Bandgap tuning in ZnxCd1-xTe superlattices through variable atomic ordering.

We explore the entire search space of 32-layer ZnxCd1-xTe superlattices to find the structures that minimize and maximize the bandgap at each possible zinc concentration. The searching is accomplished through an accurate and efficient combination of valence force field dynamics, the empirical pseudopotential method, and the folded spectrum method. We also describe the use of an alternate preconditioner that improves the robustness and efficiency of the locally optimal preconditioned conjugate gradient's solutions to the folded spectrum method. The physical properties of these superlattices, such as their formation energies, bandgaps, densities of states, effective masses, and optical response functions, are investigated with density functional theory paired with hybrid functionals and compare well to available experimental measurements. It is revealed that the bandgap of ZnxCd1-xTe may change by up to 0.2 eV depending on how the layers in the superlattice are ordered. Stacking order has a large, irregular effect on the effective masses, but optical response functions seem insensitive to it.

Electron and hole transfer from indium phosphide quantum dots.

Electron- and hole-transfer reactions are studied in colloidal InP quantum dots (QDs). Photoluminescence quenching and time-resolved transient absorption (TA) measurements are utilized to examine hole transfer from photoexcited InP QDs to the hole acceptor N,N,N',N'-tetramethyl-p-phenylenediamine (TMPD) and electron transfer to nanocrystalline titanium dioxide (TiO2) films. Core-confined holes are effectively quenched by TMPD, resulting in a new approximately 4-ps component in the TA decay. It is found that electron transfer to TiO2 is primarily mediated through surface-localized states on the InP QDs.

Development of sleep spindle bursts during the first year of life.

Sleep spindles, as defined in the international electroencephalography (EEG) glossary, are distinguished from spindle bursts seen in the EEGs of premature infants. Classical sleep spindles do not occur in prematures. They first appear clearly in the EEG during slow wave sleep from the 4th week postterm (44 weeks conceptional age) and are normally present in all infants' EEGs by 9 weeks postterm. During the first year of life they may be of high voltage relative to their appearance in older children and adults and are commonly characterized by variable degrees of interhemispheric asymmetry and asynchrony. Individual differences are great. Some features of clinical significance are discussed.

Relationship of genetically transmitted alpha EEG traits to anxiety disorders and alcoholism.

We tested the hypothesis that a heritable EEG trait, the low voltage alpha (LV), is associated with psychiatric disorders. Modest to moderate evidence for genetic linkage of both panic disorder and the low voltage alpha trait to the same region of chromosome 20q has recently been reported, raising the issue of whether there is a phenotypic correlation between these traits. A total of 124 subjects including 50 unrelated index subjects and 74 relatives were studied. Alpha EEG power was measured and EEG phenotypes were impressionistically classified. Subjects were psychiatrically interviewed using the SADS-L and blind-rated by RDC criteria. Alcoholics were four times more likely to be LV (including so-called borderline low voltage alpha) than were nonalcoholic, nonanxious subjects. Alcoholics with anxiety disorder are 10 times more likely to be LV. However, alcoholics without anxiety disorder were similar to nonalcoholics in alpha power. An anxiety disorder (panic disorder, phobia, or generalized anxiety) was found in 14/17 LV subjects as compared to 34/101 of the rest of the sample (P < 0.01). Support for these observations was found in the unrelated index subjects in whom no traits would be shared by familial clustering. Lower alpha power in anxiety disorders was not state-dependent, as indicated by the Spielberger Anxiety Scale. Familial covariance of alpha power was 0.25 (P < 0.01). These findings indicate there may be a shared factor underlying the transmissible low voltage alpha EEG variant and vulnerability to anxiety disorders with associated alcoholism. This factor is apparently not rare, because LV was found in approximately 10% of unrelated index subjects and 5% of subjects free of alcoholism and anxiety disorders.

Efficacy of sleep deprivation as an activation procedure in epilepsy patients.

The primary purpose of this article is to determine whether sleep deprivation (SD) of 24 h or longer has been demonstrated to be an effective procedure for eliciting epileptiform and/or seizure patterns in the EEG in epilepsy patients as compared with those of nonepileptics. The relevant literature is reviewed, first chronologically and then critically. Methodological problems are discussed. Although no absolutely definitive study has been done, the preponderance of evidence supports the conclusion that SD is effective in activating the EEGs of epileptics. Recommendations concerning the application of SD are offered.

Event-related perturbations in an electrophysiological measure of auditory function: a measure of sensitivity during orienting?

