Michael E. Brandt

Estimation of CSF, white and gray matter volumes in hydrocephalic children using fuzzy clustering of MR images.

An algorithm and set of procedures for measuring volumes of cerebrospinal fluid (CSF), white matter, and gray matter from transaxial magnetic resonance images (MRI) of the brain are described. The algorithm is a variant of the fuzzy c-means clustering method for texture identification. This technique is used mainly to solve the problem of volume averaging of tissue compartments, but also has other advantages. It is fast, accurate, and relatively operator independent. Furthermore, it does not depend on statistical assumptions such as data normality, nor does it require any a priori heuristics. The procedure was tested successfully on imaged phantoms of known volume composition and compared with results achieved using a standard morphometric measurement approach. The procedure was also applied to brain MRIs of three clinically normal children and three age-matched children with hydrocephalus using both proton density and T2-weighted images. The algorithm was able to detect the expected increased amounts of CSF and decreased amounts of white matter characteristic of the hydrocephalic brain.

A neurophysiologically-based mathematical model of flash visual evoked potentials

Evidence is presented that a neurophysiologically-inspired mathematical model, originally developed for the generation of spontaneous EEG (electroencephalogram) activity, can produce VEP (visual evoked potential)-like waveforms when pulse-like signals serve as input. It was found that the simulated VEP activity was mainly due to intracortical excitatory connections rather than direct thalamic input. Also, the model-generated VEPs exhibited similar relationships between prestimulus EEG characteristics and subsequent VEP morphology, as seen in human data. Specifically, the large correlation between the N1 amplitude and the prestimulus alpha phase angle, and the insensitivity of P2 to the latter feature, as observed in actual VEPs to low intensity flashes, was also found in the model-generated data. These findings provide support for the hypothesis that the spontaneous EEG and the VEP are generated by some of the same neural structures and that the VEP is due to distributed activity, rather than dipolar sources.

Pre-stimulus spectral EEG patterns and the visual evoked response

The relationship between the latencies and amplitudes of the N1 and P2 components of the visual evoked potential (VEP) and the psychophysiological state of the brain immediately preceding the time of the stimulus has been investigated in 7 male subjects. Power spectral measures in the delta, theta, alpha and beta bands of the 1 sec pre-stimulus EEG were used to assess the brain state, and low intensity flashes, delivered randomly between 2 and 6 whole seconds, were used as the stimuli. Trials were ranked separately according to the relative amounts of pre-stimulus power in each EEG band and were partitioned into groups by an equal pre-stimulus spectral power criterion. Averaged EPs were computed from these groups and multiple regression analysis was used to relate pre-stimulus spectral power values to EP features. Five of the 7 subjects displayed consistent increases in N1-P2 amplitude as a function of increasing pre-stimulus relative alpha power. The between-subjects effect of pre-stimulus EEG on N1 latency was small, but was moderate for P2 latency (both significant). Both N1 and P2 latency were found to decrease with increasing amounts of pre-stimulus relative delta and theta power.

Feedback control of a biodynamical model of HIV-1

Describes a continuous differential equation model of the interaction dynamics of HIV-1 and CD4 and CD8 lymphocytes in the human body. The authors demonstrate several methods of stable control of the HIV-1 population using an external feedback control term that is analogous to the introduction of a therapeutic drug regimen. They also show how the immune system components can be bolstered against the virus through a feedback control approach.

The Relationship Between Prestimulus Alpha Amplitude and Visual Evoked Potential Amplitude

Root-mean-square (RMS) amplitude derived from power spectral measures in the alpha band of the 1 s prestimulus EEG were related to the peak-to-peak amplitude of the N1 and P2 components (N1P2PP) of the visual evoked potential (VEP) in 7 male subjects. Stimuli were low intensity flashes delivered randomly between 2 and 6 whole seconds. Trials were rank ordered according to the levels of prestimulus alpha amplitude and were partitioned into groups of 40 trials each (25 groups per data set). Averaged VEPs were computed from these groups and scattergrams of N1P2PP and enhancement factor (following the approach by Başar, 1980) vs. prestimulus alpha amplitude were produced. There was a correlation of 0.74 (p less than .0001) between prestimulus alpha amplitude and N1P2PP, and all seven subjects displayed a general inverse relationship between VEP enhancement and prestimulus alpha amplitude, replicating the results of Başar. However, we observed an exponential relationship, rather than the linear relationship reported by Başar.

