Search PubMed⌕ Search

Biomedical subjects

S Mateeff

Publications and source records attributed to S Mateeff.

At least 19 recordsLinked to original sources
PubMed2001

Speed decrements are seen better than increments through small apertures.

Five subjects observed a random dot pattern that moved horizontally within a rectangular, 40 deg x 4 deg invisible aperture. The long side of this aperture was either parallel or perpendicular to the direction of motion. A two-interval forced-choice procedure was employed for measuring the thresholds for detection of 100-ms speed increments and decrements superimposed on uniform motion of 1.1 s duration and base speeds of 8 deg/s, 16 deg/s, 24 deg/s, and 32 deg/s. The aperture orientation had almost no effect on the detection of decrements. With increasing base speed, the thresholds for increments within the perpendicular aperture rose markedly as compared to those for increments within the parallel aperture and to those for decrements. The results are interpreted in the context of facilitatory interactions between the motion detectors along the trajectory of motion of each individual dot. It is shown how, having rapid motion within the perpendicular aperture, increment detection can severely deteriorate owing to the short lifetime and the small visible path of the dots, whereas decrement detection may be almost unaffected.

Humans↗
PubMed2000

Mechanisms of simple and choice reaction to changes in direction of visual motion.

Experiments are presented in which a random dot pattern moved vertically upwards (velocity vector V(1)) and then abruptly changed its direction of motion by the angle alpha (velocity vector V(2)), either to the left or to the right, without changing the speed. Subjects performed simple reactions to the direction change, disregarding its sign. In another experiment choice reactions to the same stimuli were performed: the subjects pushed a left button when the direction change was to the left and a right button when the change was to the right. The simple reaction time decreased monotonically with alpha increasing from 11 degrees to 169 degrees, whereas, within the same range of angles, a U-shaped curve described the function of the choice reaction time versus alpha. Both types of reaction time increased with decreasing the base speed. Difficulties are outlined which occur when the angle of change alpha is considered as 'intensity' of the stimulus. Instead, the parameter mid R:V(2)-V(1)mid R:, the absolute value of the difference between the velocity vectors before and after the change, is shown to be a meaningful 'intensity' parameter for the simple reaction task. The parameter V(2N), the speed of the velocity component normal to the initial velocity vector V(1), is suggested as an 'intensity' parameter for the choice reaction task. It is shown that the simple and choice reactions to changes in direction of visual motion are performed by two distinct mechanisms which seem to work in parallel and may be nearly equally fast for small angles of change, when mid R:V(2)-V(1)mid R: approximately V(2N).

Adult↗
PubMed2000

The discrimination of abrupt changes in speed and direction of visual motion.

A random dot pattern that moved within an invisible aperture was used to present two motions contiguously in time. The motions differed slightly either in speed (Experiments 1 and 3) or in direction (Experiments 2 and 4) and the subject had to discriminate the sign of the change (e.g. increment or decrement). The same discrimination task was performed when the two motions were temporally separated by 1 s. In Experiments 1 and 2 discrimination thresholds were measured with motion durations of 0.125, 0.25, 0.5 and 1.0 s and mean speeds of 2, 4, 8, and 16 degrees/s. In Experiments 3 and 4 thresholds were measured with aperture widths of 5 and 20 cm. The discrimination of contiguous motions progressively deteriorated with decreasing duration and mean speed of motion. For the lowest value of duration the Weber fraction for contiguous speeds was more than three times as the Weber fractions for separate speeds. For the same low value of duration the thresholds for discrimination of direction of contiguous motions were only about 50% higher than the thresholds for separate motions. The Weber fraction for contiguous speeds was ca. three times higher with the smaller aperture than with the larger one, provided the ratio 'aperture width mean speed' (i.e. the lifetime of the moving dots) was less than 0.3 s. Aperture width did not affect the discrimination of direction of contiguous motions. The discrimination of contiguous motions is discussed together with the known data for detection of changes in speed and direction. It is suggested that both, detection of changes in speed and discrimination of the sign of speed changes, may be performed by a common visual mechanism.

Adult↗
PubMed1999

The simple reaction time to changes in direction of visual motion.

