Saffman-Taylor fingers with kinetic undercooling.

The mathematical model of a steadily propagating Saffman-Taylor finger in a Hele-Shaw channel has applications to two-dimensional interacting streamer discharges which are aligned in a periodic array. In the streamer context, the relevant regularization on the interface is not provided by surface tension but instead has been postulated to involve a mechanism equivalent to kinetic undercooling, which acts to penalize high velocities and prevent blow-up of the unregularized solution. Previous asymptotic results for the Hele-Shaw finger problem with kinetic undercooling suggest that for a given value of the kinetic undercooling parameter, there is a discrete set of possible finger shapes, each analytic at the nose and occupying a different fraction of the channel width. In the limit in which the kinetic undercooling parameter vanishes, the fraction for each family approaches 1/2, suggesting that this "selection" of 1/2 by kinetic undercooling is qualitatively similar to the well-known analog with surface tension. We treat the numerical problem of computing these Saffman-Taylor fingers with kinetic undercooling, which turns out to be more subtle than the analog with surface tension, since kinetic undercooling permits finger shapes which are corner-free but not analytic. We provide numerical evidence for the selection mechanism by setting up a problem with both kinetic undercooling and surface tension and numerically taking the limit that the surface tension vanishes.

Types of foot problems seen by Australian podiatrists.

BACKGROUND: Understanding frequency of foot problems can assist health care planners with resource deployment to new and emerging services such as paediatric podiatry and focus future research on the most salient foot conditions. METHODS: A review of 2187 patient consultations during a three month period was conducted. Patient medical and podiatric history was coded using industry standards. All patients were recruited for convenience from a metropolitan university podiatry clinic. RESULTS: 392 new patients were identified with mean age 40.6 years old (range 1-95), with 65% being female. Arthritic diseases, asthma, hypertension and allergies were the most common medical conditions reported. The frequency of new consultations in younger people (n=102; 27%) exceeded those of the elderly (n=75; 20%). Conversely, the elderly were nearly three times more prevalent in this cohort (n=910; 43%) compared to younger people (n=332; 16%). CONCLUSION: This study illustrates the diverse nature of pathology seen by podiatrists. Knowledge that skin lesions are highly prevalent is of relevance to health departments, given the aging nature of most populations. Moreover there appears to be a growing trend in the number of young people who present for care, however government funded access to these services are limited.

A prospective observational study of the use of the Proseal laryngeal mask airway for postpartum tubal ligation.

BACKGROUND: Though controversial, the risk of pulmonary aspiration during general anaesthesia in the immediate postpartum period appears low. The efficacy of the Proseal laryngeal mask airway was studied prospectively in a group of patients undergoing postpartum tubal ligation. METHODS: The Proseal laryngeal mask airway was employed for airway management in 90 fasted patients undergoing tubal ligation via minilaparotomy at least 8 h after normal vaginal delivery (mean 36.5, range 8-96 h). Gastric volume and pH were measured, using aspiration through a gastric tube. RESULT: Proseal laryngeal mask airway insertion was successful in all patients, requiring one attempt in 75 patients (83%). The median (range) leak pressure was 35 (23-40) cmH2O. Twenty-two patients (25%) had a leak pressure of 40 cmH2O or greater. Gastric tube placement was successful in all patients, described as easy in 79 (87%), and difficult in 11 (13%). The mean initial volume of gastric aspirate was 10.7 (0-64) mL and the final volume 15.6 (0-71) mL. The mean pH of the gastric aspirate was 2.6 (1.2-6.6). There were no incidents of suspected fluid regurgitation or aspiration, but two patients required intubation during surgery. Ten patients (11.1%) complained of sore throat in the recovery room, nine of which were described as mild. All patients reported being satisfied with their anaesthesia. CONCLUSION: The Proseal laryngeal mask airway provides an effective airway for general anaesthesia in fasted patients undergoing tubal ligation from 8 h after normal vaginal delivery. While the safety of an unprotected airway in this population remains uncertain, this study suggested a low risk of regurgitation, especially in the first 24 h post partum.

Visual neuroscience: Resonating to natural images.

Visual neurons may be optimized to produce sparse, distributed responses to natural scenes. This proposal, along with recent results from monkey fMRI and electrophysiology, may force us to re-interpret many neuroimaging results.

