Optic flow is used to control human walking.

How is human locomotion visually controlled? Fifty years ago, it was proposed that we steer to a goal using optic flow, the pattern of motion at the eye that specifies the direction of locomotion. However, we might also simply walk in the perceived direction of a goal. These two hypotheses normally predict the same behavior, but we tested them in an immersive virtual environment by displacing the optic flow from the direction of walking, violating the laws of optics. We found that people walked in the visual direction of a lone target, but increasingly relied on optic flow as it was added to the display. The visual control law for steering toward a goal is a linear combination of these two variables weighted by the magnitude of flow, thereby allowing humans to have robust locomotor control under varying environmental conditions.

Experimental non-O group 1 Vibrio cholerae gastroenteritis in humans.

In this study, 27 volunteers received one of three non-O group 1 Vibrio cholerae strains in doses as high as 10(9) CFU. Only one strain (strain C) caused diarrhea: this strain was able to colonize the gastrointestinal tract, and produced a heat-stable enterotoxin (NAG-ST). Diarrhea was not seen with a strain (strain A) that colonized the intestine but did not produce NAG-ST, nor with a strain (strain B) that produced NAG-ST but did not colonize. Persons receiving strain C had diarrhea and abdominal cramps. Diarrheal stool volumes ranged from 154 to 5,397 ml; stool samples from the patient having 5,397 ml of diarrhea were tested and found to contain NAG-ST. The median incubation period for illness was 10 h. There was a suggestion that occurrence of diarrhea was dependent on inoculum size. Immune responses to homologous outer membrane proteins, lipopolysaccharide, and whole-cell lysates were demonstrable with all three strains. Our data demonstrate that V. cholerae of O groups other than 1 are able to cause severe diarrheal disease. However, not all strains are pathogenic for humans: virulence of strain C may be dependent on its ability both to colonize the intestine and to produce a toxin such as NAG-ST.

Diagnostic value of plasmid analyses and assays for virulence in Yersinia enterocolitica.

The possession of a 42- to 48-megadalton plasmid alone does not appear to be predictive of virulence in Yersinia species. Twelve of 100 Yersinia enterocolitica strains contained a 42 to 48-megadalton plasmid, and 4 of 30 Y. enterocolitica-like strains contained a 42- to 48-megadalton plasmid. Seven strains of Y. enterocolitica contained the 42- to 48-megadalton plasmid plus an 82-megadalton plasmid, and these were the only study strains lethal for mice. Based on restriction endonuclease digestion, the 42- to 48-megadalton plasmid DNA from these seven strains were similar and were not similar to the 42- to 48-megadalton plasmids present in the other nine strains. The ability to invade guinea pig eye tissues, calcium dependency, autoagglutination, and colonial morphology at 37 degrees C were also associated with plasmid DNA, but the relationships were either variable or not reciprocal. Neither tissue culture invasiveness nor heat-stable toxin production was associated with plasmid DNA. It was concluded that biochemical speciation and a total plasmid profile in combination with enzyme digests are predictive of virulence in Y. enterocolitica as it is measured by mouse lethality.

Space-time behavior of single and bimanual rhythmical movements: data and limit cycle model.

How do space and time relate in rhythmical tasks that require the limbs to move singly or together in various modes of coordination? And what kind of minimal theoretical model could account for the observed data? Earlier findings for human cyclical movements were consistent with a nonlinear, limit cycle oscillator model (Kelso, Holt, Rubin, & Kugler, 1981) although no detailed modeling was performed at that time. In the present study, kinematic data were sampled at 200 samples/second, and a detailed analysis of movement amplitude, frequency, peak velocity, and relative phase (for the bimanual modes, in phase and antiphase) was performed. As frequency was scaled from 1 to 6 Hz (in steps of 1 Hz) using a pacing metronome, amplitude dropped inversely and peak velocity increased. Within a frequency condition, the movement's amplitude scaled directly with its peak velocity. These diverse kinematic behaviors were modeled explicitly in terms of low-dimensional (nonlinear) dissipative dynamics, with linear stiffness as the only control parameter. Data and model are shown to compare favorably. The abstract, dynamical model offers a unified treatment of a number of fundamental aspects of movement coordination and control.

