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OBM Neurobiology

(ISSN 2573-4407)

OBM Neurobiology is an international peer-reviewed Open Access journal published quarterly online by LIDSEN Publishing Inc. By design, the scope of OBM Neurobiology is broad, so as to reflect the multidisciplinary nature of the field of Neurobiology that interfaces biology with the fundamental and clinical neurosciences. As such, OBM Neurobiology embraces rigorous multidisciplinary investigations into the form and function of neurons and glia that make up the nervous system, either individually or in ensemble, in health or disease. OBM Neurobiology welcomes original contributions that employ a combination of molecular, cellular, systems and behavioral approaches to report novel neuroanatomical, neuropharmacological, neurophysiological and neurobehavioral findings related to the following aspects of the nervous system: Signal Transduction and Neurotransmission; Neural Circuits and Systems Neurobiology; Nervous System Development and Aging; Neurobiology of Nervous System Diseases (e.g., Developmental Brain Disorders; Neurodegenerative Disorders).

OBM Neurobiology publishes a variety of article types (Original Research, Review, Communication, Opinion, Comment, Conference Report, Technical Note, Book Review, etc.). Although the OBM Neurobiology Editorial Board encourages authors to be succinct, there is no restriction on the length of the papers. Authors should present their results in as much detail as possible, as reviewers are encouraged to emphasize scientific rigor and reproducibility.

Publication Speed (median values for papers published in 2023): Submission to First Decision: 7.5 weeks; Submission to Acceptance: 15.9 weeks; Acceptance to Publication: 7 days (1-2 days of FREE language polishing included)

Current Issue: 2024  Archive: 2023 2022 2021 2020 2019 2018 2017
Open Access Review

The Origin of Natural Neurostimulation: A Narrative Review of Noninvasive Brain Stimulation Techniques

Igor Val Danilov 1,2,*

  1. RTU Liepaja Academy, Liela iela, 14 Liepaja LV-3401, Latvia

  2. Academic Center for Coherent Intelligence, via Alberico II-11, Rome, Italy

* Correspondence: Igor Val Danilov

Academic Editor: Fady Alnajjar

Received: September 08, 2024 | Accepted: November 24, 2024 | Published: November 29, 2024

OBM Neurobiology 2024, Volume 8, Issue 4, doi:10.21926/obm.neurobiol.2404260

Recommended citation: Val Danilov I. The Origin of Natural Neurostimulation: A Narrative Review of Noninvasive Brain Stimulation Techniques. OBM Neurobiology 2024; 8(4): 260; doi:10.21926/obm.neurobiol.2404260.

© 2024 by the authors. This is an open access article distributed under the conditions of the Creative Commons by Attribution License, which permits unrestricted use, distribution, and reproduction in any medium or format, provided the original work is correctly cited.

Abstract

This narrative review of the literature on noninvasive brain stimulation techniques observes four neurostimulation domains: light therapy, photobiomodulation, a group of techniques within transcranial electric current and magnetic field stimulations, low-frequency sound stimulations, including vibroacoustic therapy, and rhythmic auditory stimulation. The review aims to determine whether or not different brain stimulation approaches rely upon a similar physicochemical sequence of actions. The study identifies relevant hypotheses about processes at the cellular level underlying noninvasive brain stimulation. The data analysis reveals that mitochondria activity will likely play a central role in brain stimulations implemented by different approaches. Additionally, the mother-fetus neurocognitive model analysis gives insight into conditions of the natural neurostimulation of the fetal nervous system during pregnancy. Drawing on these findings, the article supposes the hypothesis of the origin of neurostimulation during gestation. The article presents the requisites of the nature-based brain stimulation technique, called Acoustic Photonic Intellectual Neurostimulation (APIN), derived from studying natural neurostimulation components.

Keywords

Neurostimulation; neuromodulation; acoustic; photonocs; bioengineering; mother-fetus neurocognitive model

1. Introduction

This narrative review of the literature on the noninvasive brain stimulation techniques observes four neurostimulation domains: light therapy (LT) or phototherapy; laser brain stimulation, so-called photobiomodulation (PBM); a group of methods within transcranial electric current and magnetic field stimulations; low-frequency sound stimulations including vibroacoustic therapy (VAT) and rhythmic auditory stimulation (RAS). Because these brain stimulation methods are noninvasive, capable of modulating neuronal plasticity independent of underlying diseases, and can be personalized, they have gained significant interest in the scientific community.

The narrative review is outlined as follows. The Introduction concisely presents each observed approach's definition, treatment scope, and technical characteristics. The 1.5 Subsection presents the research problem of the review. The review method is formulated in the Materials and Methods Section. The Results Section contains theoretical explanations of the therapeutic effect of different noninvasive brain stimulation techniques. This Section tries “cataloging” the relevant hypotheses from the whole topic of noninvasive brain stimulation. In the Discussion section, the current study revises the found hypotheses about the neurostimulation grounds to add their explanation with insight into the essential role of mitochondria in each. Finally, this Section highlights the physicochemical grounds of natural nervous system stimulation and the characteristics of the nature-based brain stimulation technique.

1.1 Light Therapy for Brain Stimulation

Light therapy (LT, also known as Phototherapy or Luxtherapy) refers to the body's exposure to direct sunlight or intensive artificial light at controlled wavelengths to treat various medical disorders, including those associated with the brain:

  • Depression. Systematic review studies showed that treatment with LT significantly reduced the rigor of depression [1,2,3,4]).
  • Chronic pain. Evidence supported the beneficial effect of LT on chronic pain management [1].
  • Post-traumatic stress disorder. The recent systematic review revealed promising results in treating PTSD symptoms [5]. 
  • Insomnia. A systematic review showed the effectiveness of sleep care in insomnia disorders [6].
  • Cognitive functions. Clinical data support the beneficial effects of LT on cognitive dysfunction [1].

Research also revealed that light therapy may reduce agitation in dementia patients [7,8]; bright LT is commonly used to regulate seasonal affective disorder and other circadian rhythm-related disorders [9].