Critical flicker frequency responses in visual cortex.

Critical flicker frequency (CFF) threshold is defined as the frequency at which a flickering light is indistinguishable from a steady, non-flickering light. CFF is useful for assessing the temporal characteristics of the visual system. While CFF responses are believed to reflect activity in the central visual system, little is known about how these temporal frequencies are processed in the visual cortex. The current paper estimated the CFF threshold for cells in the rat visual cortex by recording single unit responses to flickering stimuli. Results showed that: (1) there was a broad range of temporal tuning, (2) CFF threshold was lower in simple cells than in complex and hypercomplex cells, and (3) there was no significant difference in CFF threshold between areas 17 and 18.

The effects of aging on visual memory: evidence for functional reorganization of cortical networks.

Recent evidence suggests that the mature human brain is capable of substantial functional reorganization following injury. The fact that the brain retains a great deal of plasticity raises the possibility that cortical reorganization may occur during normal aging. We examined this issue by using positron emission tomography (PET) to measure the brain activity associated with short-term memory for simple visual attributes in young and old observers. A two-interval forced choice procedure was used to measure spatial frequency discrimination thresholds for sine wave gratings presented at different inter-stimulus intervals (ISI). Memory load was manipulated by varying the duration of the ISI and by presenting an irrelevant masking stimulus in the middle of the ISI. Old and young observers performed the experiment equally well. However, the neural systems correlated with good performance differed for the two age groups. The results support the hypothesis that the functional networks that underlie visual memory undergo reorganization during aging.

The spatial distribution of inhibition of return.

Inhibition of return (IOR) refers to the finding that response times (RTs) are typically slower for targets at previously attended (cued) locations than for targets at novel (uncued) locations. Although previous research has indicated that IOR may spread beyond a cued location, the present study is the first to examine the spatial distribution of IOR with high spatial resolution over a large portion of the central visual field. This was done by using a typical IOR procedure (cue, delay, target) with 4 cue locations and 441 target locations (each separated by 1 degree of visual angle). The results indicate that IOR spreads beyond the cued location to affect the cued hemifield. However, the cues also produced a gradient of RTs throughout the visual field, with inhibition in the cued hemifield gradually giving way to facilitation in the hemifield opposite the cue.

Health-related quality of life following podiatric surgery.

This 6-month prospective study investigated the outcomes of foot surgery performed by Fellows of the Australian College of Podiatric Surgeons. The study recruited 140 patients who were treated for orthopedic, neurological, or integumentary diseases of the foot. The majority of subjects who underwent podiatric surgery experienced significant postoperative improvements in a range of health-related quality-of-life dimensions as measured by the disease-specific Foot Health Status Questionnaire (FHSQ) and the generic Short Form 36 (SF-36) questionnaire. Subjects reported a reduction in foot pain, increased levels of physical function, improved general foot health perception, and improved footwear-related quality of life. No significant adverse outcomes or unplanned re-admissions to the hospital were reported. This study demonstrates the advantage of assessing health-related quality of life as opposed to patient satisfaction.

Changes in foot function with the use of Root and Blake orthoses.

This study examined the effects of two designs of rigid foot orthoses on plantar pressure measurements and identified differences between the devices. While wearing modified Root- and Blake-style orthoses, 27 subjects were examined with the Electrodynogram (EDG) in-shoe pressure measurement system. Reliability testing was performed on the EDG data. Significant changes were observed in the temporal parameters of gait when subjects wore the orthoses. When the devices were used, the duration of some of the components of stance phase was altered, and the initiation of loading beneath the medial forefoot was delayed. A reduction in the total duration of loading at discrete sites beneath the heel and forefoot was also observed. The effects of the two orthoses were similar, with only small differences observed between the devices.

Generalized common fate: grouping by common luminance changes.

A critical step toward object recognition is the segmentation of a scene into relevant regions. One of the most important cues for segmentation is that of common fate: Elements that move together are grouped together Here we describe a new instantiation of common fate, in which elements move together not through physical space, but through luminance space. Experiment I shows that when elements of a scene become brighter together or darker together, observers group those elements together Experiments 2 and 3 show that this effect is not due to the availability of fixed luminance differences between target and background regions, but requires common changes within each region in the direction of luminance over time. The effect is differentiated from the recently discovered grouping cue of temporal synchrony, and is considered instead to be an extension of Wertheimer's original grouping factor of common fate. Common fate for luminance, or generalized common fate, is an extremely strong cue for the segmentation of a scene, yielding a tremendous advantage over grouping by fixed luminance cues.

