Abstracts

The response of cortex to thalamic stimulation through implanted deep brain stimulation (DBS) electrodes is largely unknown. We studied the relation between thalamic stimulation parameters and the morphology and topographic distribution of the cortical response in patients with intractable epilepsy undergoing DBS of the anterior thalamus. The generation of such cortical responses has been suggested to predict clinical efficacy of stimulation. Scalp-recorded cortical responses were studied in four patients who had previously undergone DBS surgery. Stimuli were delivered through contacts implanted bilaterally in the anterior thalamus (anterior and dorsal medial nuclei) using the programmable stimulation device (Medtronic ITREL II). All patients demonstrated reproducible time-locked cortical responses. The morphology of these responses, however, was quite heterogeneous, depending primarily on the site of stimulation (i.e. the contacts that were stimulated). Following bipolar 2 Hz stimulation, the typical response consisted of an ipsilateral triphasic negative-positive-negative deflection (peaking at around 25, 50 and 100 ms; maximal over the ipsilateral frontal scalp, subsequently extending to the lateral temporal scalp) followed by a broad ipsilateral biphasic positive-negative deflection (peaking at around 150 and 250 ms; maximal over the ipsilateral fronto-central scalp). The amplitude of the entire response was positively related to the strength of the stimulus (with a lower threshold for monopolar stimulation), and inversely related to the impedance of the electrode. With appropriate, high-intensity stimulation parameters, robust cortical responses of considerable inter- and intra-individual variation can be recorded from the scalp following anterior thalamus stimulation. These complex cortical responses most probably reflect mixed activation of both specific and nonspecific thalamocortical pathways. The heterogeneity of the cortical response, however, suggests a certain degree of point-to-point specificity. (Supported by Swiss National Science Foundation: PA00A-101502)