1.1.1 Techniques

Concerning brain stimulation, light therapy uses various technologies to irradiate the body with light, such as UVB lamps that emit a spectrum of ultraviolet light with wavelengths ranging from 290–320 nm and a light box that exposes the human body to intense light in a range of 2500–10000 lux of light at a specified distance, with the controlled frequency in the band of 350-750 nm (usually the diapason of the visible light is 400-700 nm) [10].

In brain stimulation, light therapy functions through the visual system of two subsystems and skin irradiation. The image-forming vision pathway involves phototransduction through photoreceptors (rods and cones) responsible for image-forming vision. Another integrator of light information is photoreceptive ganglion cells, which act as rudimentary visual brightness detectors, e.g. [1,11]. Ganglion cells transduce light stimuli to multiple brain regions that regulate brain functions unrelated to image-forming vision, including the circadian rhythm [12]. Due to melanopsin (a light-sensitive protein), these cells are intrinsically photosensitive. Ganglion cells are integrated with rod and cone input in processing information regarding the light surroundings across a broad spectrum of wavelengths and light levels. The photoreceptive ganglion cells exhibit a spectral sensitivity curve of Gaussian distribution in the 420–590 nm range with a peak spectral sensitivity at about 480 nm.

1.1.2 Limitations and Side Effects

While light therapy is an efficient tool for treating disorders associated with the brain, it can cause side effects of varying severity, including pruritus, blistering, skin erythema, xerosis, altered pigmentation, photoaging, and photocarcinogenesis [13]. Review studies also highlighted that further research is needed to refine the light parameters to develop personalized therapeutics [6].

1.2 Photobiomodulation for Brain Stimulation

Photobiomodulation (PBM) or low-level light therapy refers to directional low-power and high-fluence monochromatic or quasimonochromatic light [14]. This technique is used in the treatment of various disorders; the list below shows the areas of application:

  • Cognitive functions. Laser transcranial PBM may improve the function of traumatically injured brains [14,15,16,17,18,19] and diseased or normal brains [20,21,22,23,24,25,26]. Clinical data showed improvements in cognition, quality of life, and clinical signs of neurodegenerative conditions, such as dementia, Alzheimer’s disease, and Parkinson’s disease [14,27,28,29,30,31,32,33,34,35].
  • Depression. Research studies reported reduced depression symptoms [14,32,33,34,35].
  • Chronic pain. A literature review revealed photobiomodulation benefits chronic pain [36,37].
  • Post-traumatic stress disorder. A review revealed significant results in PTSD symptom management [38]. 
  • Insomnia. Clinical research showed that intravascular laser irradiation of blood is a beneficial and feasible treatment for insomnia [39].

Research has shown that laser transcranial PBM may lead to vasodilation and increased cerebrovascular oxygenation [20,21,22,23,24,25,26] and exert a neuroinflammatory suppressive effect [26,27]. Photobiomodulation is also an efficient treatment in inflammation management [36]. Increased cerebral oxygenation significantly impacts cognitive tasks such as memory and attention [20].

1.2.1 Techniques

This neurostimulation approach exerts its function of treating brain tissue with light using lasers (the acronym of Light Amplification by Stimulated Emission of Radiation) or light-emitting diodes (LEDs) generally in the red to near-infrared wavelengths 600–1100 nm. The laser beam has a narrow spectral width (around 1 nm), emitting the coherent light of a single wavelength. The LED's light is non-coherent and non-monochromatic; it has a spectral width of up to 80 nm., which results in a more broad, Gaussian-like spectrum of emitted light [40].

Laser devices emit coherent light with various coherence lengths depending on the bandwidth of the specific laser. Laser-coherence lengths can vary from meters for the He-Ne laser to only a few mm for diode lasers [41]. Laser and light-emitting diodes are different techniques. They don’t work the same way. While laser diodes emit focused light with a narrow beam, LED light is dispersed and multidirectional. Lasers for photobiomodulation are presented within a range of beam size, power density, and wavelength characteristics that provide tissue penetration within the method's scope [41]. For example, a laser known as HD Laser™ CG5000 has a beam size of 45 mm, a power density range of up to 1.6 W/cm2 and a 1064 nm wavelength [41].

1.2.2 Limitations and Side Effects

A review reported that clinical studies are heterogeneous in populations and therapy settings, and most lack sufficient control [38]. The literature contains poor data on the side effects of this method. Research reported changes in patients' weight and systolic blood pressure, which were insignificant; only diastolic blood pressure increased and reached statistical significance, but not clinical one [42]. Another potential side effect should become a subject of long-term research. The target, frequency, and dose of the laser beam in transcranial PBM radiation are still questions that need to be defined [43]. In brain stimulation, PBM stimulation targets relatively large areas (relatively the neuronal scale) of poorly characterized brain tissue. One cannot be sure that the target, frequency, and intensity do not give an excessive irradiation dose to healthy neurons when treating brain tissues, the outcome of which would emerge later [43].

1.3 Transcranial Current and Magnetic Therapy for Brain Stimulation

This noninvasive brain stimulation approach applies electromagnetic properties of the brain activity to provide neurostimulation by non-invasive means. It encompasses a cohort of methods, such as Cranial Electrical Stimulation (CES), Electroconvulsive therapy (ECT), Transcranial Alternating Current Stimulation (tACS), Transcranial Direct Current Stimulation (tDCS), Transcranial random noise stimulation (tRNS), and Transcranial Magnetic Stimulation (tMS). In the literature, tDCS, tACS, and tRNS are usually noted as a group with the unique name Transcranial Electrical Stimulation (TES).