Time course of amodal completion revealed by a shape discrimination task.

We measured the extent of amodal completion as a function of stimulus duration over the range of 15-210 msec, for both moving and stationary stimuli. Completion was assessed using a performance-based measure; a shape discrimination task that is easy if the stimulus is amodally completed and difficult if it is not. Specifically, participants judged whether an upright rectangle was longer horizontally or vertically, when the rectangle was unoccluded, occluded at its corners by four negative-contrast squares, or occluded at its corners by four zero-contrast squares. In the zero-contrast condition, amodal completion did not occur because there were no occlusion cues; in the unoccluded condition, the entire figure was present. Thus, comparing performance in the negative-contrast condition to these two extremes provided a quantitative measure of amodal completion. This measure revealed a rapid but measurable time course for amodal completion. Moving and stationary stimuli took the same amount of time to be completed (approximately 75 msec), but moving stimuli had slightly stronger completion at long durations.

Corticolimbic interactions associated with performance on a short-term memory task are modified by age.

Aging has been associated with a decline in memory abilities dependent on hippocampal processing. We investigated whether the functional interactions between the hippocampus and related cortical areas were modified by age. Young and old subjects' brain activity was measured using positron emission tomography (PET) while they performed a short-term memory task (delayed visual discrimination) in which they determined which of two successively presented sine-wave gratings had the highest spatial frequency. Behavioral performance was equal for the two groups. Partial least squares (PLS) analysis of PET images identified a hippocampal voxel whose activity was similarly correlated with performance across groups. Using this voxel as a seed, a second PLS analysis identified cortical regions functionally connected to the hippocampus. Quantification of the neural interactions with structural equation modeling suggested that a different hippocampal network supported performance in the elderly. Unlike the neural network engaged by the young, which included prefrontal cortex Brodmann's area (BA) 10, fusiform gyrus, and posterior cingulate gyrus, the network recruited by the old included more anterior areas, i.e., dorsolateral prefrontal cortex (BA 9/46), middle cingulate gyrus, and caudate nucleus. Recruitment of a distinct corticolimbic network for visual memory in the elderly suggests that age-related neurobiological deterioration not only results in focal changes but also in the modification of large-scale network operations.

Mutation screening of a neutral amino acid transporter, ASCT1, and its potential role in schizophrenia.

In a previous study, a genome scan of a subset of schizophrenia families from Palau, Micronesia gave evidence suggestive of linkage to microsatellite markers at 2p13-14. In addition, in a large extended multiplex pedigree (K1583), an 11 cM 2p13-14 haplotype segregated with the illness in eight distantly related schizophrenics. The haplotype region includes a neutral amino acid transporter, ASCT1. We mutation-screened the coding region, flanking intronic sequence and 5'-untranslated region of this transporter in affected members of K1583, two Palauan controls and one Caucasian control. Most polymorphisms were found to be silent or common to all samples scanned. A G/A heterozygote within intron 3 was found in one schizophrenic member of K1583, but was not found in any of the other affected members of K1583. A G/A heterozygote within intron 6 was found in two of six schizophrenics tested in K1583, and in one control. As none of the sequence polymorphisms segregated with illness in the eight schizophrenics, it is unlikely that changes in the 5'-untranslated region, coding sequence or flanking intronic sequence of the ASCT gene predispose to schizophrenia in these families.

Tryptophan hydroxylase polymorphisms in suicide victims.

Both environmental and genetic factors appear to contribute to the risk for suicide. The serotonergic system has been implicated in depression, impulsivity and suicidality. Tryptophan hydroxylase (TPH) is the rate-limiting enzyme in the synthesis of serotonin. Suicide has been associated with polymorphisms in intron 7 of the TPH gene. These alleles were studied in samples from 47 deceased Caucasian males as part of the Utah Youth Suicide Study. A 918 base pair fragment spanning the region of interest was amplified. The A218C polymorphism was visualized by restriction fragment length polymorphism (RFLP) and the A779C was sequenced. Neither A218C nor A779C appeared to be associated with suicide in this population. These results did not change when the sample was stratified by age (10-21 years, 22-31 years) or when violent suicides were selected. The complexity of the phenotype of suicide may reflect multiple biological and social etiologic factors, and poses a worthy challenge for genetic studies.