Steady-state and perturbed rhythmical movements: a dynamical analysis.

This study examined rhythmic finger movements in the steady state and when momentarily perturbed in order to derive their qualitative dynamical properties. Movement frequency, amplitude, and peak velocity were stable under perturbation, signaling the presence of an attractor, and the topological dimensionality of that attractor was approximately equal to one. The strength of the attractor was constant with increasing movement frequency, and the Fourier spectra of the steady-state trials showed an alternating harmonic pattern. These results are consistent with a previously derived nonlinear oscillator model. However, the oscillation was phase advanced by perturbation overall, and a consistent phase-dependent, phase-shift pattern occurred, which is inconsistent with the model. The overall phase advance also shows that any central pattern generator responsible for generating the rhythm must be nontrivially modulated by the limb being controlled.

Visual control of posture during walking: functional specificity.

Three experiments examined the functional specificity of visually controlled posture during locomotion by presenting large-screen displays to participants walking on a treadmill. Displays simulated locomotion down a stationary hallway, a hallway that traveled with the observer, or a frontal wall that traveled with the observer. A superimposed oscillation specified postural sway in 6 possible directions. With the wall, sway amplitude was isotropic and directionally specific in all conditions. However, with the hallways, sway was anisotropic (lateral > anterior-posterior [AP]), and diagonal responses were flattened into the lateral plane. When the treadmill was turned 90 degrees to the hallway, both the anisotropy and flattening were reversed (AP > lateral), indicating that they are determined by the visual structure of the scene. The results can be explained by postural control laws based on both optical expansion and motion parallax, yielding biases in planar environments that truncate parallax.

Enrichment for Plesiomonas shigelloides from stools.

Bile peptone broth and alkaline peptone water (pH 8.5) were examined as enrichment media for the isolation of Plesiomonas shigelloides from stools, with salmonella-shigella agar as the isolation medium. After 423 parallel examinations in two different experiments, bile peptone broth enrichment for 24 h was observed to be six times more effective (P less than 0.01) than direct plating alone on salmonella-shigella agar. Bile peptone broth was found to be twice as effective as alkaline peptone water for the recovery of P. shigelloides from stools.

Coupling of posture and gait: mode locking and parametric excitation.

We investigate the temporal coordination of human gait and posture and infer the nature of their coupling. Participants viewed a sinusoidally oscillating visual display which induced medial-lateral postural sway during treadmill walking, while display frequency was varied (0.075-1.025 Hz). First, postural responses exhibited the usual low-pass characteristic but with an additional resonance peak near the preferred stride frequency, although shifted downward by 0.12 Hz; this provides evidence of a coupling from gait to posture. Second, the step cycle adapted to mode lock with the visual driver and postural sway, as well as displaying instances of intermittency (slipping in and out of phase) and quasiperiodicity (phase wandering); this provides evidence of a coupling from posture to gait. We observed a spectrum of integer mode locks, including a large 1:1 trapping region about the stride frequency and superharmonic entrainment (stride frequency > driver frequency) at lower driver frequencies. A coupled-oscillator model that incorporates a novel parametric coupling from posture to the gait "stiffness" term reproduces these features of the data, including the resonance peak shift. Biological coordination patterns may thus emerge naturally as properties of a system of appropriately coupled oscillators.

Media for the isolation of Aeromonas hydrophila.

Isolation rates of Aeromonas hydrophila from stool samples of symptomatic and asymptomatic individuals were examined for several common enteric media. Sheep blood agar with 10 micrograms of ampicillin per ml, preceded by overnight enrichment in alkaline peptone water, yielded 2.6 times the number of isolates as the other media examined and is recommended for the isolation of A. hydrophila from humans.

Motion parallax is used to control postural sway during walking.

Three experiments tested the hypothesis that postural sway during locomotion is visually regulated by motion parallax as well as optical expansion. Oscillating displays of three-dimensional scenes were presented to participants walking on a treadmill, while postural sway was recorded. Displays simulated: (a) a cloud, in which parallax and expansion are congruent, (b) a hallway, (c) the side walls of the hallway, (d) a ground surface, (e) a wall, (f) the wall with a central hole, (g) a hall farther from the observer, and (h) a wall farther from the observer. In contrast to previous results with a hallway, responses with the cloud were isotropic and directionally specific. The other displays demonstrated that motion parallax was more effective than simple horizontal flow in eliciting lateral sway. These results are consistent with the hypothesis that adaptive control of sway during walking is based on congruent expansion and parallax in natural environments.