The growing body of literature shows the application of all these brain stimulation methods in treating similar brain disorders. The list below shows the areas of application (for further reading, the links present all the above techniques in the order listed):

Interestingly, the current study found controversy in reporting the results of treatment. On memory: While Brunyé and colleagues [63] reported that CES selectively increases performance on a recognition memory test, Michals and colleagues [69] did not. Pain relief: The experiments by Alwardat and colleagues 2020 did not support the clinical use of tDCS to reduce pain; meanwhile, the systematic review by Moshfeghinia and colleagues [53] highlighted the potential effect of tDCS in pain treatment. Some researchers, such as Saccenti and colleagues [57], Moo-Estrella and Pérez-Pichardo [44], Provencher and colleagues [61], and Ma and colleagues [62] investigated two or three neurostimulation methods within the umbrella of electric current and magnetic brain neurostimulation in one study. Research also showed other interesting results: a potential to treat epilepsy [70], smoking cessation using insula neurostimulation [71], and preliminary results in recovery from coma [72].

1.3.1 Techniques

  • Cranial electrotherapy stimulation (CES) delivering low-intensity (50 μA to 4 mA) electrical current at 0.5-100 Hz via electrodes attached to the earlobes, maxilla-occipital junction, mastoid processes, or temples [73].
  • Electroconvulsive therapy (ECT) apples 500–1000 mA. Most devices are already tuned with a default current of 800–900 mA. Some devices operate at lower currents (200-400 mA) [74]. There are at least three positions to place electrodes: Bitemporal (electrodes are placed in the left and right frontotemporal regions), Bifrontal (on frontal areas), Right (Left) unilateral (one electrode is placed 1 inch right to vertex and another on the right frontotemporal region) [74].
  • Transcranial Alternating Current Stimulation (tACS) uses a current amplitude (Peak-To-Peak) of 0.65-4 mA, a frequency range between 4.5 and 120 Hz, and electrode location depending on the scope region of treatment [75].
  • Transcranial Direct Current Stimulation (tDCS) generally applies a current intensity between 0.5 and 2 mA, although up to 4 mA has been tested in human studies [76,77].
  • Transcranial random noise stimulation (tRNS) uses an alternate current of 0.5 mA along with a random amplitude of up to 0.5 mA and frequency between 0.1 and 640 Hz [78].
  • Transcranial Magnetic Stimulation (TMS): The stimulation coils produce a magnetic field of 0.5-7.5 T with a pulse frequency in the case of repetitive tMS ranging from 0.5 Hz to 250 Hz [79]. High-frequency rTMS with a stimulus frequency greater than 1 Hz is supposed to provoke cortical firing, while a stimulus frequency less than 1 Hz is believed to inhibit it [80].

1.3.2 Limitations and Side Effects

A recent review [81] highlighted several shortcomings in the research data of these techniques: (i) sample sizes were small, often case reports or case series; (ii) outcome measures and reported benefits varied; (iii) the optimal healthy irradiation dose (intensity and duration of the interventions) was not defined; most studies did not personalize the dose; (iv) protocols were different, including different electrode sizes, shapes, and locations; [81]. The last two constraints raise an essential challenge of the inability to localize the effect of these neurostimulation methods on specific tissues in the relevant neural zones [73]. These techniques target a large area of inadequately characterized tissue, which, together with an undefined dose, can destroy healthy cells during treatment [73].

1.4 Low-Frequency Sound Brain Stimulations

Because music can be defined as a derivative of vibration and an organized sequence of sounds generally within operating frequency [82], the current review observes Music Therapy (MT), Vibroacoustic therapy (VAT), and Rhythmic auditory stimulation (RAS) within the frame of the single neurostimulation domain of low-frequency sound brain stimulations. Although they are different therapeutic methods, from the perspective of brain stimulation, they implement a similar tool: acoustic wave. They demonstrate effectiveness in various areas:

  • Depression. The literature shows significant reductions in anxiety, depression, and apathy that occurred after music therapy [83].
  • Cognitive functions [84,85,86,87,88].
  • Chronic pain. MT may provide a practical complementary approach to relieving pain [84,87,88,89].
  • Post-traumatic stress disorder. Empirical evidence shows that patients with post-traumatic stress disorder (PTSD) may benefit from MT [90]. 
  • Insomnia. This therapy may improve sleep quality in adults with insomnia symptoms [91].

1.4.1 Techniques

According to the American Music Therapy Association, MT uses music interventions with a 1-1.5 Hz rhythm frequency for personalized treatment [92].

Vibroacoustic therapy (VAT) combines low-frequency sound vibration in a range of 20-300 Hz, music listening of the music rhythm in a range of 0.6-1.5 Hz, and therapeutic interaction, generally used in rehabilitation [93,94].

Rhythmic auditory stimulation (RAS) uses the physiological effects of the rhythmic motor cueing in the tact of the rhythm in a range of 0.6-1.5 Hz [95,96].

1.4.2 Limitations and Side Effects

Therapies within the frame of the sound brain stimulation show heterogeneous results. Most studies that tested VAT's effects on depression, pain, and cognitive functions have significant limitations because of research design and small sample sizes, which reduce the validity of other studies' positive results [94]. A systematic review highlighted controversial results reported on the effects of MT on improving the overall aspects of cognition (such as memory, orientation, and registration), agitation, daily functioning, and the quality of life of patients living with dementia [85].

1.5 Research Problem

Currently, from the literature on noninvasive brain stimulation, it is still being determined how long the effects of all four brain stimulation approaches last. What should the duration of one neurostimulation session be, and how many sessions should there be? Are the benefits of all therapies long-term? The author believes that if there were a generally accepted theoretical explanation for the therapeutic effect of noninvasive brain stimulation, it would be easier to answer the above questions. The research question of the current study is even more ambitious: whether or not a unique natural physicochemical set of actions underlies different brain stimulation approaches.

2. Materials and Methods

The narrative review tried to map promising hypotheses about processes at the cellular level underlying noninvasive brain stimulation mechanisms. Several articles are aimed at analyzing the theoretical foundations of neurostimulation. At the same time, many attempts have been made, but only a few propose an idea about the comprehensive architecture of neurostimulation from an agent (molecule or cell) to the physiological effect. The author believes that a review of the processes at the cellular level underlying various techniques from a wide range of noninvasive brain stimulation methods contributes to understanding the more predictable development of artificial neurostimulation techniques.

