Superior orthonasal but not retronasal olfactory skills in congenital blindness

Olfaction and gustation in blindness: a state of the art of the literature

To date, there are quite a few studies assessing olfaction and gustation in blindness, with great variability in sample size, participants' age, blindness onset and smell and taste evaluation methods. Indeed, the evaluation of olfactory and gustatory performance can differ depending on several factors, including cultural differences. Therefore, here we analysed through a narrative review, all the works reporting a smell and taste assessment in blind individuals during the last 130 years, trying to summarize and address the knowledge in this field.

Flexible cue anchoring strategies enable stable head direction coding in both sighted and blind animals

Vision plays a crucial role in instructing the brain’s spatial navigation systems. However, little is known about how vision loss affects the neuronal encoding of spatial information. Here, recording from head direction (HD) cells in the anterior dorsal nucleus of the thalamus in mice, we find stable and robust HD tuning in rd1 mice, a model of photoreceptor degeneration, that go blind by approximately one month of age. In contrast, placing sighted animals in darkness significantly impairs HD cell tuning. We find that blind mice use olfactory cues to maintain stable HD tuning and that prior visual experience leads to refined HD cell tuning in blind rd1 adult mice compared to congenitally blind animals. Finally, in the absence of both visual and olfactory cues, the HD attractor network remains intact but the preferred firing direction of HD cells drifts over time. These findings demonstrate flexibility in how the brain uses diverse sensory information to generate a stable directional representation of space.

Vision plays an important role in the head direction cell system in animals. Here the authors recorded from head direction cells in rd1 mice that show retinal degeneration at 1 month, and find that they use smell cues to maintain stable HD tuning.

Searching for perceptual similarity within, and between, the (chemical) senses

In this narrative historical review, I want to take a closer look at the concept of perceptual similarity both as it applies within, and between, the chemical senses (specifically taste and smell). The discussion is linked to issues of affective similarity and connotative meaning. The relation between intramodal and crossmodal judgments of perceptual similarity, and the putatively special status of those odorants that happen to take on taste qualities will also be discussed. An important distinction is drawn between the interrelated, though sometimes distinct, notions of perceptual similarity and crossmodal congruency, specifically as they relate to the comparison of chemosensory stimuli. Such phenomena are often referred to as crossmodal correspondences, or by others (incorrectly in my view), as a kind of ubiquitous synesthesia.

Altered Brain Functional Connectivity at Resting-State in Patients With Non-arteritic Anterior Ischemic Optic Neuropathy

Purpose: To investigate the possible changes in functional connectivity (FC) in patients with non-arteritic anterior ischemic optic neuropathy (NAION) using resting-state functional MRI (fMRI).

Methods: Thirty-one NAION patients and 31 healthy controls were recruited and underwent resting-state fMRI scans. Regions of interest (ROIs) were defined as bilateral Brodmann’s area 17 (BA17). FC analysis was performed between the ROIs and the rest of the brain regions, and the between group comparisons of FC were performed. We conducted correlation analysis between the FC changes and the clinical variables in NAION patients.

Results: Compared with healthy controls, patients with NAION showed significantly decreased FC between the left BA17 and the right inferior frontal gyrus, left caudate nucleus. As for the right BA17, patients exhibited significantly increased FC with the left olfactory gyrus and decreased FC with the right superior frontal gyrus (SFG), right insula. Moreover, FC values between the right insula and the right BA17 were positively correlated with the right side of mean sensitivity in the central visual field (r = 0.52, P < 0.01) and negatively correlated with the right side of mean defect in the central visual field (r = −0.55, P < 0.01).

Conclusion: Our study indicated that patients with NAION showed significantly abnormal functional reorganization between the primary visual cortex and several other brain regions not directly related to visual function, which supports that NAION may not only be an ophthalmic disease but also a neuro-ophthalmological disease.

Better Olfactory Performance and Larger Olfactory Bulbs in a Mouse Model of Congenital Blindness

It is well established that early blindness results in enhancement of the remaining nonvisual sensory modalities accompanied by functional and anatomical brain plasticity. While auditory and tactile functions have been largely investigated, the results regarding olfactory functions remained less explored and less consistent. In the present study, we investigated olfactory function in blind mice using 3 tests: the buried food test, the olfactory threshold test, and the olfactory performance test. The results indicated better performance of blind mice in the buried food test and odor performance test while there was no difference in the olfactory threshold test. Using histological measurements, we also investigated if there was anatomical plasticity in the olfactory bulbs (OB), the most salient site for olfactory processing. The results indicated a larger volume of the OB driven by larger glomerular and granular layers in blind mice compared with sighted mice. Structural plasticity in the OB may underlie the enhanced olfactory performance in blind mice.

