In the Annals of Neurology, a team of physicians, led by Tony Ro of the Department of Psychology at Rice University in Houston, Texas, report the unusual case of a woman who began to feel sounds following a stroke
The woman, a 36-year-old professor, suffered a rare type of cerebrovascular accident: a lacunar infarct, in which a small blood vessel deep within the brain became blocked. This led to damage in the ventrolateral thalamic nucleus (VL) on the right side of her brain.
When first examined, some 9 months after her stroke, the woman reported significant changes in her sensations and attentional abilities. Her abilitiy to detect tactile sensations on her left foot, hand, arm and face were markedly decreased. She also reported hemispatial neglect – that is, she would occasionally bump into the left side of doorways and veer rightwards when driving.
About 1 year after her stroke, the physicians performed a series of behavioural tests on the woman. She was still neglecting the left side of her body, but also demonstrated a phenomenon called unisensory anti-extinction: when two identical visual or tactile stimuli were presented simultaneously, one to either side of her body, the woman reported that the stimulus on the left was significantly stronger. But only 50% and 73% of the tactile and sensory stimuli, respectively, were detected when presented alone to the left side.
The woman then underwent two different types of neuroimaging (functional magnetic resonance and diffusion tensor imaging; the former providing images of brain activity, the latter of white matter tracts that connect different parts of the brain). The scans showed a small area of damage to the right VL.
About one-and-a-half years after her stroke, the woman was no longer neglecting the left side of her body, but instead reported that she had begun to experience sound-touch synaesthesia. She would feel tingling sensations in her left hand and arm when she heard certain sounds, such as the voice of a particular radio presenter, and this was confirmed by further behavioural tests in which she was presented with auditory and/ or tactile stimuli.
More brain scans were then performed. The fMRI showed that the lesion was exactly the same size, and in the same location, as the previous scans. But the DTI data showed that there had been a marked reduction in the number of fibres connecting the thalamus and the cerebral cortex in the right hemisphere, and that the remaining connections were more disorganized than those on the left.
The team of physicians therefore believe that the changes in the patient’s sensory perceptions could have occured as a result of anatomical and functional re-organization of the neural connections between the thalamus and the cortex. For example, conoections damaged as a result of the stroke may have strenghtened the connections between the auditory and somatosensory cortices, which would produce the sound-touch synaesthesia demonstrated by the patient.
The VL was believed to be involved in motor function, but this study provides some evidence that it also has sensory functions. It also supports the idea that other reported cases of acquired synaesthesia following brain damage are a result of cross-wiring between sensory modalities.
Ro, T., et al. (2007). Feeling sounds after a thalamic lesion. Ann. Neurol. doi: 10.1002/ana.21219.