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.

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.

Vasopressin- and oxytocin-immunoreactive nerve cells in the aging rat hypothalamus.

Immunohistochemistry and morphometry were used to study the age-related changes in the vasopressin (AVP) and oxytocin (OXT) nerve cells in the paraventricular (PVN), supraoptic (SON) and suprachiasmatic (SCN) nuclei of 3-, 11- and 28-month-old rats. The results showed a statistically significant reduction in the mean number of AVP cells in the PVN, SON and SCN, and of OXT cells in the PVN with advancing age. Different age-related changes in the mean size of the immunoreactive cells were found in the three nuclei: a significant and transitory increase in the AVP and OXT cell sizes in the PVN, a tendency towards increasing the AVP and OXT cell sizes in the SON, and a significant and gradual decrease in the AVP cell size in the SCN. The combination of the morphometric data and staining patterns of the AVP and OXT perikarya and fibers in the PVN and SON pointed to an increased transport of AVP and OXT in 11-month-old rats as well as to a decreased production of these peptides in the PVN of 28-month-old rats. Taken together the staining pattern and the morphometric results showed a progressive loss of AVP cells in the SCN in aging.

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.

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.

[The accuracy of saccadic eye movements is associated with their horizontal and vertical direction]. / Presnost' sakadických pohybov ocí závisí od ich horizontálneho a vertikálneho smeru.

Electro-oculagraphic, photo-oculographic, and magnetoelectro-oculographic methods were used to study and to compare the accuracy of 10 degrees saccadic movements of the eyes in dependence on their direction to the right, to the left, upwards and downwards. Leftward saccades in right-handers and rightward saccades in left-handers proved to be more accurate compared to saccades in the opposite direction. This finding may be related to the functional asymmetry of the cerebral hemispheres. Downward saccades are inaccurate in comparison to upward saccades. Their inaccuracy is caused by overshooting the target. In the case of vertical saccades, the existence of different generators in the brain for triggering upward and downward saccades may play a role, along with a potential preprogramming of the change in the downward look into a sequence of two saccades. The obtained results emphasize the requirement of calibrating the electro-oculogram by saccades in that direction in which the analyzed eye movements are recorded.

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.

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.

Matching the orientation of dot patterns with real, interpolated, and extrapolated lines.

The perception of the orientation of random-dot patterns was studied using four different matching tasks. Homogeneous, elongated patterns and patterns containing Moiré effects were used. One of the tasks implied linear extrapolation and two others implied linear interpolation of the matching line. The fourth task was identical with those used in previous studies by the authors on this topic. Systematic deviations from the axes of orientation of the patterns were observed for the latter task when compared with the former ones. When a short matching line, implying linear extrapolation, was used performance by subjects tended to be more inaccurate than in the other matching tasks. The linear interpolation tasks, in which the matching line was determined by either two collinear distant short lines or by two distant dots, provided more accurate and stable performance than the other two tasks. The results are discussed from the point of view of global orientation perception derived from an image function of the stimuli.

Models for the perception of orientation in random dot patterns.

The aim of this paper is to summarize and to compare some known mathematical models of orientation perception in random dot patterns and to propose new solutions of this question. The model adequacy is judged from the previously obtained experimental results. Apart from the models based on some simple function of the coordinates of dots forming a pattern, also models derived from the so-called image function of the pattern are analysed. The latter ones were found more flexible to render the different features of experimentally obtained data, mainly in the case of orientation ambiguity for some special patterns. The stochastic variants of the deterministic models are introduced.

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.

Adaptation of suprathreshold contrast and luminance stimuli.

The temporal and spatial properties of the difference in perceived contrast and brightness of two suprathreshold stimuli presented successively in different retinal locations were determined. Stimulus onset asynchrony (SOA) was varied and the perceived contrast or brightness of the first stimulus (S1) was measured as a function of SOA by matching the contrast or luminance of the second stimulus (S2) to that of S1. The two stimuli overlapped in time for 200 ms to allow the comparison to be made. The adjusted values for S2 could well be fitted with an exponential decay function of SOA. For luminance increments and decrements the time constant for this function was 253 ms; for checkerboards with checks of size 16 min square the time constant was 164 ms. The difference in perceived contrast was dependent on initial contrast in a nonlinear fashion. It increased with increasing check size and was independent of the mean luminance and spatial proximity of the two stimuli. The phenomenon was observed with different pattern types and with dichoptic presentation, but could only be seen when direct comparison of the two stimuli was possible.

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.

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.

Estimating the duration of gratings under masking and nonmasking conditions.

Previous research has shown that the subjective duration of very short time intervals decreases with the increase of spatial frequency (SF) and this effect is supposed to be mediated by an early peripheral component of the persistence process called "retinal persistence". As the retinal components of persistence are very likely to be erased by a mask, we suggest that if retinal persistence underlies SF influence on duration estimation, the introduction of a mask would make subjective duration independent of SF. This inference was tested estimating the duration of presentation of square-wave gratings of equal mean luminance and contrast (85%) and of varying SF under two conditions: with and without a mask. Subjective duration decreased with the increase of SF under both experimental conditions It was concluded that retinal persistence does not underlie the SF effect on brief duration perception. Rather, the apparent contrast of the gratings covarying with SF could account for SF influence on perceived duration.

Influence of local orientation cues on estimating the tilt of two-dimensional patterns.

Ten subjects estimated the orientation of elongated random patterns composed of dots or short line segments. The estimated overall orientation of the patterns was influenced by the local orientation of the composing lines. Thus, when they formed an angle of 5.7 deg with the orientation of the least-squared-distances axis (LS-axis) of the pattern there was an effect of 'attraction', and when this angle was 22.9 deg, an effect of 'repulsion'. Estimated orientation coincided with the orientation of the LS-axis for angles of 11.5 and 45.8 deg, and when the patterns were composed of dots. The results point to interactions between different mechanisms involved in orientation perception.

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.