Connectivity of the amygdala, piriform, and orbitofrontal cortex during olfactory stimulation: a functional MRI study

The majority of existing functional MRI studies on olfactory perception have addressed the relationship between stimulus features and the intensity of activity in separate regions considered in isolation. However, anatomical studies as well as neurophysiological recordings in rats and insects suggest that odor features may also be represented in a sparse manner through the simultaneous activity of multiple cortical areas interacting as a network. Here, we aimed to map the interdependence of neural activity among regions of the human brain, representing functional connectivity, during passive smelling. Seventeen healthy participants were scanned while performing a blocked-design task alternating exposure to two unpleasant odorants and breathing fresh air. High efferent connectivity was detected for the piriform cortex and the amygdala bilaterally. By contrast, the medial orbitofrontal cortex was characterized by high afferent connectivity, notably in the absence of an overall change in the intensity of hemodynamic activity during olfactory stimulation. Our results suggest that, even in the context of an elementary task, information on olfactory stimuli is scattered by the amygdala and piriform cortex onto an anatomically sparse representation and then gathered and integrated in the medial orbitofrontal cortex.