Abstract
Direct cortical stimulation (DCS) in epilepsy surgery patients has a long history of functional brain mapping and seizure triggering. Here, we review its findings when applied to the insula in order to map the insular functions, evaluate its local and distant connections, and trigger seizures. Clinical responses to insular DCS are frequent and diverse, showing a partial segregation with spatial overlap, including a posterior somatosensory, auditory, and vestibular part, a central olfactory-gustatory region, and an anterior visceral and cognitive-emotional portion. The study of cortico-cortical evoked potentials (CCEPs) has shown that the anterior (resp. posterior) insula has a higher connectivity rate with itself than with the posterior (resp. anterior) insula, and that both the anterior and posterior insula are closely connected, notably between the homologous insular subdivisions. All insular gyri show extensive and complex ipsilateral and contralateral extra-insular connections, more anteriorly for the anterior insula and more posteriorly for the posterior insula. As a rule, CCEPs propagate first and with a higher probability around the insular DCS site, then to the homologous region, and later to more distal regions with fast cortico-cortical axonal conduction delays. Seizures elicited by insular DCS have rarely been specifically studied, but their rate does not seem to differ from those of other DCS studies. They are mainly provoked from the insular seizure onset zone but can also be triggered by stimulating intra- and extra-insular early propagation zones. Overall, in line with the neuroimaging studies, insular DCS studies converge on the view that the insula is a multimodal functional hub with a fast propagation of information, whose organization helps understand where insular seizures start and how they propagate.
Highlights
IntroductionThe insula, or “the fifth lobe of the brain”, was first described in 1809 by the anatomist
The insula, or “the fifth lobe of the brain”, was first described in 1809 by the anatomistJohann Christian Reil [1]
Direct cortical stimulation (DCS) permits a fair degree of selectivity of the response with a high degree of localization, since the stimulus is delivered in bipolar mode through with a high degree of localization, since the stimulus is delivered in bipolar mode through adjacent contacts only 2–5 mm away from each other and because the adjacent contacts only 2–5 mm away from each other and because the stimulated contacts are not located on the pia-arachnoid but inside the cortex, which stimulated contacts are not located on the pia-arachnoid but inside the cortex, which avoids avoids a current “leak” through the cerebrospinal fluid [23]
Summary
The insula, or “the fifth lobe of the brain”, was first described in 1809 by the anatomist. The advent of neuneuroimaging techniques allowed the progressive discovery of its wide array of functions roimaging techniques allowed the progressive discovery of its wide array of functions and and connectivity high connectivity to brain other areas, brain leading areas, leading to increasing in this structure high to other to increasing interestinterest in this structure [9,10,11,12,13,14] In this context, clinical seizure analysis as well as insular DCS studies performed durduring procedures provided important new information that helped unveil ing SEEGSEEG procedures provided important new information that helped unveil some some properties of thisbut small but highly functional and densely connected properties of this small highly functional and densely connected cortex.
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