Search

This study examined the relationship between orienting responses to novel events and subsequent exploratory behavior. The N2-P3 electrophysiologic component of the orienting response was found to be larger for novel than repetitive background stimuli. Across subjects, the amplitude of this N2-P3 response in frontal regions strongly predicted the proportional increase in the duration of viewing directed toward novel compared to background stimuli. Within subjects, larger N2-P3 amplitudes in response to novel stimuli were associated with longer viewing durations on those stimuli. These results suggest that the N2-P3 component of the orienting response reflects the activity of a neural system involving frontal networks that dynamically regulates the subsequent allocation of attentional resources to novel stimuli.

Orphans and vulnerable youth who live in group homes are at risk of poor mental health and sexual and drug-using behaviors that increase the risk of HIV transmission. This study explores factors related to this risk among youth living in group homes (“children’s homes”) for orphans and vulnerable children in South Africa, a country afflicted by high levels of parental loss due to HIV. The study explores 1) knowledge and attitudes about HIV, 2) social support, 3) communication with group home caregivers, and 4) the relevance of an existing evidence-based HIV prevention and mental health promotion program to situations where sexual and drug risk behaviors can occur. In-depth qualitative individual interviews were conducted with 20 youth (age 10 to 16 years) residing in two children’s homes in Durban, South Africa. Content analysis focused on critical themes related to coping and prevention of risk activities. Respondents exhibited inconsistent and incomplete knowledge of HIV transmission and prevention. They displayed positive attitudes toward people living with HIV, but reported experiencing or witnessing HIV-related stigma. Participants witnessed substance use and romantic/sexual relationships among their peers; few admitted to their own involvement. While relationships with childcare workers were central to their lives, youth reported communication barriers related to substance use, sex, HIV, and personal history (including parental loss, abuse, and other trauma). In conclusion, these qualitative data suggest that evidence-based HIV prevention programs that bring caregivers and youth together to improve communication, HIV knowledge, social support, youth self-esteem, and health care, reduce sexual and drug risk behaviors, and strengthen skills related to negotiating situations of sexual and substance use possibility could benefit youth and childcare workers in children’s homes.

Fluconazole pharmacokinetics were evaluated for 10 volunteers with AIDS who had no clinical evidence of gastroenteritis. Single 100-mg intravenous (i.v.) and oral (p.o.) doses were administered in a randomized, crossover design. i.v. doses were delivered by a constant-rate infusion over 30 min. Serum fluconazole concentrations were measured by gas-liquid chromatography. The i.v. and p.o. studies were modelled simultaneously by iterative two-stage analysis, which provided individual parameter estimates and a population pharmacokinetic model. Median areas under the concentration-time curves for i.v. and p.o. studies did not differ (90.6 and 99.3 micrograms/ml.h, respectively). Consistent with this finding, the median fractional bioavailability was 1.1 (range, 0.45 to 1.3), comparable to those in healthy subjects. Serum pharmacokinetics in these AIDS patients were generally similar to published data for healthy volunteers. However, following p.o. dosing, we observed a slightly delayed and highly variable time to maximum concentration in serum (median, 2 h; range, 15 min to 8 h). Data were well described by a linear, two-compartment pharmacokinetic model with first-order absorption and elimination. Repeated-measures analysis of variance found no significant differences among any of the pharmacokinetic parameters between i.v. and p.o. studies. On the basis of our findings, we suggest no change in dosage of p.o. fluconazole in patients with AIDS who show no clinical signs of enteropathy.

There has been increased interest in the ability of the adult human nervous system to reorganize and adapt to environmental changes throughout life. This ability has been termed "plasticity." Plastic changes in the cerebral cortex have been studied: (a) as modifications of sensory or motor cortical representation of specific body parts (cortical maps, body representation level); and (b) as changes in the efficacy of existing synapses or generation of new synapses (neuronal or synaptic level). In this review, we describe paradigms used to study mechanisms of plasticity in the intact human motor system, the functional relevance of such plasticity, and possible ways to modulate it.

