Language and music side by side: post-stroke aprosodia and amusia are subserved by the same brain networks.

Abstract

When I was a medical student—in the ’70s of the last century—I was taught that language is processed in the left, music in the right hemisphere. At the time this was state of knowledge, based on a history of more than 100 years of clinical case reports, such as early observations by the French neurologist Jean-Baptiste Bouillaud who documented the capacity to sing in two patients with aphasia. Remarkably, he reported on a 50-year-old composer with aphasia and verbal agraphia who was able to compose a new tune and write down the notes. The patient was also able to sing or ‘modulate his voice’, thus maintaining emotional prosody (for an excellent review on these historical cases, see [1]. Within 50 years, a much more complex picture has emerged and is still further developing. Neuropsychology is a dynamically evolving field and benefits from the enormous progress in standardized clinical testing methods as well as in neuroimaging methods. In a beautiful study in this volume of the European Journal of Neurology, Sihvonen et al. [2] have carefully assessed lesion patterns and structural connectivity in stroke patients at the 3-week subacute and 3-month post-stroke stages to investigate the neural correlates of amusia and aprosodia. Thirty-nine patients with right or left hemisphere stroke were enrolled and tested for linguistic and affective prosody and musical pitch and rhythm perception. Structural magnetic resonance imaging and machine-learning-based lesion-symptom mapping was performed in line with deterministic tractography. The results showed that both aprosodia and amusia are strongly correlated and associated with comparable lesion patterns in right fronto-insular and striatal structures. Multiple regression models revealed fractional anisotropy changes and lower tract volume in the right inferior fronto-occipital fasciculus as the strongest predictors for both neuropsychological deficits over time. This is a convincing paper, demonstrating the core neural commonality of amusia and aprosodia. Both arise from damage and disconnection of the right ventral auditory stream, which accordingly seems to be crucial for integrating rhythmic and melodic acoustic information in prosody and music perception. The fact that deficits in emotional prosody recognition and melody processing are highly correlated is per se not surprising. The term ‘speech melody’ is used colloquially for affective prosody. Patel [3] has formulated the OPERA thesis, which essentially emphasizes the anatomical overlap of brain networks that process acoustic features used in both music and speech, for example waveform periodicity, amplitude envelope or fundamental frequency modulations. However, the detailed identification of anatomical structures underlying these common perceptual faculties is new. Another result is worth mentioning: out of the 24 patients with aprosodia or amusia in the subacute phase, 16 did not show these deficits any more in the 3-month follow-up. Obviously, there is an enormous re-organization occurring in the first 3 months after the lesion, behaviourally as well as functionally. For amusia, this has been demonstrated in a study by the same group [4]. Similarly, in an earlier behavioural study no more auditory deficit was found after 1 year in two out of eight patients suffering from severe amusia, demonstrating compensating functional neuroplasticity [5]. Although music induced emotions were not directly tested in this study, it is highly probable that deficits in melody and rhythm processing will result in reduced appreciation of music. This has consequences for the rehabilitation of stroke patients. If patients are unable to respond emotionally to music, they will most probably not benefit from the enormous effect preferred music has in the rehabilitation of cognitive and emotional deficits following stroke. Work from the same group by Särkämö et al. [6], has convincingly demonstrated that listening to favourite musical excerpts improves short-term memory, executive functions, language impairment or depression in stroke patients. Consequently, nowadays preferred music listening is offered to stroke victims to facilitate neuropsychological rehabilitation in many stroke units. Similarly neurological music therapy in stroke patients with motor deficits, applying musical sonification of arm movements to promote motor rehabilitation, is driven by the motivational and emotional power of music [7]. Given such a therapeutic potential of music, in the case of amusia, music therapists and neuropsychologists should establish therapies, exercises and other means to re-enable emotional appreciation of Apollo’s gift. This is a task that has not yet been tackled. Open access funding enabled and organized by ProjektDEAL. The authors have no conflict of interest. Eckart Altenmüller: Conceptualization (equal); writing—original draft (equal); writing—review and editing (equal).