Are networks for residual language function and recovery consistent across aphasic patients?

Abstract

Objectives: If neuroplastic changes in aphasia are consistent across studies, this would imply relatively stereotyped mechanisms of recovery which could guide the design of more efficient noninvasive brain stimulation treatments. To address this question, we performed a meta-analysis of functional neuroimaging studies of chronic aphasia after stroke.

Methods: Functional neuroimaging articles using language tasks in patients with chronic aphasia after stroke (n = 105) and control subjects (n = 129) were collected. Activation likelihood estimation meta-analysis determined areas of consistent activity in each group. Functional homology between areas recruited by aphasic patients and controls was assessed by determining whether they activated under the same experimental conditions.

Results: Controls consistently activated a network of left hemisphere language areas. Aphasic patients consistently activated some spared left hemisphere language nodes, new left hemisphere areas, and right hemisphere areas homotopic to the control subjects' language network. Patients with left inferior frontal lesions recruited right inferior frontal gyrus more reliably than those without. Some areas, including right dorsal pars opercularis, were functionally homologous with corresponding control areas, while others, including right pars triangularis, were not.

Conclusions: The network of brain areas aphasic patients recruit for language functions is largely consistent across studies. Several recruitment mechanisms occur, including persistent function in spared nodes, compensatory recruitment of alternate nodes, and recruitment of areas that may hinder recovery. These findings may guide development of brain stimulation protocols that can be applied across populations of aphasic patients who share common attributes.

Figure 1. Activation likelihood estimation (ALE) maps in all aphasic patients and control subjects

Control ALE clusters are in blue-green scale, and show left hemisphere language and motor activity. ALE clusters for the group of all aphasic patients are in red-yellow scale. Significant areas in ALE maps represent locations at which peak activity is highly likely to occur in functional imaging experiments, not functional activity per se. ALE maps are overlaid on the standard Colin brain in Montreal Neurological Institute (MNI) space, using a false discovery rate q = 0.01 critical threshold, and minimum cluster size of 100 mm³. Slices are in radiologic orientation, with the corresponding MNI Z coordinate.

Figure 2. Activation likelihood estimation (ALE) maps of inferior frontal cortex (IF)+ and IF− groups

IF− ALE clusters are in blue-green scale. IF+ ALE clusters are in red-yellow scale. ALE maps are overlaid on the standard Colin brain in Montreal Neurological Institute (MNI) space, using a false discovery rate q = 0.01 critical threshold, and minimum cluster size of 100 mm³. Slices are in radiologic orientation, with the corresponding MNI Z coordinate.

Figure 3. Algorithms used for interpretation and schematics of main results

Careful consideration of the response properties of areas identified in the aphasia activation likelihood estimation (ALE) analyses suggested reasonable hypotheses regarding the specific adaptive mechanisms to account for the activity. We interpreted the areas identified in the aphasia ALE maps algorithmically based on localization of activity relative to normal control activity, functional homology to corresponding normal control sites, and relationship of activity to lesion location. (A) The algorithm used for left hemisphere (LH) areas. (B) The algorithm used for right hemisphere (RH) areas. (C) Schematics of the main results. Since the right pre/postcentral gyrus activity occurred only in the inferior frontal cortex (IF)+ group, we assumed that the left pre/postcentral gyri were lesioned with the left inferior frontal gyrus (IFG) in most cases (this appeared to be true based on available figures in the papers). If this assumption were false, the correct interpretation would be g. *See Discussion for caveats. MTG=middle temporal gyrus; POp=pars opercularis; POrb=pars orbitalis; PTr=pars triangularis; MFG=middle frontal gyrus.