Intervention

In each intervention week, regular SLT sessions are replaced by daily 45-minute sessions of word-finding therapy, combined with either anodal tDCS over the left IFG (1 mA, 20 minutes; experimental condition) or sham-tDCS over the same region (control condition). Therapy is provided by speech and language therapists of the participating centers. The cathode is placed on the contralateral supra-orbital region. The intensity of 1 mA tDCS for 20 minutes and the frequency of five sessions per week is in line with most studies applying tDCS in the chronic stage [11, 13–16]. tDCS is combined with word-finding therapy, because most people with aphasia have word-finding difficulties [34]. The word-finding therapy protocol is based on Cueing Hierarchy Therapy [35]. The participant’s task is to name a picture and, based on the protocol, the therapist uses cueing techniques to help the participant to retrieve and produce the target word correctly. The cue of low stimulus power is presented first, followed by increasingly powerful cues until the correct word is retrieved and produced. Basically, the following cueing hierarchy is used: (1) “What is this?”(e.g., show picture of a tree), (2) “Can you write the word down?”, (3) graphemic cueing (e.g., provide the number of letters), (4) phonological cueing (e.g., provide the first sound, /t/), (5) semantic associations (e.g., “can you tell where you can find these”), (6) therapist says the word (e.g., “tree”), (7) repetition of the target word.

As the relative power of the cues differs across participants with aphasia, the exact cueing hierarchy is personalized. For each picture, even if the picture is named without cues, the participant is encouraged to write or copy the correct word form or, in case of inability to write, to perform an anagram task. The rationale for incorporating production of the written word, is the evidence that activating the written word has a beneficial effect on retrieving spoken words [36].

To ensure relevance of the training material for each participant, stimuli are selected on the basis of individual naming performance at baseline using the European Data Bank (EDB) for oral picture naming [37]. The first 68 items the participant is unable to name correctly within 20 s are selected. These items are divided in two sets of 34 items, matched for word length and word frequency: a therapy set, trained during the word-finding therapy, and a control set, to evaluate generalization effects to untrained items. In the first session 10 items are trained. Then, during each session new items are added, with eight new items in the second session; six new items in the third and fourth session, and four new items in the final session. For the second intervention week a new training set is selected in the same way.