Novel measure of olfactory bulb function in health and disease

Present neuroimaging techniques are capable of recording the neural activity from all over the brain but the olfactory bulb (OB). The OB is the first olfactory processing stage of the central nervous system and the site of insult in several neurological disorders, particularly Parkinson’s disease (PD). It has been suggested that the OB has a pivotal role in the olfactory system anal-ogous to primary visual cortex (V1) and thalamus in the visual system. However, due to the existing technical limitations, there has not been any non-invasive technique that can reliably measure the OB function in humans, consequently limiting its functional recording to one in-tracranial study dating back to the 60s. Initially in Study I, a non-invasive method of measuring the function of human OB is devel-oped, so-called electrobulbogram (EBG). In line with previous animal literature as well as the only intracranial study in human OB, it was demonstrated that gamma oscillations on the EBG electrodes occurred shortly after the odor onset. Subsequently, applying source recon-struction analysis provided evidence that observed oscillations were localized to the OB. Ad-ditionally, the OB recording with the EBG method showed a test-retest reliability comparable with visual event related potentials. Notably, the detected gamma oscillations were demon-strated to be insensitive to habituation, the OB’s marked characteristic which has previously been demonstrated in rodents. Last, but not least, assessing the EBG response in an individual who did not have the bilateral OB indicated that the lack of OB results in disappearance of gamma oscillations in the EBG electrodes. Given that Study I determined the possibility of reliably measuring the function of the OB using the EBG, in Study II, I assessed the functional role of OB’s oscillations in the pro-cessing of the odor valence. Odor valence has been suggested to be linked to approach–avoidance responses and therefore, processing of odor valence is thought to be one of the core aspects of odor processing in the olfactory system. Consequently, using combined EBG and EEG recording, OB activity was reconstructed on the source level during processing of odors with different valences. Gamma and beta oscillations were found to be related to va-lence perception in the human OB. Moreover, the early beta oscillations were associated with negative but not positive odors, where these beta oscillations can be linked to preparatory neural responses in the motor cortex. Subsequently, in a separate experiment, negative odors were demonstrated to trigger a whole-body motor avoidance response in the time window overlapping with the valence processes in the OB. These negative odor-elicited motor re-sponses were measured by a force plate as a leaning backward motion. Altogether, the results from Study II indicated that the human OB processes odor valence sequentially in the gamma and beta frequency bands, where the early processing of negative odors in the OB might be facilitating rapid approach-avoidance behaviors. To further evaluate the functional role of the OB in odor processing, in Study III, OB’s communication with its immediate recipient, namely piriform cortex (PC), was assessed. These two areas are critical nodes of the olfactory system which communicate with each other through neural oscillations. The activity of the OB and the PC were reconstructed using a combination of EBG, EEG, and source reconstruction techniques. Subsequently, the cross spectrogram of the OB and the PC was assessed as a measure of functional connectivity where temporal evolution from fast to slow oscillations in the OB–PC connectivity was found during the one second odor processing. Furthermore, the spectrally resolved Granger causal-ity analysis suggested that the afferent connection form the OB to the PC occurred in the gamma and beta bands whereas the efferent connection from the PC to the OB was concen-trated in the theta and delta bands. Notably, odor identity could be deciphered from the low gamma oscillatory pattern in the OB–PC connectivity as early as 100ms after the odor onset. Hence, findings from this study elucidate on our understanding of the bidirectional infor-mation flow in the human olfactory system. Olfactory dysfunction, due to neurodegeneration in the OB, commonly appears several years earlier than the occurrence of the PD-related characteristic motor symptoms. Consequently, a functional measure of the OB may serve as a potential early biomarker of PD. In Study IV, OB function was assessed in PD to answer whether the EBG method can be used to dissociate individuals with a PD diagnosis from healthy age-matched controls. The spectrogram of the EBG signals indicated that there were different values in gamma, beta, and theta for PDs compared with healthy controls. Specifically, six components were found in the EBG re-sponse during early and late time points which together dissociate PDs from controls with a 90% sensitivity and a 100% specificity. Furthermore, these components were linked to med-ication, disease duration and severity, as well as clinical odor identification performance. Overall, these findings support the notion that EBG has a diagnostic value and can be further developed to serve as an early biomarker for PD. In the last study, Study V, the prevalence of COVID-19 was determined using odor intensity ratings as an indication of olfactory dysfunction. Using a large sample data (n = 2440) from a Swedish population, odor intensity ratings of common household items over time were found to be closely associated with prevalence prediction of COVID-19 in the Stockholm region over the same time-period (r = -.83). Impairment in odor intensity rating was further correlated with the number of reported COVID-19 symptoms. Relatedly, individuals who progressed from having no symptoms to having at least one symptom had a marked decline in their odor intensity ratings. The results from this study, given the relatively large sample size, provided a concrete basis for the future studies to further assess the potential association between the deficits in the OB function and olfactory dysfunction in COVID-19. In conclusion, our proposed method for non-invasive measurement of the OB function was shown to provide a reliable recording with a potential as a diagnostic tool for PD. Combining EBG and EEG allowed for reconstruction of the OB signal at the source level, where specific oscillations were found to be critical for odor valence processing and rapid avoidance re-sponse. Moreover, oscillations in different frequency bands were found to be critical for the OB reciprocal communications and transfer of odor identity information to higher order ol-factory subsystems. Finally, COVID-19 was found to be associated with a decline in olfactory acuity which might originate from damage to the patient’s OB. In conclusion, the results from the studies within this thesis provide a new perspective on the functional role of oscillations in the human OB

