The impact of speech rhythm and rate on comprehension in aphasia

Speech comprehension impairment in post-stroke aphasia is influenced by speech acoustics. This study investigated the impact of speech rhythm (syllabic isochrony) and rate on comprehension in people with aphasia (PWA). Rhythmical speech was hypothesised to support comprehension in PWA by reducing temporal variation, leading to enhanced speech tracking and more appropriate sampling of the speech stream. Speech rate was hypothesised to influence comprehension through auditory and linguistic processing time.

One group of PWA (n = 19) and two groups of control participants (n = 10 and n = 18) performed a sentence-verification. Sentences were presented in two rhythm conditions (natural vs isochronous) and two rate conditions (typical, 3.6 Hz vs slow, 2.6 Hz) in a 2 × 2 factorial design. PWA and one group of controls performed the experiment with clear speech. The second group of controls performed the experiment with perceptually degraded speech.

D-prime analyses measured capacity to detect incongruent endings. Linear mixed effects models investigated the impact of group, rhythm, rate and clarity on d-prime scores. Control participants were negatively affected by isochronous rhythm in comparison to natural rhythm, likely due to alteration in linguistic cues. This negative impact remained or was exacerbated in control participants presented with degraded speech. In comparison, PWA were less affected by isochronous rhythm, despite producing d-prime scores matched to the degraded speech control group. Speech rate affected all groups, but only in interactions with rhythm, indicating that slow-rate isochronous speech was more comprehendible than typical-rate isochronous speech.

The comprehension network in PWA interacts differently with speech rhythm. Rhythmical speech may support acoustic speech tracking by enhancing predictability and ameliorate the detrimental impact of atypical rhythm on linguistic cues. Alternatively, reduced temporal prediction in aphasia may limit the impact of deviation from natural temporal structure. Reduction of speech rate below the typical range may not benefit comprehension in PWA.