A new radiochemical method for determining the concentration of F centres in X-or y-ray coloured crystals of alkali halides is described. The method is based on the fact that

P reservation of calculation processing during brain surgery is crucial for patients' quality of life. Over the last decade, surgical electrostimulation proved effective in identifying and preserving the areas involved in calculation processing in the left hemisphere. 13 This method has been recently validated in the right cortex as well. 4 Subcortical connectivity among different areas involved in calculation processing remains unclear, and the role of surgery in this domain has not yet been explored. We present the first report concerning the subcortical mapping of calculation processing during resection of brain tumors in the right parietal lobe. Methods Neuropsychological assessment and Surgical StrategyBecause of the location of the tumor in the right parietal lobe, awake intraoperative functional mapping was performed before and during resection, to reduce the risk of unfavorable outcomes on calculation. Patients gave their written informed consent. A full assessment of calculation skills was performed before surgery according to the methods of Delazer et al., 3 assessing different counting abilities, number comprehension, numerical transcoding, calculation, arithmetic reasoning, and conceptual knowl- Preservation of calculation processing in brain surgery is crucial for patients' quality of life. Over the last decade, surgical electrostimulation was used to identify and preserve the cortical areas involved in such processing. Conversely, subcortical connectivity among different areas implicated in this function remains unclear, and the role of surgery in this domain has not been explored so far. The authors present the first 2 cases in which the subcortical functional sites involved in calculation were identified during right parietal lobe surgery. Two patients affected by a glioma located in the right parietal lobe underwent surgery with the aid of MRI neuronavigation. No calculation deficits were detected during preoperative assessment. Cortical and subcortical mapping were performed using a bipolar stimulator. The current intensity was determined by progressively increasing the amplitude by 0.5-mA increments (from a baseline of 1 mA) until a sensorimotor response was elicited. Then, addition and multiplication calculation tasks were administered. Corticectomy was performed according to both the MRI neuronavigation data and the functional findings obtained through cortical mapping. Direct subcortical electrostimulation was repeatedly performed during tumor resection. Subcortical functional sites for multiplication and addition were detected in both patients. Electrostimulation interfered with calculation processing during cortical mapping as well. Functional sites were spared during tumor removal. The postoperative course was uneventful, and calculation processing was preserved. Postoperative MRI showed complete resection of the tumor. The present preliminary study shows for the first time how functional mapping can be a promising method to intraoperatively identify the subcortical functional sites inv...

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All electrostimulation studies on arithmetic have so far solely reported general errors. Nonetheless, a classification of the errors during stimulation can inform us about underlying arithmetic processes. The present electrostimulation study was performed in a case of left parietal glioma. The patient's erroneous responses suggested that calculation was mainly applied for addition and a combination of retrieval and calculation was mainly applied for multiplication. The findings of the present single-case study encourage follow up with further data collection with the same paradigm.