Laminar fMRI: applications for cognitive neuroscience

The cortex is a massively recurrent network, characterized by feedforward and feedback connections between brain areas as well as lateral connections within an area. Feedforward, horizontal and feedback responses largely activate separate layers of a cortical unit, meaning they can be dissociated by lamina-resolved neurophysiological techniques. Such techniques are invasive and are therefore rarely used in humans. However, recent developments in high spatial resolution fMRI allow for non-invasive, in vivo measurements of brain responses specific to separate cortical layers. This provides an important opportunity to dissociate between feedforward and feedback brain responses, and investigate communication between brain areas at a more fine- grained level than previously possible in the human species. In this review, we highlight recent studies that successfully used laminar fMRI to isolate layer-specific feedback responses in human sensory cortex. In addition, we review several areas of cognitive neuroscience that stand to benefit from this new technological development, highlighting contemporary hypotheses that yield testable predictions for laminar fMRI. We hope to encourage researchers with the opportunity to embrace this development in fMRI research, as we expect that many future advancements in our current understanding of human brain function will be gained from measuring lamina-specific brain responses

Endogenous and exogenous hemodynamic signals in primary visual cortex of alert non-human primates

The advent of neuroimaging techniques in particular the ones suitable for studies in alert humans has disseminated fast. Research in fields involving neuro-correlates of cognitive processes has flourished. Still the neural underpinnings of the neuroimaging signals remain to be fully characterized; this field is an active topic of research. In the context of behavior/cognition, the interpretation of neuroimaging signals is even more intricate

Cortical Layer-Dependent Hemodynamic Regulation Investigated by Functional Magnetic Resonance Imaging

