Quantum Monte Carlo computations of phase stability, equations of state, and elasticity of high-pressure silica.

Silica (SiO(2)) is an abundant component of the Earth whose crystalline polymorphs play key roles in its structure and dynamics. First principle density functional theory (DFT) methods have often been used to accurately predict properties of silicates, but fundamental failures occur. Such failures occur even in silica, the simplest silicate, and understanding pure silica is a prerequisite to understanding the rocky part of the Earth. Here, we study silica with quantum Monte Carlo (QMC), which until now was not computationally possible for such complex materials, and find that QMC overcomes the failures of DFT. QMC is a benchmark method that does not rely on density functionals but rather explicitly treats the electrons and their interactions via a stochastic solution of Schrödinger's equation. Using ground-state QMC plus phonons within the quasiharmonic approximation of density functional perturbation theory, we obtain the thermal pressure and equations of state of silica phases up to Earth's core-mantle boundary. Our results provide the best constrained equations of state and phase boundaries available for silica. QMC indicates a transition to the dense alpha-PbO(2) structure above the core-insulating D" layer, but the absence of a seismic signature suggests the transition does not contribute significantly to global seismic discontinuities in the lower mantle. However, the transition could still provide seismic signals from deeply subducted oceanic crust. We also find an accurate shear elastic constant for stishovite and its geophysically important softening with pressure.

New mechanism for the alpha to omega martensitic transformation in pure titanium.

We propose a new direct mechanism for the pressure driven alpha-->omega martensitic transformation in pure titanium. A systematic algorithm enumerates all possible pathways whose energy barriers are evaluated. A new, homogeneous pathway emerges with a barrier at least 4 times lower than other pathways. The pathway is shown to be favorable in any nucleation model.

Neuropsychological assessment of relatively intact individuals: pychometric lessons from an HIV+ sample.

The assessment of relatively intact individuals poses special problems for neuropsychologists, because fine discriminations are often needed to identify subtle deficits or gradual declines in performance. Many neuropsychological tests, however, are not well-suited for making such exact discriminations. 117 HIV+ individuals were administered 26 different neuropsychological tests that produce 48 scores. Measures of skewness and kurtosis were used to identify nonnormal sampling distributions. While many tests showed good sampling distributions, several demonstrated ceiling effects and other restrictions of range. This included some tests, such as Boston Naming and Lafayette Grooved Pegboard, that are not ordinarily considered screening instruments. Such nonnormal distributions distort the interpretation of clinical and research data, and indicate a need to use tests that are suited to the abilities of the population being assessed.

Vitamin B12 deficiency and nervous system disease in HIV infection.

BACKGROUND: Vitamin B12 deficiency may result in a number of neurological and neuropsychiatric disorders. Patients with human immunodeficiency virus type 1 (HIV-1) infection may have a high rate of vitamin B12 deficiency and nervous system disease. Vitamin B12 deficiency may contribute to neurological disease in HIV-1-infected individuals. OBJECTIVE: To evaluate the possible contribution of vitamin B12 deficiency to neurological disease in HIV-1-infected individuals. MAIN OUTCOME MEASURES: Comparison of serum vitamin B12 levels with neurological, neuropsychological, and mood state abnormalities in 153 HIV-1-positive subjects and 57 high-risk seronegative controls. A subgroup of 67 subjects underwent additional extensive clinical neurophysiological, cerebrospinal fluid, and magnetic resonance imaging evaluations. RESULTS: No statistically significant relationships were noted between vitamin B12 levels and abnormalities on any of the measures examined. CONCLUSIONS: This study does not indicate an important role for vitamin B12 deficiency in the neurological disease of HIV-1 infection.

Cerebrospinal fluid analysis in human immunodeficiency virus infection.

Cerebrospinal fluid (CSF) analytes were evaluated in 59 human immunodeficiency virus (HIV+) individuals to assess neurological involvement. Glucose, total protein, cell counts, p24 antigen, CSF: serum albumin/IgG ratios, and oligoclonal bands were measured. Eighty percent of samples showed abnormalities in one or more analyte. In some patients samples, these abnormalities could mimic those of secondary opportunistic infection when none was present. The presence of oligoclonal banding in CSF (31 percent) and disturbances in CSF: serum albumin/IgG ratio (30 percent) were related to decreases in serum CD4+ lymphocytes. Disturbances in CSF: Serum albumin/IgG ratio were also related to severity of non-neurological HIV disease staging. Cerebrospinal fluid oligoclonal bands were distinct from that found in serum in the same subjects. Since immune complexes between immunoglobulins and enzymes are observed in these same patients, these oligoclonal bands may result in artifactually elevated enzyme results secondary to decreased clearance leading to erroneous clinical decisions. There was no significant relationship between any abnormalities and the presence of neurologic disease as established by a wide variety of other studies. It is important to recognize the limits of CSF interpretation in this patient group.

