Caspar Stephani

Functional neuroanatomy of the insular lobe.

The insula is the fifth lobe of the brain and it is the least known. Hidden under the temporal, frontal and parietal opercula, as well as under dense arterial and venous vessels, its accessibility is particularly restricted. Functional data on this region in humans, therefore, are scarce and the existing evidence makes conclusions on its functional and somatotopic organization difficult. 5 patients with intractable epilepsy underwent an invasive presurgical evaluation with implantation of diagnostic invasive-depth electrodes, including insular electrodes that were inserted using a mesiocaudodorsal to laterorostroventral approach. Altogether 113 contacts were found to be in the insula and were stimulated with alternating currents during preoperative monitoring. Different viscerosensitive and somatosensory phenomena were elicited by stimulation of these electrodes. A relatively high density of electrode contacts enabled us to delineate several functionally distinct areas within the insula. We found somatosensory symptoms to be restricted to the posterior insula and a subgroup of warmth or painful sensations in the dorsal posterior insula. Viscerosensory symptoms were elicited by more anterior electrode contacts with a subgroup of gustatory symptoms occurring after stimulation of electrode contacts in the central part of the insula. The anterior insula did not show reproducible responses to stimulation. In line with previous studies, we found evidence for somato- and viscerosensory cortex in the insula. In addition, our results suggest that there is a predominantly posterior and central distribution of these functions in the insular lobe.

Prophylactic treatment in menstrual migraine: A proof-of-concept study

The present study aimed to investigate the efficacy of repetitive cathodal direct current stimulation (rctDCS) over the visual cortex as a prophylactic treatment in patients with menstrual migraine. 20 female patients were recruited in this double-blind, placebo-controlled study and were assigned to receive either cathodal or sham stimulation. Over 3 menstrual cycles, tDCS with 2mA intensity and 20 min duration was applied to the visual cortex of the patients, in 5 consecutive sessions 1-5 days prior to the first day of their menstruation. The primary endpoint of the study was the frequency of the migraine attacks at the end of the treatment period, however, additional parameters, such as the number of migraine related days and the intensity of pain were also recorded 3 months before, during and 3 months post-treatment. Visual cortex excitability was determined by measuring the phosphene thresholds (PTs) using single pulse transcranial magnetic stimulation (TMS) over the visual cortex. Sixteen patients completed the study. A significant decrease in the number of migraine attacks (p=0.04) was found in the cathodal group compared to baseline but not compared to sham (p=0.053). In parallel the PTs increased significantly in this group, compared to the sham group (p<0.05). Our results indicate that prophylactic treatment with rctDCS over the visual cortex might be able to decrease the number of attacks in patients with menstrual migraine, probably by modifying cortical excitability.

A white matter tract mediating awareness of speech

Objective: To investigate the effects of extraoperative electrical stimulation of fiber tracts connecting the language territories. Methods: We describe results of extraoperative electrical stimulation of stereotactic electrodes in 3 patients with epilepsy who underwent presurgical evaluation for epilepsy surgery. Contacts of these electrodes sampled, among other structures, the suprainsular white matter of the left hemisphere. Results: Aside from speech disturbance and speech arrest, subcortical electrical stimulation of white matter tracts directly superior to the insula representing the anterior part of the arcuate fascicle, reproducibly induced complex verbal auditory phenomena including (1) hearing ones own voice in the absence of overt speech, and (2) lack of perception of arrest or alteration in ongoing repetition of words. Conclusion: These results represent direct evidence that the anterior part of the arcuate fascicle is part of a network that is important in the mediation of speech planning and awareness likely by linking the language areas of the inferior parietal and posterior inferior frontal cortices. More specifically, our observations suggest that this structure may be relevant to the pathophysiology of thought disorders and auditory verbal hallucinations.

Differences of Intracranial Electrical Stimulation Thresholds in the Human Brain

BACKGROUND The thresholds of current intensities required to evoke a clinical response during intracranial electrical stimulation mapping vary. OBJECTIVE We wanted to test if there is systematic variation of threshold currents between different brain areas. METHODS Electrode contacts of depth electrodes in a series of patients undergoing presurgical evaluation for epilepsy surgery were stimulated. The brain regions we studied included the insular cortex, the pericentral cortex, and the intervening white matter. RESULTS Current intensities that elicited clinical responses in central subcortical white matter tracts were significantly lower than those needed to evoke a clinical response in the cortex. CONCLUSION We found systematic differences of threshold stimulation intensities between cortical and subcortical structures in the human brain. In addition to known neurophysiologic differences between white and grey matter, the systematic difference in stimulation thresholds may be due to the direction of the white matter tracts with respect to the electrode orientation and to inherent properties of specific brain regions.

The effect of current flow direction on motor hot spot allocation by transcranial magnetic stimulation

The objective of this study was to investigate the significance of pulse configurations and current direction for corticospinal activation using transcranial magnetic stimulation (TMS). In 11 healthy subjects (8 female), a motor map for the motor evoked potentials (MEPs) recorded from the first dorsal interosseus (FDI), abductor digiti minimi (ADM), extensor carpi radialis, and biceps brachii (BB) muscles of the dominant side was established. Starting from a manually determined hot spot of the FDI representation, we measured MEPs at equal oriented points on an hexagonal grid, with 7 MEPs recorded at each point, using the following pulse configurations: posteriorly directed monophasic (Mo‐P), anteriorly directed monophasic (Mo‐A), biphasic with the more relevant second cycle oriented posteriorly (Bi‐P) as well as a reversed biphasic condition (Bi‐A). For each pulse configuration, a hot spot was determined and a center of gravity (CoG) was calculated. We found that the factor current direction had an effect on location of the CoG‐adjusted hot spot in the cranio‐caudal axis but not in the latero‐medial direction with anteriorly directed pulses locating the CoG more anteriorly and vice versa. In addition, the CoG for the FDI was more laterally than the cortical representations for the abductor digiti minimi (ADM) and extensor carpi radialis (ECR) which were registered as well. The results indicate that direction of the current pulse should be taken into account for determination of the motor representation of a muscle by TMS.