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

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

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...

T

An alternative to the classical coherence measure is to separate phase and amplitude information in the signals and analyze the phase locking or phase synchrony as a measure of connectivity between neural assemblies. Phase locking between two oscillators is defined as n,m(t) < const,pn,m(t) n i(t) mfi2(t), where n and m are integers and i(t) and (t) are the instantaneous phases of the signals (Tass, et al., 1998). In order to compute any measure of phase synchrony, the instantaneous phase of...

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...