from baseline, e.g., 2 or 3 days after the final t-PBM session) and at long-term follow-up (after 10 weeks from baseline, e.g., 4 weeks after the final t-PBM session).

3. Study Design

This is an interventional, parallel-group, two-arm, double-blind (participant, investigator), sham-controlled randomized clinical trial to test the biological effects of t-PBM with NIR in people with DS. We intended to enroll 23 subjects diagnosed with DS with the aim of 16 completers. Medications, augmentative devices, and other interventions were recorded at enrollment and throughout the study.

3.1. Inclusion Criteria

Inclusion criteria include 1. individuals between the ages of 16 and 35 and 2. diagnosis of DS (i.e., clinical diagnosis of Trisomy 21 or presumed Complete Unbalanced Translocation of Chromosome 21).

3.2. Exclusion Criteria

Exclusion criteria include: 1. diagnosis of seizure disorder, 2. diagnosis of dementia, 3. inability to complete study procedures, 4. English as a second language, 5. speech as the secondary mode of communication, 6. speech of less than two-word utterances, 7. changes in medications, augmentative devices, and other intervention two weeks before baseline testing and throughout study completion at Principal Investigator’s (PI) discretion, 8. untreated obstructive sleep apnea (OSA), 9. participation in other clinical research trials that may influence primary outcomes or adherence to the proposed study, as assessed by the PI and the co-investigators (Co-Is), 10. candidates who have a current diagnosis of cancer and/or are currently undergoing treatment for cancer (in full remission is not exclusionary), 11. history of migraine with an aura in the past six months, and 12. current pregnancy. Additionally, all measures will be taken to avoid exposing the thyroid region of the neck, areas with open wounds, or areas that may result in direct irradiation to the eye.

As the MRI is an exploratory aim, subjects with contraindications to MRI will be excluded from MRI but continue to be involved in the study. Additionally, any subject who is unwilling to undergo an MRI can opt out of this procedure and still participate in the study.

3.3. Source of Subjects

The study will take place at MGH and participants will be recruited from the surrounding communities (Greater Boston and New England areas). Based on the composition of the patient population at MGH and Boston (Boston, MA, USA), we anticipate that at least 50% of the participants will be women. The percentage of minority participants is expected to be at least 20%. We are only enrolling English-speaking participants in this study since the measures used are developed and validated in the English language and because non-English-speaking subjects may not be able to understand study procedures, which could undermine the validity of the results.

4. Study Procedures

The timeline for the study procedures is summarized in Figure 1. Further details about each study step are described in the following subsections.