Recently Dzhafarov et al. presented a model explaining data on simple reaction time (RT) to unidimensional velocity changes. The authors suggested that having a motion with an initial velocity V0, the velocity change detection system is reinitialized by means of a "subtractive normalization" process. Therefore, any abrupt change from V0 to V1 is detected as if it were the onset of motion with a speed equal to /V1-V0/. They derived that the RT is a function of /V1-V0/(-2/3). We tested this model for the case of two-dimensional velocity changes. Our subjects observed a random dot pattern that moved horizontally, then changed the direction of motion by an angle alpha in the range between 6 degrees and 180 degrees without changing the speed V. Speeds of 4 and 12 deg/s were used. The subjects reacted as quickly as possible to the direction change. The RTs asymptotically decreased with increasing alpha; with 12 deg/s speed the RTs were shorter than those obtained with 4 deg/s. It was shown that the data can be well described as a function of /V1-V0/(-2/3)=(2Vsin(alpha/2))(-2/3). An extension of the "subtractive normalization" hypothesis for the case of two-dimensional velocity changes is proposed. It is based on the assumption that the velocity vector V1 after the change is decomposed into two orthogonal components. Alternative explanations based on the use of position or orientation cues are shown to contradict the data.

Humans↗
PubMed1998

The time it takes to detect changes in speed and direction of visual motion.

We studied the ability of human observers to detect abrupt changes in velocity of motion of a random dot pattern. The pattern moved horizontally for 0.9 s at velocity V0, then changed to V1 either in speed, or in direction for a time T and returned to the initial motion. The threshold duration for detection of the change was measured for initial speeds of 2, 4, 8 and 16 deg/s. The time to detect a velocity reversal was equal to that for detection of an increase in speed by a factor of three. The time to detect an abrupt cessation of motion was equal to the time for detection of an increase in speed by a factor of two. The time to detect a direction change, the speed being constant, decreased gradually with increasing angle between V0 and V1 from 12 to 180 degrees and with increasing V0; the detection time was a function of (V1-V0) almost independent of the value of V0. This finding supports the hypothesis of Dzhafarov et al. (Percept Psychophys 1993;54:373-750), that the visual system effectively reduces the detection of velocity changes (from V0 to V1) to the presumably more simple detection of a motion onset, from 0 to (V1-V0). The characteristics of the detection process in the cases of uni- and two-dimensional velocity changes are discussed.

Analysis of Variance↗
PubMed1996

Perception of visual motion with modulated velocity: effects of viewing distance and aperture size.

Subjects observed a random dot pattern that moved horizontally with modulated velocity within an invisible aperture. The velocity contrast, (V2-V1)/V1, was 2/3. Two different percepts occurred while observing this stimulus. At lower modulation frequencies, between 2 and 12 Hz, velocity changes were clearly seen; this percept is called "motion irregularity". At frequencies higher than 20 Hz velocity changes were no longer visible; the moving pattern appeared to be divided into stationary columns of different luminance. We call this percept "pattern irregularity". The critical frequency for detection of motion irregularity was independent of viewing distance; it was an inverted U-shaped function of the linear rather than the angular mean velocity of the pattern. At higher mean velocities the critical frequency increased with increasing aperture size; at lower mean velocities it was not affected by the size of the aperture. It is shown that detection performance is a function of the relative velocity of the pattern, i.e. of the ratio between the mean velocity in deg/sec and the aperture size in deg. Pattern irregularity could be detected at modulation frequencies even above 100 Hz. The critical frequency increased with increasing velocity and with decreasing viewing distance. It is suggested that detection of motion irregularity is determined by two distinct processes that are based on spatial analysis of motion at low relative velocities and temporal analysis at high relative velocities; both processes provide constancy of detection performance regardless of viewing distance. On the other hand, pattern irregularity seems to be detected on the basis of an analysis of the retinal luminance distribution at high modulation frequencies.

Adult↗
PubMed1996

What determines the detection of changes in motion velocity? A comment on Dzhafarov, Sekuler, and Allik (1993)

We comment on a recent model aimed at explaining data on speed of reaction to motion onset and to changes in motion velocity. The model is based on calculating the running variance of the stimulus positions passed during the motion. We show that although the model is successful in explaining data on motion onset and suprathreshold velocity changes, it may not be able to explain data on time of reaction to changes in velocity when these are near the detection threshold.

Acceleration↗
PubMed1995

Temporal thresholds and reaction time to changes in velocity of visual motion.