Functional magnetic resonance imaging (fMRI) is currently one of the most widely used non-invasive neuroimaging modalities for mapping brain activation. Techniques such as blood oxygenation level dependent (BOLD) fMRI or cerebral blood volume (CBV)-weighted fMRI are based on the assumption that hemodynamic responses are tightly regulated by neural activity. However, the relationship between fMRI responses and neural activity is still unclear. To investigate this relationship, the unique properties of temporal frequency tuning of primary visual cortex neurons was used as a model since it can be used to separate the neural input and output activities of this area. During moving grating stimuli of 1, 2, 10 and 20 Hz temporal frequencies, two fMRI studies, areal and laminar studies, were conducted with different spatial resolution in a 9.4-T Varian spectrometer. In areal studies, BOLD fMRI was able to detect the difference in tuning properties between area 17 (A17), area 18 (A18) and lateral geniculate nucleus. In A17, the BOLD tuning curve seemed to reflect the local field potential (LFP) low frequency band (<12 Hz) rather than spiking activity and LFP gamma band (25-90 Hz). In laminar studies, a high spatial resolution protocol was adopted to resolve the different cortical layers in A17. In addition to BOLD fMRI, CBV-weighted fMRI was performed to eliminate the contamination from the superficial draining veins. These results showed that BOLD and CBV tuning curves do not reflect the underlying spiking activity or the LFP activity at infragranular layers (the bottom layer of three cortical layers). This implies that the hemodynamic response may not be regulated on a laminar level. Therefore, caution should be taken when interpreting BOLD responses as the sole indicator of different aspects of neural activity in areal and laminar scales

Resting-state Functional Network Disruptions in a Rodent Model of Mesial Temporal Lobe Epilepsy (TLE)

Mesial temporal lobe epilepsy (TLE) is the most common form of drug-refractory epilepsy. The clinical application of non-invasively mapped networks using resting-state functional magnetic resonance imaging (rsfMRI) in humans has been rather limited due to heterogeneous (varying etiology, drugs, onset, latent period, etc.) patient groups. We employed a pharmacological (kainic acid) rodent model of TLE to measure the extent of functional network disruptions using rsfMRI, and study selected behaviors and olfactory to hippocampus transmission. Graph theoretical network modelling and analysis revealed significant increase in functional connectivity connectivity to the temporal lobe (hippocampus) in epileptic-rats compared to controls in the limbic (nucleus accumbens, medial dorsal thalamus), and ``default mode’’ network (retrosplenial, sensorimotor, auditory and posterior parietal cortices). Loss in righting reflex that occurred in response to a lower isoflurane concentration in kainate-treated rats compared to controls was also revealed. These results suggest extensive disruptions in brain networks affected by TLE

Hemodynamic Responses Evoked by Neuronal Stimulation via Channelrhodopsin-2 Can Be Independent of Intracortical Glutamatergic Synaptic Transmission

Maintenance of neuronal function depends on the delivery of oxygen and glucose through changes in blood flow that are linked to the level of ongoing neuronal and glial activity, yet the underlying mechanisms remain unclear. Using transgenic mice expressing the light-activated cation channel channelrhodopsin-2 in deep layer pyramidal neurons, we report that changes in intrinsic optical signals and blood flow can be evoked by activation of a subset of channelrhodopsin-2-expressing neurons in the sensorimotor cortex. We have combined imaging and pharmacology to examine the importance of glutamatergic synaptic transmission in this form of neurovascular coupling. Blockade of ionotropic glutamate receptors with the antagonists CNQX and MK801 significantly reduced forepaw-evoked hemodynamic responses, yet resulted in no significant reduction of channelrhodopsin-evoked hemodynamic responses, suggesting that stimulus-dependent coupling of neuronal activity to blood flow can be independent of local excitatory synaptic transmission. Together, these results indicate that channelrhodopsin-2 activation of sensorimotor excitatory neurons produces changes in intrinsic optical signals and blood flow that can occur under conditions where synaptic activation of neurons or other cells through ionotropic glutamate receptors would be blocked

Role of Astrocytes in Neurovascular Coupling

Neural activity is intimately tied to blood flow in the brain. This coupling is specific enough in space and time that modern imaging methods use local hemodynamics as a measure of brain activity. In this review, we discuss recent evidence indicating that neuronal activity is coupled to local blood flow changes through an intermediary, the astrocyte. We highlight unresolved issues regarding the role of astrocytes and propose ways to address them using novel techniques. Our focus is on cellular level analysis in vivo, but we also relate mechanistic insights gained from ex vivo experiments to native tissue. We also review some strategies to harness advances in optical and genetic methods to study neurovascular coupling in the intact brain

