Open Access

Felix Zahnert, Urs Johannes Kleinholdermann, Marcus Belke, Boris Rudolfo Keil, Katja Menzler, David José Pedrosa Carrasco, Lars Timmermann, Tilo Kircher, Igor Nenadić, Susanne Knake

• Highlights • Piriform cortex and amgydala can be separated based on their distinct structural connectivity. • Similar to histological findings, the connectivity of the piriform cortex suggests posterior frontal and temporal subregions. • Subregions of the piriform cortex have distinct connectivity profiles. • Anterior PC extended into ventrotemporal PC posteriorly, which has not been described before, requiring further investigation. • All parcellations were made publicly available. Abstract The anatomy of the human piriform cortex (PC) is poorly understood. We used a bimodal connectivity-based-parcellation approach to investigate subregions of the PC and its connectional differentiation from the amygdala. One hundred (55 % female) genetically unrelated subjects from the Human Connectome Project were included. A region of interest (ROI) was delineated bilaterally covering PC and amygdala, and functional and structural connectivity of this ROI with the whole gray matter was computed. Spectral clustering was performed to obtain bilateral parcellations at granularities of k = 2–10 clusters and combined bimodal parcellations were computed. Validity of parcellations was assessed via their mean individual-to-group similarity per adjusted rand index (ARI). Individual-to-group similarity was higher than chance in both modalities and in all clustering solutions. The amygdala was clearly distinguished from PC in structural parcellations, and olfactory amygdala was connectionally more similar to amygdala than to PC. At higher granularities, an anterior and ventrotemporal and a posterior frontal cluster emerged within PC, as well as an additional temporal cluster at their boundary. Functional parcellations also showed a frontal piriform cluster, and similar temporal clusters were observed with less consistency. Results from bimodal parcellations were similar to the structural parcellations. Consistent results were obtained in a validation cohort. Distinction of the human PC from the amygdala, including its olfactory subregions, is possible based on its structural connectivity alone. The canonical fronto-temporal boundary within PC was reproduced in both modalities and with consistency. All obtained parcellations are freely available.