Neural Bases of Language Recovery After Stroke Can Only Be Fully Understood Through Longitudinal Studies of Individuals.

Despite decades of intense interest and investment in cognitive science, there remains a not only incomplete but also highly inconsistent body of evidence regarding how adult brains recover from even the most focal injuries associated with stroke. In this paper, I provide a broad narrative review of the studies of post-stroke aphasia recovery that have sought to identify the mechanisms of language recovery through longitudinal functional imaging. I start with studies that used functional imaging in groups of neurotypical individuals that have revealed areas of the brain that are reliably activated by language tasks and are functionally connected, referred to here as the "language network." I then review group studies in which functional imaging data were averaged across groups of people with post-stroke aphasia to characterize the neurobiology of recovery. These group studies of post-stroke aphasia have yielded very different results and have led to conflicting conclusions. Subsequently, I examine results of studies of single subjects that have employed longitudinal functional imaging to identify mechanisms of language recovery. Together, these single subject studies make it clear that mechanisms of neural recovery are far from uniform, even in people with very similar lesions and time since stroke. On this basis, I argue that it is not justifiable to average functional imaging data across individuals with post-stroke aphasia to draw meaningful insights into how brain networks change to support language recovery. Each individual's brain networks change over time, but in divergent ways that depend on the extent of disruption to the normal language network, interventions to facilitate recovery, the health of the intact brain, and other variables yet to be identified. While averaging results across participants with post-stroke aphasia might be able to identify certain changes in the networks that are correlated with specific language gains, uncovering the range of mechanisms and dynamics of language recovery after stroke requires longitudinal imaging of individuals.