Listening to the brain: new techniques in intraoperative brain mapping.

For surgically resectable lesions in proximity to motor and speech cortex, mapping eloquent cortex in and around the tumor is essential for optimizing clinical outcomes. This is especially true in the treatment of medically intractable epilepsy, tumors, or arteriovenous malformations. 1–4 Because of the high degree of interpatient variability in the anatomic location of motor and language areas, correct localization of these sites is critical. 5 The current gold standard for clinical mapping is electrocortical stimulation (ECS). There are, however, technical limitations to this technique that lessen its ease and utility to both the surgeon and the patient. ECS mapping can be complicated by the induction of afterdischarges and seizures, which can introduce a source of error in mapping and even possibly make mapping untenable. 6 Although effective in identifying essential sites of functional cortex, stimulation is also relatively inefficient. Cortical locations must be interrogated in series, which can result in a prolonged mapping procedure. Another tool that is limited to identification of motor cortex is the use of somatosensory evoked potentials. 7 This technique can identify the boundary between motor and sensory cortex in patients under general anesthesia. The procedure is safe, fast, and easy to apply in the operating room. In the presence of certain anesthetic agents, however, its sensitivity and specificity can sometimes be limited. 8-10 Although considered a useful tool for identification of sensorimotor regions, the utility of somatosensory evoked potentials is limited to the sensorimotor cortex because of its unique anatomic and functional configuration. Efforts to extend the mapping potential of such cortical electrophysiological recordings to other eloquent areas and higher associative regions were explored by Sidney Goldring in the early 1990s. 11 These more advanced approaches, however, have had limited success. Thus, taken together, ECS and the use of somatosensory evoked potentials have been the sole intraoperative techniques used for decades (summarized in Figure very-low-frequency oscillations (< 0.5 Hz), known as SCPs. 24 Given that these networks are durable through sleep/wake cycles and anesthesia, 31–33 they were recently evaluated as a possible tool for mapping sensorimotor cortex. Breshears et al 32 studied 8 subjects undergoing surgical treatment for intractable epilepsy. SCPs were recorded from the cortical surface while awake and under propofol anesthesia. To test brain-mapping utility, SCP networks were identified using data-driven (seed-independent) and anatomy-driven (seed-based) approaches. The sensitivity and specificity of these networks for identifying sensorimotor cortex (as determined by ECS) were calculated. The study demonstrated that these SCP-defined networks identified with a data-driven approach in patients under anesthesia and awake were 90% and 93% sensitive and 58% and 55% specific for sensorimotor cortex, respectively (example in Figure 3B). These results suggest that resting-state networks may be useful for tailoring stimulation mapping and could provide a means of identifying eloquent regions in patients while under anesthesia.