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Biomedical subjects

L Mitrani

Publications and source records attributed to L Mitrani.

At least 19 recordsLinked to original sources
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↗
PubMed1979

Localization of the change in intensity of a visually pursued light target.

The aim of the subjects was to track a moving light target and to locate the change in its intensity (its disappearance or dimming). It is shown experimentally that: (1) mislocation of target disappearance is smaller than mislocation when the target is dimmed three times; (2) the distance at which the eyes move after the target is darkened does not influence localization. The data disprove possible influence of "overtracking" on localization. They are in favour of the hypothesis about the dependence of localization on stimulus intensity.

Adult↗
PubMed1977

Localization of the disappearance of a light target during tracking eye movements. II.

A light target moving with constant velocity and disappearing at a place determined by the experimenter, is presented to the subjects who track it visually and determine the locus of its disappearance with respect to a scale mounted on the screen. The result is that the disappearance of the target is systematically mislocated in the direction of the eye movement. The value of the mislocation depends on the target intensity. Under different experimental conditions the subjects localize target disappearance by steady fixation of the eyes. The resulting mislocation has entirely different characteristics compared with the first conditions. The theory that the phenomenon of mislocation is due only to wgnal latency in the visual pathways is rejected. A hypothesis is proposed concerning the participation of psychological factors in the process of localization.

Eye Movements↗
PubMed1977

Visual extrapolation of a line segment to the point of its intersection with a straight line. II.

The article describes three experiments to study visual extrapolation of a line segment to the point of its intersection with a straight line. Eye movements are shown to play no significant role in solving problems involving visual spatial extrapolation. It is also indicated that systematic mislocations of the point of intersection sought in cases of acute angles are preserved even when the motor response is substituted by verbal estimation. When the test line segment is presented for a very short time (tau =20 ms), the estimations of the subjects manifest great individual differences and considerable dispersion. The results of the experiments show that the ability of visual extrapolation is influenced by different and numerous factors and this ability does not reflect only the functioning of simple detectors for direction in the visual system.

Adult↗
PubMed1977

Estimation of duration of an array of time interval and of its separate elements.

It is shown that when a human subject estimates verbally the duration of an array consisting of three intervals - two light intervals divided by a dark one - considerable underestimation is observed compared with estimates of the same duration in the control experiment. In case of separate estimation of the three intervals composing the array the sum of the estimates is much more precise even compared with estimates in the control. Perhaps in this case the system for time intervals estimation operates with two "internal clocks", each of which reads the duration of the dark and of the light intervals. In estimating the array of three intervals the subjects estimate the total array and they do not summate their separate estimations. It may be assumed that the system uses only one "clock" for measuring the duration of the total array.

Adult↗
PubMed1977

Estimation of short time intervals under different experimental conditions.

A complex experiment is carried out in order to study the human ability to estimate and reproduce short time intervals in the 100-1000 ms range presented with light stimuli. The estimation task consists in verbal nomination of the time intervals presented to the subject in random order after preliminary training. In the production task the subject is expected to press a telegraph key simultaneously with the end of the light interval presented the duration of which is announced to him in advance. In the reproduction task the subject is to press the key a given time after the onset of the stimulus which duration is of 2 sec. The results show that in spite of the relatively good achievements in tasks involving verbal estimation of the duration of the intervals, the reproduction tasks are performed with systematic errors. In the production task the mean values of the response times decrease upon increasing of the required interval, while in the reproduction task they increase parallel with the increase of the required interval. Consequently, one and the same short time interval is perceived differently depending on the task to be performed, namely: verbal nomination, coincidence between hand movement and the end of the interval, or reproduction after verbal nomination.

Adult↗
PubMed1977

Estimation of time intervals--elements of an array of three intervals.

The article reports the results of experiments on human ability to estimate time intervals separately or in groups of two intervals as elements of a three-interval array presented by two light signals separated by a dark interval. The first light interval is shown to be systematically overestimated compared with the second one. Moreover, the subjects usually ascribe a constant value to the dark interval. The double interval (light + dark or dark + light) proves to be underestimated compared with the case when estimation of the same intervals was required without estimation of a third interval. The additional task changes the estimations of the subjects. It may be claimed that the complexity of the problem is connected with definite changes in the average estimations of the subjects.

Darkness↗
PubMed1977

Visual extrapolation of a line segment to the point of its intersection with a straight line. I.

A study is made of the capacity of fifty subjects to estimate the position of the point of intersection between a straight line and the visually extrapolated extension of a line segment at three different angles between the line and the segment: 30 degrees, 60 degrees and 90 degrees. The results show systematic deviations in the estimation of acute angles. The point of intersection of the straight line and the segment is misperceived to be shifted inwards in the angle between them. It is also shown that the set of imaginary extensions of a line segment with a given length, until its intersection with the given straight line, is determined only by the angle between the straight line and the segment and does not depend on the distance between them. Various possible mechanisms which could determine the solution of the task facing the subjects are discussed.

Adult↗
PubMed1977

Localization of disappearance of a light target during tracking eye movements. I.

A light target moving at a constant velocity in horizontal direction and disappearing at a locus determined by the experimenter is presented to the subjects in the experiments. The task of the subjects is to track its movement with their eyes and to determine the place of its disappearance using a scale fixed on the screen. Under these conditions they systematically mislocate the locus of disappearance in the direction of the eye movement. The mislocation is in linear dependence on the velocity of tracking and, moreover, it depends on the locus of disappearance of the target. The error is smaller when the target disappears at the end of tracking. The theory that mislocation is due to perception time is rejected.

Adult↗