4.1. Study Visits

• Pre-screen (week -1): Prior to enrolling in the study, interested individuals will be called by trained study staff for a pre-screening or sent a link to an electronic version of the eligibility survey. The trained study staff will review study details and procedures and will ask a series of questions used to assess eligibility for the study. If the individual is eligible for the study, they will be invited to a screening visit. Screening (week zero): Subjects will review the consent form with the study clinician (PI or Co-I; MDs) and be given the opportunity to ask questions. Adult subjects will then complete an Informed Consent Survey (ICS) that assesses their understanding of the study procedures. The study clinician will evaluate each adult subject’s capacity for consent. If the subject is deemed unable to provide their consent, surrogate consent may be obtained from a parent, legal guardian, health care proxy, family or spouse involved in their care, or person with durable power of attorney. Subjects will be asked a series of questions to assess eligibility for the study, including demographic information and concomitant medications and therapies. A medical history will be taken. The subject may also try on the MedX 1100 and/or EEG (without powering the devices) if there is concern that the subject may not be able to tolerate these devices, as some individuals with DS are particularly sensitive to wearing items on their heads. If the subject is eligible, they will be invited to continue to the baseline visit.
• Baseline visit (week 1): Subjects will complete a neuropsychological test battery consisting of the Kaufman Brief Intelligence Test-II (KBIT-2), Language (Semantic Verbal Fluency and Picture Naming), Memory (List Learning, List Recall, List Recognition), and Attention (Forward Digit Span and Coding) subtests of the Repeatable Battery for the Assessment of Neuropsychological Status (RBANS), the Goldman–Fristoe Test of Articulation (GFTA), the Reaction Time (RTI), Paired Associates Learning (PAL), and Motor Screening Task (MOT) subtests of the Cambridge Neuropsychological Test Automated Battery (CANTAB), and a Standardized Narrative Task (i.e., Wordless Picture Book). Subjects will also complete the Obstructive Sleep Apnea-18 (OSA-18). The NIS-Skin Color Scale and Clinical Global Impressions—Severity (CGI-S) Scale will be administered, since skin color is important for studies involving light. Additionally, subjects will undergo resting state EEG to measure gamma neuronal oscillations. Subjects will also have their vital signs recorded and undergo an optional resting state functional connectivity magnetic resonance imaging (rs-fMRI) scan. Caregivers will complete the Cognitive Scale for Down Syndrome (CS-DS), the PedsQL Young Adult Quality of Life Inventory (Acute Version), the Burden Scale for Family Caregivers—Short Version, and the Reiss Screen for Maladaptive Behavior (RSMB).
• Subjects will then be randomized (1:1) to t-PBM-NIR or t-PBM-Sham. t-PBM-NIR will be administered using MedX 1100, with five LED cluster heads for simultaneous treatment locations (i.e., two midline placements and three left side placements over cortical language areas). For subjects with head sizes too small to accommodate five diodes, one of the left side placements will be omitted. The omitted diode is the one over the left angular gyrus. For people with smaller heads, four diodes cover the same approximate area as five diodes on a larger head. The t-PBM-Sham device is identical to the t-PBM-NIR device except it does not deliver NIR.
• Treatment visits (weeks 2–7): Subjects will undergo three treatments per week for a continuous six-week period, preferably with 48 h between each treatment. Subjects will complete a total of 18 treatment sessions. The Perceived Blinding Questionnaire (PBQ) will be completed at treatments 9 and 18. Study staff will complete an intervention tracking form at each treatment visit. The Clinical Global Impressions—Severity and Improvement Scales (CGI-S) and (CGI-I) will be administered once per week during the treatment phase. A “week” is defined as three treatment visits. Short-term follow-up study visit (week 8): The short-term follow-up study visit takes place within approximately one week of the final treatment session. Subjects will complete a neuropsychological test battery consisting of the KBIT-2, the Language, Memory, and Attention subtests of the RBANS, GFTA, the RTI, PAL, and MOT subtests of the CANTAB, and a Standardized Narrative Task (i.e., Wordless Picture Book). Subjects will also complete the OSA-18. Additionally, subjects will undergo resting state EEG to measure gamma neuronal oscillations. Subjects will also have their vital signs recorded and undergo an optional rs-fMRI scan. Caregivers will complete the CS-DS, the PedsQL Young Adult Quality of Life Inventory (Acute Version), and the Burden Scale for Family Caregivers—Short Version. The CGI-S and CGI-I will be administered. Participants who opted in to the MRI portion of the study will undergo an rs-fMRI scan.
• Long-term follow-up study visit (week 12): The long-term follow-up study visit takes place approximately 4 weeks after the final treatment session. The subject and caregivers will repeat the exact same neuropsychological testing, imaging, and surveys that were completed at the short-term follow-up. Participants who opted in to the MRI portion of the study will undergo an rs-fMRI scan.
• Optional Open-Label (OL) treatment: Participants who did not show improvement in language from the study (as determined by the PI and Co-I’s) will be offered twice weekly in-office treatments for up to 6 weeks at no cost to the participant or their family. At the conclusion of OL treatment, caregivers will complete the CS-DS, the PedsQL Young Adult Quality of Life Inventory (Acute Version), and the Burden Scale for Family Caregivers—Short Version. The t-PBM device used in the OL phase is the LightForce^® EXPi Deep Tissue Laser Therapy^TM System, Transcranial PhotoBioModulation-1000 (tPBM-2.0), which is described in further detail in the next section. The OL is used as an incentive for participants who may be reluctant to participate due to the possibility of being assigned a sham treatment. The OL provides them guaranteed, active t-PBM, assuming they complete all study procedures and do not show significant improvement in language. The parameters are based on existing parameters used in other t-PBM studies.

4.2. t-PBM Administration

The t-PBM-NIR device used in the study is the MedX 1100 with three LED cluster heads. The device will have the visible red diodes disabled and will have both continuous and pulsing options with a duty cycle of 50% (not the default 80%). The t-PBM-sham device is identical except the diodes are disabled and will produce mild warmth to mimic the sensation of the t-PBM-NIR device. The treatment program (using the parameters outlined below) will be programmed into the device. The console displays the time and energy delivered for each port separately and automatically turns itself off once the treatment is completed.

The randomization (1:1) to NIR or sham involves three applications per week for 6 weeks, with at least 48 h between the treatments, with the MedX, Model 1100, with three LED cluster heads for simultaneous treatment locations—e.g., two mesial (midline) placements; and two to three left side of head placements over cortical language areas, with parameters corresponding to adequate t-PBM exposure in adults. The sham device is identical, except it delivers no NIR. See

Table 1. t-PBM parameters.