A random dot pattern moved at a velocity V1. The velocity then increased or decreased abruptly to another value V2 for some time and again returned to V1. The temporal threshold, i.e. the duration of V2 that was necessary to detect the change was measured. Thresholds for the detection of the same velocity increment, V2 = 2 x V1, were shorter when the baseline velocity V1 increased from 1 to 8 deg/sec (Expt 1). The temporal threshold decreased as the velocity contrast (V2 - V1)/(V1 + V2) increased from 0.33 to 0.77. The thresholds for the detection of velocity decrements were in general longer than those for the detection of increments (Expt 3). In Expts 2 and 4 the random-dot pattern moved with velocity V1, which abruptly increased or decreased to V2, without returning to V1. The reaction time to the change was measured for the same velocity pairs as those used in the temporal threshold measurements. There was a good correspondence between changes in the reaction times and changes in the thresholds under the various conditions. The data are interpreted on the basis of two hypotheses: higher velocities are detected by mechanisms that respond more rapidly; and integration of velocities occurs when temporally-adjacent motions are presented.

Adult↗
PubMed1993

Multi-aperture viewing: perception of figures through very small apertures.

A new phenomenon of viewing objects through multiple apertures is reported. When a solid figure is hidden behind a sieve of very small apertures (pinholes), only its general shape may be perceived. Outline figures may be entirely invisible behind the sieve. Motion of the figure markedly improves the visibility of the outline figures and of the small details and edges of the solid figures. Two experiments are presented. The first demonstrates that the size of a dark stimulus moving behind a rear-illuminated row of holes is perceived with an accuracy that may be considerably better (i.e. the error is smaller) than the interhole distance. The results of the second experiment show that the visibility of an outline figure, a ring, is significantly better when the figure smoothly moves than when presented at random positions behind a two-dimensional sieve. Changing the velocity within the range of 5.18-10.36 deg/sec does not affect the visibility of the figure while it moves; however, a lower rate of discrete presentation leads to significantly better visibility. An explanation of the phenomenon is presented in terms of integration of information about the relative positions of the covered/uncovered holes, with a possible involvement of motion analyzing mechanisms.

Adult↗
PubMed1991

Selective directional sensitivity in visual motion perception.

We present two experiments demonstrating that: (i) the latency of perception of the position of a small visual target moving towards the fovea is shorter than that of the same target moving away from the fovea; (ii) the reaction time (RT) to onset of motion of the same type of target is also shorter when it moves towards the fovea; and (iii) the RT to onset of motion away from the fovea may be shorter when larger, textured stimuli are employed. The relation of the findings to the existence of two systems for visual motion information processing and to recent neurophysiological findings is discussed.

Adult↗
PubMed1991

Perceptual constancy during ocular pursuit: a quantitative estimation procedure.

Perceptual constancy of visual motion is usually described as the degree of correspondence between physical and perceived characteristics of motion in the external world. To study it, one has to assess the relationship between physical motion, its retinal image, and its perception. We describe a quantitative estimation procedure for a measure K denoting the degree of perceptual constancy of background target motions noncollinear to the eye movements during ocular pursuit. The calculation of K is based on three vectors describing the target motion (1) as it is physically, (2) as it is mapped to the retina, and (3) as it is perceived, but only the direction of the perceptual motion vector has to be determined experimentally. K allows for quantitative comparison between experiments with a variety of parameters in visual motion displays.

Eye Movements↗
PubMed1990

Visual localization and estimation of extent of target motion during ocular pursuit: a common mechanism?

The ability to localize a visual target and to estimate the distance through which it moves was studied during ocular pursuit. In the first experiment observers had to localize the position of a visually tracked moving target when they heard an acoustic signal. The signal was sounded near the beginning or near the end of the motion. The distance between the perceived positions was shorter than the distance between the corresponding physical positions of the target. The 'shortening' became more pronounced with higher tracking velocity. In another condition the observers estimated the length of the motion path between two successive sound signals, one presented near the beginning and one near the end of the motion. The length of path travelled was underestimated, the effect being stronger with higher tracking velocity. In the second experiment this effect of velocity on the underestimation of distance was shown to exist only during ocular pursuit and not during steady fixation. The hypothesis that localization and estimation of distance during ocular pursuit share a common mechanism is discussed.

Acceleration↗
PubMed1989

The role of the adjacency between background cues and objects in visual localization during ocular pursuit.

Subjects used eye movements to pursue a light target that moved from left to right with a velocity of 15 deg s-1. The stimulus was a sudden five-fold decrease in target intensity during the movement. The subject's task was to localize the stimulus relative to either a single stationary background point or the midpoint between two points (28 deg apart) placed 0.5 deg above the target path. The stimulus was usually mislocated in the direction of eye movement; the mislocation was affected by the spatial adjacency between background and stimulus. When an auditory, rather than a visual, stimulus was presented during tracking, target position at the time of stimulus presentation was visually mislocated in the direction opposite to that of eye movement. The effect of adjacency between background and target remained the same. The involvement of processes of subject-relative and object-relative visual perception is discussed.

Adult↗