Behavioral Evidence for Enhanced Olfactory and Trigeminal Perception in Congenitally Deaf Individuals

Sensory deprivation, particularly hearing loss, is an excellent model to study neuroplasticity in the human brain and the adaptive behaviors that support the daily lives of deprived individuals. In adaptation to their hearing loss, deaf individuals rely on their other intact senses. Visual and tactile abilities are enhanced in deafness, but few studies have evaluated the olfactory function. This study aimed to compare the impact of congenital deafness on olfactory capacities using psychophysical tasks. Methodological issues raised in previous studies, such as homogeneous onset of deafness and cognitive function assessment, were considered. Eleven individuals with bilateral severe-to-profound deafness since birth were compared to 11 hearing non-signers similar in age (age range = 20-51 years old) and sex (7 women). The deaf subjects were assessed using various standardized neuropsychological tests to ascertain typical cognition. Olfactory functions were evaluated using the Sniffin Sticks battery test, which measures olfactory detection threshold, odor discrimination, and odor identification. Further, accuracy and response time were examined for the identification and localization of two odors to disentangle olfactory sensitivity from sensitivity in the trigeminal system. The Sniffin Sticks test demonstrated superior performances in the deaf participants to discriminate and identify odors. In line with this, they also showed higher sensitivity when both identifying and localizing odors. These findings suggest that congenital deafness is associated with superior performance in higher-level olfactory processing and increased sensitivity in the trigeminal system.