Measure (Unit)	Value
Wavelength (nm)	870, NIR
Pulse rate (Hz)	40
Duty cycle (%)	50
Exposure time	30 min, for each site (5 sites), delivered simultaneously NOTE: without pulsing, it requires 22.5 s to deliver 1 J/cm2; with a 50% duty cycle it requires twice as much time: 40 J/cm2 × 22.5-s × 2 = 1800 s or 30 min at each site.
Window, spot size (cm2)	22.48 cm2 × each site (diameter 5.345 cm)
Total area exposed (cm2)	~112 cm2 (22.48 × 5 sites)
Peak Irradiance, Peak Power Density (mW/cm2)	44.48
Fluence (J/cm2) per LED cluster head	40 NOTE: The study clinician will have the option to taper participants up from 20 J/cm2 to 40 J/cm2 over the first few treatments to allow participants time to acclimate to t-PBM. The clinician may continue treatment at 20 J/cm2 or 30 J/cm2 if a participant cannot tolerate 40 J/cm2.
Energy Density (J/cm2) per LED cluster head	40 J/cm2 × 22.48 cm2 = 900 J per cluster head
Total Energy per session (kJ)	~3.6–4.5
Power per LED cluster head (mW)	1000
Device	MedX Health, Console Model 1100 with three LED cluster heads that can be used simultaneously

Abbreviations: nm = nanometers; Hz = Hertz; NIR = near-infrared; cm² = square centimeter; mW = milliwatt; mW/cm² = milliwatt per square centimeter; J/cm² = Joule per square centimeter; LED = light-emitting diode.

for a detailed report of treatment parameters to be used in the study and

Table 2. t-PBM device placements.

Target Brain Area	Diode Placement
Area 1: Mesial prefrontal cortex	At the center, front forehead—at the junction with front hairline
Area 2: Precuneus cortex	Junction of midsagittal and lambdoid suture lines
Area 3: Broca Speech area	Left temple
Area 4: Wernicke area/left temporal cortex	Above left ear (T3)
Area 5: Left angular gyrus area (BA39) NOTE: This diode will be omitted if the participant’s head is too small to accommodate all five diodes.	Posterior and superior to T3

Abbreviations: BA39 = Brodmann area 39; T3 = temporal lobe 3.

for specific placement on the head of the LED probes [39], which are also shown in Figure 2.

For the optional OL treatment, we will use an investigational device based on LiteCure’s LightForce^® EXPi Deep Tissue Laser Therapy^TM System. The device is manufactured and supplied by LiteCure LLC, 101 Lukens Dr, Suite A, New Castle, DE 18720. The t-PBM-2.0 is considered a Class II medical device per 21 CFR 890.5500 and 878.4810 and is manufactured per 21 CFR 820. It utilizes a laser diode source with a maximum continuous (CW) output of ≤30 W at a wavelength of 808 nm and a nominal beam diameter of 40 mm at the outside aperture. This device uses a laser source to apply NIR light through two diodes, located at F3 and F4. Each diode has a 12 cm² irradiation area (for a total area of 24 cm²). Participants will be treated using the following parameters: an irradiation close to 300 mW/cm² (3.5 W per diode) with a wavelength of 808 nm, irradiated continuously for 330 s. The first of the twelve OL sessions will be given at 1.5 W to allow the participant to accommodate the treatment. The dosage can be adjusted based on clinician discretion, with an acceptable treatment range of 1.0 W to 3.5 W. The application of NIR laser light is relatively safe. Based on human clinical trial experience to date, the following adverse events have been reported with t-PBM as local reactions at the application site: erythema, mild local pain, application site discomfort, and headaches.

