The electrochemical activity of neurons allows it to respond to a stimulus in one millisecond [1]. This responsiveness allows a large number of neurons to emit action potential synchronously in response to external stimuli.

Thus, originally Siever [2] and Vieira [3] said that the use of modern technologies based on the reactivity of the brain to specific stimuli may be used to achieve cure and prevention of certain diseases, such as Parkinson's and Alzheimer’s diseases through technologies such as brain stimulation, which is based on the production of brain waves through light and sound stimuli. Specifically, the brain stimulation analyzed in this study uses equipment with sunglasses fitted with LED lamps in the inner face of the lens, headphones and an electronic device capable of sending synchronized visual and auditory stimuli in a specific frequency. The visual stimulation occurs through strobe lights, and auditory stimulation is made by binaural beats. These two techniques may be combined and enforced on the nervous system at frequencies that can range from 1 to 30 Hz producing states of consciousness compatible with the brain waves delta (1-3 Hz), theta (4-7 Hz), alpha (8-12Hz), and beta (13-30 Hz) [4, 5].

Calomeni et al. [6] put the technology of brain stimulation by light and sound between the biofeedback techniques as it allows the individual after a period of training to develop the ability to produce mental favorable states to performance in specific situations. This theory is enhanced by Vieira [3], who defines the biofeedback as an immediate return of information through sensitive electronic equipment able to capture sensory responses amplifying and transforming physiological reasons in signals. In this context, the synthesis of brain waves fits this definition because this technology is able to stimulate the brain externally, to the brain to principle follows the frequency imposed, and after producing the desired brain waves.

In this sense, studies based on photo stimulation that highly corroborate with the possibility of brain waves can be induced through externally stimulated frequencies, and these frequencies can change the state of consciousness [5] depending on external factors such as time of stimulation, culture, and expectations of the individual who will be stimulated [5, 7]. Regarding auditory stimulation through binaural beats, França et al. [5] says that the brain’s response in relation to the applied frequency is doubled.

To Calomeni et al. [8], the theoretical basis that guides the technique of brain stimulation by light and sound is structured in hemispheric balance and equalization of cortical pattern, favorable to the performance of a task. Yet for these authors, this effect is attached by the bombardment of the retina and the olivary nucleus through photic and auditory stimuli at a specific frequency, which causes a standardization of the cortex in a frequency that is compatible with a brainwave, when perceived through the thalamus. Thus, the photic and auditory stimulation can be classified as a type of deep stimulation because its effects into the cortical level will occur through the structures of the brainstem and the thalamus [6].

Therefore, this study aimed to seek scientific research that used the photic and auditory stimulation as a way to enhance cerebral activity.

Within the included studies for qualitative synthesis, three were experimental studies, and one was a case report. In this way, the first study selected, after applying the criteria defined in the methodology, used the photic and auditory stimulation associated with motor practice. In this study, Vieira [3] observed the effects of photic and auditory stimulation associated with a complex motor practice in mental functions of 37 individuals, both genders and ages between 68 and 78 years, with Alzheimer’s disease. The sample was randomly divided into 3 groups that received different levels of stimulation. The first of those groups received the photic and auditory stimulation associated with practice of psychomotor activities dependent on high levels of memory. Another group participated in a psychomotor training associated with a placebo of photic and auditory stimulation. Finally, the third group of the study maintained the common treatment applied to patients with Alzheimer’s disease and received the photic and auditory stimulation. The results obtained after training show a gain in cortical performance of the overstimulated group when compared with the less stimulated one, with an increase in the cortical activity in specific regions of the brain as presented in brain mapping results (EEG).

Another study conducted by Calomeni and colleagues [8] verified the effects of photic and auditory stimulation associated with imagery in the heart rate variable. For this, the authors stimulated a frequency of 20 Hz (beta wave) in the cortex of 10 sportsmen, with mean age 14 ± 0.7 years. The total time of stimulation was 10 minutes and was requested from individuals that mentally simulate (imagery) situation that occurs in training and normal games. All process of stimulation occurred 1 h before the start of a training routine, which guarantees the absence of stressors related to training or competitive situation. Even under these conditions, the results show physiological adaptations even without the influence of these stressors.

In the same sense, Ribeiro Júnior et al. [1] observed the effect of photic and auditory stimulation in the variable reaction time and skilled motor-cognitive efficiency of 20 football players. The athletes were divided into two groups of 10 individuals that performed a task: kicking a ball in a specific quadrant marked in the goal area in response to a random light stimulus. The performance in the task and the speed of mental processing through the motor reaction time were evaluated, that is, the time that each athlete lingered to identify and hit the indicated quadrant by light signal. In the treatment, the soccer players who formed the experimental group had brain activity equalized at 11 Hz (alpha wave) during 20 minutes by a period of 2 weeks. The results show that the experimental group obtained significant gains in improving the reaction time when compared with the control group, and inside the experimental group, the combined training produced significant improvements if compared pretest and posttest.

