Identifying a Superfluid Reynolds Number via Dynamical Similarity.

The Reynolds number provides a characterization of the transition to turbulent flow, with wide application in classical fluid dynamics. Identifying such a parameter in superfluid systems is challenging due to their fundamentally inviscid nature. Performing a systematic study of superfluid cylinder wakes in two dimensions, we observe dynamical similarity of the frequency of vortex shedding by a cylindrical obstacle. The universality of the turbulent wake dynamics is revealed by expressing shedding frequencies in terms of an appropriately defined superfluid Reynolds number, Re(s), that accounts for the breakdown of superfluid flow through quantum vortex shedding. For large obstacles, the dimensionless shedding frequency exhibits a universal form that is well-fitted by a classical empirical relation. In this regime the transition to turbulence occurs at Re(s)≈0.7, irrespective of obstacle width.

Onsager-Kraichnan condensation in decaying two-dimensional quantum turbulence.

Despite the prominence of Onsager's point-vortex model as a statistical description of 2D classical turbulence, a first-principles development of the model for a realistic superfluid has remained an open problem. Here we develop a mapping of a system of quantum vortices described by the homogeneous 2D Gross-Pitaevskii equation (GPE) to the point-vortex model, enabling Monte Carlo sampling of the vortex microcanonical ensemble. We use this approach to survey the full range of vortex states in a 2D superfluid, from the vortex-dipole gas at positive temperature to negative-temperature states exhibiting both macroscopic vortex clustering and kinetic energy condensation, which we term an Onsager-Kraichnan condensate (OKC). Damped GPE simulations reveal that such OKC states can emerge dynamically, via aggregation of small-scale clusters into giant OKC clusters, as the end states of decaying 2D quantum turbulence in a compressible, finite-temperature superfluid. These statistical equilibrium states should be accessible in atomic Bose-Einstein condensate experiments.

Characteristics of two-dimensional quantum turbulence in a compressible superfluid.

Fluids subjected to suitable forcing will exhibit turbulence, with characteristics strongly affected by the fluid's physical properties and dimensionality. In this work, we explore two-dimensional (2D) quantum turbulence in an oblate Bose-Einstein condensate confined to an annular trapping potential. Experimentally, we find conditions for which small-scale stirring of the condensate generates disordered 2D vortex distributions that dissipatively evolve toward persistent currents, indicating energy transport from small to large length scales. Simulations of the experiment reveal spontaneous clustering of same-circulation vortices and an incompressible energy spectrum with k(-5/3) dependence for low wave numbers k. This work links experimentally observed vortex dynamics with signatures of 2D turbulence in a compressible superfluid.

Observation of vortex dipoles in an oblate Bose-Einstein condensate.

We report experimental observations and numerical simulations of the formation, dynamics, and lifetimes of single and multiply charged quantized vortex dipoles in highly oblate dilute-gas Bose-Einstein condensates (BECs). We nucleate pairs of vortices of opposite charge (vortex dipoles) by forcing superfluid flow around a repulsive Gaussian obstacle within the BEC. By controlling the flow velocity we determine the critical velocity for the nucleation of a single vortex dipole, with excellent agreement between experimental and numerical results. We present measurements of vortex dipole dynamics, finding that the vortex cores of opposite charge can exist for many seconds and that annihilation is inhibited in our trap geometry. For sufficiently rapid flow velocities, clusters of like-charge vortices aggregate into long-lived multiply charged dipolar flow structures.

Use of surface-wave spectroscopy to characterize tilt modes of a vortex in a Bose-Einstein condensate.

A vortex in a condensate in a nonspherical trapping potential will in general experience a torque. The torque will induce tilting of the direction of the vortex axis. We observe this behavior experimentally and show that by applying small distortions to the trapping potential, we can control the tilting behavior. By suppressing vortex tilt, we have been able to hold the vortex axis along the line of sight for up to 15 sec. Alternatively, we can induce a 180 degrees tilt, effectively reversing the charge on the vortex as observed in the lab frame. We characterize the vortex nondestructively with a surface-wave spectroscopic technique.

Watching dark solitons decay into vortex rings in a Bose-Einstein condensate.

We have created spatial dark solitons in two-component Bose-Einstein condensates in which the soliton exists in one of the condensate components and the soliton nodal plane is filled with the second component. The filled solitons are stable for hundreds of milliseconds. The filling can be selectively removed, making the soliton more susceptible to dynamical instabilities. For a condensate in a spherically symmetric potential, these instabilities cause the dark soliton to decay into stable vortex rings. We have imaged the resulting vortex rings.