The effects of salient foreground stimuli in evoked potentials to weak background probe stimuli were examined in situations requiring passive observation or discriminative judgments of foreground tone stimuli. The background probe stimuli consisted of a continual train of weak acoustic stimuli presented at a rate of about 40 stimuli per second. Under such conditions, a 40-Hz steady-state rhythm (SSR) is established, which has been proposed to consist of the algebraic summation of individual middle-latency components evoked by stimuli in the train. The 40-Hz SSR was averaged over trials and extracted from the composite event-related potential signal using narrow-band digital filtering, for continuous examination of latency and amplitude during the course of the period immediately preceding and following the foreground stimulus. The foreground stimulus was followed by a brief period (peaking at about 200 ms) during which the latency of response to the background probe stimuli was reduced. The extent of this latency reduction was in proportion to the magnitude of the simultaneous slow-wave ERP responses and, to a lesser extent, heart rate responses. It is proposed that the results may reflect a transient period of sensitization during orienting, at a presumably early level in the auditory system, and that the method thus offers a means for determining the extent and temporal course of such effects.

Normal somatosensory evoked potentials in a child in persistent coma.

Somatosensory evoked potentials have been useful in predicting coma outcome in children. We present a patient who failed to recover consciousness after a near-drowning episode and has remained in a vegetative state. The initial and subsequent somatosensory evoked potentials have been normal, illustrating that normal results do not necessarily portend a favorable outcome.

Event-related perturbations in an electrophysiological measure of auditory sensitivity: effects of probability, intensity and repeated sessions.

It is often held that novel or salient stimuli are followed by a brief period of orienting or alerting during which sensory processes are facilitated. Evidence for such a period of facilitation was sought in a paradigm in which evoked responses to weak auditory probe stimuli were examined when given in the presence of salient foreground stimuli, which were varied in probability and intensity, and which were given in two replicate sessions. The background probe stimuli consisted of a continuous train of auditory pip stimuli delivered at a rate of 40 pips per second. Under such conditions of repetitive stimulation a steady-state rhythm (SSR), which is believed to reflect summated early and middle latency evoked responses, is established in the EEG at a corresponding frequency of 40 Hz. The 40 Hz SSR was extracted using a digital averaging and filtering technique and examined continuously for changes in amplitude and latency. The rhythm showed a brief episode during which the latencies of response were decreased. The reduction in latency was greatest at 186 ms after the foreground stimulus, at which time the latencies of individual peaks in the rhythm were reduced by about 3.5 ms. The magnitude of the latency reduction response was larger for intense and for rare stimuli, and showed long-term decrement during the second session. Event-related potential and heart rate responses to the foreground stimulus were also affected by probability, intensity and session, but not in the same pattern. It was hypothesized that the latency shift in the 40 Hz SSR reflects a brief period of sensitization during alerting or orienting responses.

Recovery of cognition from persistent vegetative state in a child with normal somatosensory evoked potentials.

The absence of bilateral early cortical SEPs in a PVS due to nontraumatic coma is usually associated with failure to recover cognition or awareness, although rarely patients with bilaterally absent cortical SEPs in posttraumatic PVS may regain cognition. On the other hand, normal cortical SEPs in nontraumatic coma may be related to favorable outcomes as shown in this patient and other reports. Our patient is unique in that he had had serial normal SEPs, was in a PVS for 7 1/2 months, and recovered cognition, but not without cost in terms of damage to intellectual capability. Further long-term clinical follow-up studies to correlate clinical outcome with serial SEP data may be indicated.

Effects of chronic ethanol consumption on male Syrian hamster hepatic, microsomal mixed-function oxidases.

Chronic alcohol consumption significantly increases the risk of drug interactions. We have described its effects on hamster microsomal monooxygenases. Male Syrian hamsters (85 g) were given 10% ethanol in water and food ad lib for up to 6 weeks. Microsomal electron transport components and metabolism of ethylmorphine, benzphetamine, aniline, and acetaminophen were measured. At 4 weeks, SDS-PAGE of ethanol microsomes showed an induced band with an Mr of 53,900 daltons and there was a 2-3 fold stimulation of aniline and acetaminophen metabolism. Cytochrome P-450 increase was not significant. For the six week period, Caloric intake (3 weeks, p less than 0.001), liquid consumption (3 weeks, p less than 0.05) and body weights (6 weeks, p less than 0.05) of ethanol animals were significantly greater than controls; kidney weights were significantly less (p less than 0.05). Ethanol consumption increased from 20% of the daily caloric intake (week 1) to 31% (week 6). Induction of specific substrate metabolism without apparent deleterious physiological changes establishes hamsters fed 10% ethanol in drinking water as a biochemical model for the study of chronic alcohol consumption and specific drug interactions.