Deriving behavioural receptive fields for visually completed contours.

The visual system is constantly faced with the problem of identifying partially occluded objects from incomplete images cast on the retinae. Phenomenologically, the visual system seems to fill in missing information by interpolating illusory and occluded contours at points of occlusion, so that we perceive complete objects. Previous behavioural [1] [2] [3] [4] [5] [6] [7] and physiological [8] [9] [10] [11] [12] studies suggest that the visual system treats illusory and occluded contours like luminance-defined contours in many respects. None of these studies has, however, directly shown that illusory and occluded contours are actually used to perform perceptual tasks. Here, we use a response-classification technique [13] [14] [15] [16] [17] [18] [19] [20] to answer this question directly. This technique provides pictorial representations - 'classification images' - that show which parts of a stimulus observers use to make perceptual decisions, effectively deriving behavioural receptive fields. Here we show that illusory and occluded contours appear in observers' classification images, providing the first direct evidence that observers use perceptually interpolated contours to recognize objects. These results offer a compelling demonstration of how visual processing acts on completed representations, and illustrate a powerful new technique for constraining models of visual completion.

Effects of aging on the useful field of view.

Previous research has shown that the useful field of view (UFOV) is a useful tool in predicting driving ability, and the UFOV also seems to decline with age. The goals of the current study were first, to examine UFOV changes systematically as a function of age (15-84), and, second, to determine the effect of dividing attention on the UFOV. Our results show that the deterioration in the UFOV begins early in life (by 20 years, or younger). This deterioration is best conceptualized as a decrease in the efficiency with which observers can extract information from a cluttered scene, rather than by shrinking of the field of view per se. The diminished efficiency among elderly observers is exacerbated when conditions require the division of attention between central and peripheral tasks.

Holey fibers with random cladding distributions.

We provide what is to our knowledge the first direct confirmation that light can be guided in a holey fiber with randomly distributed air holes in the cladding. We also show that many of the features previously attributed to periodic holey fibers, in particular, single-mode guidance at all wavelengths, can also be obtained with random holey fibers. We provide insight into exactly how sensitive a holey fiber's optical properties are to the details of the cladding profile.

Recruitment of unique neural systems to support visual memory in normal aging.

The performance of many cognitive tasks changes in normal aging [1] [2] [3]. Recent behavioral work has identified some tasks that seem to be performed in an age-invariant manner [4]. To understand the brain mechanisms responsible for this, we combined psychophysical measurements of visual short-term memory with positron emission tomography (PET) in young and old individuals. Participants judged the differences between two visual stimuli, and the memory load was manipulated by interposing a delay between the two stimuli. Both age groups performed the task equally well, but the neural systems supporting performance differed between young and old individuals. Although there was some overlap in the brain regions supporting performance (for example, occipital, temporal and inferior prefrontal cortices, and caudate), the functional interconnections between these common regions were much weaker in old participants. This suggests that the regions were not operating effectively as a network in old individuals. Old participants recruited unique areas, however, including medial temporal and dorsolateral prefrontal cortices. These unique areas were strongly interactive and their activity was related to performance only in old participants. Therefore, these areas may have acted to compensate for reduced interactions between the other brain areas.

Effects of aging on calculation efficiency and equivalent noise.

Contrast sensitivity under photopic conditions declines with age; however, the cause of this decline remains unknown. To address this issue, we measured detection thresholds for sine wave gratings in noise, under various conditions of spatial-frequency uncertainty, and estimated observers' internal noise and calculation efficiency. Statistical analyses revealed that efficiencies were lower for old (median age at 68 years) than for young (median age at 22 years) observers; no significant differences in internal noise were found. A control experiment ruled out the possibility that reduced retinal illuminance causes the decline in efficiency with age. Our results demonstrate that age-related neural changes play a major role in the decline in contrast sensitivity with age. Possible contributing mechanisms are considered.