Functional mapping.

Direct cortical electrostimulation during brain surgery has been the basis of much of our understanding of functional regions of the cortex. The classic electrostimulation studies of Wilder Penfield and Herbert Jasper still guide our approaches to eloquent brain regions today. Direct cortical electrostimulation has also traditionally been the litmus test of whether neurological deficits would be incurred by tissue removal. This principle guides neurosurgeons removing lesions in the vicinity of languagebearing or sensorimotor cortex. It is also clear that removal of other known functional brain regions could result in specific neurological dysfunction (such as visual loss in occipital resection, alexia in dominant posterior temporal resection, and Gerstmann syndrome in dominant parietal lobe resection). The accompanying study by Della Puppa et al. documents an unexpected finding of mathematical processing dysfunction elicited by direct cortical electrostimulation of the nondominant parietal lobe, in the vicinity of the intraparietal sulcus.1 Data was obtained from 3 patients who were undergoing awake surgery of this area, and the authors found a reproducible dysfunction of multiplication and addition functions induced by direct cortical stimulation in the nondominant parietal lobe at relatively low amplitudes. These regions were spared and the patients had no deficit in mathematical processing postoperatively. Given that conventional wisdom holds that computational processing is housed in the dominant inferior parietal lobule, the results of this study contradict that widely held belief. Theirs is therefore an interesting and potentially important observation. It is not clear, however, that there is a 1-to-1 correlation between stimulation-induced function and focal deficit after resection. For example, the basal temporal lobe cortex on the dominant side commonly harbors language sites, yet resection of this area is routine in temporal lobe resections, and results in no detectable deficit. One must therefore interpret the results of the present study with caution. It must be remembered that mathematical processing—like many of the more complex brain functions—is likely a distributed function. The finding of a “node” in this network that could be stimulated and cause interruption in this function does not mean that this is the only such node, or even that it is a necessary one. Just like the basal temporal lobe and its relationship to language, it is possible that these areas in the nondominant parietal lobe could be safely resected despite the findings on direct cortical electrostimulation mapping. This study nicely illustrates a fairly highly localized area whose stimulation results in specific mathematical dysfunction. In order to improve our understanding of the significance of this finding, it should be followed up with focused functional MRI studies in an attempt to corroborate the significance of the direct cortical electrostimulation findings. Functional MRI could also help answer questions relating to the possibility of other brain regions involved in this function, and perhaps provide an answer to the question of where division and subtraction went. (http://thejns.org/doi/abs/10.3171/2013.4.JNS13560)