The role of central and peripheral vision in postural control during walking.

Three hypotheses have been proposed for the roles of central and peripheral vision in the perception and control of self-motion: (1) peripheral dominance, (2) retinal invariance, and (3) differential sensitivity to radial flow. We investigated postural responses to optic flow patterns presented at different retinal eccentricities during walking in two experiments. Oscillating displays of radial flow (0 degree driver direction), lamellar flow (90 degrees), and intermediate flow (30 degrees, 45 degrees) patterns were presented at retinal eccentricities of 0 degree, 30 degrees, 45 degrees, 60 degrees, or 90 degrees to participants walking on a treadmill, while compensatory body sway was measured. In general, postural responses were directionally specific, of comparable amplitude, and strongly coupled to the display for all flow patterns at all retinal eccentricities. One intermediate flow pattern (45 degrees) yielded a bias in sway direction that was consistent with triangulation errors in locating the focus of expansion from visible flow vectors. The results demonstrate functionally specific postural responses of both central and peripheral vision, contrary to the peripheral dominance and differential sensitivity hypotheses, but consistent with retinal invariance. This finding emphasizes the importance of optic flow structure for postural control regardless of the retinal locus of stimulation.

New virulence-associated plasmid in Yersinia enterocolitica.

In a study of 103 strains of Yersinia enterocolitica, 10 strains were found to be lethal for mice and to possess 42- and 82-megadalton plasmids. This association was statistically significant (P much less than 0.001). Serotypes of Y. enterocolitica previously considered avirulent were found to possess these plasmids and to be lethal for mice. A spontaneous derivative of one strain contained only the 82-megadalton plasmid and was lethal for mice anyway. This virulence-associated plasmid is a potential diagnostic tool for the clinical or public health laboratory which must delineate pathogenic strains of Y. enterocolitica.

Infant bouncing: the assembly and tuning of action systems.

We outline a theory of infant skill acquisition characterized by an assembly phase, during which a task-specific, low-dimensional action pattern emerges from spontaneous movement in the context of task constraints, and a tuning phase, during which adjustment of the system parameters yields a more energetically efficient and more stable movement. 8 infants were observed longitudinally when bouncing while supported by a harness attached to a spring. We found an initial assembly phase in which kicking was irregular and variable in period, and a tuning phase with more periodic kicking, followed by the sudden appearance of long bouts of sustained bouncing. This "peak" behavior was characterized by oscillation at the resonant frequency of the mass-spring system, an increase in amplitude, and a decrease in period variability. The data are consistent with a forced mass-spring operating at resonance.

Enterotoxin and cytotoxin production by enteroinvasive Escherichia coli.

It has long been suspected that besides their ability to invade enterocytes, enteroinvasive Escherichia coli (EIEC) strains have the ability to elaborate an enterotoxin. We tested 35 EIEC strains for cytotoxins and 9 (1 per serogroup) for enterotoxins. All 35 strains exhibited low levels of Vero cell cytotoxins that are immunologically and genetically distinct from Shiga-like toxin I or II of enterohemorrhagic E. coli. Sterile supernatants and cell lysates of two EIEC strains were tested in rabbit ileal loops, and both stimulated moderate fluid accumulation (circa 0.5 ml/cm) without tissue damage; secretory activity was confirmed in Ussing chambers, where these two strains and the seven others tested significantly increased short circuit current without altering tissue conductance. Curing the 140-MDa invasiveness plasmid from an EIEC strain did not diminish enterotoxin production. Culture in minimal Fe2+ medium is necessary to detect expression of the enterotoxin which is circa 68 to 80 kDa in size and is distinct from the EIEC cytotoxin.

Prevalence of Plesiomonas shigelloides among diarrhoeal patients in Bangladesh.

