Biomedical subjects

N Bocheva

Publications and source records attributed to N Bocheva.

15 recordsLinked to original sources

PubMed2000

The contributions of slant and tilt to the detection of local surface orientation in structure from motion.

The contribution of slant and tilt to the detection of differences in local surface orientation was examined for structure-from-motion (SFM) displays of a complex sinusoidal surface. Observers judged whether an elliptical SFM gauge figure appeared to be lying on the surface or intersecting it. The gauge figure orientation either matched the local surface orientation or differed from it in slant, tilt, or both. Similar sensitivity was found for deviations in slant and tilt, but greater biases and variability were found when the gauge figure deviated from the local surface orientation in slant, depending on the sign of the difference between the gauge figure and local surface orientation and the position of the gauge figure. The results are consistent with Stevens' (Biological Cybernetics, 46 (1983) 183-195) discussion of the computational advantages of slant and tilt contributing independently to the detection of differences in local surface orientation. The effects of changes in perceived surface slant and tilt during rotation and of the misperception of surface depth on the detection of local orientation in dynamic images are discussed.

Female↗

PubMed1998

Perceptual grouping of strip dot patterns.

Grouping of one-dimensional (strip) dot patterns was investigated, using an adjustment procedure to evaluate the distance (adjusted gap) at which two patterns appear to merge in a single strip. The size of the patterns had only a scaling effect on the adjusted gap. The adjusted gap and the variance of the estimates were influenced by the number of dots and the regularity (similarity, symmetry) of the patterns in the pair. Models of proximal grouping failed to account for the results obtained. A two-stage process implying determination of dot location and evaluation of inter-dot separation by means of "eclectic units" (Morgan, Hole & Glennerster, 1990) was considered to explain the findings.

Adult↗

PubMed1996

Different mechanisms may underlie the performance in relative localization tasks.

Two different groups of subjects had to adjust two-dimensional stimuli, differing in size, shape and type (dot patterns or irregular contour figures), within a reference circle. The two groups performed under two different instructions. The first instruction stressed matching the centres of the stimulus and the circle, while the second required simply positioning the test stimulus in the middle of the reference circle. In two control experiments the subjects had to determine the position of the centres of each stimulus and of the reference circle. Under the first instruction the accuracy of performance, estimated by the variance of the responses, depended on the stimulus size, shape and type in agreement with previous results and models of relative localization. Under the second instruction, however, accuracy remained invariant. Possible mechanisms of relative localization that might differ at their first stages of localization of the separate stimuli are considered.

Adult↗

PubMed1993

Perceived location of two-dimensional patterns.

The study attempted to test the possibility that the center of gravity of two-dimensional patterns is the cue used by a human observer for their localization. Four experiments were carried out. The first, using a matching procedure, required the localization of the center of irregular dot patterns, contour and filled polygons which varied in size and orientation. In the other three experiments the subjects had to point to briefly exposed dot patterns in which overall shape (convex and concave in Expts 2 and 3) and dot density (Expt 4) were manipulated. The performance of these direct localization tasks was found to be as accurate as the performance in previous studies of indirect localization or regular patterns. The results consistently supported the claim that information about position of the center of gravity is used for the localization of visual objects.

Adult↗

PubMed1993

The relative roles of vertices and sides in determining perceptual centres within shapes.

In the main experiment, subjects estimated the centre of five polygons in three variants: whole contour, only sides, or only corners. The perceived centre was closer to the area barycentre than to any other mathematical centre. Errors decreased as the number of sides of the figures increased. Larger mean deviations and variability were observed for the corner-only variant. Errors did not differ between the other two variants. However, this could be due to the fact that in the side-only variant only a small portion of the sides was erased, making in this way this variant to be similar to the complete contour. To exclude this possibility, in a Control Experiment side-only figures with different lengths erased around the corners were used. Since there was no worsening of the performance with longer erasings, it is concluded that, in finding the centre, the visual system relies mainly on the information provided by the sides, while a role of the corners is not evident.

Form Perception↗

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↗

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↗

PubMed1985

Factors influencing the response time in extrapolation of motion studies.