4.3. Assessments

• Kaufman Brief Intelligence Test-II (KBIT-2): The KBIT-2 is a brief, individually administered assessment of verbal and nonverbal intelligence that produces a verbal, non-verbal, and composite IQ score [69].
• Repeatable Battery for the Assessment of Neuropsychological Status (RBANS): The RBANS is a brief, individually administered battery to measure cognitive function across several domains including language, attention, visuospatial/constructional, immediate memory, and delayed memory. The current study will use subtests that assess language, memory, and attention sections. Two expressive language subtests compose the Language Index: Picture Naming and Semantic Fluency. These subtests assess an individual’s ability to name pictures and to retrieve and produce express words when given a semantic in a timed format. Low scores on these subtests indicate impaired expressive language skills. Two subtests compose the Attention Index: Digit Span and Coding. These subtests assess an individual’s ability to attend to and repeat a series of digits forward and to focus and quickly draw simple designs that are associated with a specific number. Low scores on these subtests indicate impairments in auditory attention and brief focused visual attention and processing speed. Two verbal memory subtests compose the Immediate Memory Index: List Learning and Story Memory. These subtests assess an individual’s ability to learn and recall unrelated information presented within a brief word list and related information presented in a story consisting of two sentences in length. For this study, only the List Learning subtest will be completed. Subjects will also complete List Recall and List Recognition from the Delayed Memory Index. Low scores on these subtests indicate significant difficulty use same terminology with learning, retaining, and recalling verbal information [70].
• Goldman–Fristoe Test of Articulation (GFTA): The GFTA is a brief, individually administered assessment of speech sound abilities in articulation [71]. We will use raw data because it is administered in a modified manner, not scored or administered according to the standardized procedures outlined in the published manual. This modification allows for the analysis of articulation without comparison to normative standards.
• Reiss Screen for Maladaptive Behavior (RSMB): The RSMB is a brief, individually administered assessment of maladaptive behaviors such as aggression, destruction, and attention-seeking in individuals with intellectual disabilities. Maladaptive behaviors are rated as not a problem, problem, or major problem. The measure is used to screen for the type and severity of psychiatric comorbidities [72].
• Cambridge Neuropsychological Test Automated Battery (CANTAB): The CANTAB is a computerized measure of cognitive function across several domains including working memory, learning and executive function, visual, verbal, and episodic memory, attention, information processing, reaction time, social and emotion recognition, and decision making. The current study will only use the Reaction Time, Paired Associates Learning, and Motor Screening subtests. The Reaction Time subtest measures movement time, reaction time, response accuracy, and impulsivity. The Paired Associates Learning subtest assesses visual memory and new learning. The Motor Screening subtest measures sensorimotor deficits or lack of comprehension of the task and will be used to validate data collected from subjects [73,74].
• Expressive Sampling (ELS)—Narration: In this task, a sample of the participant’s spoken language is collected as they produce a narrative for a wordless pisture book they are shown by an examiner. The examiner follows a script and provides minimal scaffolding so that the result reflects what the subject can say independently. Participants are asked to look at the wordless picture book without talking to get a sense of the story as the examiner guides the participant through the exercise. The participant is then shown the wordless picture book a second time and asked to tell the story. Outcome measures derived from this task have been shown to be feasible and have excellent psyhcometric properties and be associated with meaningful adaptive skills for individuals with DS. Examiners are trained to achieve predetermined levels of fidelity with the administration following well-established procedures by the investigative team.

A digital audio recorder is used to record the participant’s storytelling. The digital audio recordings are transcribed and analyzed using Systematic Analysis of Language Transcripts (SALT), which entails creating test files in which the orthographic transcription is prepared following standard conventions in child language research regarding segmentation and coding of linguistic elements. For example, the talk is segmented into C-units (Communication-units), which can range from a single word to an independent clause and its modifiers. SALT also allows automatic computation of a range of variables. Transcription involves a consensus process and two transcribers to ensure a high degree of accuracy and good agreement among independent transcribing teams. Training procedures for transcribers have been manualized. All transcription and coding are performed at UC Davis, Sacramento, CA, USA. The measures computed for this study have all been shown to have strong test–retest reliability, minimal practice effects on repeated administrations and good construct validity. The measures computed are as follows: 1. Lexical Diversity, which reflects the size of the participant’s expressive vocabulary and is defined as the number of different word roots in 50 complete and fully intelligible C-units (or the full sample of complete and fully intelligible C-units if the participant produces fewer than 50 C-units). Higher scores are indicative of more advanced expressive vocabulary. 2. Syntax, which reflects expressive syntactic complexity and is computed as the mean length of C-unit measured in morphemes (MLU) for complete and fully intelligible C-units. Higher scores are indicative of more advanced expressive syntax. 3. Unintelligibility, which indexes speech articulation problems and is computed as the proportion of the total C-units that are partly or fully unintelligible to the transcriber. Higher scores indicate more problems with articulation.

• Obstructive Sleep Apnea-18 (OSA-18): The OSA-18 is a brief, individually administered quality-of-life instrument given to caregivers to assess how obstructive sleep apnea affects the subject’s life in the areas of sleep disturbance, physical symptoms, emotional distress, daytime function, and caregiver concerns [75].
• New Immigrant Survey—Skin Color Scale (NIS-SCS): The scale is an 11-point scale, ranging from zero to 10, with zero representing albinism, or the total absence of color, and 10 representing the darkest possible skin. The ten shades of skin color corresponding to points 1 to 10 on the Massey and Martin Skin Color Scale are depicted in a chart, with each point represented by a hand, of identical form, but differing in color. The respondent never sees the chart. We gather this data for use as a potential covariate in analyses as skin color can affect the absorption of NIR.
• The Perceptions of Blinding Questionnaire (PBQ): The PBQ is a self-report questionnaire to determine the degree to which the participant believes they are receiving the treatment or the sham. Due to the needs of the study population, questions will be asked by the study staff and answered by the participants instead of using the written form.
• Cognitive Scale for Down syndrome (CS-DS): The CS-DS is a written 62-item informant-rated scale that collects information on executive function, memory, and language abilities in adults with DS. Sixty-one items are scored from 0 to 2 and the final question is open-ended. This scale will provide important information regarding the subject’s everyday functioning over the past one to two months.
• PedsQL Young Adult Quality of Life Inventory (Acute Version): This 23-item informant-rated scale assesses physical, emotional, social, and work-related functioning in young adults with Down syndrome. Items are rated on a scale of 0 (“Never”) to 4 (“Almost Always”). The period is the past seven days.
• Clinical Global Impressions—Severity and Improvement (CGI-S, CGI-I): These two instruments are scored 1-7 by the clinician based on the assessment of the subject’s overall clinical status. They measure, based on history and scores on other instruments, (a) severity of cognitive impairment (CGI-S) and (b) clinical improvement (CGI-I).
• Burden Scale for Family Caregivers—Short Version (BSFC-s): This 10-item caregiver-rated scale assesses the subjective burden of participating in the study placed upon family caregivers. Items are rated on a scale of 1 (“Strongly Disagree”) to 3 (“Strongly Agree”); higher scores indicate higher subjective burden.

