Improved spatial accuracy of functional maps in the rat olfactory bulb using supervised machine learning approach.

Functional MRI (fMRI) is a popular and important tool for noninvasive mapping of neural activity. As fMRI measures the hemodynamic response, the resulting activation maps do not perfectly reflect the underlying neural activity. The purpose of this work was to design a data-driven model to improve the spatial accuracy of fMRI maps in the rat olfactory bulb. This system is an ideal choice for this investigation since the bulb circuit is well characterized, allowing for an accurate definition of activity patterns in order to train the model. We generated models for both cerebral blood volume weighted (CBVw) and blood oxygen level dependent (BOLD) fMRI data. The results indicate that the spatial accuracy of the activation maps is either significantly improved or at worst not significantly different when using the learned models compared to a conventional general linear model approach, particularly for BOLD images and activity patterns involving deep layers of the bulb. Furthermore, the activation maps computed by CBVw and BOLD data show increased agreement when using the learned models, lending more confidence to their accuracy. The models presented here could have an immediate impact on studies of the olfactory bulb, but perhaps more importantly, demonstrate the potential for similar flexible, data-driven models to improve the quality of activation maps calculated using fMRI data.

Direct detection of a single evoked action potential with MRS in Lumbricus terrestris.

Functional MRI (fMRI) measures neural activity indirectly by detecting the signal change associated with the hemodynamic response following brain activation. In order to alleviate the temporal and spatial specificity problems associated with fMRI, a number of attempts have been made to detect neural magnetic fields (NMFs) with MRI directly, but have thus far provided conflicting results. In this study, we used MR to detect axonal NMFs in the median giant fiber of the earthworm, Lumbricus terrestris, by examining the free induction decay (FID) with a sampling interval of 0.32 ms. The earthworm nerve cords were isolated from the vasculature and stimulated at the threshold of action potential generation. FIDs were acquired shortly after the stimulation, and simultaneous field potential recordings identified the presence or absence of single evoked action potentials. FIDs acquired when the stimulus did not evoke an action potential were summed as background. The phase of the background-subtracted FID exhibited a systematic change, with a peak phase difference of (-1.2 ± 0.3) × 10(-5) radians occurring at a time corresponding to the timing of the action potential. In addition, we calculated the possible changes in the FID magnitude and phase caused by a simulated action potential using a volume conductor model. The measured phase difference matched the theoretical prediction well in both amplitude and temporal characteristics. This study provides the first evidence for the direct detection of a magnetic field from an evoked action potential using MR.

Neuromapping olfactory stimulation using magnetoencephalography - visualizing smell, a proof-of-concept study.

Importance: Currently, clinical assessment of olfaction is largely reliant on subjective methods that require patient participation. The objective method for measuring olfaction, using electroencephalogram (EEG) readings, can be supplemented with the improved temporal resolution of magnetoencephalography (MEG) for olfactory measurement that can delineate cortical and peripheral olfactory loss. MEG provides high temporal and spatial resolution which can enhance our understanding of central olfactory processing compared to using EEG alone. Objective: To determine the feasibility of building an in-house portable olfactory stimulator paired with electrophysiological neuroimaging technique with MEG to assess olfaction in the clinical setting. Design setting and participants: This proof-of-concept study utilized a paired MEG-olfactometer paradigm to assess olfaction in three normosmic participants. We used a two-channel olfactory stimulator to deliver odorants according to a programmed stimulus-rest paradigm. Two synthetic odorants: 2% phenethyl alcohol (rose) and 0.5% amyl acetate (banana) were delivered in increasing increments of time followed by periods of rest. Cortical activity was measured via a 306-channel MEG system. Main outcomes and measures: Primary outcome measure was the relative spectral power for each frequency band, which was contrasted between rest and olfactory stimulation. Results: Compared to rest, olfactory stimulation produced a 40% increase in relative alpha power within the olfactory cortex bilaterally with both odorants. A 25%-30% increase in relative alpha power occurred in the left orbitofrontal cortex and precentral gyrus with phenethyl alcohol stimulation but not amyl acetate. Conclusion and relevance: In this proof-of-concept study, we demonstrate the feasibility of olfactory measurement via an olfactometer-MEG paradigm. We found that odorant-specific cortical signatures can be identified using this paradigm, setting the basis for further investigation of this system as a prognostic tool for olfactory loss.

Time-dependent spatial specificity of high-resolution fMRI: insights into mesoscopic neurovascular coupling.

High-resolution functional magnetic resonance imaging (fMRI) is becoming increasingly popular because of the growing availability of ultra-high magnetic fields which are capable of improving sensitivity and spatial resolution. However, it is debatable whether increased spatial resolutions for haemodynamic-based techniques, like fMRI, can accurately detect the true location of neuronal activity. We have addressed this issue in functional columns and layers of animals with haemoglobin-based optical imaging and different fMRI contrasts, such as blood oxygenation level-dependent, cerebral blood flow and cerebral blood volume fMRI. In this review, we describe empirical evidence primarily from our own studies on how well these fMRI signals are spatially specific to the neuronally active site and discuss insights into neurovascular coupling at the mesoscale. This article is part of the theme issue 'Key relationships between non-invasive functional neuroimaging and the underlying neuronal activity'.

Neural and hemodynamic responses to optogenetic and sensory stimulation in the rat somatosensory cortex.

Introducing optogenetics into neurovascular research can provide novel insights into the cell-specific control of the hemodynamic response. To generalize findings from molecular approaches, it is crucial to determine whether light-activated circuits have the same effect on the vasculature as sensory-activated ones. For that purpose, rats expressing channelrhodopsin (ChR2) specific to excitatory glutamatergic neurons were used to measure neural activity, blood flow, hemoglobin-based optical intrinsic signal, and blood oxygenation level-dependent (BOLD) functional magnetic resonance imaging (fMRI) during optogenetic and sensory stimulation. The magnitude of the evoked hemodynamic responses was monotonically correlated with optogenetic stimulus strength. The BOLD hemodynamic response function was consistent for optogenetic and sensory stimuli. The relationship between electrical activities and hemodynamic responses was comparable for optogenetic and sensory stimuli, and better explained by the local field potential (LFP) than the firing rate. The LFP was well correlated with cerebral blood flow, moderately with cerebral blood volume, and less with deoxyhemoglobin (dHb) level. The presynaptic firing rate had little impact on evoking vascular response. Contribution of the postsynaptic LFP to the blood flow response induced by optogenetic stimulus was further confirmed by the application of glutamate receptor antagonists. Overall, neurovascular coupling during optogenetic control of glutamatergic neurons largely conforms to that of a sensory stimulus.