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Biomedical subjects

L Mitrani

Publications and source records attributed to L Mitrani.

At least 19 recordsLinked to original sources
PubMed1998

[Prevalence of human immunodeficiency virus infection and behaviors associated with its transmission among parenteral drug users selected on the street].

BACKGROUND: To estimate the prevalence of HIV infection and the associated risk behaviours among intravenous drug users not receiving treatment for their drug dependence. PATIENTS AND METHODS: A cross-sectional study of 200 intravenous drug users recruited from the streets of Barcelona in 1993. Information about the socio-demographic aspects and behaviours was obtained through a personal interview using a standardised questionnaire which was carried out by three ex-drug users. Saliva samples were used to determine anti HIV antibodies. RESULTS: The prevalence of HIV infection was 51%. 57% borrowed used syringes, 65% lended their syringes, and 41% practised back of frontloading. 85% if those who shared syringes always cleaned them, however 59% only used water. 78% had heterosexual relations, in 33% of the cases with a non-injecting sexual partner. 65% always used condoms with sexual clients although only 26% always used them with stable sexual partners and 36% with casual sexual partners. 78% had performed the HIV test and among those who knew they were seropositive, 40% had received some kind of health control in the last 6 months. Not having a legal income, injecting speedball or barbiturates, unknowing self HIV negative status and practicing forms of indirect sharing were found to be independently associated with syringe sharing. CONCLUSIONS: The high prevalence of HIV infection and associated risk behaviours highlights the need to increase and adapt the prevention efforts and investigation to this group. Social marginalization and poliuse of drugs should be addressed in HIV prevention programs.

Adult↗
PubMed1993

Model of visual localization.

The position of an object in the visual field is a global characteristic of the object which must be determined shortly after its appearance. This characteristic is useful for organizing actions towards the object or for its better examination. A model of visual localization based on the concept of "image function" is proposed. It predicts that the centroid of a visual object determines its position. The centroid could be easily extracted from the maximum in the image function under certain conditions. Symmetry and size may influence the accuracy (absolute and relative) in locating visual objects.

Humans↗
PubMed1990

Perceiving the center of irregular contour quadrangles.

This investigation examined how subjects perceived and localized the centers of irregular quadrangles. Five contour figures in four orientations were used. Seventeen subjects localized the centers of the figures in each orientation, three times. The estimated positions were found to be distributed according to the two-dimensional normal law. The mean position of the perceived center was very close to the centroid of the figures if they were considered flat homogeneous bodies. The orientation of the figures influenced the distributions of the estimates. The axis of maximal variance of the estimates was very close to the axis of orientation of the figures.

Adult↗
PubMed1989

Errors in estimating the orientation of dot patterns.

The error in estimating the orientation of a dot pattern was measured as the difference between the orientation of the least-squared-distances line (LS-line) of the pattern and the orientation of a line adjusted by the subject to match the perceived orientation of the pattern. Analysis of the mean errors (averaged over ten subjects) obtained for one hundred patterns confirmed that the orientation of the LS-line represents the orientation of elongated dot-patterns. It is shown that estimated orientation was systematically biased towards the nearest 45 degrees oblique meridian. This bias points to the importance of the +/-45 degrees directions as natural norms for left- and right-side tilt in the frontoparallel plane.

Adult↗
PubMed1989

Perception time and reaction time.

Two experiments were performed in order to examine the possible contribution of visual perception time in motor response time under conditions of pursuit eye movements. Subjects had to follow with their eyes a light spot moving horizontally along a reference scale at a constant velocity of 14 degrees/s. Stimuli (the disappearance of the moving spot) were presented randomly at distances of 11.3, 17.7 and 24.2 angular degrees from the onset of motion. In Experiment 1 subjects had to report verbally the scale division at which the stimuli were presented. In Experiment 2 subjects had to press a button as quickly as possible after stimulus presentation. No relationship between visual perception time and motor reaction time was found.

Adult↗
PubMed1989

Is the 45 degrees-oblique a third dominant direction?