4.4. EEG Testing

This study will primarily use the Neuroelectrics^® StarStim 32-Channel EEG. If this device cannot be used, the StatX24 Advanced Brain Monitoring^® 20-channel EEG will be used in its place. The Neuroelectrics StarStim 32 device is a battery-powered wearable 32-channel tES stimulator and EEG monitor intended for simultaneous tES stimulation and EEG collection. For the current study, we will only be utilizing the device to collect EEG. The StarStim 32 provides an efficient and effective approach for wireless acquisition and recording of EEG signals. The system utilizes a neoprene headcap with 39 potential EEG placeholder locations and screwable-hybrid tES/EEG sensors which can record high-quality EEG. The full setup can be obtained with less than fifteen min of set-up time and without scalp abrasion. The StarStim 32 acquires 32 channels of monopolar EEG recordings with a linked reference to the right mastoid. The StarStim 32 set-up consists of the following: 1. a neoprene headcap with 39 potential EEG placeholder locations, 2. the tES/EEG device itself, which can transmit data both wired and wirelessly to a linked PC workstation, and 3. a set of 32 screwable-hybrid tES/EEG sensors that record the EEG in real-time. We will be utilizing the following EEG locations in the current study: FP1, FP2, AF3, AF4, Fz, F3, F4, F7, F8, FC1, FC2, FC5, FC6, T7, T8, Cz, C3, C4, CP1, CP2, CP5, CP6, Pz, P3, P4, P7, P8, PO3, PO4, Oz, O1, and O2. The placement of EEG electrodes follows the International system, which labels them based on the areas of the cerebral cortex they cover. These labels correspond to the lobe or specific area of the brain being recorded: fronto-polar (FP), frontal (F), central (C), temporal (T), parietal (P), and occipital (O). Regarding their lateralized location, odd numbers (1, 3, 5, 7) refer to electrodes placed on the left hemisphere, and even numbers (2, 4, 6, 8) refer to those on the right hemisphere. Electrodes over the midline (zero line) are labeled with the letter “z”. The StarStim 32 device collects signals from the screwable hybrid sensors placed on the patient and performs analog-to-digital conversion, encoding, formatting, and transmitting of all signals. Communication between the StarStim device and the study-specific PC is made either via a direct USB connection or via Bluetooth connection.

The participant will be fitted with an EEG sensor headset to start their resting state data acquisition. For resting state acquisition with eyes open, the participant will be asked to relax, sit still, and focus on a cross for 5 min while EEG data is acquired. The participant will be instructed to try to limit blinking and any face movement including clenching their jaw. Several acquisitions and/or longer acquisitions may be necessary if the participant is particularly restless. The participant will then be asked to relax and close their eyes for 5 min during the eyes-closed resting state acquisition. Multiple and/or longer eyes-closed acquisitions may be necessary if the participant has difficulty or is unable to sit still for 5 min. Any issues, including the inability to remain still for the acquisition, will be noted.

4.5. MRI Scanning

The collection of MRI images will take place in a 3T magnet scanner with multi-channel receivers. Participants will undergo a conventional high-resolution 3D T1-weighted MPRAGE scan, a resting state functional connectivity MRI sequence, and a diffusion tensor imaging (DTI) sequence if possible. We will examine areas of language function like primary and secondary visual cortices, Frontal Eye Fields (FEF), Intraparietal Sulcus (IPS), Wernicke’s Area, and mid-temporal areas. We will also examine network-specific nodes like the Medial Prefrontal Cortex (mPFC), Posterior Cingulate Cortex (PCC), Angular Gyrus (AG) Lateral Parietal Cortex (LPC), from the DMN; Dorsal Anterior Cingulate Cortex (dACC), Anterior Insula (AI), and Supplementary Motor Area (SMA), from the Salience Network (SN); and Dorsolateral Prefrontal Cortex (dlPFC) and Posterior Cingulate Gyrus (PCG) from the Central Executive Network (CEN).