Event-related potentials in human immunodeficiency virus infection. A prospective study.

P3 event-related evoked potentials (ERP) were recorded from 47 human immunodeficiency virus (HIV)-positive subjects examined twice and 29 HIV-positive subjects examined three times at 6-month intervals. The P3 latency significantly increased over time for asymptomatic subjects and subjects with acquired immunodeficiency syndrome (AIDS) and AIDS-related complex. N2 latency was prolonged relative to control values in both HIV-positive groups but did not increase with time. The P3 latency correlated with neuropsychologic measures of motor control and speed of mental processing. Confounding factors (active or previous substance abuse, developmental disabilities, and history of closed head injury or epilepsy) did not significantly affect ERP latencies. Endogenous ERP components are frequently abnormal in HIV-positive subjects and the P3 latency progressively increases over time. Continued follow-up is required to determine the clinical utility of ERP studies in the HIV-positive population.

Peripheral neuropathy in a cohort of human immunodeficiency virus-infected patients. Incidence and relationship to other nervous system dysfunction.

A cohort of 94 patients infected with human immunodeficiency virus was evaluated clinically and electrophysiologically for the presence of peripheral neuropathy, and the results were compared with evaluations of central nervous system function. Thirty-two (34%) had some degree of peripheral neuropathy; 18 (19%) (six [12%] of the 49 asymptomatic patients, five [45%] of the 11 patients with acquired immunodeficiency syndrome [AIDS], and seven [21%] of the 34 patients with AIDS-related complex) had neuropathy on clinical examination; and 21 (23%) (eight [16%] asymptomatic, four [36%] AIDS, and nine [26%] AIDS-related complex) had neuropathy on electrophysiologic evaluation. There was a significant correlation between the presence of neuropathy and evidence of central nervous system dysfunction.

Implications of self-reported cognitive and motor dysfunction in HIV-positive patients.

OBJECTIVE: The authors examined HIV-positive subjects to determine the relationship of patient complaints of cognitive and motor dysfunction to psychiatric status and performance on established cognitive and motor function tests. METHOD: HIV-positive volunteers (N = 77) were evaluated at entry into a longitudinal neurological study. Forty were asymptomatic, 29 had AIDS-related complex, and eight had AIDS. The subjects were not selected for the presence or absence of cognitive or motor complaints. Complaints of cognitive and motor dysfunction were assessed with items from the AIDS Clinical Trials Group Macro Neurologic Exam. Current depression and anxiety were assessed with the Profile of Mood States. Psychiatric status was assessed with the NIMH Diagnostic Interview Schedule, a structured interview that provides DSM-III psychiatric diagnoses. Actual cognitive and motor performance was measured with standard neuropsychological tests known to be sensitive to the effects of HIV. RESULTS: Cognitive complaints were found in 38 (49%) of the subjects. These complaints were associated with psychiatric symptoms but not with cognitive performance. Motor complaints, found in 12 (16%) of the subjects, were associated with poorer motor performance but not with psychiatric symptoms. The overall frequency of psychiatric diagnosis was high. CONCLUSIONS: Self-reports of cognitive and motor dysfunction were common in this unselected group and are of concern to health care providers. Potentially treatable psychiatric conditions were also common, particularly in subjects with cognitive complaints, and appropriate treatment referrals are indicated. Patients who report motor dysfunction should be neurologically evaluated for treatable causes.

The importance of confounding factors in the evaluation of neuropsychological changes in patients infected with human immunodeficiency virus.

There are conflicting reports on the early effects of human immunodeficiency virus (HIV) infection on the nervous system. Some studies have suggested that there may be early cognitive impairment, while others have refuted this. We describe the results of extensive neuropsychological testing in a group of 40 infected subjects. These indicate that the degree of impairment is closely related to confounding factors other than the infection itself. Our conclusion is that the early stages of HIV disease are not associated with a high frequency of cognitive impairment if these confounding variables are taken into consideration.