Mental arithmetic during human thalamic stimulation

Abstract

The effect of electrical stimulation of the human thalamus on a simple mental arithmetic task, counting backwards by three's, was observed during therapeutic stereotaxic operations under local anesthesia. Performance during stimulation was compared with matched trials where no stimulation occurred as to latency and accuracy of identifying the initial number, rate of counting backwards by three's, and the accuracy of these calculations. With left thalamic stimulation the rate of counting was accelerated, with an increase in calculation errors but no significant change in the latency of identifying the initial number. With right thalamic stimulation, the rate of counting was significantly slowed with a significant increase in the latency of identifying the initial number, and an increase in calculation errors. The changes in the test of simple mental arithmetic were also compared with the performance of these same patients with stimulation at these same sites and current levels on tests of object naming and short term verbal memory. The findings were interpreted as suggesting that both right and left thalamus, or structures passing through them, function in processes involved in mental arithmetic but in a differential way. Stimulation of the left thalamus evokes a specific alerting response, accelerating many higher mental processes, especially memory. Significant stimulation effects relate right thalamus to number reading, and arithmetic calculations, in particular calculations involving numbers less than ten.