Electrocorticographic sensorimotor mapping

Abstract

Localizing eloquent cortex during neurosurgical resection planning is critical to minimizing postoperative neurologic deficits. Despite a host of sensorimotor mapping technologies including somatosensory evoked potentials (SSEP’s), functional magnetic resonance imaging (fMRI), and electrocorticography (ECoG), the accepted gold standard is still considered to be electrical stimulation mapping (ESM; Haglund et al., 1994, Keles et al., 2004). ESM can be performed intraoperatively, which requires an awake, cooperative patient for language mapping, or appropriate anesthetic and absence of muscle relaxant for motor mapping. However, ESM can elicit afterdischarges and seizures, both of which can impair subsequent ESM testing. ESM can also generate painful stimuli due to activation of dural and trigeminal nociceptive afferents. In the extra-operative setting, ESM requires some degree of cooperation so that movements can be differentiated as evoked or spontaneous, and sensory responses can be elicited with patient feedback. Unlike ESM, ECoG records spectral changes in various frequency bands due to normal cortical function during overt or imagined motor activity. ECoG not only provides clinical recordings for epilepsy monitoring on an unparalleled spatiotemporal scale (Toole et al., 2007), but also is also able to resolve task-associated spectral changes in high frequency bands that may reflect local cortical activity (Miller et al., 2007 a, b, Szhuraj et al., 2005, Crone et al., 1998 a, b, Leuthardt et al., 2007). Newer approaches also include observations of spectral changes in slow cortical potentials during resting states. A number of ECoG studies (Table 1) attempt to compare ECoG with ESM, with varying results. Table 1 Studies comparing sensorimotor mapping with ECoG versus ESM. ECS(+) = Cortical stimulation responsive, ECoG(+) = Significant spectral change identified by ECoG in given frequency band, LFB = Low frequency band, HFB = High frequency band. ERS = event-related ... Review of current paper In this issue of Clinical Neurophysiology, Vansteensel and colleagues report their effort to use ECoG signals to map motor cortex in patients with subdural electrodes implanted for epilepsy surgery planning (Vansteensel et al., 2013). The authors confirm the work of a few other centers (Table 1) showing that high frequency (here, 65–95Hz) band power (HFB) has the highest sensitivity and specificity relative to electrical stimulation mapping sites. Their novel contribution is the use of evoked signals during epochs of movement defined from the patient’s own spontaneous movement. One can appreciate the advantage of this approach in the uncooperative or impaired patient, where cued-based movement might have a variable response delay, or even not be performed. Most methods of behavioral mapping would not work in such a situation, whereas this post-hoc approach does succeed. However, this limitation has not necessarily been an overwhelming obstacle, as even in the pediatric population, band frequency mapping can be applied successfully (Wray et al., 2012). In addition, the method described requires lengthy video reviewing to tag the epochs of movement, and thus is not feasible in an intraoperative environment. Integrating the activity-recorded videos with relevant signal processing also introduces a delay in obtaining the mapping results in a situation where results are usually desired in hours or minutes. However, the fact that this approach utilizes signals generated during patient self-directed, spontaneous and unrestricted movements, rather than as directed by the examiner, is philosophically appealing. It remains to be seen if this is an additional advantage. There is some discrepancy between the frequency mapping and the stimulation results, which tended to manifest as more widespread frequency changes. The authors eloquently discuss the implications of this, questioning the ‘gold-standard’ status of ESM and adding insight into the discussion on its role in optimizing the balance between neurosurgical treatment and functional outcomes.