fNIRS neuroimaging in olfactory research: A systematic literature review

Abstract

There are a number of key features which make olfaction difficult to study; subjective processes of odor detection, discrimination and identification, and individualistic odor hedonic perception and associated odor memories. In this systematic review we explore the role functional near-infrared spectroscopy (fNIRS) has played in understanding olfactory perception in humans. fNIRS is an optical neuroimaging technique able to measure changes in brain hemodynamics and oxygenation related to neural electrical activity. Adhering to PRISMA guidelines, results of this search found that generally the majority of studies involving healthy adult subjects observed increased activity in response to odors. Other population types were also observed, such as infants, individuals with autism, attention deficit hyperactivity disorder (ADHD), post-traumatic stress disorder (PTSD), mild cognitive impairment (MCI) and dysosmia. fNIRS coverage heavily favored the prefrontal cortex, temporal and parietal regions. This review finds that odor induced cortical activation is dependent on multiple factors, such as odorant type, gender and population type. This review also finds that there is room for improvement in areas such as participant diversity, use of wearable fNIRS systems, physiological monitoring and multi-distance channels.

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

“…Whilst the peak of activity may be beyond the depth of fNIRS monitoring abilities, task-based changes in regional cerebral blood flow may occur across a larger area within the cited region which may be detectable with fNIRS within the superficial aspects of these cortical regions. As summarised in Gunasekara et al (2022) , have used fNIRS to monitor the OFC during olfactory stimulation. Twelve of these studies reported fNIRS signal changes within the interrogated regions of the OFC.…”

“…Using a well-established task which is known to involve olfactory processing, olfaction can be studied using fNIRS, with regions of interest accessible in the frontal and parietal lobes. As summarised in Gunasekara et al (2022) , multiple studies have already used fNIRS to study olfaction. These studies predominantly considered prefrontal regions of interest.…”

“…In particular, this review aims to identify key brain regions associated with these cognitive processes, and the common methodological approaches used, to determine whether neuroimaging with fNIRS would be a suitable tool for research into olfaction, olfactive imagery and crossmodal visual-olfactive correspondences. We have recently summarised and reviewed in Gunasekara et al (2022) , the current status of using fNIRS in olfaction. We now seek to expand this to other neuroimaging modalities and assess the use of neuroimaging approaches and paradigms within olfactive imagery and crossmodal visual-olfactory integration, and advise best practise when applying fNIRS technology in these domains.…”