Can tDCS enhance item-specific effects and generalization after linguistically motivated aphasia therapy for verbs?

Abstract

Background: Aphasia therapy focusing on abstract properties of language promotes both item-specific effects and generalization to untreated materials. Neuromodulation with transcranial Direct Current Stimulation (tDCS) has been shown to enhance item-specific improvement, but its potential to enhance generalization has not been systematically investigated. Here, we test the efficacy of ACTION (a linguistically motivated protocol) and tDCS in producing item-specific and generalized improvement in aphasia.Method: Nine individuals with post-stroke aphasia participated in this study. Participants were pre-tested with a diagnostic language battery and a cognitive screening. Experimental tasks were administered over multiple baselines. Production of infinitives, of finite verbs and of full sentences were assessed before and after each treatment phase. Nonword repetition was used as a control measure. Each subject was treated in two phases. Ten daily 1-h treatment sessions were provided per phase, in a double-blind, cross-over design. Linguistically-motivated language therapy focusing on verb inflection and sentence construction was provided in both phases. Each session began with 20 min of real or sham tDCS. Stimulation site was determined individually, based on MRI scans.Results: Group data showed improved production of treated and untreated verbs, attesting the efficacy of behavioral treatment, and its potential to yield generalization. Each individual showed significant item-specific improvement. Generalization occurred in the first phase of treatment for all subjects, and in the second phase for two subjects. Stimulation effects at the group level were significant for treated and untreated verbs altogether, but a ceiling effect for Sham cannot be excluded, as scores between real tDCS and Sham differed only before treatment.Conclusion: Our data demonstrate the efficacy of ACTION and suggest that tDCS may enhance both item-specific effects and generalization.

Improving Production of Treated and Untreated Verbs in Aphasia: A Meta-Analysis

“…are stored independently from the corresponding phonological form (Miozzo and Caramazza, 1997 ; Roelofs et al, 1998 ; Bastiaanse and Van Zonneveld, 2004 ), and are retrieved even when verbs are produced as isolated words. While some studies suggest that sentence level treatment may be more conducive to generalization (Links et al, 2010 ; Thompson et al, 2013 ; de Aguiar et al, 2015 ), in the current study, the level of input (receiving cues as single words, sentences, or both) and the level of output (producing single words, sentences, or both) did not feature in the interactions of variables predicting either outcome. Rather, syntactic features may be accessed implicitly even when treatment does not overtly require it and even if it is provided at the single word level.…”

“…In the intact language system, different verbs share information about the syntactic structures in which they occur (Pickering and Branigan, 1998 ), and this facilitates production of shared constructions (structural priming: e.g., Bock, 1986 ). In aphasia, generalization is reported in the lexical retrieval of verbs after treatment of argument structure (Thompson et al, 2013 ), and of tense production in sentences (Links et al, 2010 ; de Aguiar et al, 2015 ). Our findings are in line with these studies, as patients with grammatical impairment who did not receive morphological training were less likely to improve in the production of untreated verbs than those who did.…”

“…A different outcome can be expected when information shared by many items is unavailable. As discussed above, restoration of grammatical information using a small set of treated items can make the same knowledge available for untreated verbs (Links et al, 2010 ; Thompson et al, 2013 ; de Aguiar et al, 2015 ). However, patients may only benefit from this transfer of knowledge if they are lacking it before treatment.…”

“…As suggested by Dickey and Yoo ( 2010 ) improvement in production of treated and untreated items relies on different mechanisms. Generalization 1 is seldom reported in the aphasia rehabilitation literature, though it was observed after treatment of argument structure (Thompson et al, 2013 ), and of tense production in sentences (Links et al, 2010 ; de Aguiar et al, 2015 ). It has been proposed that generalization is constrained by the underlying cognitive impairment (it would be more likely in the event of semantic impairment, than in the event of lexeme-level damage; Miceli et al, 1996 ) and/or by characteristics of the therapy task (more likely when abstract semantic or syntactic features are treated; e.g., Boyle and Coelho, 1995 ; Thompson and Shapiro, 2005 ).…”

“…In addition, it has been proposed that generalization is also influenced by an interaction of linguistic and extra-linguistic computations: with practice, the treatment task becomes easier and the cognitive load of task-specific computations is reduced. Consequently, more processing resources can be allocated to lexical retrieval when treated and untreated items are presented in the same task (de Aguiar et al, 2015 ).…”