Transcranial direct current stimulation in poststroke aphasia recovery
Lippincott Williams and Wilkins
School of Medical and Health Sciences
Transcranial direct current stimulation (tDCS), a form of noninvasive brain stimulation originally studied for its effect on motor limb physiology, has been investigated for its use in the treatment of aphasia since 2008. The experimental use of tDCS for aphasia, however, began differently from those paradigms established for poststroke motor recovery, both conceptually and in method. Not only is aphasia research a relative newcomer to the field of tDCS experimentation, it has thus far been somewhat of an outlier in its limited use of tDCS autonomously.
Theoretically understood to be vastly more complex than our intricate motor systems, cortical language representation has most recently been conceptualized as a dual stream, diffuse network, with language processing subcomponents evolved from nonlinguistic primates. In the dual stream model, human language functions are lateralized primarily in the left hemisphere, with Broca’s area comprising the left complement of a bilateral dorsal stream network devoted to naming and articulation. Conversely, Wernicke’s area constitutes the origin of a bilateral ventral stream in which semantic meaning is attached to components of speech sounds. Additional activation in homologous right hemisphere language areas seems to be determined by lexical necessity, with increased articulatory demands activated within the bilateral dorsal stream and the decoding of unfamiliar words activated in the bilateral ventral stream network. Complex as it may be to optimally prime the motor cortex for poststroke limb rehabilitation using tDCS, it may be considered even more challenging to modulate the cortical plexus which encodes and produces language in all of its richness. The theoretical mechanisms of brain activation during tDCS protocols suggest that tDCS primes the brain for enhanced outcomes in behavioral therapies, which may have led to the appeal of combining methods concurrently.