Stereo and motion cues in preattentive vision processing--some experiments with random-dot stereographic image sequences

Low-level preattentive vision processing is of special interest since it seems the logical starting point of all vision processing. Exploration of the human visual processing system at this level is, however, extremely difficult, but can be facilitated by the use of stroboscopic presentation of sequences of random-dot stereograms, which contain only local spatial and temporal information and therefore limit the processing of these images to the low level. Four experiments are described in which such sequences were used to explore the relationships between various cues (optical flow, stereo disparity, and accretion and deletion of image points) at the low level. To study these relationships in more depth, especially the resolution of conflicting information among the cues, some of the image sequences presented information not usually encountered in 'natural' scenes. The results indicate that the processing of these cues is undertaken as a set of cooperative processes.     

The binocular representation of uniform motion

In the model of motion perception proposed by Swanston, Wade, and Day (1987, Perception 16 143-159) it was suggested that retinocentric motion and eye movement information are combined independently for each eye, to give left and right orbitocentric representations of movement. The weighted orbitocentric values are then added, to give a single agocentric representation. It is shown that for a physical motion observed without pursuit eye movements this formulation predicts a reduction in the perceived extent of motion with monocular as opposed to binocular viewing. This prediction was tested, and shown to be incorrect. Accordingly, a modification of the model is proposed, in which the left and right retinocentric signals are weighted according to the presence or absence of stimulation, and combined to give a binocular retinocentric representation. In a similar way left-eye and right-eye position signals are combined to give a single binocular eye movement signal for version. This is then added to the binocular retinocentric signal to give the egocentric representation. This modification provides a unified account of both static visual direction and movement perception.     

A microgenetic study of the Müller-Lyer illusion.

In two experiments a decomposed Müller-Lyer pattern was used to measure the time course of the illusion. A partial report procedure was used to prevent the subjects from focusing only on parts of the pattern and to maximize visual processing. The Müller-Lyer figure was decomposed into two parts, its angles and its line. A configuration of three pairs of angles, each corresponding to a row in the usual partial report arrangement, was used. A line that did or did not fit the gap was shown with a variable delay (interstimulus interval, ISI). By this procedure the relevant row (line gap) was cued. The subject had to decide whether the line fitted or was too short/long. Two exposure times for the angles were used, 50 or 200 ms in one experiment and 50 or 500 ms in the other. The result of the first experiment, with outward-pointing fins, was a stable illusion for all values of ISI (25-400 ms) and all exposure times, with one significant exception: exposure for 50 ms with an ISI of 50 ms yielded an illusion peak. It was shown that this was not caused by a reduction in length-discrimination performance. In the second experiment, with inward-pointing fins, no such peak occurred. There was only a tendency for the illusion to vanish with zero ISI. The results are discussed with respect to 'global to local' theories of visual processing.     

Colour changes as a function of luminance contrast.

When spectral light increases in luminance, the hues change. Normally, long-wavelength light becomes increasingly yellow, and short-wavelength light turns blue or blue-green. This is known as the Bezold-Brücke hue shift. Less notice has been paid to the change in relative chromatic content (saturation or chromatic strength) that accompanies these shifts in hue. As luminance contrast increases from zero, chromatic strength increases to reach a maximum at a luminance that is wavelength dependent. Short-wavelength blueish light reaches this maximum at low relative luminances, whereas midspectral yellowish stimuli need several log units higher luminance. Red and green are somewhere in between. For luminances above this maximum, the chromatic content usually diminishes, and most light becomes more whitish in appearance. In this study it is demonstrated how the combined chromatic appearance of hue and chromatic strength change with intensity. Both phenomena find a common physiological interpretation in the nonlinear and nonmonotonic responses of colour-opponent P cells in the retina and lateral geniculate nucleus of the primate. A model that combines the outputs of six P-cell types accounts for observers' estimates of hue and chromatic strength.     

Imitation in rats: Initial responding and transfer evidence

In two experiments, rats observed a conspecific demonstrator pushing a single manipulandum, a joystick, to the right or to the left for food reward before being given access to the joystick, for the first time, from a different orientation. In Experiment 1, rats that had observed left-pushing made more of their first three responses to the left than did rats that had observed right-pushing. In Experiment 2 the axis of joystick movement was rotated through 90° between observation and testing for half of the animals. These rats, like those that were tested with the joystick in the position it had occupied during demonstrator responding, showed a significant tendency to push the joystick in the same direction relative to their own bodies as had their demonstrators. These results, which cannot be explained in terms of stimulus-reinforcer learning, provide evidence that rats are capable of imitation, i.e. response, or response-reinforcer, learning by observation.     

On the relation between feeling of knowing and lexical decision: persistent subthreshold activation or topic familiarity?

Experiment 1 replicated Yaniv and Meyer's (1987) finding that lexical decision and episodic recognition performance was better for words previously yielding high-accessibility levels (a combination of feeling-of-knowing and tip-of-the-tongue ratings) in comparison with those yielding low-accessibility levels in a rare word definition task. Experiment 2 yielded the same pattern even though lexical decisions preceded accessibility estimates by a full week. Experiment 3 dismissed the possibility that the Experiment 2 results may have been due to a long-term influence from the lexical decision task to the rare word judgment task. These results support a model in which Ss (a) retrieve topic familiarity information in making accessibility estimates in the rare word definition task and (b) use this information to modulate lexical decision performance.