Aphasia refers to a degraded language capability due to lesions in the cortical language areas or association pathways. Expressive aphasia (also known as non-fluent, anterior, or motor aphasia) affects spoken language (Broca's area) while receptive aphasia (also known as fluent, posterior, or sensory aphasia) affects language interpretation and memory (Wernicke's area). Total (or global) aphasia is the consequence of large lesion(s) in the perisylvian area, extending deep into the subjacent white matter. It can involve the frontal, temporal, and parietal lobes, resulting in the loss of all of nearly all speech and language functions. Transcortical motor aphasia (also called anterior isolation syndrome) is caused by damage in the cortical areas around Broca's area, but sparing the arcuate fasciculus and Wernicke's area while transcortical sensory aphasia (also called posterior isolation syndrome) results from damage in the cortical areas around Wernicke's area, but sparing the arcuate fasciculus and Broca's area.
Melodic intonation therapy is a type of speech-language therapy that has been used to treat non-fluent aphasia. During MIT sessions a person with non-fluent aphasia is encouraged to hum, and then to sing words or phrases they find hard to recall, while tapping out a rhythm (NHS, 2011). MIT typically consists of speaking with a simplified and exaggerated prosody, characterized by a melodic component (2 notes, high and low) and a rhythmic component (2 durations, long and short). This technique has been shown to increase the number of words a person can recall. This may be due to the fact that the part of the brain we use to recall song lyrics and music is different from the part of the brain we use to recall spoken language. So MIT teaches a person with non-fluent aphasia an alternative way to recall words and phrases. However, most aphasic patients do not respond to melodic intonation therapy. This includes those with global and transcortical aphasia and almost all with evidence of significant posterior language area involvement.
There is some evidence that melodic intonation therapy is effective for treating the speech-related problems of children and adults when diagnosed with Broca's aphasia (AAN, 1994). However, the technique is now being used with children diagnosed with developmental apraxia of speech, which is defined as a speech articulation disorder not involving sensory or motor paralysis.
A systematic review of the evidence by Roper (2003) concluded that “there is little to no evidence available supporting the effectiveness of Melodic Intonation Therapy (MIT) for children with apraxia.” Based on this evidence-review, the Center for Evidence-Based Practices of the Orelena Hawks Puckett Institute (2003) made the following recommendation: "Use of modified forms of melodic intonation therapy to assist the speech development of young children who have apraxia of speech can not be recommended as a beneficial intervention until the practice is studied more carefully and thoroughly."
A Medicare National Coverage Determination (1983) states that melodic intonation therapy is a covered service only for nonfluent aphasic patients unresponsive to conventional therapy, and only when the conditions for coverage of speech pathology services are met.
In a pilot study, Tamplin (2008) examined the effects of vocal exercises and singing on intelligibility and speech naturalness for subjects with acquired dysarthria following traumatic brain injury or stroke. A multiple case study design was used, involving pre-, mid-, and post-treatment assessments of intelligibility, rate, naturalness, and pause time for 4 subjects with dysarthria. Each subject participated in 24 individual music therapy sessions over 8 weeks involving oral motor respiratory exercises, rhythmic and melodic articulation exercises, rhythmic speech cuing, vocal intonation therapy, and therapeutic singing using familiar songs. Results were measured using a standardized dysarthric speech assessment -- the Sentence Intelligibility Test, waveform analysis, and ratings of speech naturalness. Statistically significant improvements in functional speech intelligibility were achieved but improvements in rate of speech were insignificant. Speech naturalness improved post-treatment and a reduction in the number and length of pauses was verified via waveform analysis. The author stated that these preliminary findings suggested that a program of vocal exercises and singing may facilitate more normative speech production for people with acquired dysarthria and supported the need for further research in this area.
Vines et al (2011) noted that research has suggested that a fronto-temporal network in the right hemisphere may be responsible for mediating MIT’s positive effects on speech recovery. These researchers investigated the potential for a non-invasive brain stimulation technique, transcranial direct current stimulation (tDCS), to augment the benefits of MIT in patients with non-fluent aphasia by modulating neural activity in the brain during treatment with MIT. The polarity of the current applied to the scalp determines the effects of tDCS on the underlying tissue: anodal-tDCS increases excitability, whereas cathodal tDCS decreases excitability. These investigators applied anodal-tDCS to the posterior inferior frontal gyrus of the right hemisphere, an area that has been shown both to contribute to singing through the mapping of sounds to articulatory actions and to serve as a key region in the process of recovery from aphasia, particularly in patients with large left hemisphere lesions. The stimulation was applied while patients were treated with MIT by a trained therapist. A total of 6 patients with moderate-to-severe non-fluent aphasia underwent 3 consecutive days of anodal-tDCS + MIT, and an equivalent series of sham-tDCS + MIT. The 2 treatment series were separated by 1 week, and the order in which the treatments were administered was randomized. Compared to the effects of sham-tDCS + MIT, anodal-tDCS + MIT led to significant improvements in fluency of speech. The authors concluded that these results support the hypothesis that, as the brain seeks to re-organize and compensate for damage to left hemisphere language centers, combining anodal-tDCS with MIT may further recovery from post-stroke aphasia by enhancing activity in a right hemisphere sensorimotor network for articulation. These preliminary findings need to be validated by well-designed studies.
Chrysikou and Hamilton (2011) stated that aphasia is a common consequence of unilateral stroke, typically involving perisylvian regions of the left hemisphere. The course of recovery from aphasia after stroke is variable, and relies on the emergence of neuroplastic changes in language networks. Recent evidence suggested that rehabilitation interventions may facilitate these changes. Functional re-organization of language networks following left-hemisphere stroke and aphasia has been proposed to involve multiple mechanisms, including intra-hemispheric recruitment of peri-lesional left-hemisphere regions and transcallosal inter-hemispheric interactions between lesioned left-hemisphere language areas and homologous regions in the right hemisphere. Moreover, it is debated whether inter-hemispheric interactions are beneficial or deleterious to recovering language networks. Transcranial magnetic stimulation (TMS) and tDCS are non-invasive procedures that can be applied clinically to modulate cortical excitability during post-stroke language recovery. Intervention with these non-invasive brain stimulation techniques also allows for inferences to be made regarding mechanisms of recovery, including the role of intra-hemispheric and inter-hemispheric interactions. The authors reviewed recent evidence that suggests that TMS and tDCS are promising tools for facilitating language recovery in aphasic patients.