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.

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.

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.

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.

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.

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.

The influence of membrane cholesterol on the GABAA receptor.

1. Neurosteroids such as pregnanolone have been established as potent modulators of the GABAA receptor in both electrophysiological and binding studies. Since cholesterol is present in substantial amounts in the neuronal membranes, we have sought evidence for possible interactions of cholesterol with the neurosteroid site and more generally, with the GABAA channel. 2. Synaptosomal membranes were prepared from rat whole brain, cerebral cortex, cerebellum and spinal cord. These membranes were enriched with cholesterol to about double the original level by incubation with liposomes comprised of 50 phosphatidylcholine: 50 cholesterol in the presence of 1% BSA. The additional cholesterol formed a homogeneous mixture with the endogenous cholesterol. 3. The effects of cholesterol and modulatory drugs on the GABAA channel were assessed from the changes induced in [3H]-flunitrazepam (FNZ) binding. Cholesterol enrichment did not affect FNZ binding itself; however, the enhancement of [3H]-FNZ binding by pregnanolone was affected. In membranes from cerebral cortex, the potency of pregnanolone was reduced by a factor of 3.2 following cholesterol enrichment. By contrast, in membranes from spinal cord, the potency of pregnanolone was increased by a factor of 8.4 following cholesterol enrichment. In membranes from cerebellum, there was little overall change in pregnanolone potency although the effects of threshold concentrations were increased. 4. The enhancement of [3H]-FNZ binding by propofol in whole brain membranes was reduced in cholesterol-enriched membranes, similar to the effects of pregnanolone. Experiments with muscimol resulted in an increase in its potency as a potentiator of [3H]-FNZ binding, following cholesterol enrichment. 5. These results provide little evidence for a selective competition between cholesterol and pregnanolone at its binding site. Rather, they suggest an influence of membrane cholesterol on the functional coupling between the benzodiazepine site and the other specific drug sites on the GABAA channel. The detailed pattern of influence depended upon the region of CNS and may be related to the subunit composition of the GABAA channels present.

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.

The performance characteristics of an intra-arterial oxygen electrode.

The performance characteristics of intravascular oxygen electrodes used during and after open heart surgery are described. The pO2 recorded continuously by the electrodes correlated extremely well with corresponding measurements made on samples of arterial blood. The results show that intravascular pO2 electrodes are accurate and can be readily used during surgery and intensive care of adult patients.

The effects of contrast, spatial scale, and orientation on foveal and peripheral phase discrimination.

We examined the effects of contrast, spatial scale, and orientation, on phase discrimination thresholds. In expt I, the ratio of thresholds for 180 deg shifts in F + 2F gratings remained invariant across a wide range of fundamental contrasts. Experiment II demonstrated that random fluctuations in overall pattern contrast did not affect discrimination. Experiment III found that foveal, but not peripheral, thresholds were roughly independent of spatial scale; foveal-peripheral differences in phase sensitivity could not be eliminated by scaling stimulus size. Finally, expt IV found that thresholds for some phase shifts varied significantly with orientation in the periphery; in general, peripheral sensitivity was greatest for radially-oriented gratings. The implications of these findings for models of phase discrimination are discussed.

Masking of spatial frequency in visual memory depends on distal, not retinal, frequency.

Spatial frequency discrimination thresholds measured in a two-interval forced choice paradigm are virtually constant across inter-stimulus intervals ranging from 400 to 30,000 msec, demonstrating that an accurate representation of spatial frequency is maintained in short-term memory. This representation can be degraded by briefly flashing a grating during the retention interval. Moreover, this memory masking effect varies with the spatial frequency of the mask, suggesting that the mechanisms used to store spatial frequency in memory are similar to low-level visual filters. In this paper we replicate those previous findings and extend them by showing (1) that accurate memory for spatial frequency lasts as long as 1 min; (2) that memory masking is based on distal (c/cm), not retinal (c/deg), spatial frequency.

The effects of luminance on FPL and VEP acuity in human infants.

Grating acuity was measured in 16-week-old human infants. Three measurement techniques were used: forced-choice preferential-looking (FPL), and two visual-evoked-potential (VEP) techniques. The stimuli were counterphase flickering sinewave gratings with a space-average luminance of -1.0 or 2.0 log cd/m2. Slightly different luminance-dependent changes occur between FPL and VEP acuities, suggesting that some factor influences the two methods differently as stimulus luminance varies. A comparison between FPL acuities and VEP acuities within infants suggests a quantitative relationship between techniques. Infant's acuity for sinewave gratings with a space-average luminance of -2.0, -1.0, 0.0, 1.0 and 2.0 log cd/m2 was also measured using a single VEP paradigm. The results are compared to the same measurements in adults and to infant and adult ideal observers. VEP acuity in this group of infants improves by about 0.5 log units between -2.0 and 0.0 log cd/m2 and remains asymptotic between 0.0 and 2.0 log cd/m2. This result suggests that luminance-dependent changes in infant acuity cannot be fully accounted for by immaturities in the optics and photoreceptor spacing and efficiency.

