Neural and behavioral plasticity in olfactory sensory deprivation

Abstract

The human brain has a remarkable ability to reorganize as a consequence of altered demands. This ability is particularly noticeable when studying the neural effects of complete sensory deprivation. Both structural and functional cerebral reorganization have repeatedly been demonstrated in individuals with sensory deprivation, most evident in cortical regions associated with the processing of the absent sensory modality. Furthermore, sensory deprivation has been linked to altered abilities in remaining sensory modalities, often of a compensatory character. Although anosmia, complete olfactory sensory deprivation, is our most common sensory deprivation, estimated to affect around 5 % of the population, the effects of anosmia on brain and behavior are still poorly understood. The overall aim of this thesis was to investigate how the human brain and behavior are affected by anosmia, with a focus on individuals with congenital (lifelong) sensory deprivation. Specifically, Study I and Study IV assessed potential behavioral and neural multisensory compensatory abilities whereas Study II and Study III assessed potential reorganization beyond the processing of specific stimuli; the latter by determining morphological and resting-state functional connectivity alterations.

Integration of information from different sensory modalities leads to a more accurate perception of the world around us, given that our senses provide complementary information. Although an improved ability to extract multisensory information would be of particular relevance to individuals deprived of one sensory modality, multisensory integration has been sparsely studied in relation to sensory deprivation. In Study I, multisensory integration of audio-visual stimuli was assessed in individuals with anosmia using two different experimental tasks. First, individuals with anosmia were better than matched controls in detecting multisensory temporal asynchronies in a simultaneity judgement task. Second, individuals with congenital, but not acquired, anosmia demonstrated indications of an enhanced ability to utilize multisensory information in an object identification task with degraded stimuli. Based on these results, the neural correlates of audio-visual processing and integration were assessed in individuals with congenital anosmia in Study IV. Relative to matched normosmic individuals, individuals with congenital anosmia demonstrated increased activity in established multisensory regions when integrating degraded audio-visual stimuli; however, no compensatory cross-modal processing in olfactory regions was demonstrated. Together, Study I and IV suggest that complete olfactory sensory deprivation is linked to enhanced audio-visual integration performance that might be facilitated by increased processing in multisensory regions. In Study II, whole-brain gray matter morphology was assessed in individuals with congenital anosmia. Both increases and decreases in the orbitofrontal cortex, a region associated with olfaction and sometimes referred to as secondary olfactory cortex, were observed in individuals with congenital anosmia in relation to matched controls. However, in contrast to our expectations, no sensory deprivation-dependent effects were demonstrated in piriform cortex, a region commonly referred to as primary olfactory cortex. Furthermore, Study III revealed an absence of differences in resting-state functional connectivity between individuals with congenital anosmia and normosmic individuals within the primary olfactory cortex (including piriform cortex) as well as between core olfactory processing regions.

In conclusion, the studies presented within this thesis suggest the existence of a potential multisensory compensatory mechanism in individuals with anosmia, but demonstrate a striking lack of morphological and functional alterations in piriform (primary olfactory) cortex. These results demonstrate that complete olfactory deprivation is associated with a distinct neural and behavioral reorganization in some regions but also a clear lack of effects in other regions; the latter underline the clear differences between our senses and suggest that extrapolating from individual senses should be done cautiously.