Olfactory dysfunction is a prevalent non-motor symptom of Parkinson’s disease (PD). This dysfunction is a result of neurodegeneration within the olfactory bulb (OB), the first processing area of the central olfactory system, and commonly precedes the characteristic motor symptoms in PD by several years. Functional measurements of the OB could therefore potentially be used as an early biomarker for PD. Here, we used a non-invasive method, so-called electrobulbogram (EBG), to measure OB function in PD and age-matched healthy controls to assess whether EBG measures can dissociate PDs from controls. We estimated the spectrogram of the EBG signal during exposure to odor in PD (n = 20) and age-matched controls (n = 18) as well as identified differentiating patterns of odor-related synchronization in the gamma, beta, and theta frequency bands. Moreover, we assessed if these PD-EBG components could dissociate PD from control as well as their relationship with PD characteristics. We identified six EBG components during the initial and later stages of odor processing which dissociated PD from controls with 90% sensitivity and 100% specificity with links to PD characteristics. These PD-EBG components were related to medication, disease duration, and severity, as well as clinical odor identification performance. These findings support using EBG as a tool to experimentally assess PD interventions, potentially aid diagnosis, and the potential development of EBG into an early biomarker for PD.

Removing function from a developed and functional sensory system is known to alter both cerebral morphology and functional connections. To date, a majority of studies assessing sensory-dependent plasticity have focused on effects from either early onset or long-term sensory loss and little is known how the recent sensory loss affects the human brain. With the aim of determining how recent sensory loss affects cerebral morphology and functional connectivity, we assessed differences between individuals with acquired olfactory loss (duration 7-36 months) and matched healthy controls in their grey matter volume, using multivariate pattern analyses, and functional connectivity, using dynamic connectivity analyses, within and from the olfactory cortex. Our results demonstrate that acquired olfactory loss is associated with altered grey matter volume in, among others, posterior piriform cortex, a core olfactory processing area, as well as the inferior frontal gyrus and angular gyrus. In addition, compared to controls, individuals with acquired anosmia displayed significantly stronger dynamic functional connectivity from the posterior piriform cortex to, among others, the angular gyrus, a known multisensory integration area. When assessing differences in dynamic functional connectivity from the angular gyrus, individuals with acquired anosmia had stronger connectivity from the angular gyrus to areas primary responsible for basic visual processing. These results demonstrate that recently acquired sensory loss is associated with both changed cerebral morphology within core olfactory areas and increase dynamic functional connectivity from olfactory cortex to cerebral areas processing multisensory integration.

Neuronal oscillations route external and internal information across brain regions. In the olfactory system, the two central nodes -the olfactory bulb (OB) and the piriform cortex (PC) -communicate with each other via neural oscillations to shape the olfactory percept. Communication between these nodes have been well characterized in non-human animals but less is known about their role in the human olfactory system. Using a recently developed and validated EEG-based method to extract signals from the OB and PC sources, we show in healthy human participants that there is a bottom-up information flow from the OB to the PC in the beta and gamma frequency bands, while top-down information from the PC to the OB is facilitated by delta and theta oscillations. Importantly, we demonstrate that there was enough information to decipher odor identity above chance from the low gamma in the OB-PC oscillatory circuit as early as 100 ms after odor onset. These data further our understanding of the critical role of bidirectional information flow in human sensory systems to produce perception. However, future studies are needed to determine what specific odor information is extracted and communicated in the information exchange.

Determining the valence of an odor to guide rapid approach–avoidance behavior is thought to be one of the core tasks of the olfactory system, and yet little is known of the initial neural mechanisms supporting this process or of its subsequent behavioral manifestation in humans. In two experiments, we measured the functional processing of odor valence perception in the human olfactory bulb (OB)—the first processing stage of the olfactory system—using a noninvasive method as well as assessed the subsequent motor avoidance response. We demonstrate that odor valence perception is associated with both gamma and beta activity in the human OB. Moreover, we show that negative, but not positive, odors initiate an early beta response in the OB, a response that is linked to a preparatory neural motor response in the motor cortex. Finally, in a separate experiment, we show that negative odors trigger a full-body motor avoidance response, manifested as a rapid leaning away from the odor, within the time period predicted by the OB results. Taken together, these results demonstrate that the human OB processes odor valence in a sequential manner in both the gamma and beta frequency bands and suggest that rapid processing of unpleasant odors in the OB might underlie rapid approach–avoidance decisions.

Measures of neural processing can be obtained non-invasively from all areas of the human brain but one, the olfactory bulb. Here, the authors show that signals obtained from EEG electrodes at the nasal bridge represent responses from the human olfactory bulb, the so-called Electrobulbogram. Current non-invasive neuroimaging methods can assess neural activity in all areas of the human brain but the olfactory bulb (OB). The OB has been suggested to fulfill a role comparable to that of V1 and the thalamus in the visual system and have been closely linked to a wide range of olfactory tasks and neuropathologies. Here we present a method for non-invasive recording of signals from the human OB with millisecond precision. We demonstrate that signals obtained via recordings from EEG electrodes at the nasal bridge represent responses from the human olfactory bulb - recordings we term Electrobulbogram (EBG). The EBG will aid future olfactory-related translational work but can also potentially be implemented as an everyday clinical tool to detect pathology-related changes in human central olfactory processing in neurodegenerative diseases. In conclusion, the EBG is localized to the OB, is reliable, and follows response patterns demonstrated in non-human animal models.

In response to the coronavirus disease 2019 (COVID-19) pandemic, countries have implemented various strategies to reduce and slow the spread of the disease in the general population. For countries that have implemented restrictions on its population in a stepwise manner, monitoring of COVID-19 prevalence is of importance to guide the decision on when to impose new, or when to abolish old, restrictions. We are here determining whether measures of odor intensity in a large sample can serve as one such measure. Online measures of how intense common household odors are perceived and symptoms of COVID-19 were collected from 2440 Swedes. Average odor intensity ratings were then compared to predicted COVID-19 population prevalence over time in the Swedish population and were found to closely track each other (r = -0.83). Moreover, we found that there was a large difference in rated intensity between individuals with and without COVID-19 symptoms and the number of symptoms was related to odor intensity ratings. Finally, we found that individuals progressing from reporting no symptoms to subsequently reporting COVID-19 symptoms demonstrated a large drop in olfactory performance. These data suggest that measures of odor intensity, if obtained in a large and representative sample, can be used as an indicator of COVID-19 disease in the general population. Importantly, this simple measure could easily be implemented in countries without widespread access to COVID-19 testing or implemented as a fast early response before widespread testing can be facilitated.

Although stress is an increasing global health problem in cities, urban green spaces can provide health benefits. There is, however, a lack of understanding of the link between physiological mechanisms and qualities of urban green spaces. Here, we compare the effects of visual stimuli (360 degree virtual photos of an urban environment, forest, and park) to the effects of congruent olfactory stimuli (nature and city odours) and auditory stimuli (bird songs and noise) on physiological stress recovery. Participants (N = 154) were pseudo-randomised into participating in one of the three environments and subsequently exposed to stress (operationalised by skin conductance levels). The park and forest, but not the urban area, provided significant stress reduction. High pleasantness ratings of the environment were linked to low physiological stress responses for olfactory and to some extent for auditory, but not for visual stimuli. This result indicates that olfactory stimuli may be better at facilitating stress reduction than visual stimuli. Currently, urban planners prioritise visual stimuli when planning open green spaces, but urban planners should also consider multisensory qualities.