The Retinotopy of Visual Spatial Attention

AbstractWe used high-field (3T) functional magnetic resonance imaging (fMRI) to label cortical activity due to visual spatial attention, relative to flattened cortical maps of the retinotopy and visual areas from the same human subjects. In the main task, the visual stimulus remained constant, but covert visual spatial attention was varied in both location and load. In each of the extrastriate retinotopic areas, we found MR increases at the representations of the attended target. Similar but smaller increases were found in V1. Decreased MR levels were found in the same cortical locations when attention was directed at retinotopically different locations. In and surrounding area MT+, MR increases were lateralized but not otherwise retinotopic. At the representation of eccentricities central to that of the attended targets, prominent MR decreases occurred during spatial attention

Sub-millimeter fMRI reveals multiple topographical digit representations that form action maps in human motor cortex

The human brain coordinates a wide variety of motor activities. On a large scale, the cortical motor system is topographically organized such that neighboring body parts are represented by neighboring brain areas. This homunculus-like somatotopic organization along the central sulcus has been observed using neuroimaging for large body parts such as the face, hands and feet. However, on a finer scale, invasive electrical stimulation studies show deviations from this somatotopic organization that suggest an organizing principle based on motor actions rather than body part moved. It has not been clear how the action-map organization principle of the motor cortex in the mesoscopic (sub-millimeter) regime integrates into a body map organization principle on a macroscopic scale (cm). Here we developed and applied advanced mesoscopic (sub-millimeter) fMRI and analysis methodology to non-invasively investigate the functional organization topography across columnar and laminar structures in humans. Compared to previous methods, in this study, we could capture locally specific blood volume changes across entire brain regions along the cortical curvature. We find that individual fingers have multiple mirrored representations in the primary motor cortex depending on the movements they are involved in. We find that individual digits have cortical representations up to 3 ​mm apart from each other arranged in a column-like fashion. These representations are differentially engaged depending on whether the digits’ muscles are used for different motor actions such as flexion movements, like grasping a ball or retraction movements like releasing a ball. This research provides a starting point for non-invasive investigation of mesoscale topography across layers and columns of the human cortex and bridges the gap between invasive electrophysiological investigations and large coverage non-invasive neuroimaging

High-resolution CBV-fMRI allows mapping of laminar activity and connectivity of cortical input and output in human M1

Layer-dependent fMRI allows measurements of information flow in cortical circuits, as afferent and efferent connections terminate in different cortical layers. However, it is unknown to what level human fMRI is specific and sensitive enough to reveal directional functional activity across layers. To answer this question, we developed acquisition and analysis methods for blood-oxygen-level-dependent (BOLD) and cerebral-blood-volume (CBV)-based laminar fMRI and used these to discriminate four different tasks in the human motor cortex (M1). In agreement with anatomical data from animal studies, we found evidence for somatosensory and premotor input in superficial layers of M1 and for cortico-spinal motor output in deep layers. Laminar resting-state fMRI showed directional functional connectivity of M1 with somatosensory and premotor areas. Our findings demonstrate that CBV-fMRI can be used to investigate cortical activity in humans with unprecedented detail, allowing investigations of information flow between brain regions and outperforming conventional BOLD results that are often buried under vascular biases

Modulation of oxidative metabolism in human visual cortex studies with positron emission tomography

This dissertation addresses the question of whether increases in neural activity caused by sensory stimulation require a significant acceleration of aerobic metabolism in the brain. A new technique for measuring cerebral oxygen consumption with Positron Emission Tomography (PET) was used to assess focal changes in oxidative metabolism in different conditions of visual stimulation. Specifically, positron emission tomography was used to determine whether statistically significant increases in regional cerebral oxygen metabolism are produced by different lengths and types of visual stimulation. These experiments employed a single-bolus inhalation of [15O]O2 to measure oxygen uptake into cortical tissue.In one experiment, oxygen consumption during a monocular presentation of a stimulus restricted to a single visual hemi-field (after 3 minutes and 8 minutes of stimulation) was compared to oxygen consumption measured during a simple fixation baseline. Oxygen consumption increased by 20.5% after 3 minutes. In a second experiment similar procedure was used to examine the changes in oxygen metabolism produced by two different visual stimuli. Regional oxygen metabolism increased by 23% for both stimulus conditions, a reversing yellow-blue checkerboard and a rapidly blinking white-disk. In conclusion, focal activation of human visual cortex is accompanied by significant increases in oxygen metabolism

A high-order finite element method for Tokamak plasma equilibria /

A numerical method for the solution of the axisymmetric, free-boundary, Tokamak equilibrium problem is described. The method uses high-order polynomials defined over a mesh of triangular finite elements to solve the magnetohydrodynamic equilibrium (Grad-Shafranov) equation. Arbitrary coil and plasma current configurations can be specified. The formulation incorporates a nonlinear procedure for computing the coil currents required to place the plasma in a desired position. The solution to the nonlinear Grad-Shafranov equation is computed using a modified Newton's method. The inner-most system of sparse, linear equations is solved using a preconditioned, conjugate gradient algorithm. A computer program, PLEQUI (PLasma EQUIlibrium), was written in a portable FORTRAN dialect to implement the method. The method was tested using both fixed-boundary and free-boundary plasma problems. The program was validated by comparing the results to analytic solutions, by examining the flux plots, or by comparing the solution to the output of another finite-element code

Detecting and estimating the regions of activation in CBF activation studies in human brain

This paper has proposed a test for regions of activation based on the maximum value. It suffers from the same problem of over-correction as all tests, such as a Bonferroni-corrected test, which correct for searching over a volume of possible locations: the critical values of our test and estimator are much higher than the critical value for a pre-determined point in the image. The price paid for not knowing the location of the region of activation is therefore quite severe. In practice the investigator knows much more about which regions are likely to be activated. Experimenters using this test are thus faced with declaring that no activation has taken place in regions where activation seems `meaningful' based on prior knowledge of brain function, but where the maximum of the image falls below the critical value. It is possible to overcome this by reducing the search volume to a smaller region containing just these likely regions, thus lowering the critical value, but these critical values do not decrease appreciably

Topographical and laminar distribution of audiovisual processing within human planum temporale

The brain is capable of integrating signals from multiple sensory modalities. Such multisensory integration can occur in areas that are commonly considered unisensory, such as planum temporale (PT) representing the auditory association cortex. However, the roles of different afferents (feedforward vs. feedback) to PT in multisensory processing are not well understood. Our study aims to understand that by examining laminar activity patterns in different topographical subfields of human PT under unimodal and multisensory stimuli. To this end, we adopted an advanced mesoscopic (sub-millimeter) fMRI methodology at 7 T by acquiring BOLD (blood-oxygen-level-dependent contrast, which has higher sensitivity) and VAPER (integrated blood volume and perfusion contrast, which has superior laminar specificity) signal concurrently, and performed all analyses in native fMRI space benefiting from an identical acquisition between functional and anatomical images. We found a division of function between visual and auditory processing in PT and distinct feedback mechanisms in different subareas. Specifically, anterior PT was activated more by auditory inputs and received feedback modulation in superficial layers. This feedback depended on task performance and likely arose from top-down influences from higher-order multimodal areas. In contrast, posterior PT was preferentially activated by visual inputs and received visual feedback in both superficial and deep layers, which is likely projected directly from the early visual cortex. Together, these findings provide novel insights into the mechanism of multisensory interaction in human PT at the mesoscopic spatial scale