fNIRS a novel neuroimaging tool to investigate olfaction, olfactory imagery, and crossmodal interactions: a systematic review

Abstract

Olfaction is understudied in neuroimaging research compared to other senses, but there is growing evidence of its therapeutic benefits on mood and well-being. Olfactory imagery can provide similar health benefits as olfactory interventions. Harnessing crossmodal visual-olfactory interactions can facilitate olfactory imagery. Understanding and employing these cross-modal interactions between visual and olfactory stimuli could aid in the research and applications of olfaction and olfactory imagery interventions for health and wellbeing. This review examines current knowledge, debates, and research on olfaction, olfactive imagery, and crossmodal visual-olfactory integration. A total of 56 papers, identified using the PRISMA method, were evaluated to identify key brain regions, research themes and methods used to determine the suitability of fNIRS as a tool for studying these topics. The review identified fNIRS-compatible protocols and brain regions within the fNIRS recording depth of approximately 1.5 cm associated with olfactory imagery and crossmodal visual-olfactory integration. Commonly cited regions include the orbitofrontal cortex, inferior frontal gyrus and dorsolateral prefrontal cortex. The findings of this review indicate that fNIRS would be a suitable tool for research into these processes. Additionally, fNIRS suitability for use in naturalistic settings may lead to the development of new research approaches with greater ecological validity compared to existing neuroimaging techniques.

Keywords: crossmodal visual-olfactory integration; fNIRS; neuroimaging; olfaction; olfactory imagery; systematic review.

Figure 1

PRISMA flow-chart depicting the literature screening process, including number of articles found via keyword searches and additional sources, number of articles excluded, and number of articles retained.

Figure 2

Distribution of research methodologies employed for research into olfaction, olfactive imagery and crossmodal interactions.

Figure 3

Distribution of publications related to olfaction, odour imagery and crossmodal visual-olfactory integration by year. For the purposes of this review, the search range was restricted to 2003–2023.

Figure 4

A schematic view of the human olfactory system. The primary and secondary olfactory regions are represented in blue and green, respectively. Amy, amygdala; Ento, entorhinal cortex; Hipp, hippocampus; OFC, orbitofrontal cortex; PC, piriform cortex; Thal, thalamus. Retrieved from Saive et al. (2014).

Figure 5

A schematic view of the human olfactory system. The primary, secondary and tertiary olfactory regions are represented in blue, purple and green respectively.

Figure 6

A schematic representation of commonly cited regions involved in olfaction as identified in this review.

Figure 7

A schematic representation of commonly cited regions associated with olfactory imagery as identified in this review.

Figure 8

(A) A table of modality general regions identified by McNorgan (2012). (B) The general imagery network (cool colours) identified using ALE analysis. Conjunction analysis of studies comparing complex and resting-state baseline conditions identified nine clusters (hot colours) that were active across all imagery conditions, regardless of baseline task. L, left; R, right; SMA, supplementary motor area; Med, medial; BA, Brodmann area. Retrieved from McNorgan (2012).

Figure 9

A schematic representation of commonly cited regions associated with crossmodal visual-olfactory integration identified in this review.

Figure 10

Effective connectivity model for olfactory-visual stimulation identified using DCM by Stickel et al. (2019). Amy, amygdala; C, cuneus; IPS, inferior parietal sulcus. Retrieved from Stickel et al. (2019).