Purpose of review
Functional neuro-imaging studies of aphasic stroke offer the potential for a better understanding of the neuronal mechanisms that sustain language recovery. Conclusions, however, have been hampered by a set of unexpected challenges related to experimental design and interpretation. In this review of studies published between January 2004 and February 2005, we discuss imaging studies of speech production and comprehension in patients with aphasia after left hemisphere stroke.
Studies of speech production suggest that recovery depends on slowly evolving activation changes in the left hemisphere. In contrast, right hemisphere activation changes have been interpreted in terms of transcallosal disinhibition that do not reflect recovery because they occur early after stroke, in areas homologous to the lesion, and do not appear to correlate with the level of recovery. There have been few studies of auditory speech comprehension, but unlike speech production, recovery of speech comprehension appears to depend on both left and right temporal lobe activation.
Together, recent studies provide a deeper appreciation of how the neuronal mechanisms of recovery depend on the task, the lesion site, the time from insult and the distinction between neuronal reorganization that does and does not sustain recovery. Although many more studies of aphasic stroke are required with larger patient numbers and more focal lesion sites, we also argue that clinical diagnosis and treatment requires a better understanding of the normal variability in functional anatomy and the many neuronal pathways that are available to sustain each type of language task.