In the last study included for systematic review, Calomeni et al. [6] conducted a case study that applied the photic and auditory stimulation associated with the imagery to produce a functional improvement in deteriorated movement patterns as a result of stroke. To achieve this objective, the studies stimulated the alpha brainwave in the frequencies 8, 9, and 11 Hz alternately in 4 subjects of the sample, associated with the mental practice of motor gesture evaluation (imagery) and the visualization of people performing the same movement without any functional limitations. After the 12 30-minute sessions of the proposed intervention, it was found that all individuals evaluated achieved significant gains in the function of the motor gesture evaluated both in terms of runtime as the functional quality of movement. According to the authors, this means that the intervention proposed in the study can be used as an adjunctive therapy in the recovery of motor sequels through increasing the efficiency of neuronal function (Table 1).

Although the study conducted by França [5] was not included in the systematic review because of failure to meet the criteria of the adopted inclusion, it represents an important milestone because it sought to develop an inducer of brain waves at lower costs and with greater possibility of customization. Due to bring knowledge that were fundamental to this study.

The study conducted by Vieira [3] proposed the investigation of the behavior of brain frequencies in the performance of cognitive motor skills and the potential memory in individual with Alzheimer's disease. The author justify the utilization of photic and auditory stimulation techniques because of wide applicability to the prevention and treatment of some diseases such as Parkinson's and Alzheimer's disease until the use in training for sportive practice and learning. It was concluded that psychomotor activities associated with photic and auditory stimulation possibly incited neuroplastic processes through new neural interconnections and that this brain's ability to self-organize is also present in elders with Alzheimer’s disease.

Differently of Vieira [3], Calomeni et al. [8] determined whether the photic and auditory stimulation associated with the imagery would produce significant changes in heart rate of basketball players even in the absence of physical stimuli or stressor agents. The authors concluded that, although many studies have agreed with the idea of great susceptibility of heart rate to motor events in potentially stressful situations, no paper has been found that observed the variation of heart rate when the only influencing factor was a mental task, and in this sense, they checked that the photic and auditory stimulation associated with imagery produced significant changes in heart rate even in the absence of physical stimuli or stressor agents and that this effect can be used as a means for preparation of individuals that aim to improve the blood supply before an intense activity or a stressful situation without performing specific physical movements.

Still in the sports context, Ribeiro Júnior et al. [1] sought to determine the effect of photic and auditory stimulation in the variable reaction time and skilled cognitive-motor efficiency in soccer players. The adopted procedure produced in the players of experimental group a statistically significant improvement in the motor performance and consequently in the mental processing. Important fact registered by Ribeiro Júnior et al. [1] was that before intervention, the experimental group had lower performance when compared with the control group. After stimulation, the experimental group started to have higher performance compared with the control group. Thus, they concluded that this study opens a great possibility of the emergence of a new technique to motor-cognitive learning, based on the methodology developed, supposing that athletes can have a transfer of the registered gains for any sport that requires improvement in motor efficiency.

In a recent study, Calomeni et al. [6] applied the photic and auditory stimulation associated to imagery with the objective of producing a functional improvement on the patterns of deteriorated movements by stroke. At the end of the study, they found that all evaluated individuals have obtained gains on functionality of motor gesture and that these gains are related to the increase in the efficiency of neuronal function. Also, according to these authors, the efficiency of neuronal function is more related to the reduction in the variability of activation of neurons, represented by reduction of standard deviation in the signal of EEG than by the increase on the number of recruited neurons for task, as represented by the increase of electrical activity registered on EEG.

Apparently, regardless of the application of the improvement of sportive performance or the physical/mental health provided by analyzed studies to photic and auditory stimulation, the effectiveness of the shown results, which is common for all studies, is based on the fact that the stimuli of light and sound reach the structures of brainstem first. This characterizes the deep stimulation [3, 6, 9] and may explain the changes produced in the brain and inferred through the cortical activity. In this sense, the idea that more studies about this stimulation seems consistent; its applications and consequences could be performed to produce more consistent theoretical and practical stuff about its effects and utility.

In the analysis of scientific papers that composed the universe of the study, we can conclude that brain stimulation through photic and auditory stimuli seems to be a noninvasive method to induce specific mental states. In all reviewed studies, the individuals who have passed by brain stimulation associated or not with other strategies they obtained gains when compared to control groups.

These considerations suggest the hypothesis that the technique of brain stimulation can be applied to induce favorable mental states to maximize interventions on several disorders that affect humans in a safe and noninvasive method.

That is why it seems plausible to suggest that new clinical investigations about photic and auditory stimulation should be developed and that other positive results can possibly arise through the association of photic and auditory stimulation with treatments for mental disorders.