Moreover, low (30-70 Hz) and high (>70 Hz) gamma range cortical oscillations have been shown to display higher spatial selectivity than the lower alpha (∼7–12 Hz) and beta (∼15–30 Hz) cortical rhythms –. Evidence from a growing body of research investigating event-related dynamics of brain activity in human and non-human primates has revealed that the neural processes underlying motor behavior, perception and higher-order cognitive tasks are associated with neural oscillations in various frequency bands (e.g. Passive intracranial recordings provide a further attractive setting for functional mapping performed by engaging the patients in various cognitive protocols and analyzing the signals recorded at all the implanted electrodes in search for possible task-related responses.
įurthermore, throughout the presurgical evaluation period, the patient's cerebral activity is continuously recorded via implanted electrodes in order to detect the area of seizure onset. The direct effect (be it excitatory or disruptive) of electrical stimulation on the processes carried by the target neural population allows for the mapping of various functional systems such as the sensorimotor system, the vestibular system and neural processes involved in memory and language –, –. Alternatively, stimulations can also trigger positive neural responses, for instance in the sensori-motor cortex where they are used to map the sensori-motor homonculus. Also, they generally reproduce reversibly the consequence of focal cortical lesions, which effect on behaviour, such as speech disruption, can be observed by the experimenter and reveal eloquent areas. Stimulations may reproduce aura symptoms, induce electroclinical seizures, and provide valuable information regarding the organization of the epileptogenic network. In general, ECS are performed under continuous video-EEG control in several sessions after electrode implantation throughout the pre-surgical evaluation period.
INTRACRANIAL EEG AND HUMAN BRAIN MAPPING SERIES
) consists in delivering series of short (∼1 ms) low intensity (∼1 mA) current pulses through the intracerebral electrodes to produce localized current flows which interfere with local neural activity. The gold standard for assessing the risk of neurological impairment following epilepsy surgery is electrical cortical stimulations (ECS) mapping.
Given the large interindividual variability in functional cortical organisation, especially when lesions are present, the precise mapping between the implanted cortical structures and their functions must be performed for each patient individually and may not be solely inferred from previous neurological reports and neuroimaging studies. This requires not only the identification of the underlying epileptogenic network but also a detailed understanding of the functional organisation of the cortical areas in which it is embedded, so that eloquent cortical regions can be spared –. Becoming seizure-free and maintaining normal neurocognitive functions after surgery are both fundamental in order to improve the patient's quality of life. For patients who undergo brain surgery to treat medically intractable epilepsy, the outcome crucially depends on the accurate definition of the brain volume that has to be resected.