4.6. Data Management

A secure comprehensive web-based data acquisition and management system, Research Electronic Data Capture (REDCap), was programmed by the researchers to process, edit, and store study data in a centralized database [76,77]. All data collected throughout the course of the study are to be directly documented on REDCap at the time of acquisition or stored in a primary secure repository as the resource document before subsequent transfer to REDCap. Each study subject will be allocated an individual file within the system. This file will encompass information gathered during pre-screening, screening, 18 treatment visits, and follow-ups. Demographics, medical history, adverse events, and outcomes will be uploaded, monitored, and validated by designated and trained study staff members. The final data clean-up process is scheduled to be concluded shortly after the last data collection time point. The ultimate locked study database will then be provided to the study statistician for subsequent analyses.

4.7. Statistical Analyses

• Changes in EEG gamma power (40 Hz): EEG will be tested from baseline to after completion of all 18 t-PBM sessions (after 6 weeks from baseline, e.g., within a week after the final t-PBM session); and at long-term follow-up (after 10 weeks from baseline, e.g., 4 weeks after the final tPBM session).
• Changes in intelligibility, vocabulary, and syntax: Language testing will be performed at baseline, after completion of all 18 t-PBM sessions (after 6 weeks from baseline, e.g., within a week after the last t-PBM session) and at long-term follow-up (after 10 weeks from baseline), through standardized tasks and semi-computerized signal detection.
• Changes in RTI, PAL, and MOT: The RTI, PAL, and MOT tests will be administered at baseline, after completion of all 18 t-PBM sessions (after 6 weeks from baseline, e.g., within a week after the last t-PBM session) and at long-term follow-up (after 10 weeks from baseline), through computerized CANTAB [78].
• Qualitative and quantitative physician—and caregiver—report: Progress, tolerability, and safety will be assessed throughout the study. Of note, the same assessment by a caregiver will be repeated after 4 weeks from the end of all 18 t-PBM sessions (after 10 weeks from baseline).

To test our Aim 1, we will compare groups for change in EEG gamma power (40 Hz) before and after all 18 t-PBM sessions (from baseline to after 6 weeks), and for long-term effects (after 10 weeks from baseline), for specific effects on brain oscillations (independent t-test). Alpha and beta EEG power will also be collected and analyzed. To test Aim 2, we will perform similar comparisons for the effects on language before and after all 18 t-PBM sessions (from baseline to after 6 weeks) and for long-term effects (after 10 weeks from baseline), except that the dependent variables will be intelligibility, vocabulary, syntax, and grammar (independent t-test). To test Aim 3, we will perform similar comparisons as in Aim 2 (including time points) except that the dependent variables will be the reaction times as well as the memory retrieval latency and number of correct trials at the delayed match to sample (independent t-test). To test Aim 4, we will use linear regression models with EEG gamma power and language-cognitive outcomes as the independent and dependent variables, respectively.

With a sample size of n = 16 individuals with DS (completers), this study will only have sufficient power (80%) to detect as significant (p < 0.05) an effect size of 1.5; however, it will allow us to detect the magnitude of the effect size and to plan for adequately powered studies in the future. As an exploratory measure, a resting state functional-connectivity MRI (rs-afcMRI) scan will be performed at baseline and after completion of all 18 t-PBM sessions (after 6 weeks from baseline, e.g., within a week after the last t-PBM session) and for long-term effects (after 10 weeks from baseline). rs-fcMRI will be used to study the DMN, as well as the SN, CEN, and Language Network, before and after repeated t-PBM sessions. Prior literature in different patient populations supports the hypothesis of greater brain connectivity underlying clinical improvements with t-PBM. In addition to rs-fcMRI sequences, the team will strive to acquire DTI for all study subjects at baseline and after 6 weeks.

4.8. Demographic Data on Existing Participants

As of 14 March 2024, we have enrolled 18 subjects, of whom 14 were randomized and 13 have completed the study. From the participants enrolled, the median age was 22.5 years (range: 17–32), with 67% self-identifying as female and 33% as male. The majority self-identified as White (78%), 6% as Asian, and 16% as having more than one race (Asian/White, Native American/Alaskan Native/White, and African American/White). At the time, all participants stated they had never been married. All subjects completed at least ten years of education, with a mean of 13 years (standard deviation: 2). Employment status varied, including full-time students (22%), part-time employment (17%), combined full-time study and part-time employment (28%), and unemployment (33%). With regard to handedness, most of the subjects were right-handed (50%), while 44% were left-handed, and one participant was ambidextrous (6%).

