Topographic Plasticity of Language Sites across Time

Convincing evidence of individual variability in location of language exists from intraoperative cortical stimulation results during awake craniotomies in patients with lesions in eloquent areas. However, little information is available about whether the localization of these areas is stable over time or if plastic changes allow a topographic shift of the functional units in the adult brain. The purpose of this investigation was to find out whether the localization of language-associated areas in the inferior frontal gyrus was reproducible in a 23-year-old, right-handed man who presented with a history of febrile seizures and was diagnosed with CT and MRI scans suggesting a left temporal low-grade astrocytoma. Neurological examination was normal. Surgery and reoperation for recurrence 4 years later was performed in local anesthesia with a bone flap of 2.5cm in diameter to expose the superior temporal gyrus and the perisylvian cortex of the fontal lobe. Brain mapping techniques involved cortical stimulation, electrocorticography for detecting after discharges during stimulation and simultaneous neuropsychological testing including object naming, reading and free talking and the retrieval of newly learnt material. Using a constant-current stimulus isolation unit connected to a bipolar electrode with the tips 2mm apart stimulation was performed systematically, starting at low current flows and increasing the current up to 20 mA (50Hz, 0.2 ms). In the first operation reversible speech arrest or slowing of speech during stimulation could be provoked in only two circumscribed small areas in the inferior frontal gyrus, an area which was not infiltrated by the tumor macroscopically and radiographically. These effects were reproducible after tumor removal at the end of the first operation. At reoperation 4 years later the former area of the stimulation effect could be easily identified because the same surgical approach was used. In contrast to the first operation no language site was detectable with stimulation currents up to 20 mA. This area was still free of tumor. The patient showed no aphasia at any time. These results suggest a reorganization of the network involved in the processing of language. However, it cannot be distinguished whether this effect was caused by the temporal tumor or the tumor removal, or whether these plastic changes occur under physiological conditions, too.