Enhancement of verb retrieval: Neuromodulation, repetition priming, and aphasia rehabilitation

There is a need to increase the efficacy and efficiency of aphasia rehabilitation protocols. In order to do this, we may rely on knowledge about how the language system may be changed by experience, including it’s plasticity to behavioral training and to neuromodulation. In this thesis, I study mechanisms of language facilitation in healthy individuals and mechanisms of language recovery in individuals with aphasia. I use both behavioral modification techniques (repeated naming in healthy individuals, and linguistically motivated aphasia therapy in individuals with aphasia), and neuromodulation (transcranial direct current stimulation, tDCS). The data in this thesis indicate that, in healthy individuals, language facilitation by repetition priming reflects changes in implicit processing of stimuli (at the level of lexical retrieval), and explicit episodic retrieval of the prior occurrence of the stimuli. In patients with aphasia, the data indicates that the observation of item-specific improvement after treatment depend on pre-treatment levels accessibility to the lexeme level via semantics, and also on short term memory skills and/or post-lexical processing skills. Generalization may occur when damage to abstract features is present(semantic and/or grammatical), and when knowledge of abstract features is engaged during treatment. tDCS has been successfully used in prior research to enhance language training and rehabilitation effects. However, our data from healthy individuals did not reveal effects of neuromodulation, and the data with patients was ambigous: it is not possible to discern whether tDCS indeed incrteased the effects of therapy or wether the data are better explained as reflecting a ceiling effect.