Third-order neurons send projections to the dorsomedial nucleus of the thalamus, the basal forebrain, and the limbic system. The thalamic connections are thought to serve as a conscious mechanism for odor perception, while the amygdala and the entorhinal area are limbic system components and may be involved in the affective components of olfaction. Regional cerebral blood flow investigations have demonstrated a significant increase in the amygdaloid nucleus with the introduction of a highly aversive odorant stimulus, and this has been associated with subjective perceived aversiveness. The pyriform lobe includes the olfactory tract, the uncus, and the anterior part of the parahippocampal gyrus. The entorhinal area is known as the secondary olfactory cortex and also is included in the pyriform lobe.
Olfaction is the only sense that has direct cortical projections without a thalamic relay nucleus.
The dorsomedial nucleus of the thalamus does receive some olfactory fibers that ultimately reach the orbitofrontal cortex. The anterior olfactory nucleus receives collateral fibers from the olfactory tract and projects to the contralateral olfactory bulb and anterior olfactory nucleus via the anterior commissure. The region of anterior perforated substance contains cells that receive direct mitral cell collaterals and input from the anterior olfactory nucleus, amygdaloid nucleus, and the temporal cortex. This area ultimately projects to the stria medullaris and the medial forebrain bundle. Using the uncinate fasciculus, the entorhinal area sends projections to the hippocampal formation, the anterior insula, and the frontal cortex.