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The potential role of rhythmic entrainment and music therapy intervention for individuals with autism spectrum disorders

Autism spectrum disorder (ASD) is characterized by social and interpersonal communication disabilities and repetitive motor activities. A deficit in social interaction may be due to motor and synchronization disabilities in individuals with ASD. These disabilities serve as a hindrance for the progression of day-to-day life. ASD individuals are known to have variations in the neural network contributing to changes in their oral-motor activity. As the brain has experience-dependent structural plasticity, these changes in the neural network can probably be reversed with appropriate treatment Music playing a universal role in human life has been studied for its therapeutic potential in rehabilitation of ASD individuals. Music and rhythm have shown a significant potential in improving the oral-motor activities of people affected by ASD. Music based interventions are being used for children diagnosed with ASDs to improve their social communication and motor skills. This article represents the possible role of rhythmic cueing for sensorimotor regulation in ASD individuals. This can serve as a base for further research for the impact of musical therapy on coordination and oral-motor synchronization of individuals diagnosed with ASD.


INTRODUCTION
Autism spectrum disorder (ASD) is a multifaceted neurodevelopmental disorder that encompasses a group of behavioral conditions, frequently characterized by social and interpersonal communication disabilities, associated with repetitive motor activities. Although, Kanner (1943) described ASD for the first time in 1943 and Asperger reported about autistic psychopathy in 1944 (Fitzgerald, 2001), until 1980's it was treated as a psychiatric disorder and children were misdiagnosed as being schizophrenic (Chisholm et al., 2015). Individuals with ASD may exhibit sensory dysfunction and problems in oral motor movements. Sensory dysfunction is prevalent in at least 70% of individuals with ASD (Tomchek and Dunn, 2007). The hearing and language disorders in children with autism are predominantly expressed as sensitive hearing which is exhibited by closing the ears, anger, crying, irritation and throwing objects while hearing certain noises. Sensory processing deficits typically manifest as hyper or hypo-reactivity to sensory input such as sensory-seeking or sensory-avoiding behaviors (Baranek et al., 2006;Baranek et al., 2007). Although ASD children with expressive language deficits are found to have motor and oral-motor impairments which are closely linked with speech production, fluency and clarity, language deficits are not described as a significant feature of ASD in DSM-V (Benítez- Dalton, 2017).
Toddlers and young children with motor difficulties tend to remain nonverbal or exhibit problems with expressive speech and language, implying that speech discrepancies maybe secondary to oral-motor deficits. While ASD is considered as a developmental disorder, disruptions of social communication is rarely accepted as a result of challenges in speech and language. Further, research investigating the efficacy of alternative multisystem therapies for children with ASD is still growing, but interventions that specifically aim the oral-motor features in ASD are still scarce. These therapies may be necessary to effectively improve the social abilities of affected individuals. Current research suggests that, listening to music and rhythmic patterns improves attention in children with ASD, which may be a sign of improved sensory integration. Among the following four major music therapy (MT) interventions for ASD including auditory motor mapping training, melodic intonation therapy, improvisational music therapy and rhythm training, the potential of rhythm training to improve sensorimotor functioning in ASD is gaining momentum. In this review, we discuss about the prospective roles of music, rhythm, rhythmic entrainment, how rhythm enhances cortical plasticity and the effect of rhythm based music intervention as a rehabilitation method for ASD individuals.

MUSIC, BRAIN, AND CONNECTIVITY
Music is known to regulate arousal as well as attention in the brain and has the capacity to engage different areas in the brains of individuals with neurological conditions. Perception and production of auditory rhythms involves the subcortical and cortical brain networks consisting of the auditory cortex, basal ganglia, supplementary motor area (SMA), premotor cortices, and cerebellum (Koelsch, 2014). The architecture of the auditory system detect temporal patterns in auditory signals with great precision and speed (Shelton and Kumar, 2010). The whole brain responds to music and the pitch of the music is processed in the right temporal lobes which also governs speech, making it possible to use music to improve interpersonal communications. Moreover, the ASD brain is characterized as consisting of short range and long range communications. The short range consists of cortico-cortical connections and long range includes brain regions like the frontal, temporal, parietal, and subcortical areas (O'Reilly et al., 2017) Khan et al. (2013) have reported that both the short and long range connectivity are reduced among ASD and the disability of long-range networks are claimed to cause the social-emotional and communication deficiencies of autism. However, as the brain has experience-dependent structural plasticity, these changes in the neural network can probably be reversed with appropriate treatment.
Autistic individuals can possess "splinter skills" or "islets of ability" and approximately 10% of autistic entities have been re-ported to exhibit savant abilities in music, drawing, or calculation. The distinctive stimulus of music can offer ASD children the opportunity to interact socially and strive toward nonmusical social outcomes. Interestingly, studies had shown that autistic children were interested and perhaps more talented in music as compared to their unaffected counterparts, further exhibiting the possibility of developing MT (Applebaum et al., 1979;Molnar-Szakacs and Heaton, 2012). Traditional MT has been employed to cater to the social, communicative, and cognitive needs of ASD children (LaGasse, 2017). Cochrane meta-analysis suggested that music-based training significantly improves verbal and gestural communication abilities compared to placebo therapy in individuals with ASD (Gold et al., 2006). In many countries, MT has been practiced as a useful intervention to improve social interaction, verbal communication and socio emotional reciprocity (Geretsegger et al., 2014). Quintin et al. (2011) have further claimed that the ability of ASD individuals to recognize the emotions behind music were similar to that of other individuals, making music a potential therapeutic strategy.