No Olfactory Compensation in Food-related Hazard Detection Among Blind and Deaf Adults: A Psychophysical Approach

The exposure-driven olfactory compensation associated with sensory loss is likely to be observed in assessment of food-related dangers. Therefore, in the current study we tested the hypothesis that olfactory compensation occurs in the case of protection from food-related hazards. We compared thresholds for detection of an unpleasant rotten food odor (fermented fish sauce) in four groups of subjects: blind subjects (n = 100), sighted controls (n = 100), deaf subjects (n = 74) and hearing controls (n = 99). Overall, we observed no significant differences in smell acuity between the blind and deaf groups and their matched control samples. However, the sensory deprived subjects assessed their sensitivity as higher than did control groups. The present study is yet another example of research among large samples of sensory deprived individuals that shows no evidence of olfactory compensation. This result is consistent with a growing number of studies suggesting no sensory compensation in simple, absolute sensitivity tasks.

Odor lateralization and spatial localization: Null effects of blindness

People can navigate through an environment using different sensory information, including olfactory cues. Correct intranasal localization and external location of odors can be learned, and some people are able to lateralize olfactory stimuli above chance, which raises the question: What determines the spectrum of olfactory localization abilities. Here, we explored whether odor lateralization and localization abilities are increased in the course of sensory compensation. In a series of studies, we combined two different aspects of odor localization. Study 1 compared abilities of 69 blind people (M_age = 41 ± 1.6 years; 32 females) and 45 sighted controls (M_age = 38.3 ± 2.1 years; 25 females) to correctly lateralize eucalyptol, an odorant with a strong trigeminal component, presented to either nostril. Studies 2 and 3 involved a more ecologically valid task, namely spatial localization of olfactory stimuli. In Study 2, 13 blind individuals (M_age = 28.5 ± 3.5 years; seven females) and 16 sighted controls (M_age = 34.9 ± 3.2 years; ten females) tried to localize a single odorant, while in Study 3, 97 blind individuals (M_age = 43.1 ± .5 years; 48 females) and 47 sighted controls (M_age = 38.7 ± .7 years; 27 females) attempted to localize a single target odor in an experimental olfactory space comprising four different odorants. Blind and sighted subjects did not differ in their abilities to lateralize and to localize odors, and their performance across all tasks suggests that odor lateralization and localization are important for navigation in an environment regardless of visual status.

Body-odor based assessments of sex and personality - Non-significant differences between blind and sighted odor raters

People exhibit different sensitivity to the signaling properties of body odors in the social context. Here, we aimed to investigate whether visual status modulates sensitivity to socially-relevant cues carried by body odors and whether it affects psychophysical ratings of such smells. We compared abilities of 19 early-blind, 9 late-blind and 13 sighted people to accurately assess sex, neuroticism and dominance of odor donors based on body odor samples. We showed that early blind, late blind and sighted people do not differ in accuracy of sex and personality assessments based on body odor samples. Additionally, the three participating groups perceived the presented body odor samples as similarly intense, pleasant and attractive. We discuss our findings in the context of interpersonal olfactory communication and olfactory compensation.

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

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.

Voice-based assessments of trustworthiness, competence, and warmth in blind and sighted adults

The study of voice perception in congenitally blind individuals allows researchers rare insight into how a lifetime of visual deprivation affects the development of voice perception. Previous studies have suggested that blind adults outperform their sighted counterparts in low-level auditory tasks testing spatial localization and pitch discrimination, as well as in verbal speech processing; however, blind persons generally show no advantage in nonverbal voice recognition or discrimination tasks. The present study is the first to examine whether visual experience influences the development of social stereotypes that are formed on the basis of nonverbal vocal characteristics (i.e., voice pitch). Groups of 27 congenitally or early-blind adults and 23 sighted controls assessed the trustworthiness, competence, and warmth of men and women speaking a series of vowels, whose voice pitches had been experimentally raised or lowered. Blind and sighted listeners judged both men’s and women’s voices with lowered pitch as being more competent and trustworthy than voices with raised pitch. In contrast, raised-pitch voices were judged as being warmer than were lowered-pitch voices, but only for women’s voices. Crucially, blind and sighted persons did not differ in their voice-based assessments of competence or warmth, or in their certainty of these assessments, whereas the association between low pitch and trustworthiness in women’s voices was weaker among blind than sighted participants. This latter result suggests that blind persons may rely less heavily on nonverbal cues to trustworthiness compared to sighted persons. Ultimately, our findings suggest that robust perceptual associations that systematically link voice pitch to the social and personal dimensions of a speaker can develop without visual input.

No Sensory Compensation for Olfactory Memory: Differences between Blind and Sighted People

Blindness can be a driving force behind a variety of changes in sensory systems. When vision is missing, other modalities and higher cognitive functions can become hyper-developed through a mechanism called sensory compensation. Overall, previous studies suggest that olfactory memory in blind people can be better than that of the sighted individuals. Better performance of blind individuals in other-sensory modalities was hypothesized to be a result of, among others, intense perceptual training. At the same time, if the superiority of blind people in olfactory abilities indeed results from training, their scores should not decrease with age to such an extent as among the sighted people. Here, this hypothesis was tested in a large sample of 94 blind individuals. Olfactory memory was assessed using the Test for Olfactory Memory, comprising episodic odor recognition (discriminating previously presented odors from new odors) and two forms of semantic memory (cued and free identification of odors). Regarding episodic olfactory memory, we observed an age-related decline in correct hits in blind participants, but an age-related increase in false alarms in sighted participants. Further, age moderated the between-group differences for correct hits, but the direction of the observed effect was contrary to our expectations. The difference between blind and sighted individuals younger than 40 years old was non-significant, but older sighted individuals outperformed their blind counterparts. In conclusion, we found no positive effect of visual impairment on olfactory memory. We suggest that daily perceptual training is not enough to increase olfactory memory function in blind people.