The linear dependence between the response time and the concealment time in prediction of motion studies is confirmed. The parameters of the linear relationship could be considered as individual characteristics of the subjects. Our and other experimental results show that the parameters of the linear relationship could be manipulated selectively. Factors, which influence velocity perception and the subject's knowledge of the results, change mainly the slope value. Factors, changing the concealment time uncertainty, e. g. the moment of target presentation, affect the intercept value. It is supposed that the slope value reflects the subjective velocity scale and that the intercept value is related to the individual peculiarities of the subjects in organization and execution of a response in a definite moment.

Humans↗

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↗

PubMed1982

Half-field asymmetry in visual perception of distance.

Distances were presented to the right and left halves of the visual field during steady fixation of a central point. Two methods of evaluation were used: direct scaling and the method of limits. A small but statistically significant difference was found between the estimates of distances presented to the left and to the right of the fixation point by direct scaling. No difference was found when the method of limits was used. The results are interpreted as evidence for the existence of functional asymmetry connected with the computation-and-speech ability of the hemispheres. No evidence for perception asymmetry could be found.

Adult↗

PubMed1981

Visio-motor prediction of collision.

Visually presented moving stimuli were used in a collision prediction experiment aimed at revealing the ability of human subjects to estimate the velocity of motion. Spatial extent, velocity and exposure duration of movement were varied so that the correlation between these characteristics of the stimuli was low (about 0.5). The moving stimulus disappeared at a certain place on the screen. By pressing a button the subject had to estimate the moment the stimulus would reach a target in the visual field. A linear dependence between estimated and actual duration of concealed motion was obtained. It is concluded that spatial extent and duration of motion do not influence the performance of the task. Velocity estimation and individual characteristics of the mechanisms controlling the motor response are supposed to determine the linear dependences obtained for all subjects tested.

Accidents↗

PubMed1981

Scaling spatial extent, velocity and duration of motion.

Experiments on magnitude estimation of spatial extent, velocity and duration of 11 moving stimuli which had low correlations between the stimulus attributes revealed psychophysical curves of different slopes and shapes for the three attributes of motion. A psychophysical power function with an exponent almost equal to 1.0 described the magnitude estimation of distance. Deviations in velocity judgments were observed with shortening the spatial extent of motion. Duration estimation determined by moving stimuli was found to differ significantly from the estimation of time intervals of the same range presented with stationary light stimuli (Marinova, Mitrani, Yakimoff, 1980). The results indicate that velocity cannot be regarded as a directly perceived attribute of motion. Distance only seems to be directly perceived.

Distance Perception↗

PubMed1981

Estimating the velocity of briefly presented moving stimuli.

The ability of human subjects to estimate the velocity of briefly presented moving stimuli was studied in a prediction-of-collision experiment. After the disappearance of the moving target the subject had to press a button at the moment the target would reach a predetermined position in the visual field. Four velocities of the moving target were used -6 degrees/s, 10 degrees/s, 14 degrees/s, and 20 degrees/s. For each velocity there were five exposure distances: 0.5, 1.0, 2.0, 3.0 and 4.0 degrees and four concealment distances: 6, 8, 10 and 12 degrees. For the large exposure distances a linear dependence of the response time on the concealment time was observed. This dependence was not linear for exposure durations less than 100 ms. The response time increased in these cases. Despite the common character of the results obtained with different subjects there were clear individual differences most probably due to differences in subjective velocity scales and individual mechanisms of motion extrapolation and organization of motor responses.

Humans↗

PubMed1980

Some temporal characteristics of eye tracking.

The frequency of occurence and the latency of eye tracking determined by moving stimuli of different velocities (5, 10 and 15 degrees/s) and different durations (between 50 and 400 ms) were measured. The result was that eye movement can occur only if the duration of the stimulus exceeds a critical value. This value depends on the stimulus velocity. The difference between the latency of the movement and the critical duration of the stimulus however remains constant. This difference is interpreted as the time necessary for selecting and releasing the appropriate programme for eye movement.

Adult↗