Perception

Gestalt grouping rules imply a process or mechanism for grouping together local features of an object into a perceptual whole. Several psychophysical experiments have been interpreted as evidence for constrained interactions between nearby spatial filter elements and this has led to the hypothesis that element linking might be mediated by these interactions. A common tacit assumption is that these interactions result in response modulation which disturbs a local contrast code. We addressed this possibility by performing contrast discrimination experiments using two-dimensional arrays of multiple Gabor patches arranged either (i) vertically, (ii) in circles (coherent conditions), or (iii) randomly (incoherent condition), as well as for a single Gabor patch. In each condition, contrast increments were applied to either the entire test stimulus (experiment 1) or a single patch whose position was cued (experiment 2). In experiment 3, the texture stimuli were reduced to a single contour by displaying only the central vertical strip. Performance was better for the multiple-patch conditions than for the single-patch condition, but whether the multiple-patch stimulus was coherent or not had no systematic effect on the results in any of the experiments. We conclude that constrained local interactions do not interfere with a local contrast code for our suprathreshold stimuli, suggesting that, in general, this is not the way in which element linking is achieved. The possibility that interactions are involved in enhancing the detectability of contour elements at threshold remains unchallenged by our experiments.

Flank transparency is the perception of a colored transparent filter evoked by apparent-motion displays containing as few as two colors. Displays of flank transparency contain a random array of line segments placed on a uniform background. Small flanks are added to the line segments if the segments fall in the interior of a moving virtual shape, such as a virtual disk. This leads to the perception of a colored transparent disk with well-defined boundaries moving over the array of lines. Current qualitative and quantitative models of luminance and color conditions for perceptual transparency do not account for flank transparency as they require displays containing at least three different colors.

We compared the contribution and the effectiveness of modulating the orientation perception of two types of visual information: the visual frame and the visual polarity of objects. In experiment 1, we examined the effect of a square frame, a mouse, an elephant, and a map of France on the apparent vertical. In the upright position, despite the presence of tilted individual component features, the visual objects had no illusory visual tilt effects. When tilted, these objects had a substantial effect on the direction that appeared to be vertical. However, rod-setting errors were smaller in the inducing objects than when observed with the frame display. In the second experiment, the results of experiment 1 were replicated with a meaningful circular contour—a porthole and a clock. The presence of the external circular contour did not abolish the illusion on the apparent vertical. Moreover, in experiment 3, a clock whose numbers were displaced and not tilted—to avoid the possible tilt influence of visual cues—was also able to deflect the subjective visual vertical. This finding suggests that through top–down processing shapes can act as a framework which serves as a reference influencing the perceived orientation of the inner objects.

The effect of voluntary attention on afterimage fragmentation was explored in two experiments. The afterimage, in the form of a 30°-tilted star of David, was generated after prolonged steady fixation in the first experiment, and with a brief and intense flash in the second experiment. Subjects were instructed to select various target shapes in the afterimage for attention and, at the same time, observe what was visible or invisible. Verbal reports and manual responses to afterimage changes were analyzed. Attended shapes were found to disappear from awareness faster than unattended ones (experiment 1), and complementary shapes were found to predominate visual awareness when one of the pair was selected for attention (experiment 2). Voluntary attention was also found to affect closure (filling-in of enclosed regions) and smoothing of line figures in afterimages.

Ternus stimuli give rise to two mutually exclusive visual experiences: with long interstimulus intervals (ISIs) the elements in the stimulus are perceived as moving together as a group (‘group movement’), while at shorter ISIs only a single element appears to be moving (‘element movement’ or ‘end-to-end movement’). It has been hypothesized that group and element movements, respectively, reflect magnocellular and parvocellular activity. On this basis, Ternus tests have been used to assess magnocellular function in dyslexic individuals. This use of Ternus stimuli is examined in the present report. On the basis of amplitude spectra of the stimuli and of a review of previous studies it is concluded that to use Ternus tests to assess magnocellular function is problematic.

The visual-search paradigm provides a controlled and easy to implement experimental situation in which to study the search process. However, little work has been carried out in humans to investigate the extent to which traditional visual-search tasks are similar to more general search or foraging. Here we report results from a task in which search involves walking around a room and leaning down to inspect individual locations. Consistent with more traditional search tasks, search time increases linearly with display size, and the target-present to target-absent search slope is 1:2. However, although rechecking of locations did occur, compared to more traditional search it was relatively rare, suggesting an increased role for memory.

The pattern of motion in the retinal image during self-motion contains information about the person's movement. Pursuit eye movements perturb the pattern of retinal-image motion, complicating the problem of self-motion perception. A question of considerable current interest is the relative importance of retinal and extra-retinal signals in compensating for these effects of pursuit on the retinal image. We addressed this question by examining the effect of prior motion stimuli on self-motion judgments during pursuit. Observers viewed 300 ms random-dot displays simulating forward self-motion during pursuit to the right or to the left; at the end of each display a probe appeared and observers judged whether they would pass left or right of it. The display was preceded by a 300 ms dot pattern that was either stationary or moved in the same direction as, or opposite to, the eye movement. This prior motion stimulus had a large effect on self-motion judgments when the simulated scene was a frontoparallel wall (experiment 1), but not when it was a three-dimensional (3-D) scene (experiment 2). Corresponding simulated-pursuit conditions controlled for purely retinal motion aftereffects, implying that the effect in experiment 1 is mediated by an interaction between retinal and extra-retinal signals. In experiment 3, we examined self-motion judgments with respect to a 3-D scene with mixtures of real and simulated pursuit. When real and simulated pursuits were in opposite directions, performance was determined by the total amount of pursuit-related retinal motion, consistent with an extra-retinal ‘trigger’ signal that facilitates the action of a retinally based pursuit-compensation mechanism. However, results of experiment 1 without a prior motion stimulus imply that extra-retinal signals are more informative when retinal information is lacking. We conclude that the relative importance of retinal and extra-retinal signals for pursuit compensation varies with the informativeness of the retinal motion pattern, at least for short durations. Our results provide partial explanations for a number of findings in the literature on perception of self-motion and motion in the frontal plane.

Does thirst make you more likely to think you see water? Tales of thirsty desert travelers and oasis mirages are consistent with our intuitions that appetitive state can influence what we see in the world. Yet there has been surprisingly little scrutiny of this appetitive modulation of perception. We tested whether dehydrated subjects would be biased towards perceptions of transparency, a common property of water. We found that thirsty subjects have a greater tendency to perceive transparency in ambiguous stimuli, revealing an ecologically appropriate modulation of the visual system by a basic appetitive motive.