Wait, we can do that? In short, yes, but only under very specific circumstances.
Measuring human intracranial activity involves, of course, opening the skull and inserting electrodes or sensors which can record brain activity into the brain. This activity can be recorded either from single neurons or from entire neuronal populations.
This is a medical procedure which needs to be conducted in certain cases. There are two types of situations when this can occur: either in acute situations (during brain surgery), or in chronic cases.
In acute situations, patients are woken up during surgery, and some areas are stimulated, while simultaneously recording responses. This procedure is necessary, for example, when the surgery involves critical areas, such as those related to language. Surgeons are able to do this because the brain doesn’t have any pain receptors.
The most common chronic cases refer to patients who suffer from treatment-resistant epilepsy, for whom the best option is to find the seizure focus and remove it. As such, electrodes are implanted in regions suspected to be the focus and the patients are continuously monitored for a couple of weeks.
There are both advantages and disadvantages with respect to this procedure. Advantages include the high spatial and temporal resolution, the high signal-to-noise ratio, and, most importantly, the chance to directly observe neuronal activity during processes which are unique to humans, such as speech. Other than invasiveness, disadvantages include the heterogeneity of the patient group, as well as the fact that they are usually taking medication which can influence the results, the limited study time and the limited number of regions sampled.
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Mukamel, R., & Fried, I. (2012). Human intracranial recordings and cognitive neuroscience. Annual review of psychology, 63, 511-537.