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Amplitude Differences (amplitude + difference)
Selected AbstractsCompartment size estimation with double wave vector diffusion-weighted imagingMAGNETIC RESONANCE IN MEDICINE, Issue 1 2008Martin A. Koch Abstract Double wave vector diffusion weighting uses gradients along two different directions between excitation and acquisition. It has been shown theoretically that for restricted diffusion the signal amplitude in such an experiment can depend on the angle between the two gradient vectors. The highest amplitude is obtained with antiparallel orientation, and the amplitude difference between parallel and antiparallel gradient orientations depends on the compartment size. The validity of this description is experimentally tested for water between polymer beads, for radish, and for porcine spinal cord, using a clinical MR system with limited gradient strength. The results indicate that the phenomenon is observable; however, the size of the signal difference is considerably diminished when compared with theory. This is attributed to violations of the approximating conditions underlying the theoretical description and to free diffusion contributions. It is concluded that the effect could successfully be used as a basis for developing a new noninvasive method for assessing cell size. Magn Reson Med, 2008. © 2008 Wiley-Liss, Inc. [source] Anomalous signal indicators in protein crystallographyACTA CRYSTALLOGRAPHICA SECTION D, Issue 11 2005P. H. Zwart A Monte Carlo procedure is described that generates random structure factors with simulated errors corresponding to an X-ray data set of a protein of a specific size and given heavy-atom content. The simulated data set can be used to estimate Bijvoet ratios and figures of merit as obtained from SAD phasing routines and can be used to gauge the feasibility of solving a structure via the SAD method. In addition to being able to estimate results from phasing, the simulation allows the estimation of the correlation coefficient between |,F|, the absolute Bijvoet amplitude difference, and FA, the structure-factor amplitude of the heavy-atom model. As this quantity is used in various substructure-solution routines, the estimate provides a rough estimate of the ease of substructure solution. Furthermore, the Monte Carlo procedure provides an easy way of estimating the number of significant Bijvoet intensity differences, denoted as the measurability, and is proposed as an intuitive measure of the quality of anomalous data. [source] Adaptive subtraction of multiples using the L1 -normGEOPHYSICAL PROSPECTING, Issue 1 2004A. Guitton ABSTRACT A strategy for multiple removal consists of estimating a model of the multiples and then adaptively subtracting this model from the data by estimating shaping filters. A possible and efficient way of computing these filters is by minimizing the difference or misfit between the input data and the filtered multiples in a least-squares sense. Therefore, the signal is assumed to have minimum energy and to be orthogonal to the noise. Some problems arise when these conditions are not met. For instance, for strong primaries with weak multiples, we might fit the multiple model to the signal (primaries) and not to the noise (multiples). Consequently, when the signal does not exhibit minimum energy, we propose using the L1 -norm, as opposed to the L2 -norm, for the filter estimation step. This choice comes from the well-known fact that the L1 -norm is robust to ,large' amplitude differences when measuring data misfit. The L1 -norm is approximated by a hybrid L1/L2 -norm minimized with an iteratively reweighted least-squares (IRLS) method. The hybrid norm is obtained by applying a simple weight to the data residual. This technique is an excellent approximation to the L1 -norm. We illustrate our method with synthetic and field data where internal multiples are attenuated. We show that the L1 -norm leads to much improved attenuation of the multiples when the minimum energy assumption is violated. In particular, the multiple model is fitted to the multiples in the data only, while preserving the primaries. [source] The Effects of Fetal Alcohol Syndrome on Response Execution and Inhibition: An Event-Related Potential StudyALCOHOLISM, Issue 11 2009Matthew J. Burden Background:, Both executive function deficits and slower processing speed are characteristic of children with fetal alcohol exposure, but the temporal dynamics of neural activity underlying cognitive processing deficits in fetal alcohol spectrum disorder have rarely been studied. To this end, event-related potentials (ERPs) were used to examine the nature of alcohol-related effects on response inhibition by identifying differences in neural activation during task performance. Methods:, We recorded ERPs during a Go/No-go response inhibition task in 2 groups of children in Cape Town, South Africa (M age = 11.7 years; range = 10 to 13),one diagnosed