To better understand apathy and disengagement in patients with Alzheimer's disease (AD), the authors investigated possible behavioral mechanisms underlying diminished novelty-seeking activity in patients with probable AD.Apathy and disengagement have been shown to be the most common behavioral changes associated with AD.Patients and age-matched normal controls had their eye movements recorded while pairs of line drawings pitting an incongruous figure against a congruous figure were shown on a screen for 12 seconds. Characteristics of a subset of AD patients who were indifferent to novel visual stimuli as measured by exploratory eye movements were compared to those of a subset of AD patients who were attracted to novel stimuli to a degree similar to that of normal controls.The indifferent patients were judged by informants, who completed a personality questionnaire, to exhibit a greater degree of apathy. The two AD groups did not differ in overall dementia severity or performance on a Saccade-to-Target Task that required shifts of attention and gaze. In a separate task, the indifferent patients were able to accurately identify the more novel stimuli in 97.5% of trials. Normal control subjects exhibited a strong bias toward processing novel stimuli, directing a higher proportion of their first fixations and dwell time to the incongruous stimuli whether the analysis was run for 3, 6, or 12 seconds of viewing. Indifferent patients did not direct their initial fixation toward novel stimuli and distributed their looking time evenly between incongruous and congruous stimuli throughout all measured intervals.The results suggest that the indifference to novelty observed in some patients with probable AD cannot simply be attributed to global cognitive decline, more elementary attentional deficits, more rapid habituation of response to novel stimuli, or an inability to discriminate upon demand between stimuli of varying degrees of novelty. It is more likely that their behavior reflects a disruption, by AD pathology, of neural systems that modulate behavioral engagement and maintain attentional bias toward novel events in the environment.

We studied the effects of coil orientation, stimulus intensity, and shape of the induced current pulse on the amplitudes of motor evoked potentials in the left abductor pollicis brevis of 10 normal adults who had transcranial magnetic stimulation. The optimal direction of currents induced in the brain is approximately perpendicular to the central sulcus, flowing diagonally from back to front. The most effective coil orientation depends on the shape of the induced current pulse and, when the first and second phases of the pulse are of similar size, also on the intensity of stimulation. Optimal mapping of the human motor cortex with magnetic stimulation requires knowledge of the influences of all these factors.

We studied motor evoked potentials to transcranial magnetic stimulation in patients with unilateral upper limb amputations, complete T10-T12 spinal cord transection, and congenital mirror movements and in controls. Different muscles in the trunk and upper and lower extremities were evaluated at rest. In controls, muscles could be activated with stimulation of regions several centimeters wide. These areas overlapped extensively when muscles studied were from the same limb and shifted positions abruptly when muscles were from different limbs. Distal muscles were easier to activate than proximal muscles and normally evidenced exclusively a contralateral representation. Congenital defects in motor control in patients with mirror movements resulted in marked derangement of the map of outputs of distal hand muscles with enlarged and ipsilateral representations. Peripheral lesions, either acquired (amputations) or congenital (congenital absence of a limb), resulted in plastic reorganization of motor outputs targeting muscles immediately proximal to the stump. Central nervous system lesions (i.e., spinal cord injury producing paraplegia) also resulted in enlargement of the map of outputs targeting muscles proximal to the lesion. These results indicate that magnetic stimulation is a useful non-invasive tool for exploring plastic changes in human motor pathways following different types of injury.

The electric field induced in tissue during magnetic stimulation is calculated. There are two sources of the electric field: charge and a time-dependent magnetic field: both sources are important in magnetic stimulation. Charge accumulation on the tissue surface tends to shield the nerve from the stimulus. The induced electric field is generally parallel to the tissue surface. Simulations of both peripheral and central nervous system stimulation are presented.

Temperature effects on refractive index have been studied by monitoring pulse propagation delays in fiber-optic lightguides for temperatures between −40 and 67°C. Loss and dispersion properties did not change significantly from their room temperature values.

Loss measurements have been made on glass fibers immersed in index matching oil. The measurements include state of polarization analyses of the light leaving fiber ends. The character of the beam was determined by using α λ/4 plate in conjunction with a polaroid analyser to measure the components of unpolarized and polarized light. The data indicates that unclad fibers attenuate light much more than similar glass clad fiber waveguides. Mode purity within a multimode glass fiber was estimated from measurements of the admixture of modes leaving its end. Light guided along unclad fibers was considerably less distorted than light propagating along similar clad waveguides.

