Aphasia is a common result of stroke, affecting over one million Americans. Currently, intensive speech therapy is the mainstay of treatment, although its efficacy has been variable at best. Recent years have seen the emergence of non-invasive brain stimulation, specifically Transcranial Magnetic Stimulation (TMS) and Transcranial Direct Current Stimulation (tDCS), as potential treatments for post-stroke aphasia. A growing body of investigations has shown the efficacy of both modalities in facilitating recovery from chronic aphasia, while data regarding subacute aphasia are much more limited and evidence in the acute post-stroke phase are still lacking. Much remains unknown about how these techniques cause clinical improvement or about their long-term efficacy, side-effects, and safety. In this article, we examine the data demonstrating the safety and efficacy of TMS and tDCS, discuss the major differences between them, and consider how those differences may inform the use of each modality. We also consider the different models of neuroplasticity in the setting of post-stroke aphasia and how these models may influence when and in which patients each modality would impart the most benefit.

While converging evidence implicates the right inferior parietal lobule in audiovisual integration, its role has not been fully elucidated by direct manipulation of cortical activity. Replicating and extending an experiment initially reported by Kamke et al. (2012), we employed the sound-induced flash illusion, in which a single visual flash, when accompanied by two auditory tones, is misperceived as multiple flashes (Wilson, 1987; Shams et al., 2000). Slow repetitive (1 Hz) TMS administered to the right angular gyrus, but not the right supramarginal gyrus, induced a transient decrease in the Peak Perceived Flashes (PPF), reflecting reduced susceptibility to the illusion. This finding independently confirms that perturbation of networks involved in multisensory integration can result in a more veridical representation of asynchronous auditory and visual events and that cross-modal integration is an active process in which the objective is the identification of a meaningful constellation of inputs, at times at the expense of accuracy.

Recent studies have found preferential responses for brief, transient visual stimuli near the hands, suggesting a link between magnocellular visual processing and peripersonal representations. We report an individual with a right hemisphere lesion whose illusory phantom percepts may be attributable to an impairment in the peripersonal system specific to transient visual stimuli. When presented with a single, brief (250 ms) visual stimulus to her ipsilesional side, she reported visual percepts on both sides – synchiria. These contralesional phantoms were significantly more frequent when visual stimuli were presented on the hands versus off the hands. We next manipulated stimulus duration to examine the relationship between these phantom percepts and transient visual processing. We found a significant position by duration interaction, with substantially more phantom synchiric percepts on the hands for brief compared to sustained stimuli. This deficit provides novel evidence both for preferential processing of transient visual stimuli near the hands, and for mechanisms that, when damaged, result in phantom percepts.

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After being presented with an ipsilesional stimulus, individuals with synchiria report both ipsilesional and contralesional sensation. Most published reports of synchiria are limited to a single modality. We report an individual with a right frontoparietal lesion (KG_591) who demonstrated synchiria subsequent to both visual and tactile stimulation. We tested KG_591 with various tasks in which she was presented with a stimulus on the left side, right side, both sides simultaneously, or no stimulation. On tactile trials, she reported tactile synchiric percepts on approximately 40% of right hand stimulation trials. Next, we projected visual stimuli either onto or away from her hands, in order to examine whether her phantom visual sensations were limited to personal space on the body. KG_591's synchiric percepts remained constant on or off of her hands, suggesting that her deficit was not limited to her body. Furthermore, as she does not report seeing phantoms in everyday life, we examined the effect of stimulus length on synchiric perception. Phantom synchiric percepts were most frequent after 250 millisecond visual stimuli, becoming less frequent with longer stimulus times. We discuss the implications of these and other results with regards to multisensory perception and the functional architecture that underlies synchiric perception.