with fetal alcohol syndrome (FAS) or partial FAS (FAS/PFAS; n = 7); the other, a control group whose mothers abstained or drank only minimally during pregnancy (n = 6). Children were instructed to press a "Go" response button to all letter stimuli presented except for the letter "X," the "No-go" stimulus, which occurred relatively infrequently. Results:, Task performance accuracy and reaction time did not differ between groups, but differences emerged for 3 ERP components,P2, N2, and P3. The FAS/PFAS group showed a slower latency to peak P2, suggesting less efficient processing of visual information at a relatively early stage (,200 ms after stimulus onset). Moreover, controls showed a larger P2 amplitude to Go versus No-go, indicating an early discrimination between conditions that was not seen in the FAS/PFAS group. Consistent with previous literature on tasks related to cognitive control, the control group showed a well-defined, larger N2 to No-go versus Go, which was not evident in the FAS/PFAS group. Both groups showed the expected larger P3 amplitude to No-go versus Go, but this condition difference persisted in a late slow wave for the FAS/PFAS group, suggesting increased cognitive effort. Conclusions:, The timing and amplitude differences in the ERP measures suggest that slower, less efficient processing characterizes the FAS/PFAS group during initial stimulus identification. Moreover, the exposed children showed less sharply defined components throughout the stimulus and response evaluation processes involved in successful response inhibition. Although both groups were able to inhibit their responses equally well, the level of neural activation in the children with FAS/PFAS was greater, suggesting more cognitive effort. The specific deficits in response inhibition processing at discrete stages of neural activation may have implications for understanding the nature of alcohol-related deficits in other cognitive domains as well. [source] Neural Correlates of Face and Object Recognition in Young Children with Autism Spectrum Disorder, Developmental Delay, and Typical DevelopmentCHILD DEVELOPMENT, Issue 3 2002Geraldine Dawson This study utilized electroencephalographic recordings to examine whether young children with autism spectrum disorder (ASD) have impaired face recognition ability. High-density brain event-related potentials (ERPs) were recorded to photos of the child's mother's face versus an unfamiliar female face and photos of a favorite versus an unfamiliar toy from children with ASD, children with typical development, and children with developmental delay, all 3 to 4 years of age (N= 118). Typically developing children showed ERP amplitude differences in two components, P400 and Nc, to a familiar versus an unfamiliar face, and to a familiar versus an unfamiliar object. In contrast, children with ASD failed to show differences in ERPs to a familiar versus an unfamiliar face, but they did show P400 and Nc amplitude differences to a familiar versus an unfamiliar object. Developmentally delayed children showed significant ERP amplitude differences for the positive slow wave for both faces and objects. These data suggest that autism is associated with face recognition impairment that is manifest early in life. [source] Preterm and at term children: morphological and functional analysis of optic nerve and visual pathway with OCT, HRT and pVEPACTA OPHTHALMOLOGICA, Issue 2009R ANGELI Purpose To compare optic nerve morphology and visual function in 14 preterm children(mean gestational age:33,38) with normal cerebral ultrasound at birth and 15 at term children (mean gestational age:39,67). Methods Mean preterm age was 7,6 years, mean at term was 9,1 years. Children underwent to complete functional and morphological evaluation of Optic Nerve Head (ONH) with Heidelberg Retinal Tomography and Optical Coherence Tomography. The same children were evaluated at birth and at the time of morphological examinations by mean of transient and steady state pVEP. Results Mean visual acuity was 9,85 in the preterm, 10 in the at term children. HRT in preterm and at term children at term(n=28) preterm(n=25) P value mean sd mean sd Disc Area 2,5 0,7 2,34 0,68 0,38 Cup Area 0,50 0,44 0,58 0,47 0,51 Rim Area 2 0,45 1,76 0,37 0,047 C/D Area Ratio 0,18 0,12 0,22 0,14 0,23 C/D Linear Ratio 0,4 0,15 0,45 0,16 0,30 Mean RNFL Thick 0,2 0,07 0,24 0,2 0,26 Tab.1 A statistically significant reduction was found in preterm children as regard rim area (p=0,047,tab 1) at HRT and superior RNFL thickness (p=0,01), temporal and inferior inner macular thickness (p=0,03) at OCT. Differences in the pVep latencies were found at 3 and 8 months after birth, but not at the time of morphological examination. Steady State pVEP amplitudes differences were not statistically significant. Conclusion Morphologic optic disk and macula differences between term and preterm children seems not functionally affect the visual pathway. [source] | ||||||||||