Perception

Two hypotheses proposed as alternatives by Rock—frame of reference and hierarchical organisation of perception—were tested in a series of experiments with the use of the rod-and-frame illusion. This illusion produces errors in the apparent vertical due to the presence of a tilted frame surrounding the test rod. The apparent vertical is shifted in the direction of the frame tilt. When an upright square was added inside the tilted frame, rod-setting errors varied according to the visual characteristics of the display. In the case of a large display presented in the dark (experiment 1), there continued to be large errors in the direction of the outer-square tilt. This finding supports the frame-of-reference hypothesis, which proposes that the orientation of all objects in the visual field is dominated by the most peripheral reference. In the case of a small display presented in a lit environment (experiments 2 and 3), the direction of errors was the opposite. This latter finding was taken to indicate that the rod was set with reference to the perceived tilt of the inner upright square. Thus, according to a hierarchical-organisation hypothesis, the orientation of an object in the visual field is influenced by objects in the immediate surroundings not by outermost reference.

Overall, the results confirm the presence of two qualitatively different classes of orientational phenomena: one is concerned with the definition of egocentric coordinates and one with an object-centred visual representation.

The strength of the Poggendorff illusion has been determined by a nulling method for the classical as well as other configurations of the central inducing region. Compared to a uniform field, an inducing rectangle with very low contrast produces a marked illusion, which saturates at a Michelson contrast of about 0.1. With virtual borders of the Kanizsa type there is a weak illusion and this effect is attenuated when the ‘pacman’ sectors are occluded. Texture borders without luminance contrast induce a stronger illusion. These results are discussed in relation to earlier data for contrast dependence of Vernier acuity and for the orientation discrimination and tilt illusion with real and virtual borders.

We examined (i) perceived slant of a textured surface about a vertical axis as a function of disparity magnitude for horizontal-size disparity, vertical-size disparity, and overall-size disparity; and (ii) interactions between patterns with various types and magnitudes of size disparity and superimposed or adjacent zero-disparity stimuli. Horizontal-size disparity produced slant which increased with increasing disparity, was enhanced by superimposed zero-disparity stimuli, and induced contrasting slant in superimposed or adjacent zero-disparity stimuli. Vertical-size disparity produced opposite slant (induced effect) which was reduced to near zero by a superimposed zero-disparity pattern and both patterns appeared as one surface. Adjacent vertical-size-disparity and zero-disparity patterns appeared as separate surfaces with a wide curved boundary. Overall-size disparity produced slant which was enhanced by a superimposed zero-disparity pattern and less so by a zero-disparity line, and induced more slant in a zero-disparity line than in a zero-disparity pattern. The results are discussed in terms of depth underestimation of isolated surfaces, depth enhancement, depth contrast, and the processing of deformation disparity.

This paper demonstrates the use of active fixation on both fixed and moving fixation points to guide a robot vehicle by means of a steering rule which, at large distances, sets the steering angle directly proportional to the deviation of gaze direction from translation direction. Steering a motor vehicle around a winding but otherwise uncluttered road has been observed by Land and Lee to involve repeated periods of visual fixation upon the tangent point of the inside of each bend. We suggest that proportional rule devised for steering in the robotic example appears applicable to the observed human performance data, providing an alternative explanation to the quadratic rule proposed by Land and Lee.

Can people with different forms of low vision use motion parallax to improve depth judgments? We used a staircase method to compare depth thresholds using motion parallax and static viewing. We tested eighteen normal-vision subjects with a range of simulated deficits in acuity, contrast sensitivity, and simulated peripheral-field loss, and ten low-vision subjects with a wide range of acuity, contrast sensitivity, and field loss. Subjects viewed three vertical cylinders monocularly and indicated which one was at a different depth from the other two. For motion-parallax trials, observers moved their heads (in a viewing assembly on rollers) from side to side over a range of 6 – 12 cm. For static trials, the viewing assembly was fixed in place. Normal-vision subjects' depth thresholds with motion parallax were significantly smaller than those with static viewing by an average factor of 1.95 (p < 0.05) across all levels of acuity and contrast. For low-vision observers, the depth thresholds exhibited large individual differences; however, the motion-parallax thresholds were smaller than the static thresholds by an average factor of 2.05 (p < 0.01). These findings indicate that motion parallax can provide useful depth information for people with low vision.

Boundary conditions for perception of biological motion were explored with the use of computer-generated point-light animation sequences. Perception of this unique form of structure from motion is immune to variations in dot contrast polarity, dot disparity, and spatial-frequency filtering. Biological motion is perceived in texture-defined animation sequences that presumably stimulate only second-order motion pathways, and it is undisturbed by dichoptic presentation of portions of the animation tokens separately to the two eyes.

Paradoxical absence of motion in a rotating pattern is reported. The effect requires that the motion of the paradoxically stationary figure be underspecified by local motion signals, and that the paradoxically stationary figure be well segregated. This is consistent with proposals that figural segregation affects the integration of local motion signals.

We examined whether faces can produce a ‘pop-out’ effect in visual search tasks. In the first experiment, subjects' eye movements and search latencies were measured while they viewed a display containing a target face amidst distractors. Targets were upright or inverted faces presented with seven others of the opposite polarity as an ‘around-the-clock’ display. Face images were either photographic or ‘feature only’, with the outline removed. Naive subjects were poor at locating an upright face from an array of inverted faces, but performance improved with practice. In the second experiment, we investigated systematically how training improved performance. Prior to testing, subjects were practised on locating either upright or inverted faces. All subjects benefited from training. Subjects practised on upright faces were faster and more accurate at locating upright target faces than inverted. Subjects practised on inverted faces showed no difference between upright and inverted targets. In the third experiment, faces with ‘jumbled’ features were used as distractors, and this resulted in the same pattern of findings. We conclude that there is no direct rapid ‘pop-out’ effect for faces. However, the findings demonstrate that, in peripheral vision, upright faces show a processing advantage over inverted faces.

Differential thresholds for viscosity were measured in ten subjects with the use of an adaptive two-alternative forced-choice procedure. An electromagnetic linear motor was connected to each wrist and the viscosity of the motors was under computer servo control. For each block of 50 trials the viscosity of one motor was fixed at a reference value which ranged from 4 to 512 N s m⁻¹, and the viscosity of the other motor varied according to the subject's responses. On each trial subjects were required to indicate which motor had the greater viscosity and were given feedback of the correct response. By this procedure the Weber fraction for viscosity was calculated to be 19%, which is lower than the Weber fraction of 34% estimated by using the method of adjustment. The criterion used for determining the threshold differs in the two procedures (71% and 84% correct, respectively), and the results from the two studies were found to be consistent. They suggest that the Weber fraction for viscosity remains remarkably stable despite differences in the methods of measurement.