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At least 1,477 records · Page 82Linked to original sources
PubMed1992

Optical magnification as event information.

The geometrical optics of approach events is delineated. It is shown that optical magnification provides information about distance and time until collision. An experiment is described in which two objects--white styropor spheres 10 cm in diameter, seen against a white plaster wall--were moved simultaneously at equal, constant speed along straight, converging paths at eye level towards a human observer and towards a common, virtual point of collision which either coincided with the observer's station point or was placed in front of, or behind, that point. Approach events differed with regard to trajectories, distances, velocities, and times-to-collision involved. Events were observed monocularly fixating and binocularly non-fixating, without head movements. The objects always stopped before colliding, and subjects had to respond to the virtual collisions. Most responses were too early, especially for impending collisions at, or behind the observers' station point. Responses for impending collisions in front of the observers tended to be too late, especially for larger total amounts of optical magnification and higher velocities, which together imply shorter times-to-collision. Relative errors were comparatively larger for very short and very long times-to-collision throughout, where events of the first kind were overshot, the latter ones undershot. Results are interpreted with reference to biological theories and the constraints imposed by geometrical optics. Special attention is focused on the issue of unavoidable, necessary confounding of variables in time-to-collision studies.

Acceleration↗
PubMed1992

Apparent distance reduction with moving stimuli (Tandem Effect): evidence for an attention-shifting model.

When two vertical rods move through a horizontal window in close succession, the Tandem Effect can be observed. It consists of a spatial illusion (distance between the rods looking smaller than it actually is) and a temporal illusion (under certain conditions both rods are seen simultaneously in the window, though the first rod has left the window before the second rod enters it). We report six experiments that explored the distance-reduction illusion and tested an attentional model of the effect. It assumes that attention is initially focused on the first rod and then shifted to the second, when it enters the window. The percept of the pair of rods is integrated from the first rod's position at the beginning, and the second rod's position at the end, of the focus shift. Consequently their subjective distance will be smaller than their physical distance by the distance that they travel during the focus shift. Experiments 1 and 2 established the Tandem Effect as an empirical phenomenon and showed that its size depends on stimulus parameters such as window size and movement speed. Experiments 3-5 tested specific predictions from the attentional model. Experiment 6 examined a further illusion, the Fröhlich Effect, and showed that it can be subsumed under the model. The experiments produced some unexpected effects and some predictions from the model were only partly confirmed. It is shown that the main findings can be combined into two quantitative functions that describe the course of focusing. One implication is that visual attention does not "move" from one object to another; rather all attention shifts originate in the fovea. We discuss several alternative interpretations of our data and show that they are less satisfactory than the attentional model.

Adult↗
PubMed2006

Influence of the Pulfrich phenomenon on driving performance.

BACKGROUND: The Pulfrich stereoillusion occurs spontaneously in diseases inducing asymmetric visual pathway delays. Its influence on driving performance has never been investigated and was, therefore, assessed using a three-dimensional (3D) computer driving simulation. METHODS: A 3D driving scenery of a road with obstacles was visualised on an autostereoscopic 3D display. Seven normal subjects drove at a speed of 6 m/s using a steering wheel and three angles of view of the scenery (0 degrees, 45 degrees to left, and 90 degrees to left) with different interocular delays (25 ms on the right, 25 ms on the left, and no delay). One subject drove the scenery at an angle of 90 degrees without delay and with a delay of 8 ms, 16 ms, and 25 ms on the right and left, respectively, at speeds of 6 m/s, 12 m/s and 18 m/s. RESULTS: Stereo-illusion only influenced car position if the angle of view was 90 degrees (p<0.05). At this angle, increasing car speeds were associated with larger car displacements (delay on right p<0.001, on left p<0.01) and smaller delays with smaller car displacements (p<0.001). CONCLUSIONS: This study showed that Pulfrich phenomenon has an influence on car position only if the viewing angle is 90 degrees. No influence could be found if the driving direction corresponded to the visual axis of the driver. These findings are in agreement with reports of patients with spontaneous Pulfrich phenomenon who indicate that while driving, distances are only misjudged when looking sideways.

Adult↗
PubMed1981

Detection and discrimination of moving gratings.

Two techniques are described which have been used to investigate mechanisms underlying the perception of velocity. The first, subthreshold summation establishes detection thresholds of two gratings of different spatial frequencies moving at the same velocity and also the detection thresholds of combinations of these gratings. The second technique employed a detection/discrimination procedure which establishes the discriminability of two different moving gratings at detection threshold. These experiments suggest that at low velocities the detection of 2 and 6 cycle/degree gratings is mediated by independent mechanisms, this independence being diminished at higher velocities. Gratings of 2 cycles/degree moving at different velocities are only discriminably different at detection threshold when widely separated in velocity.

Differential Threshold↗
PubMed1981

Direction selectivity in human visual perception, investigated with low contrast gratings.

Velocity thresholds (VT's) of direction selective mechanism were measured with coarse low contrast gratings, visible only when moving. direction of movement was detected only in a movement perception domain (MPD), limited by upper and lower VT's and a minimum exposure time of movement. MPD expanded with increasing contrast or increasing stimulus area. MPD shifted towards higher velocities with lower spatial frequencies or with larger stimulus eccentricities. Form perception in MPD was best at optimal velocities; the spatial structure of the gratings was not evident at the VT level. These observations suggest a velocity tuned sensitivity of direction selective mechanisms and an association of movement and form perception with the activity of movement analysing neurons.

Differential Threshold↗
PubMed1981

Saccadic programming and perceived location.

The retinal location of a saccadic target was made discrepant with its perceived position by means of an induced displacement. If localizing the target required information stored in memory, the eye was directed to the perceived target position. Otherwise, it was directed to its retinal location. These findings do not conform to either a strictly retinotopic or spatiotopic model of oculomotor control. Rather they suggest that the position information used to program saccades may be either perceptual or retinal/spatial depending upon whether the information must be accessed from memory.

Eye Movements↗
PubMed1981

Localization of brief visual stimuli during pursuit eye movements.

Experimental findings concerning the properties of the phenomenon of mislocation of brief visual stimuli during smooth eye tracking are described. One of these, which cannot be explained by existing hypotheses, is that under certain conditions the mislocation magnitude tends to have zero or even negative values. A model is developed for explanation of the mislocation phenomenon. It is suggested that localization is based on: (1) information about the current eye position and (2) information about the stimulus locus on the retina. They both arrive at the localization centre with non-zero delays. The mode of information processing in this centre leads to a magnitude of mislocation which is proportional to the difference between the two delays and which could be positive, zero or negative. Factors which influence either delay should also influence the mislocation magnitude.

Discrimination, Psychological↗
PubMed1981

The apparent path of a stationary and a circularly moving spot during the smooth pursuit of another circularly moving spot.

The apparent movement path (AMP) of a circularly moving spot pursued by the eyes (spot A) and of another stationary or circularly moving spot (spot B) were examined. Experimental variables were speed of rotation; number of rotations; position of the stationary spot (experiment) 1) and phase angle between rotation of spot A and B (experiment 2). The AMP of spot A was a spirally shrinking circular or elliptic path Both speed and number of rotations had an effect on the size of AMP of spot A. The AMP of spot B was in general a circle or an ellipse in some respects corresponding with the retinal locus of the image of spot B.

Attention↗