Development of visual evoked potentials and photic driving responses in normal full term, low risk premature, and Trisomy-21 infants during the first year of life.

Stroboscopic visual evoked potentials (SVEPs) and photic driving responses were recorded during the first week of life post term and at 4, 13, 26 and 52 weeks in normal full term, low risk premature, and Trisomy-21 infants. The age effect on SVEP P2 peak latency was significant (P less than 0.001). P2 latency for the Trisomy-21 group was significantly prolonged at 1 week of age only. Voltages and interhemispheric symmetry were highly variable, with no significant age or group effects. There is considerable variability in photic driving effects, even within sessions. Maximal driving frequencies are difficult to determine; they may be near 15 Hz at term. The age effect on optimal driving frequency (3-4 Hz at week 1 increasing to 5 Hz at 12 months) was significant (P less than 0.001). The between-groups effect was not significant. The results are discussed in terms of (1) our own previous results, (2) previous results reported in the literature, and (3) significance in clinical applications.

High-power, high-repetition-rate femtosecond pulses tunable in the visible.

We demonstrate a Ti:sapphire-pumped intracavity-doubled optical parametric oscillator (OPO) that generates a total of up to 240 mW of sub-100-fs pulses tunable in the visible. The OPO consists of a 1.5-mm-thick KTiPO(4) (KTP) crystal configured in a ring cavity that is synchronously pumped by a self-mode-locked Ti:sapphire laser operating at an 81-MHz repetition rate and 2.1-W average power, producing 115-fs pulses at lambda = 790 nm. Intracavity doubling of the OPO is accomplished by inserting a 47-microm-thick beta-BaB(2)O(4) crystal into an additional focus in the OPO cavity. We demonstrate continuous tuning of the second-harmonic output from 580 to 657 nm. The potential tuning range of this intracavity-doubled KTP OPO is approximately 500 to 800 nm.

Development of EEG and daytime sleep patterns in normal full-term infant during the first 3 months of life: longitudinal observations.

EEG polygraph recordings of 1 to over 4 h duration were obtained during daytime sleeps weekly from birth to 11-13 weeks of age in 17 normal full-term newborns. Analysis of the recordings permitted more precise specification of the time courses of early developmental changes in EEG patterns related to the sleep cycle. The tracé alternant pattern of quiet sleep was seen up to 2 weeks post term in all subjects, but in none beyond 6 weeks. Active sleep onset occurred in 80% of daytime sleeps at 1-3 weeks and decreased rapidly over the next 5 weeks, but at 8-13 weeks was still seen in 5-10% of the recordings. Rolandic sleep spindle bursts appeared in some subjects as early as 4 weeks post term and were present in all beyond 8 weeks. Based upon these 3 criterion variables, the transition from 'perinatal' to 'infantile' EEG sleep patterns started at a mean age of 30.3 days and was completed at a mean age of 46.6.days. The earliest and latest completions of the transition were at 27 and 66 days, respectively. On the average the transition took just over 2 weeks. Frontal sleep transients were seen until 3 weeks post term in all subjects, but in none beyond 7 weeks. Active sleep decreased from just over 50% of total sleep time at birth to about 20% beyond 8 weeks. Percent-time quiet sleep increased proportionately. Indeterminate sleep remained relatively constant at 5-10 percent-time. A tendency for percent-time indeterminate sleep to be elevated in the presences of minor illnesses was observed. The usefulness of these, and other, EEG data in defining mature and immature EEGs in the neonatal period and the clinical significance of EEG immaturity are discussed.

Development of EEG and daytime sleep patterns in low risk premature infants during the first year of life: longitudinal observations.