The incidence of Plesiomonas shigelloides among diarrhoeal patients attending the Dhaka Treatment Centre of the International Centre for Diarrhoeal Disease Research, Bangladesh (ICDDR, B) from January through December 1987, has been reported. Using bile peptone broth (pH 8.8) as an enrichment medium and Salmonella-Shigella agar to isolate the organism, P. shigelloides was isolated from 838 (6.4%) of 13,142 patients, 523 (4.0%) of whom had no other pathogen isolated. The percentage of isolation was higher from stool (9.2%) than from rectal swab (5.9%) specimens (P < 0.005). The incidence of P. shigelloides was higher among male (64.0%) than female (36.0%) patients (P < 0.005). Isolation was highest among children less than five years. P. shigelloides was isolated most often in March (11.0%) and September (7.7%), indicating two seasonal peaks of incidence before and after the monsoons. All the strains were uniformly sensitive to chloramphenicol, furazolidon, gentamicin and trimethoprim-sulfamethoxazole. Sensitivity of the organism to ampicillin and tetracycline was 27.0% and 89.0%, respectively. This indicates that P. shigelloides may be an important agent of diarrhoea in our patient population.

Toxin production by Vibrio mimicus strains isolated from human and environmental sources in Bangladesh.

Vibrio mimicus has recently been isolated from aquatic environments of Bangladesh. A total of 125 of 300 environmental isolates, representing various biotypes, and 19 human isolates were tested for enteropathogenicity by using several models. Less than 1% of the environmental isolates and slightly more than 10% of the clinical isolates produced cholera toxin-like toxin. A significant percentage of the environmental isolates (25%) and of the human isolates (74%) induced fluid accumulation in ligated rabbit ileal loops. One environmental strain produced heat-stable toxin-like enterotoxin, whereas all of the human isolates did not. V. mimicus strains were divided into the following three groups on the basis of their activity in various toxin assays: (i) organisms which produce a heat-labile enterotoxin immunobiologically similar to cholera toxin, (ii) organisms which produce a heat-stable enterotoxin-like toxin, and (iii) organisms whose whole-cell cultures have some activity characteristic of heat-labile toxin (e.g., fluid accumulation in ligated rabbit ileal loops and positive permeability factor) but are not positive by the GM1 enzyme-linked immunosorbent assay. One isolate from this group was able to elicit these results with cell-free culture filtrates. There was no correlation of biotype with toxic activity of V. mimicus isolates.

Plasmid analysis of Shigella dysenteriae type 1 isolates obtained from widely scattered geographical locations.

Plasmid profiles and antimicrobial susceptibility patterns of 343 strains of Shigella dysenteriae type 1, obtained from 18 different geographical locations, were analyzed. Three plasmids, with molecular sizes of 140, 6, and 2 megadaltons (MDa), were present in 94, 98, and 96%, respectively, of the 343 strains isolated during either epidemic or nonepidemic periods from 1965 to 1987. In addition to these plasmids, 83% of the strains harbored a 4-MDa plasmid and 25% harbored a 20-MDa plasmid. Various plasmid profiles were observed in which the 140-, 6-, and 2-MDa plasmids occurred commonly, irrespective of the place of isolation and drug resistance pattern of the strains. Certain profiles showed significant association with drug resistance patterns. These findings suggest that three plasmids, of molecular sizes 140, 6, and 2 MDa, are unique to S. dysenteriae type 1 strains and may indicate the global spread of a pathogenic bacterial clone. Additionally, these core plasmids, plus plasmids of various other sizes, could be used to identify emerging subclones which are causing both epidemic and sporadic disease. Thus, plasmid profiles of S. dysenteriae type 1 strains can be used to monitor possible pandemic strains as well as individual epidemic strains.

Antibiotic resistance patterns of Vibrio mimicus isolated from human and environmental sources in Bangladesh.

Twenty-five environmental and 19 clinical strains of Vibrio mimicus were tested for antibiotic susceptibility patterns. Environmental strains were resistant to streptomycin, kanamycin, and trimethoprim-sulfamethoxazole; clinical strains were susceptible. Environmental strains showed variable resistance to ampicillin (44%), but clinical strains were susceptible. All strains tested were susceptible to chloramphenicol and gentamicin.