Vortex precession in Bose-Einstein condensates: observations with filled and empty cores.

We have observed and characterized the dynamics of singly quantized vortices in dilute-gas Bose-Einstein condensates. Our condensates are produced in a superposition of two internal states of 87Rb, with one state supporting a vortex and the other filling the vortex core. Subsequently, the state filling the core can be partially or completely removed, reducing the radius of the core by as much as a factor of 13, all the way down to its bare value of the healing length. The corresponding superfluid rotation rates, evaluated at the core radius, vary by a factor of 150, but the precession frequency of the vortex core about the condensate axis changes by only a factor of 2.

Assessment of life stress in adolescents: self-report versus interview methods.

OBJECTIVE: To compare the investigator-based Life Events and Difficulties Schedule (LEDS) with a self-report measure (Life Events Checklist [LEC]) for the purpose of measuring life stress in adolescents with and without a diagnosis of major depressive disorder (MDD). METHOD: Adolescents (aged 13-18 years) with a recent episode of MDD based on DSM-III-R (n = 35) and normal controls free of any Axis I lifetime psychiatric disorder (n = 35) were assessed using both the LEC and the LEDS. RESULTS: Both measures predicted membership in the depressed and nondepressed groups of adolescents. Adolescents in the depressed group were more likely to report a severe event on the LEDS (97%) than adolescents in the nondepressed group (66%) (p = .001). Similarly, subjects in the depressed group endorsed a greater number of negative events (mean = 8.1) on the LEC than subjects in the nondepressed group (mean = 3.0) (p = .0001). An examination of potential provoking agents for episodes of major depression revealed that the LEC captured only 32% of preonset severe events and 36% of preonset major difficulties identified by the LEDS. CONCLUSIONS: Interpreted in light of relative advantages and disadvantages, the results suggest that checklist and interview measures each have distinct advantages depending on the purpose for which they are being used.

Social rhythm disruption and stressful life events in the onset of bipolar and unipolar episodes.

BACKGROUND: An association between social rhythm disruption (SRD) and onset of manic episodes has recently been observed. Whether other types of bipolar (depressive and cycling) or unipolar depressive episodes are similarly related to SRD is unclear, as is the association between severely threatening life events and onset of bipolar manic, depressed and cycling episodes. METHODS: Bipolar patients with purely manic (N= 21), purely depressed (N = 21) and cycling (N = 24) episodes, and 44 patients with recurrent unipolar depression, were interviewed with the Bedford College Life Events and Difficulties Schedule. The presence of severe and SRD events during the year prior to index episode onset was then determined. RESULTS: More manic than cycling and unipolar subjects experienced SRD events during 8- and 20-week pre-onset periods, and severe events during 20-week pre-onset periods. Controlling for age and prior number of episodes left most findings unchanged. An earlier finding of more manic subjects with SRD events in an 8-week pre-onset versus control period was also replicated. CONCLUSIONS: It appears that manic onsets are influenced by stressful life events, especially those involving SRD, in a unique manner compared to onsets of other types of bipolar and unipolar episodes. Onset of bipolar cycling episodes, in contrast, seems to be relatively unaffected by SRD or severe life events. These findings refine the hypothesis that SRD may precipitate onset of affective episodes to be specific to manic onsets.

Nature of life events and difficulties in depressed adolescents.

OBJECTIVE: To examine the significance of acute life events and ongoing difficulties in adolescents with a recent major depressive disorder. METHOD: Adolescents (aged 13-18 years) with a recent episode of major depressive disorder based on DSM-III-R (n = 26) and normal controls free of any Axis I lifetime psychiatric disorder (n = 15) were assessed using the investigator-based Life Events and Difficulties Schedule (LEDS). RESULTS: Traditionally defined severe events were more likely to occur in the year prior to onset among depressed adolescents (46%) than in a comparable period among normal controls (20%), but these differences did not reach statistical significance. Expanding the definition of severe events to include those events focused on others important to the adolescent resulted in a significantly higher percentage of depressed adolescents having one or more refined "severe" events in the year prior to onset (62%) compared with normal controls (27%) (p < or = .02). It is interesting that one half of the depressed adolescents had two or more refined severe events occur during the year prior to onset compared with none of the normal controls (p < or = .01). Further analyses showed that depressed adolescents were significantly more likely to have a major difficulty precede the onset of their depression (27%) compared with normal controls (0%) (p < or = .04). CONCLUSIONS: The results suggest that depressed adolescents are exposed to high levels of stress prior to becoming depressed. Future investigations might benefit from using the LEDS with adolescents to assess acute and ongoing stressors.