Resting State fMRI Study of the Olfactory Region in Autism

This thesis was conducted to further the investigation of the Olfactory system of a typically developing individual compared to an individual with Autism. The Olfactory system is unique in that it is the only sensory system that is not relayed through the thalamus in the brain. Autism Spectrum Disorder (ASD), also known as Autism, is a developmental disorder which impairs a person's social, behavioral, developmental, cognitive and psychological aspects. Autism Spectrum Disorder can present with symptoms such as difficulty communicating, difficulty with social interactions obsessive thoughts and compulsions and repetitive behaviors. Subjects with Autism Spectrum Disorder present with an inability to process olfactory processes accurately compared to the typically developing (control) subjects. The results of this study can be used to further the understanding of the brain of a person with ASD in conjunction with developing treatments to increase the quality of life and neurological development

Neuroimaging research on olfactory rehabilitation after COVID

Els estudis i les evidències dels avantatges que aporten les teràpies de rehabilitació en relació a la neuroplasticitat son cada vegada més abundants. En aquest projecte, amb col·laboració amb el laboratori LAIMBIO (Laboratori d’Anàlisi d’Imatge Mèdica i Biometria) de la Universitat Rey Juan Carlos, s’ha fet un estudi per investigar la rehabilitació olfactiva de pacients amb anòsmia després d’haver passat COVID-19. El TFM s'ha desenvolupat dins del projecte finançat per la URJC “Nova teràpia de rehabilitació olfactòria per al tractament de persones amb seqüeles a l'olfacte per afectació pel COVID-19”. El problema principal és que la malaltia del COVID-19 és molt recent i encara es desconeixen molts dels seus efectes a nivell neuronal. En aquest projecte s’utilitzen tècniques de neuroimatge per veure l’efecte a nivell cerebral. Es comparen imatges del cervell dels pacients abans de fer teràpia de rehabilitació olfactiva, i després de fer-la. En aquest estudi s’ha treballat amb imatges de ressonància magnètica funcional (fMRI) obtingudes a l’Hospital Universitari Quironsalud Madrid. S'han inclòs quatre subjectes a l'estudi i s'han aconseguit identificar les zones cerebrals amb activació abans de fer la teràpia olfactiva i després de fer-la. Per últim, s’han obtingut resultats de les imatges a nivell de preprocessat, anàlisi estadístic (de primer i segon nivell) i anàlisis ROI (region of interest). Aquests resultats han estat interpretats i s’han extret les respectives conclusionsLos estudios y las evidencias de las ventajas que aportan las terapias de rehabilitación en relación con la neuroplasticidad son cada vez más abundantes. En este proyecto, en colaboración con el laboratorio LAIMBIO (Laboratorio de Análisis de Imagen Médica y Biometría) de la Universidad Rey Juan Carlos, se ha realizado un estudio para investigar la rehabilitación olfativa de pacientes con anosmia después de haber pasado COVID-19. El TFM se ha desarrollado dentro del proyecto financiado por la URJC “Nueva terapia de rehabilitación olfatoria para el tratamiento de personas con secuelas en el olfato por afectación por el COVID-19”. El principal problema es que la enfermedad del COVID-19 es muy reciente y todavía se desconocen muchos de sus efectos a nivel neuronal. En este proyecto se utilizan técnicas de neuroimagen para ver el efecto a nivel cerebral. Se comparan imágenes del cerebro de los pacientes antes de realizar terapia de rehabilitación olfativa, y después de realizarla. En este estudio se ha trabajado con imágenes de resonancia magnética funcional (fMRI) obtenidas en el Hospital Universitario Quironsalud Madrid. Se han incluido cuatro sujetos en el estudio y se ha conseguido identificar las zonas cerebrales con activación antes de hacer la terápia olfativa y después de hacerla. Por último, se han obtenido resultados de las imágenes a nivel de preprocesado, análisis estadístico (de primer y segundo nivel) y análisis ROI (region of interest). Estos resultados han sido interpretados y se han extraído las respectivas conclusionesObjectius de Desenvolupament Sostenible::3 - Salut i Benesta