In many animals, including humans, the sense of smell performs an important threat-detection function by helping to identify potential sources of harm, such as rotting food and toxic fumes. Little is known, however, about the initial neural mechanisms of the olfactory system when, in concert with other brain regions, one judges a scent to represent danger. Such a judgment triggers a physical avoidance response, familiar to us as the recoiling we experience upon sniffing moldy vegetables or other malodorous substances. A new study has now shed light on this process. The study has demonstrated that reactions to malodors are involuntary and rapid, rather than being a conscious, cognitive process as had long been presumed. The findings also show that the processing of unpleasant smells is prioritized over the processing of pleasant, non-threatening smells. Broadly speaking, these results offer insights into avoidance responses in animals related to the perception of chemicals in a local environment. See also: Brain; Consciousness; Olfaction
The mechanism by which an unpleasant smell sensation leads to an avoidance response in humans has been difficult to decipher given the assumed need for invasive study. Researchers avoided this problem through a novel, non-invasive experimental setup using electroencephalography, or EEG. Used in many neurological studies, EEG records faint electric currents produced by the brain through the placement of electrodes on study participants' scalps. The researchers paired this classic EEG with what they term electrobulbogram, or EBG, electrodes placed directly on participants' foreheads. EBG sensors allow for better readings of signals indicating the electrophysical activity of olfactory bulbs—twin structures that extend out from the brain overtop of the nasal cavity and where odor-sensing neurons in the nose first send information. Olfactory bulbs initially process this smell information before sending the information onward into other regions of the brain responsible for movement control and, thus, avoidance behavior. The experimental setup also captured these subsequent neural signaling and brain region activity events through standard EEG. See also: Electric current; Electroencephalography; Neuron; Nose; Sensation
The researchers presented study participants with six different odors, ranging from pleasant-smelling chemicals such as linalool—a spicy, floral scent found in citrus fruits—to foul-smelling octanoic acid that is suggestive of rancidness. The odors caused varying activity in the olfactory bulb, with unpleasant scents triggering faster responses than pleasant scents, as well as immediate signaling to the motor cortex to initiate rapid avoidance behavior. Overall, the findings suggest that avoidance behavior related to potentially harmful smells has its origins in olfactory bulb processing and has a smaller cognitive or learned component than previously realized. See also: Learning; Motor systems (neuroscience); Spice and flavoring