The topic of this narrative review is excessive; it encompasses four neurostimulation domains. Because of the broad topic, this review implemented the heuristic research approach that enabled the incorporation of essential neurostimulation elements that otherwise would be difficult to highlight. Therefore, the current study is not a systematic review. It only attempts to highlight the main hypotheses of the topic. The review does not provide a comprehensive discussion of current evidence regarding the effects of these techniques on brain functions, their contraindications, and side effects. It does not aim to provide a comprehensive view of the therapy's clinical relevance and safety profile.

The searched databases were Scopus and Web of Science. First, the study searched through key words "theory of brain stimulation", "theory of neuromodulation", "theory of light therapy", "theory of PBM", "theory of transcranial electrical stimulation", "theory of transcranial magnetic stimulation", "theory of Vibration, Acoustic and Music stimulation", etc. A further refined search included terms like “neurostimulation and adenosine”, “neurostimulation and Ca2+ and Na+ channels”, “neurostimulation and mitochondria”, “neurostimulation and mechanoreceptors”, etc.

The distinguishing of studies was motivated by an attempt to identify hypotheses about processes at the cellular level that contribute to synaptic activity during various artificial neurostimulations. The inclusion criteria used for selecting studies were as follows. Articles were selected based on (1) whether they proposed a hypothesis about the comprehensive architecture of neurostimulation from an agent (molecule or cell) to the physiological effect and (2) whether their theoretical definitions were based on empirical data.

To avoid explaining technical details when considering the hypotheses in the Results section, this section also clarifies reasons from physics (see below) that enable uniting hypotheses on the nature of transcranial electric current and magnetic field stimulations for their analysis in only one subsection, 3.3. Electricity and magnetism are two interconnected entities. In electrical circuits, a moving charge creates electric and magnetic fields. A magnetic field causes the movement of an electric charge, which creates an electric current. Human tissues have very different permittivities, which indicates their interaction with the electric field. In contrast, the magnetic permeability of tissues, except for tiny inclusions, is the same as that of air [97]. The electric and magnetic fields show the difference in interactions with biological tissue, which appears in the human body's reduced distribution of electric currents. In contrast, human tissues do not change the magnetic field propagation. Both fields induce electric fields and currents, producing similar effects on tissues [97]. Only in electrostatics does a stationary point charge induce solely an electric field without a magnetic field. From this perspective, it is not too controversial to say that, at the cellular level, current and magnetic brain stimulation may have a joint explanation.

The essential principle of the transcranial electric current and magnetic field brain stimulation approach is that neurons are both generators and recipients of electromagnetic fields; neuronal oscillations show a dual behavior in brain networks. Specifically, neuronal oscillations are influenced by spiking inputs and, in turn, affect the timing of spike outputs [98]. This fact means that neurons interact with other neurons' oscillations and, therefore, they may change their oscillations by interacting with other electromagnetic fields. Artificial (outside the brain) electric and magnetic stimulations may alter brain activity. Because transcranial electric current and magnetic field stimulations likely adhere to the same physics principles, only one theory may explain underlying processes, and they can be united for analysis in only one subsection.

3. Results

The section presents hypotheses about processes at the cellular level underlying different noninvasive brain stimulation techniques.

3.1 The Nature of LT

3.1.1 Brain Stimulation through the Image-Forming Vision Pathways

While the literature shows that LT is widely applied to treat various medical disorders associated with the brain, the mechanism behind the image-forming vision pathways (IFVP) remains largely unknown. Cones detect color and spatial detail at light levels relevant to the daytime, and rods only detect spatial detail at light levels in twilight and darker (suppressed at daytime light levels). The activation of the two groups of photoreceptors during light irradiation has no theoretical explanation yet of brain stimulation. The current review did not identify relevant hypotheses for treating brain disorders through the image-forming vision pathways in the literature.

3.1.2 Brain Stimulation through the Photoreceptive Ganglion Cells Pathways

The literature review did not find a comprehensive theory of brain stimulation through the photoreceptive ganglion cell pathways (PGCP). However, a few hypotheses on this topic are included:

  • A small fraction of secondary neurons (ganglion cells–the rudimentary visual brightness detectors) in the retina send light information to the brain through the retinohypothalamic tract, i.e., from intrinsically photosensitive retinal ganglion cells to the Suprachiasmatic nucleus. During the light part of the day, the Suprachiasmatic nucleus impacts the pineal gland to suppress the release of melatonin, reducing drowsiness. In contrast, the melatonin concentration increases due to the absence of light at night. Through this daily cycle, the Suprachiasmatic nucleus rhythmically acts on the pineal gland release of melatonin, ultimately affecting the human body’s rhythm [99]. To regulate seasonal affective disorder and other circadian rhythm-related disorders, light therapy can compensate for the seasonal daily lack of light perception, restoring a healthy balance of neural information of light perception necessary for healthy biochemical processes. 
  • Research has shown that photoreceptive ganglion cells transmit light information to the supraoptic and paraventricular nuclei [100]. This pathway may facilitate brain development. Light stimulation of this visual subset associated with photosensitive retinal ganglion cells increases the release of oxytocin from the supraoptic nucleus and the paraventricular nucleus into the cerebrospinal fluid [84].
  • Another pathway of photo-receptive ganglion cells underlies the modulatory effects of light on affecting memory. Research has shown that bright light facilitates spatial memory by activating the path to the ventral lateral geniculate nucleus and intergeniculate leaflet and nucleus reunions [1]. The latter pathway is also linked with the lateral habenula, part of the epithalamus. This pathway is associated with pain perception relief [1,101]. Specific activation of the ventral lateral geniculate nucleus and intergeniculate leaflet not only induces inhibitory post-synaptic currents in gamma-aminobutyric acid-expressing neurons but also reduces the excitatory effects of pain-related stimuli on these neurons [1,100].

3.1.3 Brain Stimulation through Skin Irradiation

Light, even when not perceived by the visual system, can trigger biochemical responses and, as a result, neuromodulation of the brain in neurological disorders, as well as disruption of the brain's stress response system and glucose homeostasis.