RHYTHM IN MUSIC
Rhythm, the primary structural feature of music denotes the division of time through distinct order. Similarly, in our human life, the natural and extemporaneous body movements are a demonstration of inner timing. The developmental defects in the brainstem and cerebellum inutero can lead to deficits in sensory perception in ASD (Trevarthen and Delafield-Butt, 2013). Trevarthen and Daniel (2005) suggested that, disorganized rhythm and synchrony in infants can be the early signs of ASD and Rett's syndrome. Chen et al. (2008) used a pulse-tapping method and have reported that, listening to music recruits motor associated regions of the brain further connecting it to movement and development. Rhythmicity plays a vital role in development and timing is critical in motor control and cognitive functions (Murphy, 2015a;Smith et al., 2014). Inutero, a fetus's neural circuits and auditory memory are forming and at five months, it can feel the rhythm of the mother's heartbeat and respiration. After birth, during development, each child finds a particular motor rhythm which will endure consistently throughout life. Rhythm also incorporates sensory perception and motor entrainment, which results into complex cognitive functions and motor adaptations. Remarkably, brain rhythms are heritable components of brain function and have been connected to computational primitives of language (Benítez-Burraco and Knyazev, 2007;Murphy, 2015b) at the brain level. Further, rhythm has also been associated with language shortfalls, neural malfunction and autism (Overy and Turner, 2009). Social interactions that include eye contact, turn taking. Synchrony and imitation can be introduced to induce and enhance social and behavioral skills in children with ASD. Furthermore, it has been reported by studies that the interaction between the auditory and motor system can be utilized in rehabilitation therapies for individuals with motor condition. Neuroscience information regarding alpha, delta and gamma rhythmic oscillations in the brain continue increasing, these oscillation being a key to arousal, anxiety and relaxation responses, must be further researched in individuals with ASD for therapeutic advancement (Thaut et al., 1991;Thaut et al., 1997).

RHYTHM AND RHYTHMIC ENTRAINMENT
Rhythmic entrainment introduced the first motor theory for the function of auditory rhythm and music in therapy. Rhythmic entrainment is known to play a role in affecting heartbeat, pain reduction and muscle relaxation, and music-mediated imagery among in MT (Bengtsson et al., 2009). Entrainment to music involves a large network of brain structures (Grahn, 2012;Phillips-Silver et al., 2010;Sihvonen et al., 2017) which involves auditory, visual, proprioceptive and vestibular perception. The complex process requires motor synchronization, attention, performance and coordination within and across individuals (Sihvonen et al., 2017). Rhythmic entrainment methods connect an individual with their own body rhythm and also connect them nonverbally with other individuals. Subsequent studies on music entrainment contributed to the necessity to codify and standardize rhythmic-musical application for motor rehabilitation in affected individuals (Galińska, 2015;Thaut et al., 2015) (Fig. 1). It has also been reported that, synchronization, entrainment of rhythmic vocalizations and bimanual motor actions can effectively stimulate the speech, motor and language networks in ASD. These findings laid the foundation for neurologic MT (NMT). NMT will use rhythm as template to accomplish complex motor tasks by activating compensatory neural networks. Music can contribute to neurological rehabilitation in various ways that include, rhythmic stimulation and entrainment, patterned information processing, and differential neurological processing of musical components by means of different brain structures, and the affective-aesthetic response include arousal, motivation and production of emotions (Thaut et al., 2009). In NMT, rhythm has stimulating, cueing and coordinative function, through cortical plasticity (Boso et al.,  2007). NMT is available as a standard treatment that has been accepted as therapy over the past decade. Since the central rhythm is disrupted in autistic individuals similar to that of patients with a damaged cerebellum, music can be used to synchronize movements and evoke emotions in affected individuals. NMT can serve as a therapeutic tool to bring natural rhythmicity to affected individuals promoting the reorganization of abnormal circuits (Chen et al., 2008;Kornysheva et al., 2010).