Identification of band-pass filtered letters and faces by human and ideal observers.

To better understand how the visual system makes use of information across spatial scales when identifying different kinds of complex patterns, we measured human and ideal contrast identification thresholds to estimate identification efficiency for 1- and 2-octave wide band-pass filtered letters and faces embedded in 2-D dynamic Gaussian noise. Varying stimulus center frequency from 1 to 70 c/object had different effects on letter and face identification efficiency. In the 2-octave conditions, identification efficiencies decreased by 0.25-0.5 log units for letters and 0.5-1.2 log units for faces as center frequency increased from 6.2 to 49.5 c/object, but only letters were identifiable at center frequencies below 6.2 c/object. In the 1-octave conditions, letter identification efficiencies increased by about 0.5 log units as center frequency increased from 1.1 to 2.2 c/object, and were nearly constant from 2.2 to 35 c/object. Letters were unidentifiable by human observers at 70 c/object. Surprisingly, face identification was impossible for human observers at all center frequencies except 8.8 c/object for one observer, and 8.8 and 17.5 c/object for a second observer. Ideal observer thresholds were obtained for both letters and faces in all conditions, so information was always available to perform the task. Thus, the failure to identify faces reflects constraints on visual processing rather than a lack of stimulus information. Selective spatial sampling may account for some of the differences between letter and face identification efficiencies.

The physical limits of grating visibility.

We examined the extent to which pre-neural factors constrain the detectability of sinusoidal gratings of different spatial frequencies and luminances. Contrast sensitivity functions were measured in two observers for foveally-presented grating patches. Spatial extent of the patches was inversely proportional to grating frequency. The observers' contrast sensitivity functions were then compared to the performance of an ideal discriminator (Geisler, 1984) which incorporated the effects of quantal fluctuations, optical transfer, ocular media transmittance, and the aperture, quantum efficiency, and spatial distribution of foveal photoreceptors. The sensitivity of the ideal discriminator was roughly 20-fold greater than that of the human observers, but the shapes of the ideal and human CSFs were quite similar from 5 to 40 c/deg and from 3.4 to 340 cd/m2. The similarity of shapes demonstrates that the high-frequency rolloff of the foveal CSF for gratings with a fixed number of cycles can be explained by the operation of pre-neural factors alone. Previous research has shown that grating summation area is inversely proportional to the square of spatial frequency. Thus, for gratings with fixed spatial extent the high-frequency rolloff can be explained by the pre-neural factors plus variations in grating summation area. These conclusions imply in turn that the neural transfer function is much flatter than previously thought and that private line connections from foveal photoreceptors to higher visual centers are common.

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.

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.

Letter localization, not discrimination, is constrained by attention.

Shaw (1984; Shaw, Mulligan & Stone, 1983) measured the probability of detecting a target letter in displays containing different numbers of items. The set size effect was significantly larger than the effect predicted by unlimited-capacity models of visual processing, and Shaw concluded that attention constrains the discrimination of complex, but not simple, patterns. We re-examined the role of attention in letter discrimination by measuring the effect of set size on the contrast needed to identify a target embedded among distractors. The results of 5 experiments show that set size effects are small for letter discrimination, but large for letter localization. The findings suggest that the large set size effect reported by Shaw (1984) was a result of asking subjects to localize the target. In addition, the results are consistent with the hypothesis that limited processing capacity constrains the perceptual processes involved in letter localization, but not discrimination.

Depressive symptoms and alterations in sucrose taste perception: cognitive bias or a true change in sensitivity?

Previous studies have reported elevated taste thresholds in depressed subjects, but those studies did not control for changes in response bias. The current study used signal detection analyses to address this shortcoming. Sucrose detection thresholds were measured (1) in subjects with high and low Hamilton Depression Rating Scale (HAM-D) scores who did not meet standard criteria for current Major Depressive Episode (MDE); and (2) in subjects who did fulfil standard criteria for MDE. Subjects with low HAM-D scores produced significantly more false alarms than the other two groups, but taste sensitivity, as indexed by d', did not vary significantly across groups. These results suggest that changes in response bias underlie previously reported increases in sucrose taste thresholds in depressed subjects.

Pressure distribution beneath the human foot.

A descriptive study was undertaken to establish the 95 percentile limits (proposed normal reference range) for pressure beneath the hallux, metatarsal heads, and heel in a group of healthy adult subjects. A new force plate device capable of accurately measuring discrete areas of pressure beneath the human foot with high temporal and spatial resolution was used. The system is capable of accurately measuring plantar foot pressure during dynamic and static foot function. The results of this research are in close agreement with other published studies of plantar foot pressure measurement with comparable systems.