  • Recent animal studies have demonstrated that trunk exposure to light can activate neurostimulation effects [102,103,104]. Red and near-infrared light (620-825 nm) is believed to induce molecules or cells in the body, such as immune cells, inflammatory mediators, and/or bone marrow-derived stem cells [105]. Skin irradiation by red and near-infrared light can even lead to brain neuromodulation by producing nerve growth factors or brain-derived neurotrophic factors [105], suggesting the potential of light to activate the body's natural healing mechanisms. When the skin is irradiated by red light near-infrared light, metal molecules in the cytochrome c oxidase (CCO) absorb photons. The light irradiation upgrades the CCO, passing it to the excited state [105,106]. Nitric oxide (NO) photo dissociates from CCO during this activation, and NO inhibits electron transportation [105]. After NO dissociation, the mitochondrial membrane potential increases [89]. This state elevates oxygen consumption levels, establishing a proton gradient that boosts Adenosine triphosphate production [105,107]. These processes cause the release of reactive oxygen species, Ca2+, and cyclic adenosine monophosphate [105,108] that can additionally activate a set of signaling pathways and transcription factors as second messengers [105]. 
  • Another hypothesis for brain stimulation through skin irradiation is based on a unique signaling system between mitochondria across different organ systems [89,105]. Mitochondria, when under stress, release a yet-to-be-identified signaling molecule called mitokine. Mitokine, encoded by both fibroblast growth factor 21 [FGF21] and mitochondrial DNA, is perspired in reaction to mitochondrial stress or the mitochondrial unfolded protein response, stimulating inter-tissue communication to manage cellular responses to mitochondrial distress [109]. This underscores the crucial role of mitochondria in the body's response to light stimulation. Cytokine release drives a mitochondrial stress response in other body parts [110]. Fibroblast growth factor 21 (FGF21) is upregulated in response to mitochondrial stress and during the mitochondrial unfolded protein response [109]. FGF21 is a hormone that significantly regulates metabolism, energy homeostasis, and mitochondrial function [109]. The overall metabolic and thermogenic effects of FGF21 may indirectly regulate mitochondrial function in various brain regions. For instance, the receptor-mediated role of FGF21 affects the hypothalamus, a critical brain region that controls energy balance, food intake, and body weight [111]. Research has shown that FGF21 exerts neuroprotective effects in various neurological disorders, such as Alzheimer's and Parkinson's [112,113]. FGF21 regulates the corticotropin-releasing factor, a vital component of the brain's stress response system [112]. FGF21 contributes to glucose homeostasis [114,115,116]. According to Liu and colleagues [105], the signaling system between mitochondria across different organ systems may contribute to modulating the brain activity that would appear in altering cognitive functions.

3.2 The Nature of Transcranial PBM

Two hypotheses rely upon enhancing mitochondrial metabolism, which is caused by, from one view, (1) an increase in mitochondrial oxygen consumption and cellular energy metabolism, and, from another, (2) modulates the production of reactive oxygen species (ROS) and causes transcriptional changes.

  • According to the received view, at the cellular level, transcranial laser radiation causes adenosine triphosphate synthesis (ATP) in mitochondria [117,118] and a release of mitochondrial reactive oxygen species and nitric oxide (NO) [23,25,26,119]. In the brain in vivo, the laser beam can reach the aimed brain tissue with established parameters' treatment power. Research reported that the primary PBM impact is the photonic oxidation of mitochondrial cytochrome c oxidase (CCO), the fourth enzyme complex in the electron transport chain within mitochondria [25,26,120,121,122]. The absorption peaks of CCO are mainly distributed in the range of 620–760 nm (red) and 780–825 nm (near-infrared) [105]. The red and near-infrared light stimulates the activity of CCO, and its excessive binding with nitric oxide (NO) is dissociated by the photons, which causes an increase in mitochondrial oxygen consumption and cellular energy metabolism [117,122]. Since it is already known that neurodegeneration and neurological dysfunction are associated with impaired mitochondrial oxidative metabolism, PBM improves the function of the brain by enhancing mitochondrial metabolism [122]. This neurostimulation secondarily leads to vasodilation (increasing oxygenation) [20,21,22,23,24,25,26] and intracellular signaling molecules [123]. 
  • Another hypothesis of transcranial PBM is that light modulates the release of reactive oxygen species (ROS) and causes transcriptional changes [122]. Research reported that light (810 nm wavelength) regulates the generation of ROS in mitochondria, induces retrograde signaling pathways from mitochondria to the nucleus, and modulates the expression of transcription factors such as NF-κB in mouse embryonic cells [122,124]. NF-κB regulates gene expression, including those with functions of antioxidant, anti-apoptotic, cell proliferation, and migration promotion [122]. According to Dong and colleagues [122], transcranial PBM can reduce NF-κB levels in activated inflammatory neuronal cells, thus exerting a neuroinflammatory suppressive effect [122]. 

3.3 The Nature of Transcranial Electric Current and Magnetic Stimulation

The nature of transcranial electric current and magnetic brain stimulation methods remains mysterious because the same stimulation, applied in different situations, can produce categorically different effects [125]. A systematic literature review emphasized the heterogeneity of these methods and outcome measures [126]. The cause of these discrepancies is likely based on the limitations of the studies mentioned in subsection 1.3.2 in the Introduction. Another reason may rely upon a poor understanding of underlying processes. This subsection aims to observe the state-of-the-art in our knowledge about them. Altering brain activity due to electrical and magnetic stimulation may have one explanation. The current review explains why it is so in section 2, Materials and Methods. In short, both electric current and magnetic fields exert similar effects on brain tissues [97]. Neuronal oscillations have a dual function in brain networks: neuronal oscillations are influenced by spiking inputs, and, in turn, they affect the timing of spike outputs [98]. These facts imply that interactions with both electric current and magnetic fields can alter neuronal activity. Therefore, this subsection observes hypotheses of the nature of all these brain stimulation techniques from the above-noted perspective. The review has found two hypotheses:

