Making Sense of the Chemical Senses

Abstract

We review our recent behavioural and imaging studies testing the consequences of congenital blind ness on the chemical senses in comparison with the condition of anosmia. We found that congenitally blind (CB) subjects have increased sensitivity for orthonasal odorants and recmit their visually de prived occipital cortex to process orthonasal olfactory stimuli. In sharp contrast, CB perform less well than sighted controls in taste and retronasal olfaction, i.e. when processing chemicals inside the mouth. Interestingly, CB do not recruit their occipital cortex to process taste stimuli. In contrast to these findings in blindness, congenital anosmia is associated with lower taste and trigeminal sensitiv ity, accompanied by weaker activations within the 'flavour network' upon exposure to such stimuli. We conclude that functional adaptations to congenital anosmia or blindness are quite distinct, such that CB can train their exteroceptive chemical senses and recruit normally visual cortical areas to process chemical information from the surrounding environment.

Cortical Plasticity and Olfactory Function in Early Blindness

“…Using a battery of 38 odorant chemicals, Gagnon et al (2015a) recently demonstrated that congenitally blind participants tended to be better and were significantly faster at identifying odors presented orthonasally whereas this was not observed when odorants were presented retronasally. Their data revealed that early-onset blind subjects “were more familiar with the orthonasal odors and used the retronasal odorants less often for cooking than their sighted counterparts”, a result that is concordant with a reduced food variety exposure in this group, which might also explain their reduced taste perception when compared to SCs (Gagnon et al, 2014, 2015b). In another study by the same group (Beaulieu-Lefebvre et al, 2011) a significantly lower odor detection threshold was observed in EB participants compared to SCs.…”

“…Noteworthy, blind subjects also showed a stronger recruitment of their occipital cortex, mainly in V2, during the odor detection task, suggesting a privileged access of olfactory stimuli to this cortex when visually deafferented from birth (Kupers et al, 2011; Kupers and Ptito, 2014). Interestingly, while EB subjects, who had increased sensitivity to orthonasal odorants, recruited their visually deprived occipital cortex to process orthonasal olfactory stimuli in a simple odor detection task, they did not recruit their occipital cortex to process taste stimuli (Gagnon et al, 2014, 2015b). This was consistent with behavioral observations, since EB performed less well than SCs in taste and retronasal olfaction (Gagnon et al, 2015a), i.e., when processing chemicals inside the mouth.…”

Attending to the Chemical Senses

“…Given that far more of the cortical real-estate is given over to the processing of visual information than it is to the processing of information in any of the other senses (see Felleman & Van Essen, 1991; see also Gallace, Ngo, Sulaitis, & Spence, 2012, Table 1), one might have expected that attentional resources would follow suit. Relevant here, Stevenson and Attuquayefilo (2013) have made a persuasive case for the claim that many of the distinctive features of olfactory consciousness/attention can be linked to the limited processing resources that are available to the olfactory system (see also Gagnon, Kupers, & Ptito, 2014;Kaas, 2013).…”

Digitizing the chemical senses: Possibilities & pitfalls

“…It is important to note that just because people may not be able to consciously report on the presence of aroma it doesn't necessarily mean that it can't still affect their perception/performance in the other senses, providing the scent is presented at a level that is suitably close to threshold (see Li, Moallem, Paller, & Gottfried, 2007). It is, though, obviously going to be much harder to convince the consumer to buy the refill if they didn't realize that they had smelled, or tasted anything in the first place (see Baus & Bouchard, 2016, for one recent example of surprisingly low olfactory detection rates when introduced in a VR kitchen setting; see also Gagnon, Kupers, & Ptito, 2014). (This links back to the fundamental misattribution error.)…”