Measurements have been made to determine the efficiency of coupling light from GaAs injection lasers (λ = 9000 A) of stripe geometry into the cores of optical fibers. Laser light was coupled into a fiber across a small air gap which separated the laser from the fiber. The power coupling efficiency was calculated by extrapolating the lasing light power emitted at the end of a short length of fiber back to its input tip and comparing it with the total laser beam power. Experiments were performed with several diffused junction (DJ) lasers and with low-threshold double-heterostructure (DH) lasers having stripes formed by proton bombardment or oxide masking. The beam profile, scanned in its near and far field region, was approximately Gaussian. The beam dimensions at the surface of the laser were estimated from far field measurements and were used to predict the coupling efficiency of single-mode and multimode fibers. Power coupling efficiencies of about 70 percent were measured for DJ lasers feeding 10-µm multimode fibers. A coupling efficiency of 25 percent, almost identical to the theoretical estimate, was achieved with a DH laser having a 1-µm × 13-µm proton-bombarded stripe, feeding the 3.2-µm core of a single-mode fiber. The coupling efficiency was greater than 40 percent when a DJ laser fed the same fiber. A cylindrical lens is proposed to increase the power coupling perpendicular to the junction plane. Permanent self-supporting couplers were made by applying epoxy between structures which supported the fiber and the laser.

This paper describes a novel technique for monitoring the outside dimensions of clad or unclad optical fibers. An oscillating mirror is used to deflect a laser beam with constant velocity across a fiber in order to measure the time interval during which the fiber intercepts the beam and casts a shadow on a photodetector. For 110 μ fibers, individual measurements made every 1/60 sec are repeatable within a spread of ± 1% from the 330 μsec mean time interval (rms deviation ≈ ± 0.6%). Results are displayed in digital form, but can be converted to an analog signal for use in a servo loop to control the pulling rate of fiber drawing machines. An additional feature of the technique is that fibers with noncircular cross sections can be monitored by making sequential diameter measurements along orthogonal axes of the fiber.

Muscular fatigue mechanisms were partially analyzed in patients with pyramidal lesions by using a computer device on line with a conventional electromyograph. Both the normal and the affected side were compared with control groups. The recordings were done throughout a ten minutes period and the maximal M wave was also measured at the starting and at the end of the procedure. The findings suggested that central events play a major role in the development of muscular fatigue in spastic muscles due to pyramidal damage.

This paper will review a systematic procedure that is being used to tailor optical properties of single-mode lightguides for specific research applications. It requires the coordinated implementation of computer-aided modeling with glass processing technology and state of the art diagnostic measurement procedures.© (1983) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Single-mode fibers serve as transmission highways for lightwave systems that transmit data at high rates over long distances. Fibers are currently being deployed with low losses and near-zero dispersion for lightwave systems applications at wavelengths near 1.3 μn. However, systems of the next generation will undoubtedly include 1.55-μm wavelength applications because intrinsic scattering losses in fused silica glasses are minimized there.

Previous work has demonstrated that group-delay spectra in single-mode fibers can be measured with 0.1 psec resolution by a white-light crosscorrelation technique. An application would be to pretest short fibers before drawing an entire preform. This could be particularly useful for achieving high draw yield for fibers whose dispersion spectra are sensitive to core diameter. This paper describes measurements of minimum-dispersion spectra that were obtained using 4 m lengths of fiber that were cut from each end of kilometer-long quadruple-clad, triangular-index, and step-index depressed-clad fibers. The axial uniformity of fiber dispersion was found to be very good. Results show that averaging dispersion spectra using short fiber segments from each end produces excellent agreement when compared with the dispersion spectrum determined from conventional pulse-delay measurements on a long length of fiber.© (1983) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

This paper will review the status of broadband single-mode lightguides that can provide low loss and high bandwidth within the 1.3 - 1.55μm spectral region.

In a group of patients chronically denervated and other with primary muscle involvement a search was made for mechanisms involved in the muscular fatigue. It was found that there was no major differences between patients chronically denervated and controls, while in those others with primary muscle disorders a loss of functional motor units during effort and a transient impairment of neuromuscular transmission play a role together with the central events normally occurring in fatigue.

Muscular fatigue was investigated in 13 healthy people by employing a simple device which comprises a computer on line with a conventional EMG equipment able to automatically measure the frequency and duration of free-run EMG recording. The procedure was carried out in the extensor digitorum brevis muscle by using surface electrodes throughout 10 minutes of voluntary maximal effort. The maximal M wave as well as the muscle response to repetitive nerve maximal stimulation was also explored at the beginning and at the end of the experiment. It was observed a decrease of the potentials frequency discharge and an increase of their duration without major changes in the M wave amplitude neither in the muscle response to the repetitive nerve discharge. The findings pointed out to the central factors as the mean responsible for the development of muscular fatigue.