A fMRI study of word retrieval in aphasia

Disrupting the brain to guide plasticity and improve behavior

Neurones may be highly stable and nonplastic cellular structures, but they are engaged in dynamically changing, intrinsically plastic neural networks that provide a most energy efficient, spatially compact, and precise means to process input signals and generate adaptable responses to a changing environment. Neural plasticity is evolution's invention to enable the nervous system to escape the restrictions of its own genome (and its highly specialized cellular specification) and thus adapt to environmental pressures, physiologic changes, and experiences. At neural system level two steps of plasticity can be identified: unmasking existing connections that may be followed by establishment of new ones possibly even with integration of new neural structures and neurons. In any case, plastic changes may not necessarily represent a behavioral gain for a given subject, as they represent the mechanism for development and learning, as much as a cause of pathology and disease. The challenge is to learn enough about the mechanisms of plasticity to be able to guide them, suppressing changes that may lead to undesirable behaviors while accelerating or enhancing those that result in a behavioral benefit for the subject or patient. Neurostimulation, including noninvasive brain stimulation techniques, provide an opportunity to modulate brain plasticity in a controlled and specific manner. Such interventions to guide behavior or treat pathological symptomatology might be more immediate in their behavioral repercussion and thus more effective than approaches intent on addressing underlying genetic predispositions.

Therapy-related reorganization of language in both hemispheres of patients with chronic aphasia

The brain processes of language recovery after stroke are poorly understood, partly because past research did not allow to differentiate the effects of spontaneous restitution processes from those of learning-related cortical reorganization. Here, we use a new approach offered by recently developed intense neuropsychological therapy methods, which allow for improving language functions within a short time period. Stroke patients with chronic aphasia received intense language therapy for 2 weeks and, over this period, improved their language performance as assessed using clinical tests. Neurophysiological activity elicited by words and pseudowords was measured before and after treatment. Over the therapy interval, early word evoked potentials (latency 250-300 ms) became significantly stronger whereas pseudoword responses did not change. Word-specific changes were documented by analyses of ERP amplitudes and root mean square values, which revealed interactions of the factors Assessment time (before vs. after therapy) and Wordness (word vs. pseudoword). Source localization using Minimum Norm Current Estimates showed that bilateral cortical sources activated by word stimuli contributed to the change, suggesting that neuronal networks distributed over both hemispheres are the substrate of cortical reorganization of language processing in intense aphasia therapy. Word-evoked differences in source strengths were significantly correlated with performance on a clinical language test, demonstrating a link between behavioral and neurophysiological changes. We suggest that the early word-evoked negativity might represent an index of reorganization of language after stroke and thus an aphasia recovery potential.

Overt naming in aphasia studied with a functional MRI hemodynamic delay design

The purpose of this study was to develop a functional MRI method to examine overt speech in stroke patients with aphasia. An fMRI block design for overt picture naming was utilized which took advantage of the hemodynamic response delay where increased blood flow remains for 4-8 s after the task [(Friston, K.J., Jezzard, P., Turner, R., 1994. Analysis of functional MRI time-series. Hum. Brain Mapp. 1, 153-171)]. This allowed task-related information to be obtained after the task, minimizing motion artifact from overt speech (Eden, G.F., Joseph, J., Brown, H.E., Brown, C.P., Zeffiro, T.A., 1999. Utilizing hemodynamic delay and dispersion to detect fMRI signal change without auditory interference: the behavior interleaved gradients technique. Magn. Reson. Med. 41, 13-20; Birn, RM., Bandettini, P.A., Cox, R.W., Shaker, R., 1999. Event-related fMRI of tasks involving brief motion. Hum. Brain Mapp. 7, 106-114; Birn, R.M., Cox, R.W., Bandettini, P.A., 2004. Experimental designs and processing strategies for fMRI studies involving overt verbal responses. NeuroImage 23, 1046-1058). Five chronic aphasia patients participated (4 mild-moderate and 1 severe nonfluent/global). The four mild-moderate patients who correctly named 88-100% of the pictures during fMRI, had a greater number of suprathreshold voxels in L supplementary motor area (SMA) than R SMA (P < 0.07). Three of these four mild-moderate patients showed activation in R BA 45 and/or 44; along with L temporal and/or parietal regions. The severe patient, who named no pictures, activated almost twice as many voxels in R SMA than L SMA. He also showed activation in R BA 44, but had remarkably extensive L and R temporal activation. His poor naming and widespread temporal activation may reflect poor modulation of the bi-hemispheric neural network for naming. Results indicate that this fMRI block design utilizing hemodynamic response delay can be used to study overt naming in aphasia patients, including those with mild-moderate or severe aphasia. This method permitted verification that the patients were cooperating with the task during fMRI. It has application for future fMRI studies of overt speech in aphasia.

