Olfactory impairments in patients with unilateral cerebellar lesions are selective to inputs from the contralesional nostril.

Functional imaging studies of olfaction have consistently reported odorant-induced activation of the cerebellum. However, the cerebellar role in olfaction remains unknown. We examined the olfactory and olfactomotor abilities of patients with unilateral cerebellar lesions, comparing performance within subjects across nostrils, as well as between subjects with age-matched and young controls. Regarding olfactory performance, initial testing revealed that patients had a contralesional impairment in olfactory identification but not olfactory detection threshold. However, when tested under conditions that prevented compensatory sniffing strategies, the patients also exhibited a contralesional olfactory detection impairment. Regarding olfactomotor function, a healthy olfactomotor system generates sniffs that are (1) sufficiently vigorous and (2) inversely proportional to odorant concentration in sniff mean airflow velocity, maximum airflow velocity, volume, and duration. Patients' sniffs were lower in overall airflow velocity and volume in comparison with control participants. Furthermore, reduced sniff velocity predicted poorer detection thresholds in patients. Finally, whereas young controls used concentration-dependent sniffs, there was a trend in that direction only for age-matched controls. Patients used sniffs that were concentration invariant. In conclusion, cerebellar lesions impacted olfactory and olfactomotor performance. These findings strongly implicate an olfactocerebellar pathway prominent in odor identification and detection that functionally connects each nostril primarily to the contralateral cerebellum.