The purpose of this study is to investigate whether transcranial direct current stimulation (tDCS) of the prefrontal cortex (PFC) influences lexical access and language production. The experimental paradigm will assess the impact of prefrontal stimulation by tDCS versus sham tDCS (S-tDCS) over the PFC of patients with chronic post-stroke aphasia during three language production tasks and a nonverbal executive functions task.

Background:

Language processing is a complex brain function supported by a large network, including domain-specific language areas as well as domain-general cognitive-control networks (Friederici & Gierhan, 2013). Noninvasive brain stimulation, such as transcranial direct current stimulation (tDCS), is increasingly being used as a promising therapeutic tool for psychiatric and neurological diseases (Tortella et al., 2015; Flöel, 2014). In the language domain, several studies revealed that tDCS over languagespecific areas induces changes in cortical function that enhances the recovery of language abilities in patients with post-stroke aphasia (Torres, Drebing & Hamilton, 2013; Monti et al., 2013). Beneficial effects of tDCS have also been found for stimulation over more domain-general cognitive control regions. Although research on non-invasive brain stimulation and aphasia reveals promising results, studies investigating the modulation of cognitive control-networks on lexical access are rare. Given the importance of a successful interplay between prefrontal and domain-specific language areas, possible therapeutic effects of tDCS over the prefrontal cortex (PFC) in aphasia can be of high value for rehabilitation and basic research.

Procedure:

A planned total of 30 patients will be included. In a first visit, the severity of aphasia, the medical history as well as inclusion/exclusion criteria will be evaluated. After this visit, patients will undergo two tDCS sessions (one tDCS and one S-tDCS session) with a one week interval between the sessions. Each session consists of an online (during stimulation) and an offline assessment (within 30 minutes after stimulation). Three language tasks and a nonverbal executive function task will be conducted online as well as offline in each of the two sessions (tDCS and S-tDCS).

Inclusion Criteria:

• Chronic aphasia due to ischemic or haemorrhagic stroke (> 6 months post-stroke)
• French as dominant language
• Right-handedness
• Left hemisphere lesion with intact bilateral PFC

Exclusion Criteria:

• Diagnosed dementia or psychiatric comorbidity
• Epileptic seizure within the last 12 months
• Metallic head implants
• Pacemaker
• Inability to understand procedures or insufficient language production abilities
• pregnancy
• strong headache on the days of the tDCS sessions
• consumption of alcohol and/or unprescribed drugs on the days of the tDCS sessions or on the day before