4.9. Risks and Discomforts

The t-PBM device emits light with a longer wavelength than the human eye can see. The staff will be provided training on basic safety procedures relative to the use of the device. The staff administering the t-PBM will be careful not to operate the LED unless it is in direct contact with the subject’s skin. Protective eyewear is not required since the device is an LED. Failure of the LED device, resulting in the cessation of investigative therapy can cause no adverse event to our knowledge. Delivery of the NIR-LED energy to an inappropriate site, such as directly over the open eye, would not pose a risk to the subject, given the use of divergent rays of light. Based on previous observations with similar LED devices, the application of the LEDs may result in a mild thermal sensation of warmth during use. The temperature of the skin is, however, kept well below the level for thermal damage. Based on human clinical trial experience to date and on sale of t-PBM devices for their intended use (e.g., Omnilux New-U), each adverse event listed below has been reported by less than 0.1% of all subjects and users: erythema, pain, discomfort, warmth, headache, or other reactions at the application site.

5. Discussion

Previous studies have provided evidence that t-PBM with NIR light is a safe, cost-effective, and user-friendly therapeutic approach that can be self-administered (for review see Salehpour et al., 2018) [38]. These studies have reported encouraging preliminary results in the treatment of neuropsychiatric disorders, demonstrating improvements in mood, memory, and cognition. Building upon these findings, we have developed a hypothesis that t-PBM may have the potential to enhance language abilities in individuals with DS when applied to specific targeted brain areas. The basis for this hypothesis lies in t-PBM’s capacity to engage with mitochondrial metabolism, influencing neural activity, particularly manifested in EEG gamma and beta wave patterns.

As mentioned earlier in preliminary data, Mannu et al. (2019) [68] reported that t-PBM sessions were well-tolerated, and no adverse experiences were reported by either the children with DS or their parents. Within weeks of beginning t-PBM, improvements in behavior were observed by physicians, parents, and caregivers. While quantitative measures were not implemented, the following qualitative improvements were noted in that study: 1. Caregivers reported enhanced dexterity, which was corroborated by more precise copying in the spiral drawing test and more detailed portrait drawings; 2. Improved verbal fluency, characterized by speaking in full sentences, was observed alongside increased attention, indicating both motor and cognitive effects of NIR light; 3. Positive changes in mood and behavior were evident, with reduced emotional volatility, less crying, agitation, or physical outbursts; 4. Parents reported increased engagement in leisure activities, such as watching TV, and observed that their children required less supervision. They described their children as calmer, less hyperactive, and more focused on current activities; 5. Parents also noted a reduced frequency of seasonal upper respiratory infections. Overall, these benefits are a key motivation for this study, with a particular emphasis on language improvement. Language is one of the most affected areas of functioning in DS and may also be the primary obstacle to independent living and meaningful community inclusion [4,79,80]. In a positron emission tomography analysis by Horwitz et al. (1990) [81], it was found that individuals with DS have a smaller left-hemisphere inferior frontal gyrus, primarily encompassing Broca’s area, compared to controls. Additionally, the authors noted that in DS, the function of Broca’s area is improperly linked with functions in other frontal and parietal regions, exacerbating the language impairments seen in this disorder. These alterations are evident as shifts in oscillatory patterns have been observed in EEG recordings of individuals with DS. Non-invasive brain stimulation techniques could help alleviate these changes, and we consider t-PBM to be a promising option.

Spera et al. (2021) [67] conducted a study demonstrating that t-PBM, delivered through a device comprising four LED clusters, significantly boosted high-frequency neural activity in the gamma and beta bands. This enhancement, believed to support advanced cognitive functions, occurred after just one 20 min exposure targeting the forebrain. During resting-state scans with participants’ eyes either open or closed, increased neural activity was observed across broad frontal-temporal areas of the scalp, reflecting typical activity patterns within these frequency bands. In the context of the 2-back working memory task, the elevated neural activity in the gamma and beta bands was statistically significant. Notably, this heightened activity was primarily concentrated in the frontal regions, attributable to the involvement of the PFC in this task [82].

In a recent study involving older adults at risk of cognitive decline, t-PBM was observed to increase gamma EEG power, with a comparatively smaller rise in beta power. These changes were specifically noted bilaterally over the temporal scalp regions. The t-PBM regimen used in the study employed a CW modality at 1064 nm, with an irradiance of 250 mW/cm², a fluence of 137.5 J/cm², and a single application covering an area of 13.6 cm². Following t-PBM, there was a sustained increase in power enhancement within the gamma frequency band [83]. In another study by Wang et al. (2019) [84], t-PBM (CW, 1064 nm, 9.72 J/cm² per min, and 106.94 J/cm² over 11 min) was applied to the right forehead of healthy participants, resulting in increased neural activity within high-frequency bands (alpha and beta), indicating an enhancement effect. Similar results were observed in a recent randomized, sham-controlled, double-blind study by Zomorrodi et al. (2019) [85]. After a single session of pulsed (40 Hz) t-PBM at 810 nm wavelength during resting state, there was a significant increase in alpha, beta, and gamma power, alongside a reduction in delta and theta power. Both sham and active modes led to increased power across all frequency bands post-stimulation compared to pre-stimulation. However, active t-PBM specifically facilitated a greater increase in alpha, beta, and gamma oscillations compared to sham mode.

