Contributions of cortical feedback to sensory processing in primary visual cortex

Closing the structure-function divide is more challenging in the brain than in any other organ (Lichtman and Denk, 2011). For example, in early visual cortex, feedback projections to V1 can be quantified (e.g., Budd, 1998) but the understanding of feedback function is comparatively rudimentary (Muckli and Petro, 2013). Focusing on the function of feedback, we discuss how textbook descriptions mask the complexity of V1 responses, and how feedback and local activity reflects not only sensory processing but internal brain states

Ultrahigh Field Functional Magnetic Resonance Electrical Impedance Tomography (fMREIT) in Neural Activity Imaging

abstract: A direct Magnetic Resonance (MR)-based neural activity mapping technique with high spatial and temporal resolution may accelerate studies of brain functional organization. The most widely used technique for brain functional imaging is functional Magnetic Resonance Image (fMRI). The spatial resolution of fMRI is high. However, fMRI signals are highly influenced by the vasculature in each voxel and can be affected by capillary orientation and vessel size. Functional MRI analysis may, therefore, produce misleading results when voxels are nearby large vessels. Another problem in fMRI is that hemodynamic responses are slower than the neuronal activity. Therefore, temporal resolution is limited in fMRI. Furthermore, the correlation between neural activity and the hemodynamic response is not fully understood. fMRI can only be considered an indirect method of functional brain imaging. Another MR-based method of functional brain mapping is neuronal current magnetic resonance imaging (ncMRI), which has been studied over several years. However, the amplitude of these neuronal current signals is an order of magnitude smaller than the physiological noise. Works on ncMRI include simulation, phantom experiments, and studies in tissue including isolated ganglia, optic nerves, and human brains. However, ncMRI development has been hampered due to the extremely small signal amplitude, as well as the presence of confounding signals from hemodynamic changes and other physiological noise. Magnetic Resonance Electrical Impedance Tomography (MREIT) methods could have the potential for the detection of neuronal activity. In this technique, small external currents are applied to a body during MR scans. This current flow produces a magnetic field as well as an electric field. The altered magnetic flux density along the main magnetic field direction caused by this current flow can be obtained from phase images. When there is neural activity, the conductivity of the neural cell membrane changes and the current paths around the neurons change consequently. Neural spiking activity during external current injection, therefore, causes differential phase accumulation in MR data. Statistical analysis methods can be used to identify neuronal-current-induced magnetic field changes.Dissertation/ThesisDoctoral Dissertation Biomedical Engineering 201

Resolving the projecjion of an occluded stimulus on the human cortical surface

The human visual system is capable of tracking multiple visual targets under a variety of task constraints and configurations. For nearly two decades, the psychophysical literature has shown that moving, occluded visual targets -- targets that are momentarily invisible as they pass behind an occluding bar -- are differentially represented by the visual system compared to their moving, non-occluded counterparts. Here, I sought to examine the neurophysiological basis of this behavioral difference in response to occluded versus non-occluded visual targets. I used brain imaging to conduct modern retinotopic mapping experiments in human participants. Once· their early visual cortices were mapped, I was able characterize the neural representations for both targets and distractors as well as during moments of occlusion and non-occlusion. The results show that, using our method, we can distinguish visual targets from distractors; furthermore, there appears to be a representation in retinotopically organized early visual cortex for visual targets that have momentarily disappeared from the visual field due to occlusion

Action Potential Monitoring Using Neuronanorobots: Neuroelectric Nanosensors

Neuronanorobotics, a key future medical technology that can enable the preservation of human brain information, requires appropriate nanosensors. Action potentials encode the most resource-intensive functional brain data. This paper presents a theoretical design for electrical nanosensors intended for use in neuronanorobots to provide non-destructive, in vivo, continuous, real-time, single-spike monitoring of action potentials initiated and processed within the ~86 × 109 neurons of the human brain as intermediated through the ~2.4 × 1014 human brain synapses. The proposed ~3375 nm3 FET-based neuroelectric nanosensors could detect action potentials with a temporal resolution of at least 0.1 ms, enough for waveform characterization even at the highest human neuron firing rates of 800 Hz.The principal author (NRBM) thanks the “Fundação para a Ciência e Tecnologia” (FCT) for their financial support of this work (grant SFRH/BD/69660/2010).info:eu-repo/semantics/publishedVersio