  • A first approximation, analyses of the literature give the insight that transcranial current stimulation methods (and also magnetic ones) alter the spiking activity of neurons [123]. Due to the bidirectional property of neurons in relationship with oscillations (as noted above), the phase of a slow oscillation modulates the amplitude of a faster rhythm, the so-called nested oscillation. Nested oscillations mean that the amplitude of higher frequency oscillations depends on the phase of slower oscillations [127]. Growing evidence shows that delta-nested gamma oscillations play an essential role in the temporal coordination of central and peripheral neural subsystems associated with cognitive functions [128,129,130]. In transcranial neurostimulation, the artificial currents [123], as well as external magnetic fields shift neurons' spikes towards certain phases of the sinusoid's waveform. But how does transcranial current stimulation or magnetic field stimulation affect the spiking activity of neurons? Given the above physics arguments, all these methods may exploit the same intrinsic qualities of physicochemical interactions, regardless of whether electric currents or magnetic fields accomplish transcranial brain stimulation. Magnetic fields may perturb Ca2+ and Na+ channel activity, considering the diamagnetic anisotropic characteristics of membrane phospholipids [131,132,133,134,135]. The voltage-gated Ca2+ channels are the primary conduits for the Ca2+ ions that cause a confluence of neurotransmitter-containing vesicles with the presynaptic membrane [135]. The timing and strength of synaptic output can be profoundly changed by alterations in the intrinsic function of these channels and their positioning within the active area [135]. The altered Ca2+ and Na+ channel activity contributes to neuronal excitability. Thus, coordinated neuronal activity may appear due to cellular properties that react in magnetic fields [131,132,133,134,135].
  • Another hypothesis argues that electromagnetic fields can cause an increase in adenosine receptors for neuronal communication [136]. According to Varani and colleagues [136], pulsed electromagnetic fields can mediate a transient and significant increase in A(2A) adenosine receptor density in cortex cell membranes. Adenosine modifies cell functioning by operating G-protein-coupled receptors (GPCR; A(1), A(2A), A(2B), A(3)). G-protein–coupled receptors set into motion chemical signaling events within the cell and enhance neuronal communication [137]. This altering appears because A(2A) regulates the production of other neurotransmitters, such as glutamate and dopamine. Interchanges between adenosine receptors and other G-protein-coupled receptors, receptors for neurotrophins, and ionotropic receptors also contribute to the adjustment of neuronal functions [136].

3.4 The Nature of Low-Frequency Sound Brain Stimulation

The hypotheses in this topic rely upon two different mechanisms of low-frequency vibrations to specific body parts: (1) vibrations alter the release of essential neurotransmitters and (2) transmitting vibrations to mechanoreceptors and pain receptors to send neurological non-pain messages to the brain:

  • The first insight, a prerequisite of a further convincing hypothesis into the nature of this brain stimulation technique, states that neurostimulation is induced by vibrations that alter the release of essential neurotransmitters, such as dopamine, serotonin, and corticosterone [138,139].
  • Another idea, a potential prerequisite of a further convincing hypothesis, proposes the transmission of vibrations to mechanoreceptors and pain receptors, such as the Pacinian Corpuscle, that can reduce chronic pain in patients and regenerate those cells [140,141]. These large mechanoreceptors in the subcutaneous and connective tissues surrounding visceral organs and joints are sensitive to pressure and can react to vibration from 60 Hz upward [140,141]. When Pacinian corpuscles are stimulated, they send neurological non-pain messages to the brain that appear to inhibit the pain impulse. A-b nerve fibers transmit information from vibration receptors Pacinian corpuscles and Meissner corpuscles. They touch receptors in the skin and stimulate inhibitory interneurons in the spinal cord that, in turn, act to reduce the number of pain signals (for further reading [140,141]).

4. Discussion

Since the narrative review aims to determine whether or not a unique natural physicochemical set of actions underlies different brain stimulation approaches, several hypotheses are acknowledged in the results section. Insofar as these hypotheses seem plausible and are based on empirical evidence, this section attempts to show that mitochondria are likely a central player in cellular processes underlying all brain stimulation techniques. The further reflections in this section based on the empirical data of other research explore whether mitochondrial metabolism pretends to become an agent of the universal endogenous neurostimulation mechanism, unique for the different brain stimulation approaches. Following this assumption, the section revises the existing hypotheses of neurostimulation to add their explanation with insight into the role of mitochondria.

The current review also highlights that:

  • All noted techniques are used in the treatment of a similar set of diagnoses related to the brain, such as depression, chronic pain, post-traumatic stress disorders, insomnia, and cognitive function disorders.
  • According to comprehensive analyses, all techniques are needed to refine the dose and develop personalized therapeutics.

These insights and findings in psychology and neuroscience allow the article in section 4.7 to propose a hypothesis of the natural brain stimulation derived from our ongoing research.

4.1 The Agent of the Universal Neuromodulation

To our knowledge, adenosine-5′-triphosphate (ATP protein) stores and transports energy in organisms. Mitochondria are the cellular "powerhouse," the energy factories of cells, which are highly proteome efficient in generating ATP. Mitochondria provide energy for normal functioning [103]. Mitochondrial ATP generation is even more proteome-efficient than glycolysis [142]. These organelles produce energy in oxidative phosphorylation: the electron transport chain and the enzyme ATP synthase convert oxygen and nutrients into ATP. Research shows three mechanisms of ATP production in animals. The mitochondria synthesize the vast majority of ATP through respiration, the so-called catabolic process [143]. The second mechanism benefits from light-harvesting chlorophyll pigments; mitochondria synthesize ATP when mixed with a light-capturing metabolite of chlorophyll [143]. Even more, research shows that red light (650-800 nm) impacts mitochondrial complex IV or cytochrome oxidase that raises ATP protein release [144,145]. Large numbers of mitochondria exist in the inner cells of the retina.

At the same time, mitochondrial output signals regulate other organelles' functions and systemically tune physiology by integrating information through dynamic, network-based physical interactions and diffusion mechanisms. [146]. According to Picard and Shirihai [146], mitochondria (being the processor of the cells) constitute the mitochondrial information processing system (MIPS), providing signal processing and integration [146].

