Projects on Bilingual Aphasia – Computational Modeling and Rehabilitation
Here is a recent presentation that summarizes our current work on bilingual aphasia rehabilitation and fMRI studies. Bilingual Aphasia
Bilingual Aphasia Rehabilitation
Recently Dr. Kiran has extended her research into rehabilitation of bilingual aphasia, a subspecialty of aphasia rehabilitation with a potential for sizable clinical and theoretical impact. In the first study to systematically examine cross-language generalization in bilingual aphasia, we demonstrated the effect of semantic-based treatment on language recovery patterns in bilingual patients with aphasia Edmonds & Kiran, (2006) Kiran & Roberts (2009)
In this study, we showed that training lexical retrieval in one language resulted in cross-language generalization (improvement in percent naming accuracy) in an organized and efficient fashion. Data from 3 bilingual patients with aphasia revealed that in an equally proficient individual, training in one language resulted in improvements in both the trained language and the untrained language. In 2 bilingual patients who were more proficient in one language pre-stroke, training the less proficient language resulted in greater generalization to the untrained more proficient language. This work provides an important contribution to rehabilitation of bilingual aphasia because most treatment studies of bilingual aphasia have been case studies or weakly controlled experimental investigations (Galvez & Hinckley, 2003; Kohnert, 2004; Watamori & Sasanuma, 1978). These results are in line with our previous work on semantic complexity (Kiran, 2007, in press, under review; Kiran & Bassetto, 2008; Kiran & Thompson, 2003b) suggesting that training complex information (i.e., the less proficient language) results in generalization to less complex information (i.e., the more proficient language). Here, we hypothesize that activation of targets in the less proficient language necessitates activation in the more proficient language.
Neuroimaging Bilingual Aphasia
This study was aimed at examining the effect of current language use/exposure on the neural representation of languages in Spanish-English stroke patients with aphasia using a semantic judgment task. Functional magnetic resonance imaging was performed on three participants with aphasia and three normal controls who had demonstrated a shift towards dominance in their second language (English). The behavioral and imaging results indicate that all participants processed their non-dominant native language (Spanish) differently compared to their dominant second language (English). Specifically, increased activation was observed in the left frontal cortex and anterior cingulate gyrus during the weaker native language processing. Further, in participants with aphasia, increased bilateral activation was observed during the weaker native language processing, indicating that decreased language usage/proficiency results in a distributed network of activation. The results of this study demonstrate that the neural substrates of language recovery in bilingual stroke patients are similar to regions engaged by normal bilinguals but include additional regions reflecting a compensatory network to sub serve successful language processing.
Current research on bilingual aphasia has only begun to inform us about the optimal rehabilitation for bilingual aphasic patients (Roberts & Kiran, 2007; Edmonds & Kiran, 2006) but the literature is still sparse in terms of interpreting the nature of naming impairments in bilingual aphasia. We have developed a computational model to simulate an English-Spanish bilingual language system in which language representations can vary by age of acquisition and relative proficiency in the two languages. This model is subsequently lesioned at specific sites by varying connection strengths between the semantic and phonological networks.
Based on recent theoretical models, the three self-organizing maps (semantic, L1 and L2) each with 30×40 neurons were trained simultaneously with the associative connections between each pair of maps. All three maps used Gaussian neighborhood functions whose width decreased exponentially (from sigma = 7.0 down to 0.2) over the course of training. Later, age of acquisition for L2 was simulated by delaying the onset of training for the L2 phonetic map and its associative connections. Differences in exposure to L1 and L2 were simulated by exposing the model to more or less phonetic input in each language.
In order to match the model’s performance in both English and Spanish to that of a group (N = 39) of individual bilingual human speakers with varying AoA and relative proficiency, the training parameters were set up to match the known ages of acquisition and exposure data as closely as possible for each test case. We then extended the model to simulate a group of bilingual aphasia patients (N = 19), by attempting to replicate the patients’ self-reported AoA and pre-stroke performance. In most cases the model usually comes close to the target performance in both languages indicating the validity of this model in simulating naming impairment in bilingual aphasia.