Stressful life events in depressed adolescents: the role of dependent events during the depressive episode.

OBJECTIVE: To examine the relationship between stressful life events and major depressive disorder (MDD) in adolescents. METHOD: Adolescents (aged 12 to 18 years) with a current episode of MDD based on Research Diagnostic Criteria (n = 39) and normal controls free of any Axis I lifetime psychiatric disorder (n = 35) were assessed using the Life Events Record. RESULTS: MDD and normal control adolescents had similar rates of total stressful life events in the year before being interviewed. Stressful life events were dichotomized into those that were most likely either independent of or dependent on the adolescent's influence/behavior. Depressed adolescents had significantly more dependent stressful life events during the previous year than did the normal controls. Further analyses showed that depressed adolescents with dependent stressful life events scored lower on symptom clusters and accordingly were less severely depressed than depressed adolescents without dependent life events. CONCLUSIONS: The results of this study indicate that depressed adolescents have an increased risk for experiencing dependent life events. Furthermore, these data suggest that dependent life events in depressed adolescents are differentially associated with the type and severity of symptom presentation. The temporal occurrence, severity, and type of stressful life events as they relate to the onset, phenotypic expression, and maintenance of depression in adolescents need to be more fully understood.

Cholinergic modulation of cortical associative memory function.

1. The effect of cholinergic modulation on associative memory function was studied in a computational model based on the physiology and anatomic structure of piriform cortex. Both the cholinergic suppression of intrinsic fiber synaptic transmission and the cholinergic changes in postsynaptic excitability described in the companion paper were examined. 2. Distributed input patterns representing odors were stored in the model with the use of a synaptic modification rule dependent on pre- and postsynaptic activity (i.e., Hebbian). Associative recall of these patterns was tested by presenting the model with degraded versions of the learned patterns and testing whether these degraded patterns evoked the same network response as the full learned input pattern. Storage was evaluated with the use of a performance measure designed to reflect how well degraded input patterns could be recognized as a particular learned input pattern. 3. When memory function was evaluated with a selective cholinergic suppression of intrinsic fiber synaptic transmission during learning, associative memory performance was greatly enhanced. Cholinergic suppression during learning prevents previously stored patterns from interfering with the storage of new patterns. 4. When memory function was evaluated with a cholinergic mediated enhancement in cell excitability during learning, the speed of learning increased, but so did the decay in performance due to interference during learning. 5. When suppression of intrinsic fiber synaptic transmission was coupled with an increase in cell excitability, the best memory performance was obtained. 6. These results provide a possible theoretical framework for linking the neuropharmacological effects of acetylcholine to behavioral evidence for a role of acetylcholine in memory function. This could help describe how memory deficits might arise from cholinergic dysfunction in diseases such as Alzheimer's dementia.

The influence of menstrual cycle changes on the tobacco withdrawal syndrome in women.

Women have lower quit rates in smoking cessation than men. There are several factors suggested which are relevant to women's difficulties in smoking programs. One factor cited is the problem that women experience during withdrawal. Similar physiological and psychological symptoms are reported after smoking cessation and during menstrual cycle changes. In this study we evaluated the association between withdrawal and reports of menstrual distress. Results showed that a significant correlation existed between menstrual distress symptoms and initial smoking withdrawal symptoms. Women who quit smoking in the last phase (Phase 2) of their menstrual cycle experienced greater withdrawal than those who quit in the early phase (Phase 1) of the cycle. When these results were compared with male quitters, the Phase 2 women experienced significantly greater withdrawal than males. These results suggest that women may have specific biological needs that should be addressed in smoking treatment programs.

Gastric adaptive changes following experimental gastroplasty.

To study the adaptive response of the stomach secondary to partial obstruction imposed by gastroplasty, groups of rabbits with calibrated midgastric constrictions created as a stoma (unreinforced, Group II), as a short channel (Group III), or as a stoma reinforced with polypropylene mesh (Group IV), were compared to controls (Group I). It was found that the unreinforced stoma dilated rapidly while the pouch dilated moderately. In both the channel and reinforced stomas, the pouch dilated markedly. Increased fibrosis was found inside of the polypropylene banding, resulting in a significant incidence of luminal obstruction. The channel dilated in stages, affecting the proximal end first; but obstructive effect remained as long as part of the channel was undilated. We conclude that a narrow stoma is incompatible with a small pouch over time. Since anatomic specifications do not define function, both function (such as emptying) and adaptive changes should be studied in animal models before introduction of any new gastric limiting procedures into clinical practice.