Semantical visual information facilitates odor imagery: a combined neurophysiological and psychological approach

Odor imagery (OI) or the ability to mentally simulate the presence of a smell, is a difficult cognitive function and is still misunderstood. Moreover, the neurophysiological approaches to OI have provided diverging results. In particular, brain activity in the olfactory network during OI seems to be influenced by the characteristics of the visual cues used to facilitate this task. In this study, we compared different types of visual cues (plain color patches, pictures and words) during OI in good odor imagers, using functional magnetic resonance imaging. Thirty right-handed participants were instructed to focus on the odors evoked by visual cues of different types. Psychological ratings of success/failure of OI as well as pleasantness and intensity of odor images were also collected. Our results show that across visual conditions, the OI task induced activity in the orbitofrontal cortex, insula, supplementary motor area and dorsolateral prefrontal cortex. Strikingly, no significant activity was found in the piriform cortex (primary olfactory brain region) despite an overall success in the OI according to psychological ratings (83% to 93% depending on the condition). This observation of successful OI without significant involvement of the primary olfactory regions in non-experts contradicts most of the existing literature on the topic. Psychological and neurophysiological comparisons between types of visual cues only revealed little effect of visual condition, although semantical cues seem to require fewer cognitive resources to generate odor images. This assumption remains to be confirmed using specific tools for the measurement of cognitive load.