RHYTHM, CORTICAL PLASTICITY, AND AUTISM SPECTRUM DISORDER
Musical training involves the cortical areas of movement such as, the precentral gyrus, SMA (Whitall et al., 2011), the cerebellum (Luft et al., 2004) auditory, occipital, sensory, frontal brain areas and the anterior corpus callosum and improves cortical plasticity and promotes structural and functional connectivity in the brain (Hardy and LaGasse, 2013). Further, Chen et al. (2008) have suggested an integral association between auditory and motor systems in the milieu of rhythm. Interestingly, Kornysheva et al. (2010) have compared the effect of desired to not desired musical rhythms on the premotor and cerebellar areas using functional magnetic resonance imaging and indicated that the desired tempo improved the activity in the ventral premotor cortex. It has also been found that structural and functional variances in the sensorimotor regions of the cerebellum and sensorimotor cerebro-cerebellar circuits can cause motor control disability and repetitive and stereotyped behaviors in ASD (D'Mello and Stoodley, 2015). Hence, music can be used as a tool to improve cortical plasticity in individuals with the disorder. It can be used to rewire sensorimotor cerebro-cerebellar circuits to improve motor control and repetitive behaviors. Rhythm can be used to couple auditory motor functioning and increase the motor function of ASD individuals.

REHABILITATION THROUGH RHYTHMIC STIMULI IN AUTISM
If rhythmic stimulus is applied systematically, it has the potential to enhance the timings of neural networks in addition to the ones associated with movement, and the interventions may have a greater impact on the patients. This may be one reason that studies on MT and ASD have demonstrated improved social (Brownell, 2002;Finnigan and Starr, 2010;Kern and Aldridge, 2006;Kim et al., 2008)) and communication skills (Lim, 2010;Wan et al., 2011). Berger's study based on the hypothesis that patterned, tempo-based, rhythm interventions, brought systemic pacing under control, eased repetitive behaviors and also reduced anxiety in ASD individuals (Berger, 2012). Corriveau and Goswami (2009) discovered the relationship between rhythmic motor entrainment in children with speech and language impairments, and also its possible implication in dyslexia. An exploratory study by Sharda et al. (2018) observed that music improves social communication and auditory motor association in individuals with ASD. Report of the study showed that, children with proprioceptive deficits in ASD performed superior in auditory and visual attention responsibilities after receiving rhythmic proprioceptive input than children who received only the proprioceptive input. Suggesting a significant role of rhythmic input in these individuals. Kalas (2012) in his report has suggested that, simple rhythm facilitates attention in all functioning levels of ASD. Additionally it has been found that predictable rhythmic structure are important for movement, and that unconscious motor response to rhythm can regulate motor activity, further reinforcing the effect of predictable rhythm in movement (Molinari et al., 2007). Moreover, Stevens and Byron (2009) have reported that Predictable rhythmic structure creates a sense of expectation, focusing on the emotional benefits of MT. As rhythm assists in facilitating motor stability, Hardy and LaGasse (2013) have found that rhythmic synchronization and systematic rhythmic cueing can increase cortical plasticity. Further, ASD individuals had also shown an increase in motor skills after receiving a biweekly rhythmic program (Srinivasan et al., 2015) accentuating the importance of MT in ASD. Interestingly, it has also been established that individuals with ASD were able to synchronize rhythm just as well as their typically developed counterparts (Tryfon et al., 2017). Thus the unique correlation (Thaut et al., 2005). Improvement in motor skills can encourage ASD individuals to demonstrate their full cognitive, social, and communicative potential to respond socially and individually.

CONCLUSIONS
Apart from social and communication deficits, movement and language differences in ASD also play a vital part in development, and hence merits further investigation. The individuals affected with ASD often lack social, interpersonal and motor skills, providing a hindrance to their everyday life. Hence, clinical treatment of ASD must focus on motor coordination and the improvement of functional motor skills along with other aspects of ASD. Auditory cues are progressively used in movement rehabilitation, but controlled clinical trials are very rare and may not yield positive results each time. Rhythmic auditory cueing can be used as an appropriate technique to stabilize the movement patterns and simplify a motor plan. Rhythmic intervention may serve as a therapeutic tool to increase the motor, language and personal skills of ASD individuals leading to a better quality of life. Hence, further research is essential to establish the association between rhythm and movement in ASD individuals.