Quantitative basis for neuroimaging of cortical laminae with calibrated functional MRI

Abstract

Layer-specific neurophysiologic, hemodynamic, and metabolic measurements are needed to interpret high-resolution functional magnetic resonance imaging (fMRI) data in the cerebral cortex. We examined how neurovascular and neurometabolic couplings vary vertically in the rat's somatosensory cortex. During sensory stimulation we measured dynamic layer-specific responses of local field potential (LFP) and multiunit activity (MUA) as well as blood oxygenation level-dependent (BOLD) signal and cerebral blood volume (CBV) and blood flow (CBF), which in turn were used to calculate changes in oxidative metabolism (CMR O2 ) with calibrated fMRI. Both BOLD signal and CBV decreased from superficial to deep laminae, but these responses were not well correlated with either layer-specific LFP or MUA. However, CBF changes were quite stable across laminae, similar to LFP. However, changes in CMR O2 and MUA varied across cortex in a correlated manner and both were reduced in superficial lamina. These results lay the framework for quantitative neuroimaging across cortical laminae with calibrated fMRI methods.spike rate | electroencephalography | glutamate | neuroenergetics | lactate T he most recognizable features of the cerebral cortex across phyla are the layers (i.e., laminae) representing different cell types that project and connect to create networks, both in the horizontal and vertical directions of the cortex (1). Functional MRI (fMRI) with high-field magnets has been used to image this complex heterogeneous system of connections across cortical laminae. Given the complexity of the blood oxygenation leveldependent (BOLD) signal (2), quantitative assessment of neurophysiologic, hemodynamic, and metabolic responses across cortical laminae is needed to interpret high-resolution fMRI data in terms of neural activity. Because synaptic density (1) and commensurate electrical and chemical activities vary across cortical layers (3, 4), it is hypothesized that hemodynamic and metabolic responses would also vary. However, there are limited results on layer-specific variations in these parameters.High magnetic fields have improved BOLD sensitivity and specificity (5), whereas other MRI developments have allowed cerebral blood volume (CBV) and flow (CBF) measurements to calibrate fMRI signal so that changes in cerebral metabolic rate of oxygen consumption (CMR O2 ) can be calculated with a biophysical model of BOLD (6). These multimodal fMRI techniques in conjunction with other related magnetic resonance spectroscopy methods have allowed quantitative insights into the molecular and cellular bases of neurovascular and neurometabolic couplings (7).In vivo recordings of neural activity with metal microelectrodes depict fluctuations of extracellular voltage, where the high-and low-frequency components, respectively, reflect multiunit activity (MUA) and local field potential (LFP) in a region (8). MUA is believed to reflect the output spiking activity of an ensemble of neurons because it reveals action potentials of large pyramidal neurons,...

Quantitative activity-induced manganese-dependent MRI for characterizing cortical layers in the primary somatosensory cortex of the rat

“…accumulation in the primary somatosensory cortex following stimulation reflect neuronal activity. Herman et al (2013) also showed that the somatosensory cortex responses acquired with various modalities were well correlated between layers (high Pearson's r score) and in particular middle and lower segments had similar responses with MUA as indicated by a slope of 1. In the present study, T 1 values between segments after the TGN stimulation stage were also well correlated in the range 0.2-1 s with slopes close to 1.0 for all the layers (not shown).…”

“…In the present study, we were able to detect cortical cytoarchitecture during a functional paradigm extending the potential of AIM-MRI. In their study, Herman et al (2013) showed a decrease of BOLD and cerebral blood volume responses in the rat somatosensory cortex from upper superficial to deep layers. On the other hand, multi-unit spike activity (MUA) and cerebral metabolic rate of oxygen (CMRO 2 ) increased.…”

“…Here, the investigation of distributions of pixels allowed characterizing different populations of pixels in low resolution images. For this reason and to examine the distribution of T 1 values in cortical layers, we investigated the barrel cortex by dividing it into three segments of identical size as performed by Herman et al (2013). The largest change in T 1 was observed in the segment corresponding to layer V-VI.…”

“…To date, it was shown that MEMRI can detect specific layers in cortical areas and that the activity-dependent properties of MEMRI were specific for localized cortical areas (Table 1). As pointed out by Herman et al (2013), it is of paramount importance to understand functional MRI signals across laminae in terms of neural activity since quantitative data on layer-specific variations are lacking. In the present study, we were able to detect cortical cytoarchitecture during a functional paradigm extending the potential of AIM-MRI.…”

“…Cortical laminae were not visually observed in all the rats. The barrel cortex was, therefore, split into three distinct segments as performed by Herman et al (2013) corresponding to cortical layers I-III, IV-V, and V-VI (Fig. 6b).…”