Functional and Structural Magnetic Resonance Imaging of Humans and Macaques

Magnetic resonance imaging (MRI) is a technique which finds use in the neurosciences both as an anatomical and functional localization tool. The traditional uses of MRI for structural analysis, such as are commonly found in medicine, can be adapted to serve in place of histological studies for identifying areas of interest in the cortex. Functional MRI (fMRI) is a rapidly developing tangent of MRI which can be used alone or in tandem with classical electrophysiological experiments to investigate neural activity. Although developed intensely for clinical and scientific studies in human subjects, MRI and fMRI have been used increasingly in the non-human primate. This document contains work exemplifying the use of fMRI in both species and methods for pre- and post-surgical anatomical MRI in the non-human primate. Serving as a solid foundation for learning the principles of block-design fMRI, a classic visual illusion, the motion aftereffect, is studied in the human by means of a hemifield visual stimulus using conventional blood oxygen level dependent (BOLD) fMRI. Primary response and levels of motion aftereffect are analyzed in visual cortex, areas pMT and pMST. A novel use of iron oxide nanoparticles as an intravascular contrast agent in the non-human primate is investigated as a method of boosting fMRI contrast, yielding an ultimate gain in contrast-to-noise at the expense of temporal resolution. While anatomical imaging served as a necessary tool for the localization of functional response in the human, further novel techniques were investigated in the non-human primate. A technique for MRI-guided implantation of multiple electrode arrays is considered, to aid the localization of sites of interest in the cortex. The use of MRI as a replacement for histological preparations for purposes of reconstructing electrode penetration sites is documented. These studies exist to aid in bridging the gap between human and non-human MRI and fMRI. Further application of these principles could be extended to the eventual placement of intracortical recording devices in the human, to benefit a patient population needing devices such as a neural prosthesis

Data-driven reconstruction of a point neuron mouse brain

In this work, we present a semi-automatic method to reconstruct a mouse whole-brain model at the point-neuron level by integrating a wide array of biological data. Our process has three parts: cell position and type assignment, connectivity mapping, and simulation. Additional validation is performed at every step of the workflow. We obtain cell positions from a voxelized data sets derived from high-resolution Nissl stained microscope image stacks in the Allen Mouse Brain reference atlas (Lein, et al. 2007), and global constraint numbers for the whole-brain (Herculano-Houzel, Ribeiro, et al. 2011). We then assign the type of each cell using In Situ Hybridization (ISH) image data from the same atlas. Cells are classified as glia subtypes, excitatory neurons, or inhibitory neurons, leaving the possibility to further expand the diversity of assigned cell types by using more genes in this step. We furthermore integrate region-specific cell densities from literature into our model. We then study cell type correlations throughout the brain and compare the resulting numbers to literature data in order to validate the process. In the second step, we use two-photon tomography images of recombinant Adeno-Associated Virus (rAAV) labeled axonal projections from the Allen Mouse Connectivity Atlas (Oh, et al. 2014) to determine the mesoscale connectivity between the neurons in different brain regions. For this step a comprehensive comparison to experimental data is difficult due to lack of similar connectivity data. We then obtain a network configuration that can be simulated with state-of the art software like Nest. We show results from a simulated whisker stimulation experiment and compare the evoked activity patterns to imaging data from Voltage Sensitive Dye (VSD) experiments (Ferezou, Haiss, et al. 2007). Finally, we build a glossary of possible whole-brain behaviors of our model, and briefly explore the possibility of a biologically plausible mapping of its input and output pathways