Improved naming after TMS treatments in a chronic, global aphasia patient--case report

We report improved ability to name pictures at 2 and 8 months after repetitive transcranial magnetic stimulation (rTMS) treatments to the pars triangularis portion of right Broca's homologue in a 57 year-old woman with severe nonfluent/global aphasia (6.5 years post left basal ganglia bleed, subcortical lesion). TMS was applied at 1 Hz, 20 minutes a day, 10 days, over a two-week period. She received no speech therapy during the study. One year after her TMS treatments, she entered speech therapy with continued improvement. TMS may have modulated activity in the remaining left and right hemisphere neural network for naming.

Test-retest reliability of fMRI during nonverbal semantic decisions in moderate-severe nonfluent aphasia patients

Cortical reorganization in poststroke aphasia is not well understood. Few studies have investigated neural mechanisms underlying language recovery in severe aphasia patients, who are typically viewed as having a poor prognosis for language recovery. Although test-retest reliability is routinely demonstrated during collection of language data in single-subject aphasia research, this is rarely examined in fMRI studies investigating the underlying neural mechanisms in aphasia recovery.

The purpose of this study was to acquire fMRI test-retest data examining semantic decisions both within and between two aphasia patients. Functional MRI was utilized to image individuals with chronic, moderate-severe nonfluent aphasia during nonverbal, yes/no button-box semantic judgments of iconic sentences presented in the Computer-assisted Visual Communication (C-ViC) program. We investigated the critical issue of intra-subject reliability by exploring similarities and differences in regions of activation during participants' performance of identical tasks twice on the same day. Each participant demonstrated high intra-subject reliability, with response decrements typical of task familiarity. Differences between participants included greater left hemisphere perilesional activation in the individual with better response to C-ViC training. This study provides fMRI reliability in chronic nonfluent aphasia, and adds to evidence supporting differences in individual cortical reorganization in aphasia recovery.

Essential language function of the right hemisphere in brain tumor patients

Neuroimaging studies of language networks in patients with brain lesions of the left language-dominant hemisphere have shown activation in the right inferior frontal gyrus (IFG). We tested the functional relevance of right IFG activation using neuroimaging-guided repetitive transcranial magnetic stimulation (rTMS) to disturb language function over bilateral IFG in right-handed patients with brain tumors and controls. All subjects were susceptible to TMS over the left IFG. In patients, this susceptibility correlated with left-sided the degree of language lateralization to the left. Those patients with lowest dominance were also susceptible to right-sided TMS proving relevant language function of the right IFG.

Spatio-temporal cortical dynamics of phonemic and semantic fluency

Hemodynamic brain imaging and lesion studies have suggested differential involvement of expressive language-related cortical regions based on the phonemic versus semantic characteristics of verbal cues. The aims of this study were: 1) to elucidate the relative timing of the activity of inferior frontal and anterior insular versus motor and supplementary motor cortex during a fluency task and 2) to assess potential differences in the location or timing of activity in anterior and posterior language areas based on letter versus category cues. Using magnetic source imaging (MSI), we found significantly earlier onset latencies and a greater number of activity sources in motor and supplementary motor compared with inferior frontal and anterior insular regions. We also observed greater left versus right hemispheric asymmetry of activation for letter compared with category cues. This study provides new insights into cortico-cortical interactions during expressive language tasks.