Hmrpe

Left Optic radiation variability as a function of threshold used to define connectivity (n 22). Right Mean FA decreased as optic radiation ROI size became larger and more dispersed, but the relation to BOLD response in visual cortex was similar. (Reprinted from Toosy et al. 2004, with permission from Elsevier) line with random-field theory (Kiebel et al. 1999). It is not yet clear whether smoothing is appropriate for analysis of DTI data, but the size of the smoothing filter can...

Acknowledgements

The authors would like to thank Mariana Lazar, Yu-Chien Wu, Alexey Samsonov, Yijing Wu, and Aaron Field at the University of Wisconsin for providing figures, as well as Steve Smith from Oxford and Greg Stanisz from Sunnybrook Health Sciences Centre, and thanks also to Ana Solodkin at the University of Chicago for lively discussions. Some of the diffusion MRI work presented here was supported by NIH grant RO1 MH 62015 and the MT and MWF data was generated from a grant from the National Multiple...

Neuronal Dynamics and Brain Connectivity

Basic Non Linear Map

Michael Breakspear1'2 and Viktor K Jirsa3'4 1 School of Psychiatry, University of New South Wales, and The Black Dog Institute, Randwick, NSW, 2031, Australia. 2 School of Physics, University of Sydney, NSW 2006, Australia 3 Theoretical Neuroscience Group (CNRS), UMR6152 Mouvement & Perception, 13288 Marseille, France 4 Center for Complex Systems & Brain Sciences, Physics Department, Florida Atlantic University, Boca Raton FL33431, USA The fluid nature of perceptual experience and the...

Regional Connectivity

Moving from connections between individual neurons to connections between brain regions, the most widespread and valuable method delivering detailed information about directed long distance connections is neuroanatomical tract tracing (for reviews see Sawchenko and Swanson 1981 Kobbert et al., 2000 Wouterlood et al. 2002). The general approach comprises now a vast range of substances with the common feature that they are taken up by neurons and spread along their projections, where the label...

Info

Data on short-term memory in topologically different parts of the PFC during delay tasks having the what-then-where design. The model contained different populations of neurons (as found experimentally) in attractor networks that responded in the delay period to the stimulus object, the stimulus position, and to combinations of both object and position information. These neuronal populations were arranged hierarchically and global inhibition mediated through inhibitory interneurons was employed...

Magnetization Transfer

Water provides the largest signal contribution to the MRI signal in brain tissues. While estimates of conductivity can be calculated from diffusion tensor data (Tuch et al. 2001), a more ideal probe of the effectiveness of white matter conduction properties would be obtained from images of myelin components. The problem is that the signals from protein and lipid molecules associated with myelin are essentially undetectable in an MRI experiment because they have ultrashort T2 values (10s of...

Integrating Neuroscientific Data Across Spatiotemporal Scales

We have already emphasized that the major goal of large-scale neural modeling is to enable one to propose a set of neural-based mechanisms that can explain particular human cognitive functions. If these mechanisms are reasonable representations of what actually occur in the brain, then simulated data generated by the model, at multiple spatiotemporal scales, should closely approximate corresponding experimental data. We will illustrate this use of modeling by discussing a model of object...

Other Large Scale Modeling Work

There have been several other recent studies in which large-scale neural modeling has been used to relate neural activity to functional neuroimaging data. We will briefly mention a few of them. Deco et al. (2004) modeled the mechanisms that underlie working memory-related activity during the execution of delay tasks that have a what-then-where design (with both object and spatial delayed responses within the same trial). They were interested in examining two notions related to the topographical...

X8

H((Ab + A- + 73 )S(xo)) - 2X8 - x7 (16) The parameter matrices AF, AB ,AL encode forward, backward and lateral connections respectively. Within each subpopulation, the dynamics of neural states are determined by two operators. The first transforms the average density of presynaptic inputs into the average postsynaptic membrane potential. This is modeled by a linear transformation with excitatory (e) and inhibitory (i) kernels parameterized by He i and Te i. He i control the maximum...

Dipole Source Modeling 41 Multipole Expansion of

Equations (3.15) and (3.18) are the general solutions for and B given an arbitrary current source density Js in the absence of boundaries. The integrand of each function involves derivatives of Js(r'), and the integration kernel 1 r r' called the Green's function (Jackson 1975). The basis of the multipole expansion is to assume that Js(r') is confined to some finite region of space, and that the point r at which the field is being computed or measured is far away compared to the size of the...

Segregation and Integration

Anatomical and functional segregation refers to the existence of specialized neurons and brain areas, organized into distinct neuronal populations grouped together to form segregated cortical areas (Shipp and Zeki, 1985 Zeki, 1993). The concept of anatomical segregation is rooted in the notion that specific brain processes or functions can be localized to specific anatomical regions of the human brain, an idea that is central to the history of neurology and cognitive neuroscience (Phillips et...

T1 and T2 Relaxometry

Contrast in most human neuroimaging studies is a function of the T1 and T2 relaxation times of the brain tissues. Consequently, regional signal differences in brain images are often caused by differences in the relaxation properties. T1 is the recovery time of the longitudinal magnetization and T2 is the decay constant associated with the transverse magnetization. Both characteristic times are highly sensitive to bulk water of the tissue and tend to increase with water content. Significant...

Integrating DTI and WMT with Function

New work is emerging that attempts to do more than simply identify differences in DTI measures as a function of some important variable such as age, disease, or performance. In these studies, the question is what are the implications of local variations in FA and or tract characteristics for behavior and brain activity Three recent studies examining correlations of local variations in FA with reaction time have found conflicting results. In an ROI analysis, FA was correlated with reaction time...

Biological Basis of EEG 21 Cortical Anatomy

Eeg Positioning

The mammalian cortex is the outer mantle of cells surrounding the central structures, e.g., brainstem and thalamus. It is unique to mammals, and is believed to be necessary for most higher-level brain functions. Topologically the cortex is comprised of two spherical shells, corresponding to the two hemispheres. The hemispheres are connected by the corpus callosum. Cortical thickness varies mostly between 2-3 mm in the human, and is folded around the subcortical structures so as to appear...

Myelin Water Fraction

Myelin Water Fraction

Although the specificity of T1 and T2 measurements are generally perceived as being poor, several investigators have recently shown that the T2 signal decay in neural tissue is multi-exponential with echo time (Menon and Allen 1991 MacKay et al. 1994 Whittall et al. 1997). Further investigation has shown that different water tissue compartments each have distinct T2 characteristics, and may be separated (see Fig. 24). In white matter, the water signal compartments are believed to originate from...

V1v2 A1a2

Network diagram of the object processing model Tagamets amp Horwitz 1998 Husain et al. 2004 . The regions of each model form a complex network of feedforward and feedback connections these interregional connections can be either excitatory excitatory-to-excitatory elements, shown as solid lines or inhibitory excitatory-to-inhibitory elements, shown as dashed lines . Regions specific to the visual model LGN, V1-V2, V4, IT are shown in red regions specific to the auditory model MGN,...

Xt 4t t t

Typically is taken to be a constant, i.e., which is reasonable for most experimental systems. It should be noted that only solutions which are asymptotically stable can be accurately approximated using numerical integration. There are two main programs available for the numerical integration of delay differential equations. The widely-used and free package XPPAUT Ermentrout, 2005 can perform numerical integration using a variety of fixed step numerical methods, including Runge-Kutta. It has a...