Mitochondria, commonly found in synaptic terminals, play a crucial role in maintaining neurotransmission by producing ATP and buffering Ca2+. This underscores their importance in neurological function [147].

Insofar as the above arguments are correct (i.e., mitochondria produce energy for cells by oxygen and light transduction, maintain neurotransmission, and provide interactive networks MIPS), these cells may contribute to a unique physicochemical set of actions underlies different brain stimulation approaches. In the following sections, this assumption is developed with support from empirical data.

4.2 Mitochondria for Brain Stimulation through the IFVP and PGCP

The physicochemical sequence of actions behind the brain stimulation through the image-forming vision pathways remains (IFVP) largely unknown (see the 3.1 section). The current work proposes a hypothesis of brain stimulation by light through these pathways. Insofar as the above arguments from the 4.1 section are correct: (i) The retina's inner cells, rich in mitochondria, serve as the powerhouse for energy transduction through the optic nerve, a crucial process in vision and brain function. (ii) Mitochondria are interactive networks of signal processing and integration (MIPS), and finally. (iii) Mitochondria play a crucial role in maintaining neurotransmission in synaptic terminals. Consequently, light can directly modulate synaptic activity in the brain through the image-forming vision pathways and the photoreceptive ganglion cell pathways by exerting its functions by stimulating ATP production of mitochondria in the retina. Indeed, there is a consensus in cognitive sciences that color lights of different wavelengths stimulate the neuronal activity of various frequency bands in multiple networks [148,149,150]. Red color elicits higher brain activity in the frontal and parietal areas [148]. In contrast, violet only evokes a response in the prefrontal cortex [150].

4.3 Mitochondria for Brain Stimulation through Skin Irradiation

This section shows that skin irradiation, by light for brain stimulation and a therapeutic effect, relies on mitochondrial activity. When the skin is irradiated by light, mitochondria release a yet-to-be-identified signaling molecule called mitokine (also see 3.3 section). Mitokine is encoded by fibroblast growth factor 21 [FGF21]. The release of mitokine triggers a mitochondrial stress response in other areas of the body [108]. Fibroblast growth factor 21 (FGF21) is up-regulated in response to mitochondrial stress and during the mitochondrial unfolded protein response [107]. FGF21 is a hormone that significantly regulates metabolism, energy homeostasis, and mitochondrial function [107]. The overall metabolic and thermogenic effects of FGF21 may indirectly regulate mitochondrial function in various brain regions. Based on the signaling system MIPS between mitochondria across different organ systems [103], FGF21 regulates their corticotropin-releasing factor, a vital component of the brain's stress response system, altering neurological functions.

4.4 Mitochondria for Transcranial PBM

The hypothesis of a therapeutic effect of transcranial laser radiation on tissues from the 3.4 section can be complemented with the experimental data that reveal the mitochondrial role in laser-to-ATP (adenosine triphosphate) synthesis. Specifically, laser radiation causes ATP production in mitochondria [115,116] and a release of mitochondrial reactive oxygen species (ROS) and nitric oxide (NO) [23,25,26,117,151]. This provides an increase in mitochondrial oxygen consumption and cellular energy metabolism. Mitochondria impact synoptic activity by producing ATP, and due to MIPS, altering neurological functions.

4.5 Mitochondria for Brain Stimulation in Electric Currents and Magnetic Fields

As noted above, electromagnetic fields can increase A(2A) adenosine receptor density in cortex cell membranes [122] (for more information, see 3.3 section above). Adenosine A(2A) receptor stimulation enhances mitochondrial metabolism [152]. Research has reported that adenosine is endogenously generated and exogenously applied [152]. Because electromagnetic fields enhance mitochondrial bioenergetics [153], mitochondria energy metabolism increases ATP release. These data can complement the hypothesis from the 3.5 section on how external electrical currents and magnetic fields affect synoptic activity, highlighting the mitochondria' role in this brain therapeutic method.

4.6 Mitochondria for Brain Stimulation by Sounds

As noted in section 3.4, empirical data have shown that sounds may exert brain stimulation due to the dopamine, serotonin, and corticosterone release, which is altered when a body receives vibrations [138,139]. However, the link between vibrations and neurotransmitters would become more apparent when considering two empirical facts: (1) sound vibrations modulate the mitochondrial functions [154], and, as already noted, (2) mitochondria activity exerts neurostimulation by altering the production of neurotransmitters such as dopamine, serotonin, and corticosterone. Interestingly, investigating changes in synaptic plasticity under different vibration protocols, research has found that only lower frequency oscillations (<30 Hz) positively modulate hippocampal synaptic plasticity, increasing cognitive functions such as memory [155].

The idea of transmitting sound vibrations to mechanoreceptors and pain receptors, such as the Pacinian Corpuscle, that can reduce chronic pain in patients and regenerate those cells [140,141], also seems plausible (see more in the 3.4 section). However, this explanation needs to be completed at the cellular level. Significantly, even during brain stimulation due to vibrations, mitochondrial activity can comprehensively explain how sounds can provide a therapeutic neurostimulation effect. Because the transduction within the Pacinian corpuscle (PC) mechanoreceptors occurs at or near the filopodia with a high concentration of mitochondria, the PC activity depends on mitochondria activity. Since it is already known that neurological dysfunction and neurodegeneration are closely related to impaired mitochondrial oxidative metabolism [120], enhancing mitochondrial metabolism enables PC to send neurological non-pain messages to a brain that appears to inhibit the pain impulse [141]. Moreover, research shows low-frequency sounds of 1-1.5 Hz (e.g., different classic music) enhance ATP [156]. These experimental data also support the insight into mitochondria' contribution to the brain stimulation of low-frequency sounds.