Attentional skills and executive functions in children with early hydrocephalus

Measures of executive functions (Tower of London, Wisconsin Card Sorting Test), focused attention (cancellation tasks), and selective attention (Stroop test) were administered to 116 school‐age children with shunted hydrocephalus, arrested (un‐shunted) hydrocephalus, and no hydrocephalus. The results revealed that children with shunted hydrocephalus solved fewer problems on the Tower of London task and achieved fewer categories on the Wisconsin Card Sorting Test than children with arrested and no hydrocephalus. However, most indices of executive functions derived from these two measures did not significantly differentiate the three groups. Children with shunted hydrocephalus also performed more poorly on the focused and selective attention tasks. However, their poorer performance was related to brain defects influencing motor speed and the transfer of information across the corpus callosum and not to specific effects of hydrocephalus. Children with hydrocephalus do demonstrate problems with attention and ...

Development and organization of the human brain tissue compartments across the lifespan using diffusion tensor imaging.

We used a diffusion tensor imaging-based whole-brain tissue segmentation to characterize age-related changes in (a) whole-brain grey matter, white matter, and cerebrospinal fluid relative to intracranial volume and (b) the corresponding brain tissue microstructure using measures of diffusion tensor anisotropy and mean diffusivity. The sample, a healthy cohort of 119 right-handed males and females aged 7–68 years. Our results demonstrate that white matter and grey matter volumes and their corresponding diffusion tensor anisotropy and mean diffusivity follow nonlinear trajectories with advancing age. In contrast, cerebrospinal fluid volume increases linearly with age.

Bifurcation control of two nonlinear models of cardiac activity

This brief describes methods based on time-delay feedback (TDF) for bifurcation control of nonlinear models of chaotic cardiac activity. We describe a nonlinear bifurcation controller that makes use of a feedback reference signal and a linear autoregressive formulation for the gain. This controller is effective at stabilizing two diverse cardiac maps to a variety of periodic orbits. We contrast our approach with the OGY method which has been used to control some chaotic biological processes and recently, some nonchaotic, stochastic ones.

Memory Functions in Children With Early Hydrocephalus

Children with arrested, shunted, and no hydrocephalus were compared on verbal and nonverbal memory tasks assessing multiple components of memory. A gradient of severity was hypothesized, with the shunted hydrocephalus group expected to exhibit the most significant memory impairments and the arrested group expected to perform more poorly than children with no hydrocephalus. Etiologies of prematurity, spina bifida, and aqueductal stenosis were represented by 157 participants. Results supported the hypothesis; the shunted hydrocephalus group performed poorer on all memory measures. Differences for the arrested group were less frequently statistically significant relative to children with no hydrocephalus. Irrespective of etiology, the shunted hydrocephalus group exhibited a pattern of performance suggestive of encoding and retrieval deficits on both verbal and nonverbal tasks, showing a pervasive disturbance of memory processes.

Morphometric evaluation of the hydrocephalic brain: relationships with cognitive development

The effects of early hydrocephalus and related brain anomalies on cognitive skills are not well understood. In this study, magnetic resonance scans were obtained from 99 children aged from 6 to 13 years with either shunted hydrocephalus (n=42) or arrested (unshunted) hydrocephalus (n=19), from patient controls with no hydrocephalus (n=23), and from normal, nonpatient controls (n=15). Lateral ventricle volumes and area measurements of the internal capsules and centra semiovale in both hemispheres were obtained from these scans, along with area measurements of the corpus callosum. Results revealed reductions in the size of the corpus callosum in the shunted hydrocephalus group. In addition, lateral ventricle volumes were larger and internal capsule areas were smaller in both hemispheres in children with shunted and arrested hydrocephalus. The centra semiovale measurements did not differentiate the groups. Correlating these measurements with concurrent assessments of verbal and nonverbal cognitive skills, motor abilities, and executive functions revealed robust relationships only between the area of the corpus callosum and nonverbal cognitive skills and motor abilities. These results support the theory of a prominent role for the corpus callosum defects characteristic of many children with shunted hydrocephalus in the spatial cognition deficits commonly observed in these children.