Stimulating and Recording Brain Signals in an Olfactory Context: Non-Canonical Brain Structures May Contribute to Olfactory Processing - A Case Series.

Unlike some other sensory systems, the sense of smell does not benefit yet from a ready-to-use or advanced olfactory implant or prosthesis. The principle of an olfactory implant rests upon volatile compounds that are captured by sensors mimicking the olfactory epithelium, translating them into different stimulation patterns which are then sent to central brain structures. The potential of stimulating a brain structure that does not seem to belong to the primary olfactory pathway, but is part of olfactory networks, has to our knowledge not been considered. We report a case series of patients with pharmacoresistant epilepsy who underwent stereoencephalography (SEEG) for diagnostic reasons, including intracranial stimulation. Firstly, the patients performed a task of passive smelling of two odors (peach and fish), during which electroencephalographic signals were recorded. In a second phase, intracranial electrical stimulation was applied. In two patients, this evoked olfactory sensations specifically after stimulation respectively of the amygdala or temporal pole. Based on these observations, a post-hoc analysis of the SEEG data from the passive smelling task was conducted, focusing on these two structures. Functional connectivity measures have been calculated in different frequency bands using the amygdala or the temporal pole as seeds. Although conclusions are limited due to the small sample size, the results along with the literature underline the interest of considering non-primary olfactory structures, such as the temporal pole, as a contributor to olfactory processing. The efficacy of a potential olfactory implant may rely not only on the primary location of stimulation, but also in the type of oscillations elicited and potential engagement of association brain structures.