4.7 Mitochondria in Universal Natural Neuromodulation

The literature analysis reveals that mitochondria participate in physicochemical interactions of different brain stimulation approaches. Since one agent plays a central role in various techniques and all techniques are used to treat the same brain-related diseases, one can ponder that these neurostimulation methods are just exceptional cases of a universal natural sequence of actions, a part of a natural flow of physicochemical interactions in nervous system development during pregnancy. However, this hypothesis is only based on known facts and has yet to be proven. Even though it relies upon empirical data from our research and other researchers, this is the only hypothesis that needs to be verified in further study. It requires more empirical evidence to be considered a robust theory. This conclusion enables further research on a natural "architect" of brain development. This section attempts to provide the first insight into the natural nervous system stimulation that appears in pregnancy and can become a prototype of the predictive therapeutic method to treat disorders related to the brain.

4.7.1 The Mother-Fetus Neurocognitive Model

Research on the first step of a child's cognitive development reveals insights into beginning cognition during pregnancy [157,158,159,160,161,162,163,164,165,166] through mother-fetus interactions [128,130,167]. According to the received view in cognitive sciences, the mother-child interplay is essential for the first step in cognition, which is associated with psychophysiological processes of shared intentionality [168]. Any inborn capacity manifests itself during lifespan. Growing empirical evidence supports emerging cognition due to interpersonal interactions through shared intentionality in young children and adults [169,170,171,172,173,174]. The comprehensive analysis of research data on child development during gestation and after birth [128,130,167] and recent findings about neurophysiological processes underlying interpersonal dynamics in neuroscience [166,175,176,177,178,179,180] allows the neurophysiological hypothesis of shared intentionality to argue about the emergence of cognitive functions as perception through heart-brain interplay during gestation [128,130,167]. Beginning from the basic pattern of stimulus-response coordination, the human capacity to appropriately detect changes within themselves and in their environments depends on the specific nervous system architecture, which evolves in a particular way during gestation [128,130,167]. The proper jumpy system development makes us humans [128,130,167]. The physicochemical properties of the mother-fetus bio-system, primarily the electromagnetic and acoustic oscillations of the mother's heart, consolidate the neuronal activity of both organisms in a choir, shaping harmony from a cacophony of distinct oscillations [128,130,167]. During the mother's intentional act, the interactions within this bio-system of acoustic fluctuations, electromagnetic waves, and chemical interactions synchronize brain oscillations, affecting neuroplasticity in the fetus [128,130,167]. At the same time, interactions with the external environment during the mother's intentional acts provide a clue to the fetus's nervous system, binding synoptic activity with appropriate stimuli [128,130,167]. The mother-fetus interaction enables the child's nervous system to evolve with adequate biological sentience. This so-called mother-fetus neurocognitive (MFN) model forms the specific architecture of the child's nervous system and contributes to the beginning of cognition [128,130,167].

4.7.2 The MFN Model Components and Experimental Validation

Recent empirical studies implemented the MFN model in psychophysiological design to verify the modulation of shared intentionality [170,171,172,173,174,175,176,177]. The research design emulated this bio-system in children and adults by applying relevant parameters with their magnitude proportional to the scale of the model. The case studies [181,182] reported the central characteristics of the model implemented to cause mitochondrial and cognitive stress in subjects: a display with color lights, an acoustic transducer, and an intellectual task. The display radiated an electromagnetic field with a frequency of 50-60 Hz and a magnetic flux density of 0.13-0.3 μT at a distance of 0.2-0.4 m, which delivered 400 lux of light. It emitted impulses of alternately flashing color lights with wavelengths of 400 nm and 700 nm (lights of violet and red, respectively) with a frequency of 1.3 Hz of alternate flashing. The sound transducer emulated the MFN model in the patient to simulate the impact of the mother's heartbeats on the fetus's nervous system. Specifically, headphones emitted a complex acoustic wave with a flux density of 0.08 T formed within the operating frequency of 1.3 Hz by the first sinusoid, the second sinusoid, and the third sinusoid. Each first, second, and third sinusoid had a period equal to the operating frequency. At the same time, the phase of the second sinusoid is shifted by a value equal to one-third of the operating frequency and the second sinusoid by two-thirds of the operating frequency. The first sine wave had an amplitude of 85 dB, and the second sine wave and the third sine wave had an amplitude of 25 dB.

As noted above, a theory argues that cognition emerges due to psychophysiological processes of shared intentionality; research shows modulation of shared intentionality in adult subjects by emulating the above-noted components of the MFN model [170,171,172,173,174]. Insofar as shared intentionality modulation is already observed experimentally and as we know that the mother-fetus interaction, specifically shared intentionality, enables evolving the child's nervous system with adequate biological sentience, it is not too controversial to suppose that environmental conditions tested in shared intentionality modulation, can also treat the injured nervous system. This natural stimulation of the fetal nervous system, the impact of the mother's heart during pregnancy, can become a prototype for "gentle" neuromodulation.

4.7.3 Establishment of Acoustic Photonic Intellectual Neurostimulation

By drawing on evidence from cognitive sciences (esp. psychology and neuroscience) and evidence from research on modulation of shared intentionality, the nature-based brain stimulation technique called Acoustic Photonic Intellectual Neurostimulation (APIN) has been developed [181,182]. The APIN technique implements three therapeutic agents derived from studying natural neurostimulation during gestation. They cooperatively stimulate specific neuronal networks of the brain, emulating the mother-fetus neurocognitive model in a patient (see above). Because mother-fetus interactions enable the evolution of the child's nervous system with adequate biological sentience, the APIN can treat the injured nervous system in patients by scaling characteristics of this bio-system [181,182]. The therapy exerts its functions by affecting neuronal plasticity and vasodilation, which increases microvascular blood flow velocity and tissue oxygenation. The APIN technique has already shown significant results in testing three cases: dysmenorrhea [181], neurodegenerative disorder [182], and phantom pain sensation (in press). The author believes this significant outcome was achieved due to fine-tuning the nervous system by modulating neural plasticity. This research has the potential to influence future studies in the field of neuromodulation research significantly.

Author Contributions

The author did all the research work of this study.

Funding

No funding.

Competing Interests

The author has declared that no competing interests exist.

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