Rhythm in disguise: why singing may not hold the key to recovery from aphasia

Abstract

The question of whether singing may be helpful for stroke patients with non-fluent aphasia has been debated for many years. However, the role of rhythm in speech recovery appears to have been neglected. In the current lesion study, we aimed to assess the relative importance of melody and rhythm for speech production in 17 non-fluent aphasics. Furthermore, we systematically alternated the lyrics to test for the influence of long-term memory and preserved motor automaticity in formulaic expressions. We controlled for vocal frequency variability, pitch accuracy, rhythmicity, syllable duration, phonetic complexity and other relevant factors, such as learning effects or the acoustic setting. Contrary to some opinion, our data suggest that singing may not be decisive for speech production in non-fluent aphasics. Instead, our results indicate that rhythm may be crucial, particularly for patients with lesions including the basal ganglia. Among the patients we studied, basal ganglia lesions accounted for more than 50% of the variance related to rhythmicity. Our findings therefore suggest that benefits typically attributed to melodic intoning in the past could actually have their roots in rhythm. Moreover, our data indicate that lyric production in non-fluent aphasics may be strongly mediated by long-term memory and motor automaticity, irrespective of whether lyrics are sung or spoken.

Figures

Figure 1

T2-weighted MRI scans (axial view) of Patients PR (A) and AS (B). Both scans show left middle cerebral artery infarctions, with only Patient PR's lesion including the left basal ganglia.

Figure 2

Schematic overview of the experimental conditions. Three lyric types are employed: original, formulaic and non-formulaic lyrics (from top to bottom). Each lyric type is produced in three experimental modalities: melodic intoning, rhythmic speech and a spoken arrhythmic control. In the conditions melodic intoning and rhythmic speech, patients sing or speak along with a playback composed of a voice to mimic and a rhythmic percussion beat, which is shown here (rhythmic). The first beat in every 4/4 measure is stressed by lowering the percussion frequency and by accentuating its intensity. In the spoken arrhythmic control, the percussion beat turns into a 3/4 stress pattern, and is shifted by an eighth note (arrhythmic).

Figure 3

Correctly produced syllables in the conditions melodic intoning (sung) and rhythmic speech (spoken) for three lyric types. Articulatory quality significantly differed for each lyric type, irrespective of whether sung or spoken (*P < 0.05; ***P < 0.001). Error bars represent confidence intervals corrected for between-subject variance (Loftus and Masson, 1994).

Figure 4

Correctly produced syllables in the conditions rhythmic speech (spoken) and the spoken arrhythmic control (arrhythmic) averaged across lyric types. The results show a significant interaction of basal ganglia (BG) lesions and rhythmicity (**P < 0.01). Nine patients with larger basal ganglia lesions (composite basal ganglia lesion score >1.5) tended to perform worse in the arrhythmic control compared with rhythmic speech. This pattern was not found in eight patients with smaller basal ganglia lesions (composite basal ganglia lesion score ≤1.5). Error bars represent confidence intervals corrected for between-subject variance (Loftus and Masson, 1994).

Figure 5

Correctly produced syllables of eight elderly patients (aged >55) and nine younger patients (aged ≤55), averaged over modalities. The results show a significant interaction of age and lyric memory (**P < 0.01). Only elderly patients showed an increased performance of original lyrics compared with formulaic lyrics. Error bars represent confidence intervals corrected for between-subject variance (Loftus and Masson, 1994).