Cortical mapping of functionally important cortex is a vital function where neurosurgical procedures are planned in sensory, motor or speech areas, and is required to identify eloquent areas and thus avoid post-surgical neurological deficit. Techniques vary but all include the placement of grids over the proposed resection site, and the observation of the clinical effects of electrical stimulation of each cortical contact. Ictal and interictal EEG can be recorded at the same time. Mapping is important because lesions commonly alter or distort the normal topography of the cerebral cortex and vascular landmarks. Acute intra-operative mapping can be carried out under local anaesthetic, but less elaborate
Figure 5.17 (a) Longitudinal depth electrode inserted in an anteroposterior fashion from the occipital lobe toward the ipsilateral hippocampus. (b) Orthogonal depth electrodes inserted transversely from lateral to medial temporal lobe at various locations. (c) Subdural strip electrodes inserted through a temporal burr hole to study lateral and medial temporal neocortex. (d) Subdural grid electrode implanted for localization of the epileptogenic zone and functional topographic mapping. (e) A percutaneous epidural screw tightened into a twist drill hole through the cranium and an epidural peg electrode placed in another twist drill hole.
functional tasks can be evaluated than in chronic longer-term pre-operative mapping. Subdural grids of electrodes have also been used for recording cortical somatosensory evoked potentials from peripheral nerve stimulation, to locate the somatosensory cortex. fMRI has the potential to replace cortical mapping in the identification of the primary motor areas, but currently cannot localize speech or language areas with sufficient accuracy to be practically useful.
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