Vertical and horizontal are widely accepted as dominant directions or norms of visual orientation in the frontoparallel plane. They are supposed to cause a normalization effect consisting in the apparent rotation of a tilted straight line towards the nearest dominant direction. The evidence for tilt normalization towards the vertical or horizontal visual meridia is indirect. On the other hand, human observers are very sensitive to departures from the vertical and horizontal, which means that most orientations in the frontoparallel plane are termed tilted rather than vertical or horizontal. By measuring directly the orientation of dot patterns we found that estimated orientation was systematically biased towards the nearest 45 degrees-oblique visual meridian. This finding is interpreted as evidence for the existence of an oblique norm in visual tilt.

Form Perception↗
PubMed1987

Effect of the localization of visual stimuli on the detection of temporal relations.

The perception of simultaneity and temporal order of a pair of light stimuli is studied in dependence on their localization in the visual field. Four different modes of presenting the light stimuli on the screen were used: (1) both stimuli in the middle of the screen, immediately above the fixation point; (2) both stimuli in the left visual half-field, 8 deg. of arc to the left of the fixation point; (3) both stimuli in the right visual half-field, at the same distance from the fixation point; (4) one stimulus in the left and the other one in the right visual half-field, accordingly at 8 deg. of arc to the left or to the right of the fixation point. The duration of each of the pair of simultaneous or consecutive light stimuli was 100 ms. The percentage of correct responses to the presentation of simultaneous or consecutive pairs of light flashes was estimated. The experimental data suggest that the perception of simultaneity of two visual events depends on the spatial position of these events in the visual field.

Adult↗
PubMed1987

A linear model for the response time in motion prediction.

A linear relationship was found to exist between the response time and the concealment time in motion extrapolation experiments. It follows that the subjects performance is characterized by the two parameters of the linear relationship rather than by the error in determining the position of the occluded moving stimulus. These two parameters can be manipulated separetely by specific changes in the experimental arrangement. A suggestion is made that one of the parameters is connected with velocity estimation, while the other with internal organization and execution of the motor response.

Humans↗
PubMed1987

Estimating the duration of briefly presented physiognomic stimuli.

The influence of complex stimuli which are supposed to induce different attitude and emotional effects in the observer on short time interval perception was studied. It is concluded that emotional and/or aesthetical factors do not interfere with short time interval perception.

Esthetics↗
PubMed1987

Spatial frequency, contrast and duration estimation.

Duration estimation of square-wave gratings presented for "short" (up to 150 ms) and "long" (above 190 ms) time intervals was studied. The spatial frequency (SF) of the gratings with equal mean luminance and contrast 85% varied from 0.7 to 12 c/deg. Duration estimates were assessed by the method of magnitude estimation. For brief presentations only the duration of high SF gratings was subjectively shorter than the duration of low SF gratings. It was shown that the apparent contrast of the square-wave gratings decreased with the increase of SF. In an experiment on duration estimation of patterns of different contrast it was demonstrated that duration estimates increased with increasing contrast for short presentations only. The apparent contrast of the gratings covarying with SF was supposed to underlie SF influence on short duration estimation. The assumption about different mechanisms operating in short and long duration perception was supported by the results.

Analysis of Variance↗
PubMed1987

Determination of the threshold of simultaneity and temporal order using the "step-ladder" method.

The article considers the sensitivity of the mechanisms for perceiving simultaneity and temporal order. A method similar to the psychophysical "staircase" method is used for determining the thresholds of the perception of simultaneity and temporal order. The simultaneity and temporal order thresholds are measured for two levels of visual adaptation of the subjects. Two clearly distinguishable thresholds are established: simultaneity threshold and temporal order threshold. The threshold values for simultaneity and temporal order are preserved, irrespective of the adaptation level, which suggests that the general illumination does not affect the mechanisms for perceiving simultaneity and temporal order.

Adaptation, Ocular↗
PubMed1987

In search of paintings influence on time judgement.

The influence of paintings on short time interval perception was studied. Three methods for time estimation were tested: direct scaling and two modifications of the method of reproduction – with and without simultaneous presentation of the painting whose duration was, reproduced. The results show that short time interval perception is influenced by such complex stimuli as paintings. The motor reproduction with simultaneous presentation of the visual stimulus was the most effective in revealing the influence of paintings on short time interval perception.