Long daytime EEG-polygraph recordings were obtained at 36, 38, and 40 weeks conceptional age, weekly thereafter for 12 weeks, and at 26 and 52 weeks post term in 5 infants born at 30--33 weeks gestation. The babies have developed normally for 18--38 months. All 74 EEGs were normal for age. Derived data were compared with similar data from 17 normal full-term infants. The tracé alternant pattern disappeared earlier in the premature than in the control group (mean ages 21.4 vs. 33.4 days post appeared earlier in the premature group (mean ages 35.4 vs. 43.8 days; P < 0.05). There were no age differences between the groups with respect to (1) the shift from active sleep onset to quiet sleep onset, (2) active sleep and quiet sleep as percentages of total sleep time, or (3) disappearance of frontal sharp waves. Overall attainment of "infantile" EEG sleep patterns occurred at 35--60 days post term (mean 43.8) for the permature group and at 27--66 days (mean 46.4) for the full-term group (difference not significant). These findings indicate that no significant errors will be made in clinical practice by applying the same maturational criteria to the post-term EEGs of prematures as to the EEGs of full-term infants.

Interhemispheric synchrony in the EEGs of full-term newborns.

Cross correlations between 4 homologous pairs of ear-reference derivations were calculated for 45-sec digitized samples of typical active and quiet sleep in 19 selected normal newborns at 21--70 h after term birth. Repeat recordings were obtained in 9 subjects 24 h after the original recordings and another set of cross correlations was calculated. For comparison, similar recordings were obtained from 4 asymptomatic young adults and cross correlations were calculated for 45-sec samples of typical stage 4 and stage REM sleep and for stage W both before and after sleep. Mean cross-correlation coefficients at zero time lag were generally higher for the adult sleep data than for the newborn data, but the differences were smaller than had been anticipated. The coefficients for active sleep data in the newborns were higher than for quiet sleep data, while in the adults the coefficients were higher for stage 4 than for stage REM. Coefficients were higher for convexity derivations than for temporal derivations in both newborns and adults. A review of the small available literature suggests that recording and analysis techniques may be significant variables determining the results obtained in such studies. The cross-correlation method of measuring interhemispheric synchrony may prove useful in identifying brain disorders in the postnatal period.

Interhemispheric amplitude symmetry in the EEGs of normal full term, low risk premature, and trisomy-21 infants.

RMS measurements of amplitude symmetry were calculated for samples of typical active and quiet sleep periods in 12 normal full term, 5 premature and 7 Trisomy-21 infants from recordings obtained at 1, 4, 12 26 and 52 weeks post term. The frequency distributions of the symmetry data for each group did not deviate significantly from the expected normal distributions. The mean symmetry ratios were not significantly different among the 3 groups. It was concluded that the EEGs of the 3 groups are characterized by amplitude symmetry in the first year of life. The possibility that symmetry data may be used for classification of abnormal EEGs was discussed.

Respiratory pauses and apnea during daytime sleep in normal infants during the first year of life: longitudinal observations.

Respiratory pauses (3-10 sec in duration), apnea (less than 10 sex in duration), and periodic respiration observed in thoracic respirograms were measured in 226 polysomnograms obtained on 17 normal infants during the first year of life. All subjects had one or more respiratory pauses in a majority of recordings; 35% had respiratory pauses in all recordings; 75% of respiratory pauses were associated with body movement. There is marked intersubject and even intrasubject variability in respiratory pause rates. The range of mean respiratory pause rates among subjects was 2.0 - 14.4/h, and for single recordings was 0.0 - 43.6/h. Their occurrence was directly related to the occurrence of periodic respiration. Rates were higher during REM and indeterminate sleep than during slow wave sleep. There was no significant trend toward increase or decrease in respiratory pause rate during the first year post term. Apnea occurred in only one of the 226 recordings (0.4%). Periodic respiration occurred in 8 of 17 subjects (47%), and in 25 of 226 recordings (11%). Its occurrence was unrelated to sleep stage. The following conclusions are considered valid on the basis of the data presented and reports in the literature: (1) Rates of respiratory interruption are higher before than after 40 weeks conceptional age. (2) There is considerable intersubject variability in rates of respiratory interruptions. (3) Respiratory pauses are common during sleep in normal human infants. (4) Respiratory pauses occur more frequently with movements than in their absence. (5) Respiratory pause rates are higher during REM sleep than during slow wave sleep. (6) Apneas of greater than 10-15 sec duration do occur in normal infants, but are rare. From the clinical viewpoint, respiratory pauses (less than 15 sec) of the central type, regardless of abundance, and periodic respiration cannot by themselves be used as evidence that a baby is at risk of anything. The occurrence of apneas (greater than 15 sec duration), especially if any are of the obstructive or mixed types (and perhaps respiratory pauses of the obstructive and mixed types), and/or if associated with bradycardia or decrease in oxygen saturation, indicate sleep apnea syndrome and suggest risk of sudden infant death.