Olfactory perception and blindness: a systematic review and meta-analysis

Abstract

Anecdotal reports suggest that blind people might develop supra-normal olfactory abilities. However, scientific evidence shows a mixed pattern of findings. Inconsistent observations are reported for both sensory-driven olfactory tasks (e.g., odor threshold) and higher-order olfactory functions (e.g., odor identification). To quantify the evidence systematically, we conducted a review and meta-analysis. Studies were included if they examined olfactory function (i.e., odor threshold, odor discrimination, free odor identification, or cued odor identification) in blind compared with a sighted control group. Articles were identified through computerized literature search. A total of 18 studies focused on olfactory threshold (n = 1227: 590 blind and 637 sighted individuals), 14 studies targeted discrimination (n = 940: 455 blind and 485 sighted), 14 studies measured cued identification (n = 968: 468 blind and 500 sighted), and 7 studies (n = 443: 224 blind and 219 sighted individuals) assessed free identification. Overall, there were no differences in effect sizes between the blind and sighted individuals after correcting the results for publication bias. We additionally conducted an exploratory analysis targeting the role played by three moderators of interests: participants' age, the proportion of women versus men in each of the studies included into meta-analysis and onset of blindness (early blind vs. late-blind). However, none of the moderators affected the observed results. To conclude, blindness seems not to affect cued/free odor identification, odor discrimination or odor thresholds.

Early-blind Individuals Show Impaired Performance in Wine Odor Categorization

“…Although early-blind individuals had a harder time to determine if two wine odors belonged to the same category, their ability to discriminate wine odors (i.e., to evaluate whether two wine odors stem from the same or from different wines) was no different from sighted controls. These results are in line with previous research in which blind individuals did not outperform sighted controls on odor discrimination tasks (Cuevas et al, 2010;Beaulieu-Lefebvre et al, 2011;Sorokowska et al, 2018), even when the tasks are more complex, as in the present study.…”

“…We also did not find any significant group differences with regards to free odor identification, despite a small advantage for the early-blind individuals, in line with Sorokowska (2018). We also examined whether early-blind individuals were better at generating odor Early-blind individuals show impaired performance in wine odor categorization, 17 descriptors (Burton et al, 2002), which may explain better free odor identification performance reported in some studies.…”

“…A recent metanalysis (Sorokowska et al, 2018) investigated this body of literature and observed, based on the data of more than thousand observations, an important publication bias, i.e., selective publishing of positive results. In fact, studies that reported significant group differences, typically had small sample sizes.…”

Self-Administered Testing of Odor Threshold and Discrimination Using Sniffin’ Sticks—Reviving the “Odor-Curves-On-Paper” Method

“…However, long-term visual impairment does not seem to affect olfactory performance in comparison to vision in healthy subjects as measured by TDI testing (Luers et al 2014). Also, a recent meta-analysis concluded blind people do not have superior olfactory abilities (Sorokowska et al 2018). Therefore, possible effects of blindfolding on testing performance within this study should only be addressed with caution.…”

Verbally Induced Olfactory Illusions Are Not Caused by Visual Processing: Evidence From Early and Late Blindness

“…In short, the majority of evidence suggests that congenitally or early blind individuals are generally less affected by multisensory illusions. However, in contrast to audition and touch, the majority of studies in olfaction show small or, in most of the cases, no differences in olfactory abilities between sighted and blind people (e.g., odor threshold, discrimination, identification, and memory), with the only difference being in odor imagery where blind individuals outperform sighted (Cornell K€ arnekull et al, 2016(Cornell K€ arnekull et al, , 2018(Cornell K€ arnekull et al, , 2020Sorokowska et al, 2019). This suggests that blind individuals would not be able to draw on enhanced odors skills to separate verbal information from the odor percept per se.…”