The ability of t-PBM to impact gamma-band neural activity in the human brain may have significant clinical implications in DS. Gamma activity has been associated with performance in complex and attention-demanding tasks and has been implicated in supporting various sensory and cognitive processes, such as perceptual processing, object representations, visual awareness, and language [86,87,88,89,90,91,92,93]. Furthermore, it has been found that the presentation of a previously learned stimulus evokes a stronger neural response in the gamma band than that of a new stimulus, suggesting a core role gamma activity may play in memory mechanisms [89]. Remarkably, clinical states such as major depressive disorder, mild cognitive impairment, dementia due to Alzheimer’s disease, and DS may alter gamma band brain activity [94,95,96].

Consideration should be given to the proposed mechanism of action of t-PBM, which is responsible for enhancing fast brain oscillations. Uncertainty regarding photon deposition at the cortical level may be considered a limitation of our study, as most human studies on t-PBM do not evaluate the anticipated NIR penetration based on study parameters, sample characteristics, light source position, and target area. Henderson and Morries (2019) [97] extensively reviewed literature on light penetration through human living tissues and cadaver models and concluded that there are scenarios where the expected photon deposition in the brain is minimal. They suggested that in such cases, observed neurophysiological or therapeutic effects of t-PBM are likely related to systemic effects of NIR, such as anti-inflammatory and antioxidant effects. Our group has experience with the therapeutic effects of systemic t-PBM on brain disorders [98]. We have speculated that, in addition to direct effects on the brain associated with photon deposition, and indirect systemic effects likely associated with blood irradiation [99], there might be a third potential mechanism related to indirect but local effects. Although speculative, we suggest that t-PBM might induce very weak skin currents, potentially leading to changes in neurophysiology, even in the absence of actual NIR deposition onto the brain. The effects of very weak skin currents on brain oscillations applied to the forehead and their potential therapeutic effects are well-documented [100,101], including modalities such as transcranial direct current stimulation (tDCS), transcranial alternating current stimulation (tACS), and cranial electrotherapy stimulation (CES) [102].

While we anticipate a direct effect of NIR light on brain function associated with photon deposition onto the brain, we acknowledge that this might not be the case in TransPhom-DS study, and either an indirect systemic and/or an indirect local effect might solely account for anticipated changes in fast brain oscillations. To produce direct effects with t-PBM, higher doses than used in TransPhom-DS would be required. Sharma et al. (2011) [103] have previously reported that a fluence of 0.3 J/cm² (810 nm) at the target tissue was effective in modulating neuronal metabolism and mitochondrial function. Additionally, our group has shown that sufficient light deposition with NIR in CW mode is expected to achieve at least 0.3 J/cm² at the cortical level [104]. Furthermore, in a follow-up study, we demonstrated that transcranial NIR light penetration is significantly greater in young adults compared to middle-aged and older adults; notably, the majority of our study subjects were in their twenties [105]. However, considering the work of Henderson and Morries (2019) [97], penetration of NIR light through the scalp and skull can be as low as 1–2% of incident light, and even non-existent, depending on selected parameters. Interestingly, even when using laser sources and high power of NIR light, remarkable interindividual variability in light penetration exists [106].

6. Conclusions

The evidence presented in previous studies underscores the potential of t-PBM with NIR light as a promising therapeutic approach for various neuropsychiatric disorders. Encouraged by these findings, we hypothesize that t-PBM can enhance language abilities in individuals with DS by targeting specific brain areas. Preliminary data indicate that t-PBM is well-tolerated and can lead to observable behavioral improvements without adverse effects. Key observations made so far highlight the multifaceted benefits of t-PBM. Given the significant language impairments in DS, t-PBM’s ability to modulate neural activity within gamma and beta bands is particularly promising. These neural oscillations are crucial for cognitive functions, including language, memory, and attention.

Despite the potential, the exact mechanism of action remains uncertain, particularly regarding photon deposition at the cortical level. While direct brain effects are anticipated, systemic and local indirect effects might also contribute to the observed benefits. Further research is needed to elucidate these mechanisms and optimize t-PBM parameters to maximize therapeutic outcomes.

In conclusion, t-PBM offers a novel, non-invasive approach that could significantly improve language and cognitive functions in individuals with DS. Continued investigation into its mechanisms and effects will be crucial in advancing this therapeutic modality.