Art↗
PubMed1986

Mechanisms and asymmetries in visual perception of simultaneity and temporal order.

Temporally overlapping, spatially separated visual stimuli were used for studying perception of simultaneity and temporal order. Pairs of flashes each of 100 ms duration were presented with stimulus onset asynchronies of 0, 30, 50, and 70 ms. Three spatial arrangements of flash presentation were tested: both flashes were presented foveally; one flash was presented foveally and the other at 8 deg in the left visual hemifield; one flash was presented foveally and the other at 8 deg in the right visual hemifield. Onset asynchronies of 30 and 50 ms were not sufficient for correct identification of temporal order although the flashes were not perceived as simultaneous. Analysis of the response distributions suggests the existence of two-independent mechanisms for evaluating temporal interrelations: one for detecting simultaneity and the other for identifying temporal order. A better detection of simultaneity was found when synchronous flashes were presented together with pairs of flashes separated by larger onset asynchronies. Reading habits may explain only part of the left-right asymmetries of the response distributions. The possible lateralization of the two suggested mechanisms within the cerebral hemispheres is discussed.

Adult↗
PubMed1985

Hemispheric asymmetry in visual perception of temporal order.

The perception of temporal order is investigated. Suitable localization of the signals is also used to study the functional asymmetry of the cerebral hemispheres during perception of temporal order. The experimental data obtained correlate well with earlier assumptions of ours, namely: (a) separate mechanism for perception of temporal order; (b) the right hemisphere is dominant for perception of the order of events in time; (e) dependence of the subjective threshold for ordering of events in time on different physical characteristics of the stimuli.

Dominance, Cerebral↗
PubMed1984

Influence of the target delay and duration on the prediction of a moving stimulus future position.

In a linear-motion prediction study an immobile target with different duration and delay with respect to the onset of the visible motion determined the motion extrapolation distance. The subject's task was to press a key at the moment when the moving stimulus would reach the position of the target if it continued its motion on the screen. The subject's response time was measured between the moment of the moving stimulus disappearance and the moment of key pressing. A linear relationship between the response time and the concealment time was obtained under all experimental conditions. It was demonstrated that the duration of the target did not affect the linear regression equation constants. The delay of the target presentation with respect to the onset of the visible motion was found to influence the free term of the linear relationship. When the target was presented with a great delay (0.56 s before the moving stimulus disappearance) the intercept in the linear relationship significantly decreased. The present results suggest that the organization of the response in the motion prediction conditions influences the temporal characteristics of the subject's performance. This influence is manifested in alternation of the free term in the linear relationship between the response time and concealment time.

Distance Perception↗
PubMed1984

Cerebral asymmetry in the perception of overlapping brief time intervals?

The perception of duration, visually presented, has been studied depending on the visual half-field in which the light stimulations have been presented. The pair of intervals of equal duration, presented for comparison in the 200-1000 ms range, was studied in three different series, in each of which the beginning of the second interval followed accordingly: (a) 10 ms after the beginning of the first interval; (b) after 1/2 of the duration of the first interval; (c) the end of the first interval is at the same time the beginning of the second one. It has been established that two light stimuli of equal duration, presented consecutively in the left and right visual half-fields, are perceived as being different due to their temporal order, provided that the duration of the first interval has elapsed either prior to or at the moment of the onset of the second interval. The temporal order does not affect the subjective duration of overlapping light stimuli. It is possible to assume some kind of hemispheric asymmetry in the duration perception, provided some critical time has elapsed between the end of the first and the beginning of the second intervals. Such an asymmetry cannot be detected in the case of the overlapping intervals studied in the present work.

Dominance, Cerebral↗
PubMed1983

Estimating the orientation of dot patterns presented tachistoscopically.

Human ability for determining the orientation of dot patterns was studied under tachistoscopic presentation. Random dot configurations with a great difference between the line determined by the most distant dots in the configuration and the line for which the sum of the squared distances between each dot of the configuration and this line is minimal were used. These lines determine the "extension" and the "elongation" of the patterns (Yakimoff, 1981). The results have shown that the elongation of a dot pattern is the essential characteristic determining its perceived orientation.

Adult↗