We combine information from our eyes, ears, and nose to understand our surroundings. When information from one sense has been limited since birth, how does the use of other senses change? Compared with vision and touch, smell has received relatively little investigation.
A study published in 2024 by Landry and colleagues compared 11 adults with congenital severe-to-profound hearing loss with 11 hearing adults matched for age and sex. Alongside odor tasks, the researchers used a task asking participants to locate stimulation inside the nose, investigating differences in both olfactory and trigeminal perception.
Sensing through the nose involves more than smell
Separate from the sense of smell that conveys floral or fruity odors, the nose has a system that conveys sensations such as cooling and stinging. This is mediated by the trigeminal nerve. Some substances with odors encountered in daily life stimulate both the olfactory and trigeminal systems.
The study included not only a task identifying odors but also one asking which nostril, left or right, had received stimulation. Performance in judging the side was used as an indicator of trigeminal perception.
Matching participants and making instructions accessible
Participants in the hearing-loss group were aged 20–51, and all had congenital bilateral severe-to-profound sensorineural hearing loss. The comparison group comprised hearing nonsigners aged 20–52. In both groups, only people whose overall smell-test scores fell within the study’s normal range were included.
Participants with hearing loss also completed tests of attention, visual memory, reasoning, and related abilities, confirming performance within standard ranges. Signed instructional videos and written communication were used to ensure that they understood the tasks before responding. Odor-identification tests involve not only odor perception itself but also understanding instructions and relevant knowledge.
The authors suggest that inconsistencies in earlier studies may reflect insufficient control for factors such as the onset of hearing loss, olfactory status, and task comprehension. The selection of participants in this study was intended to reduce those problems.
Differences in overall scores and response speed
First, participants completed smell tests using pen-shaped odor dispensers. There were three tasks: detecting how faint an odor they could find, selecting the different odor from a set of three, and identifying an odor from a list of options.
The overall score out of 48 averaged 40.16 in the hearing-loss group and 36.20 in the comparison group. This difference was statistically significant. However, there was no statistical evidence that the difference between groups varied across the three tasks. The overall difference therefore cannot simply be interpreted as a particular advantage in detecting faint odors alone.
Next, a device delivered almond-like benzaldehyde and eucalyptus-like eucalyptol to one nostril. There were 36 presentations, including air alone. Participants were asked to respond as quickly as possible to indicate either which nostril had been stimulated or which of the two odors had been delivered.
On an index combining correct responses with false alarms, the hearing-loss group performed better across the two tasks. Their response times were also shorter. Mean times in the odor-identification task were approximately 1.77 and 2.35 seconds, respectively, and in the left–right localization task approximately 1.86 and 2.39 seconds.
However, some performance in the device-based tasks was below that expected by chance. The authors highlight the small number of trials, which made individual ability difficult to estimate reliably. An additional analysis excluding low-performing participants still found an overall group difference, although the difference between mean localization scores became smaller.
A possible mechanism for sensory compensation
The authors consider the possibility that limited auditory information leads to greater processing of olfactory and trigeminal information. Identifying odors and quickly locating stimulation may help people understand the surrounding environment.
The paper emphasizes the need to examine more complex processes, such as comparing odors and interpreting their meaning, rather than focusing solely on detecting very faint odors. An important proposal is that studying both olfaction and trigeminal perception can provide a broader picture of sensory adaptation.
However, this study compared task performance; it did not measure brain activity or changes in neural circuits. The explanation that plasticity, or experience-dependent changes in brain function, produced the differences is a hypothesis consistent with the results. The study did not establish that hearing loss caused the performance differences.
The findings do not apply to every deaf or hard-of-hearing person
Besides the small sample of 11 per group, the study selected people with olfactory function in the normal range. Its findings do not represent the wider population, including people with acquired hearing loss or altered olfactory function.
Five of the 11 participants with hearing loss used cochlear implants. An exploratory comparison with nonusers found no significant differences, but the small sample does not establish that implants have no effect. The authors call for larger studies separating implant use and the age at which use began.
Even with accessible task instructions, the study did not measure all relevant knowledge of odor names or cognitive functions involved in planning and judgment. Future research needs more participants and trials, assessments suited to participants’ language and communication methods, and validation using brain imaging.
Summary
In this small study, participants with congenital severe-to-profound hearing loss and normal-range olfactory function performed better on an overall smell test and tasks involving nasal stimulation. The results suggest that differences in auditory experience may be accompanied by differences in how olfactory and trigeminal information is used.
Understanding such changes requires more than a single measure of having a good nose. Detecting faint odors, distinguishing odors, and locating stimulation should be examined separately. This perspective offers a way to understand individual sensory adaptation more concretely.
References
Landry, C., Nazar, R., Simon, M., Genest, F., Lécuyer Giguère, F., Lepore, F., & Frasnelli, J. (2024). Behavioural evidence for enhanced olfactory and trigeminal perception in congenital hearing loss. European Journal of Neuroscience, 59(3), 434–445. DOI: 10.1111/ejn.16216 / Full original article (university repository PDF). Original article: © 2023 The Authors, CC BY-NC 4.0.
