A portion of the brain that helps us respond to odors and process emotions may be malfunctioning in severely depressed individuals, say researchers who measured the brain activity of individuals presented with smells like roses and rotten butter. Because odors and emotions are processed in similar brain structures, study of the olfactory system may increase our understanding of the physiological underpinnings of depression, according to the study. Previous studies have attempted to pinpoint exactly what processes in depressed individuals' brains lead to their tendency to remember negative memories over positive ones, and to have a sense of hopelessness regarding the future. Some studies have measured brain responses to emotionally charged images or words, but these methods involve a certain amount of decoding, as individuals can respond to such images differently.
"The presentation of odors as emotional stimuli, by contrast, is powerfully direct, and odors seem to be powerful emotional stimuli," says study author Bettina M. Pause, Ph.D., of the Department of Psychology at Christian-Albrechts-University of Kiel in Germany.
Pause and colleagues investigated how 25 depressed study participants compared with a same-sized healthy group in how their senses of smell and sight responded to emotional stimuli. They also re-examined 15 of the depressed patients after their recovery to see if their responses had changed.
In addition to exposing participants to emotionally charged sights and smells, such as a compelling slide demonstration, the fragrance of roses and the fetid odor of rotten butter, the researchers also showed participants emotionally neutral color slides. Via electrodes placed on 32 scalp locations, Pause and colleagues monitored the electrical activity of the participants' brains during these exposures.
The brains of the depressed patients were less responsive to the smells and the visual stimuli than the healthy participants, the researchers found. However, while the patients showed changes in the way they processed visual cues relatively late in the perceptual process (cognitive level), they showed early deviations in registering scents (sensory level). This electrical pattern changed once their depression lifted - the 15 patients who were re-examined after their depression was successfully treated no longer exhibited any differences from the healthy participants.
The study results are published in the March issue of the journal Psychophysiology.
The researchers noted that while the depressed patients were less responsive to smells, their ability to identify smells had not suffered. This finding is in line with previous studies suggesting that while depressed individuals have no problem identifying odors, depression appears to muffle their sense of smell.
Pause and colleagues suspect that the depressed patients' reduced brain responses to odors relates to deviations in two intimately connected brain areas, called the orbitofrontal cortex and the amygdala, which play significant roles in processing emotional information and are also connected with sense of smell. Meanwhile, their reduced responsiveness to visual emotional stimuli occurs via another brain area, called the dorsal prefrontal cortex.
"Studies on the role of the orbitofrontal cortex and the amygdala in emotion strikingly parallel the studies on these brain structures in olfaction," Pause says. The amygdala may help "encode" odors and emotions, while the orbitofrontal cortex helps develop strategies for how to respond to such stimuli, according to the study.
The researchers target a dysfunctional state of the main olfactory bulb, a pea-sized structure located below the orbitofrontal cortex that receives sensory input, as potentially playing a pivotal role in depressed patients' reduced sense of smell, and well as "their intensified experience of sadness and fear," according to the study.
