Lateralized thalamic stimulation: effects on verbal memory.

This study was concerned with the effects of left- or right-thalamic stimulation on a simple verbal memory task, involving immediate free recall of lists of common words presented dichotically. The subjects were patients with Parkinson's disease undergoing stereotaxic thalamotomy. As a clinical routine, the patients also underwent a brief period of thalamic electrical stimulations before the lesion was made. The memory tests were performed during the stimulation phase of the operation and were accomplished inside the operating room. The main finding was a general deterioration of performance during thalamic stimulation compared with preoperative and postoperative testing. However, this general effect was modulated by stimulation intensity and side of stimulation. High-intensity stimulation of the left thalamus resulted in significantly fewer words being recalled compared with right thalamus stimulation and low-intensity stimulation. Finally, words presented in the first list were recalled better than words from the second and third lists, particularly during stimulation and at follow-up. The findings are discussed in relation to the thalamic activating-gating model originally proposed by Ojemann.