Rapid olfactory processing implicates subcortical control of an olfactomotor system.

Sniffs are modulated in response to odor content. Higher concentrations of odor induce lesser-volume sniffs. This phenomenon implicates a neural feedback mechanism that measures sensory input (odor concentration) and modulates motor output (sniffing) accordingly. Here we used air-dilution olfactometry to probe the time course of this olfactomotor mechanism. A stainless-steel computer-controlled olfactometer, equipped with mass flow controllers, temperature and humidity control, and on-line photo-ionization detection, was coupled to a highly sensitive pneumatotachograph that measured nasal flow. The olfactometer was used to generate four ascending concentrations of the odorants propionic acid and phenethyl alcohol. Sniff volume was inversely related to odor concentration (P > 0.0001). Sniffs were uniform and concentration independent for the initial 150 ms but acquired a concentration-dependent flowrate as early as 160 ms following sniff onset for propionic acid (P > 0.05) and 260 ms for phenethyl alcohol (P > 0.05). Considering that odorant transduction takes around 150 ms and odorant-induced cortical evoked potentials have latencies of around 300 ms, the rapid motor adjustments measured here suggest that olfactomotor sniff feedback control is subcortical and may rely on neural mechanisms similar to those that modulate eye movements to accommodate vision and ear movements to accommodate audition.