Determination of local optical properties of the rat barrel cortex during neural activation: Monte-Carlo approach to light propagation

Spatially-spectrally-resolved reflectance measurements allow in vivo measuring the optical coefficients of absorption and scattering within the cortical tissue. This method, if applied to neural tissue during enhanced activity, could allow a straightforward monitoring of the blood oxygen saturation changes occurring in the brain cortex during hemodynamic responses. Furthermore, it may provide valuable information on possible absorption and scattering changes occurring during stimulation. The feasibility of such measurements was investigated by carrying a preliminary numerical study using a Monte-Carlo light propagation routine. Experimental parameters such as the geometry of the optical probe, baseline cortex optical coefficients retrieved from the literature and anatomical characteristics of the rat barrel cortex were used as an input for the simulations. The sensitivity of the probe to the local variations of optical coefficients was investigated with this numerical approach. Additionally, the influence of the barrel cortex dimensions and the probe positioning relatively to the activated region were studied for instrumental optimization purpose. It was found that typical variations of optical coefficients can be detected if the activated region of barrel cortex has a volume of typically 1 mm3 or larger. The decay of the probe sensitivity to changes was studied as a function of the depth of the activated region. The results showed that the best sensitivity is achieved by placing the light injection fiber of the optical probe aligned onto the center of the cylindrical barrel.

Local in vivo optical measurements of neurometabolic and neurovascular coupling mechanisms in the rodent cortex

Elena Migacheva

This thesis introduces a novel approach for in vivo separation and quantification of spatio-temporal dynamics of optical coefficients (absorption, scattering), oxygen saturation (SO2) and haemoglobin concentrations within rat brain somatosensory cortex under baseline and neuronal stimulation conditions. The quantification of mentioned parameters became possible due to design of a novel optical setup for localized spatio-temporally-resolved (within ∼ 1 mm3 at 10 Hz) reflectance spectroscopy (STRReS) measurements at small source-detector separations. The possibility to automatically take into account the light propagation path lengths in tissue using Monte Carlo modeling, defines the novelty of this approach and its perspectives for brain research. This kind of corrections is currently limited or inaccurate with existing techniques like optical intrinsic signal imaging. In particular, the main aim of this thesis is to explore and better understand the local spatio-temporal characteristics of neurovascular and neurometabolic coupling mechanisms within in vivo rat cerebral cortex at initial and subsequent stages of neuronal stimulation with using STRReS technique. It has been proven theoretically (with Monte Carlo simulations) and experimentally (from in vivo rat brain cortex), that STRReS measurements are suitable from in vivo somatosensory cortex under neuronal stimulation conditions. As a part of this research, we have validated theoretically and experimentally the biological importance of new optical parameter ”gamma” (parameter of the phase function), which is nessesary to take into account for accurate optical coefficients extraction. On the other side, the STRReS technique was not found to be helpful to determine differential factors corrections for improvement of optical intrinsic signal imaging analysis due to differences in experiment’s geometries. Finally, it has been shown that temporal dynamics of reflectance is strongly influenced by light scattering variations within 500-900 nm range. Specifically, the shape of reflectance dynamics at 600 and 610 nm differs significantly from deoxy-haemoglobin concentration dynamics at initial stage of neural activation. For this reason, it is important always to take into account the variations of both components of reflectance signal: absorption and scattering. Additionally, an initial delay of 1 s was found in SO2 and haemoglobin concentrations dynamics. The interpretation of this delay, however, is strongly dependent on the origin of scattering signal. Our results are well correlated with ”metabolic” hypothesis of underlying neurovascular and neurometabolic coupling mechanisms, assuming that the scattering signal has a non-vascular origin at 500-599 nm. Nevertheless, the ”neurogenic” hypothesis is not completely excluded as far as the scattering signal has a vascular origin at 500-599 nm.

EPFL2012

Barrel cortex function

Carl Petersen, James Poulet

Neocortex, the neuronal structure at the base of the remarkable cognitive skills of mammals, is a layered sheet of neuronal tissue composed of juxtaposed and interconnected columns. A cortical column is considered the basic module of cortical processing present in all cortical areas. It is believed to contain a characteristic microcircuit composed of a few thousand neurons. The high degree of cortical segmentation into vertical columns and horizontal layers is a boon for scientific investigation because it eases the systematic dissection and functional analysis of intrinsic as well as extrinsic connections of the column. In this review we will argue that in order to understand neocortical function one needs to combine a microscopic view, elucidating the workings of the local columnar microcircuits, with a macroscopic view, which keeps track of the linkage of distant cortical modules in different behavioral contexts. We will exemplify this strategy using the model system of vibrissal touch in mice and rats. On the macroscopic level vibrissal touch is an important sense for the subterranean rodents and has been honed by evolution to serve an array of distinct behaviors. Importantly, the vibrissae are moved actively to touch - requiring intricate sensorimotor interactions. Vibrissal touch, therefore, offers ample opportunities to relate different behavioral contexts to specific interactions of distant columns. On the microscopic level, the cortical modules in primary somatosensory cortex process touch inputs at highest magnification and discreteness - each whisker is represented by its own so-called barrel column. The cellular composition, intrinsic connectivity and functional aspects of the barrel column have been studied in great detail. Building on the versatility of genetic tools available in rodents, new, highly selective and flexible cellular and molecular tools to monitor and manipulate neuronal activity have been devised. Researchers have started to combine these with advanced and highly precise behavioral methods, on par with the precision known from monkey preparations. Therefore, the vibrissal touch model system is exquisitely positioned to combine the microscopic with the macroscopic view and promises to be instrumental in our understanding of neocortical function. (C) 2012 Elsevier Ltd. All rights reserved.

Pergamon-Elsevier Science Ltd2013

Local in vivo optical measurements of neurometabolic and neurovascular coupling mechanisms in the rodent cortex

Elena Migacheva

EPFL2012

Barrel cortex function

Carl Petersen, James Poulet

Pergamon-Elsevier Science Ltd2013