Predicting online behavioural responses to transcranial direct current stimulation in stroke patients with anomia

BACKGROUND: Anomia, or difficulty naming common objects, is the most common, acquired impairment of language. Effective therapeutic interventions for anomia typically involve massed practice at high doses, requiring significant investment from patients and therapists. Aphasia researchers have increasingly looked to neurostimulation to accelerate these treatment effects, but the evidence behind this intervention is sparse and inconsistent.AIM: Here, we hypothesised that group-level neurostimulation effects might belie more systematic structure at the individual level. We sought to test the hypothesis by attempting to predict the immediate (online), individual-level behavioural effect of neurostimulation in patients with anomia.METHODS: 36 stroke patients, each with anomia at least 6 months post-onset, were asked to perform naming and judgement tasks both with concurrent neurostimulation and with sham stimulation. Using clinical, (pre-stimulation) behavioural and MRI data, and Partial Least Squares regression, we attempted to predict the effect of neurostimulation on accuracies and reaction times in both tasks. Model performance was assessed via cross-validation, and performances compared to that of a null model, which predicted the mean neurostimulation effects for all patients.RESULTS: Models derived from pre-stimulation data consistently out-performed the null model when predicting neurostimulation effects on accuracies and reaction times in both judgement and naming. Notably, we could predict declines in performance just as well as improvements.CONCLUSIONS: Inter-patient variation in online responses to neurostimulation is to some extent systematic and predictable. That declines in performance were just as predictable as improvements, implies that the effect of neurostimulation on performance in patients with anomia is unlikely to be a placebo. However, the online effect of the intervention appears to be just as likely to interfere with task performance, as it is to improve performance.