A Stimulating Look at Olfactory Hallucinations

Olfactory hallucinations tend to be unpleasant. This is true of the phantom smells associated with migraine headaches as well as epileptic seizures. The unpleasant odors are often vague but sometimes as specific as burning rubber. In his classic studies of sensations evoked by electrical stimulation of the brain, Wilder Penfield found that some patients reported brief odor perceptions—usually unpleasant— when he stimulated the olfactory bulb.

Recently a team of pediatric neurologists and neurosurgeons at Wayne State’s Detroit Medical Center took a closer look at what happens when you stimulate the brain’s olfactory areas. They examined a series of young (5 to 17 years old) epilepsy patients who had subdural electrodes implanted in order to record the source of their seizures. The electrodes were placed in various sites on the ventral surface of the frontal lobes.

The researchers used these electrodes to deliver electrical stimulation to one location at a time. They started with a low current and gradually increased it until the patient reported a sensation. Of the 16 kids tested, 11 reported a smell. Nine patients experienced an unpleasant smell (smoke, garbage, etc.) and two reported pleasant ones (strawberry, good food). This ratio is consistent with Penfield results from the 1950s, and with the broader literature on olfactory hallucinations.

Odor perceptions were triggered only by electrodes located near the midline of the brain, i.e., near the olfactory bulbs or tracts. More lateral locations produced no smells, even those these areas are known to be secondary olfactory cortex, i.e., higher-level odor processing areas.

So why do stinks outnumber nice smells in hallucinations? One thought is that the olfactory system has a built-in negative bias: it’s tuned to reject biologically hazardous smells. So when the system goes haywire (epilepsy) or is artificially stimulated, it defaults to the unpleasant side. Another possibility is that both of these abnormal situations activate an unorganized pattern of neural discharge from the otherwise finely tuned olfactory bulbs. It’s the equivalent of banging on piano keys with your fists—you’ll get a sound but usually an unpleasant one.

The study discussed here is “Olfactory hallucinations elicited by electrical stimulation via subdural electrodes: effects of direct stimulation of olfactory bulb and tract,” by Gogi Kumar, Csaba Juhász, Sandeep Sood, & Eishi Asano, published in Epilepsy & Behavior 24:264-268, 2012.