Factors affecting the ortho- and retronasal perception of flavors: A review

Flavor perception is a highly individual sensation, and is impacted by a number of factors. Olfaction is a critical element in fully experiencing flavor. In this review, we explore the differences between orthonasal (sniff) versus retronasal (mouth) olfaction, and provide a comprehensive summary of recent publications in this arena. Here we explore the complexities of flavor perception, including the role that select flavors and media have on identification and localization. We also discuss some common neural imaging techniques used in this field, as odorants activate different neural responses in diverse areas of the brain, as well as the different stimulation patterns derived from perceiving food and nonfood related odorants. The information provided will be useful for sensory scientists and industry alike for the development of novel food and beverages that positively impact the consumer experience.

Do Visually Impaired People Develop Superior Smell Ability?

It is well known that visually impaired people perform better in orientation by sound than sighted individuals, but it is not clear whether this enhanced awareness also extends to other senses. Therefore, the aim of this study was to observe whether visually impaired subjects develop superior abilities in olfactory perception to compensate for their lack of vision. We investigated the odor perception of visually impaired individuals aged 7 to 89 ( n = 99; 52 women, 47 men) and compared them with subjects of a control group aged 8 to 82 years ( n = 100; 45 women, 55 men) without any visual impairment. The participants were evaluated by Sniffin' Sticks odor identification and discrimination test. Identification ability was assessed for 16 common odors presented in felt-tip pens. In the odor discrimination task, subjects had to determine which of three pens in 16 triplets had a different odor. The median number of correctly identified odorant pens in both groups was the same, 13 of the offered 16. In the discrimination test, there was also no significant difference observed. Gender did not influence results. Age-related changes were observed in both groups with olfactory perception decreasing after the age of 51. We could not confirm that visually impaired people were better in smell identification and discrimination ability than sighted individuals.

Simultaneity judgment using olfactory-visual, visual-gustatory, and olfactory-gustatory combinations

Vision is a physical sense, whereas olfaction and gustation are chemical senses. Active sensing might function in vision, olfaction, and gustation, whereas passive sensing might function in vision and olfaction but not gustation. To investigate whether each sensory property affected synchrony perception, participants in this study performed simultaneity judgment (SJ) for three cross-modal combinations using visual (red LED light), olfactory (coumarin), and gustatory (NaCl solution) stimuli. We calculated the half-width at half-height (HWHH) and point of subjective simultaneity (PSS) on the basis of temporal distributions of simultaneous response rates in each combination. Although HWHH did not differ significantly among three cross-modal combinations, HWHH exhibited a higher value in cross-modal combinations involving one or two chemical stimuli than in combinations of two physical stimuli, reported in a previous study. The PSS of the olfactory–visual combination was approximately equal to the point of objective simultaneity (POS), whereas the PSS of visual–gustatory, and olfactory–gustatory combinations receded significantly from the POS. In order to generalize these results as specific to chemical senses in regard to synchrony perception, we need to determine whether the same phenomena will be reproduced when performing SJ for various cross-modal combinations using visual, olfactory, and gustatory stimuli other than red LED light, coumarin, and NaCl solution.

Cortical Plasticity and Olfactory Function in Early Blindness

Over the last decade, functional brain imaging has provided insight to the maturation processes and has helped elucidate the pathophysiological mechanisms involved in brain plasticity in the absence of vision. In case of congenital blindness, drastic changes occur within the deafferented “visual” cortex that starts receiving and processing non visual inputs, including olfactory stimuli. This functional reorganization of the occipital cortex gives rise to compensatory perceptual and cognitive mechanisms that help blind persons achieve perceptual tasks, leading to superior olfactory abilities in these subjects. This view receives support from psychophysical testing, volumetric measurements and functional brain imaging studies in humans, which are presented here.

Olfactory Performance in a Large Sample of Early-Blind and Late-Blind Individuals

Previous examinations of olfactory sensitivity in blind people have produced contradictory findings. Thus, whether visual impairment is associated with increased olfactory abilities is unclear. In the present investigation, I aimed to resolve the existing questions via a relatively large-scale study comprising early-blind (N = 43), and late-blind (N = 41) and sighted (N = 84) individuals matched in terms of gender and age. To compare the results with those of previous studies, I combined data from a free odor identification test, extensive psychophysical testing (Sniffin' Sticks test), and self-assessed olfactory performance. The analyses revealed no significant effects of sight on olfactory threshold, odor discrimination, cued identification, or free identification scores; neither was the performance of the early-blind and late-blind participants significantly different. Additionally, the self-assessed olfactory abilities of the blind people were no different than those of the sighted people. These results suggest that sensory compensation in visually impaired is not pronounced with regards to olfactory abilities as measured by standardized smell tests.