The olfactory cortex is separated from the outside world by only two synapses, without any relay and filter in the thalamus. The piriform cortex (PCX), part of the olfactory cortex, has been shown to play a role in odor perception, as well as in odor association and learning (Wilson and Sullivan, 2011). Nevertheless, little is known about how olfactory signals coming from the olfactory bulb (OB) are integrated in the olfactory cortex.

The PCX is composed of three layers and can be divided into two regions: the anterior PCX receives more afferents from the OB and has fewer association fibers than the posterior PCX (Bekkers and Suzuki, 2013). The anterior PCX receives not only olfactory signals, but also input from trigeminal nerve (Brand, 2006), which conveys mechanical, thermal, chemical, and/or nociceptive information. At high concentrations, most odorants can elicit intranasal sensations of burning, tingling, or pickling (Doty et al., 1978). Moreover, the odorless trigeminal stimulant, carbon dioxide (CO2), when presented with low concentration odors, can increase pungency ratings of the odors (Cain and Murphy, 1980). In addition, imaging techniques in humans have shown that even pure trigeminal stimu-