Perception

Unimodal auditory and visual apparent motion (AM) and bimodal audiovisual AM were investigated to determine the effects of crossmodal integration on motion perception and direction-of-motion discrimination in each modality. To determine the optimal stimulus onset asynchrony (SOA) ranges for motion perception and direction discrimination, we initially measured unimodal visual and auditory AMs using one of four durations (50, 100, 200, or 400 ms) and ten SOAs (40–450 ms). In the bimodal conditions, auditory and visual AM were measured in the presence of temporally synchronous, spatially displaced distractors that were either congruent (moving in the same direction) or conflicting (moving in the opposite direction) with respect to target motion. Participants reported whether continuous motion was perceived and its direction. With unimodal auditory and visual AM, motion perception was affected differently by stimulus duration and SOA in the two modalities, while the opposite was observed for direction of motion. In the bimodal audiovisual AM condition, discriminating the direction of motion was affected only in the case of an auditory target. The perceived direction of auditory but not visual AM was reduced to chance levels when the crossmodal distractor direction was conflicting. Conversely, motion perception was unaffected by the distractor direction and, in some cases, the mere presence of a distractor facilitated movement perception.

Sound and light take different amounts of time to reach their respective receptors, to be transduced, and to be transmitted to the cortex. Their processing times also vary with factors such as intensity and retinal eccentricity. We assessed the capability of subjects to perceive simultaneity correctly despite these variations. Temporal asynchronies of up to 200 ms were introduced between the components of sound/light pairs. Using the method of constant stimuli, seven subjects judged which came first. Distance, and hence the times of arrival of paired visual and auditory targets, was varied from 1 to 32 m. Visual intensity was varied by viewing the target through 1.8 dB attenuating glasses, and a retinal eccentricity of 20° was compared to central presentation. Despite large differences in reaction times, which varied in a predictable way with the stimulus parameters, the timing of sound/light pairings judged as simultaneous corresponded to when the light and sound left the source simultaneously. Almost complete compensation was found in all conditions tested, showing that these substantial but predictable variations in timing can be taken into account in creating simultaneity constancy.

Our previous research on auditory time perception showed that the duration of empty time intervals shorter than about 250 ms can be underestimated hugely if they are immediately preceded by shorter time intervals. We named this illusion ‘time-shrinking’ (TS). This study comprises four experiments in which the preceding interval, t₁, was followed by a standard interval, t₂. When t₁ ⩽ 200 ms, and t₁ ⩽ t₂, the underestimation of t₂ came into view clearly. The absolute difference between t₂ and t₁ was the crucial factor for the illusion to appear. The underestimation increased when t₂ increased from t₁ to t₁ + 65 ms, stayed at about 45 ms when t₂ was between t₁ + 65 ms and t₁ + 95 ms, and disappeared suddenly when t₂ exceeded t₁ + 95 ms. This pattern of results was observed across all values of t₁ ⩽ 200 ms. A model was fit to the data to elucidate the underlying process of the illusion. The model states that the perceived duration difference between t₁ and t₂ is reduced by cutting mental processing time for t₂; in other words, that t₂ assimilates to t₁.

In three experiments we tested the ability of children aged 8 to 12 years and adults to locate a target in an optic texture flow projected onto the ground. During the exposure phase, a static target (diode) was lit up at 6 m or 8 m on the ground in front of the subject. During the pointing phase, the subject was asked to indicate the perceived location of the target with a laser pointer as soon as the target was switched off. In the first experiment, during both phases the optic texture (environment) was either motionless or approaching the subject. Results showed that target locations were significantly more underestimated within the moving texture than within the still texture. In the second experiment, a detailed error analysis showed that the differences of performance between children and adults were not due to differences in eye height. Errors can be described by a linear fit with the retinal speed of the optic flow surrounding the targets. Distance judgments improved from the age of 8 years onwards. In the last experiment we found the same kind of results with a receding texture and without stimulation in central vision. Results are discussed in terms of subject's capacity to compensate for the effect of linear vection produced by the optic flow.

Junctions, formed at the intersection of image contours, are thought to play an important and early role in vision. The interest in junctions can be attributed in part to the notion that they are local image features that are easy to detect but that nonetheless provide valuable information about important events in the world, such as occlusion and transparency. Here I test the notion that there are locally defined junctions in real images that might be detected with simple, early visual mechanisms. Human observers were used as a tool to measure the visual information available in local regions of real images. One set of observers was made to label all the points in a set of real images where one edge occluded another. A second set of observers was presented with variable-size circular subregions of these images, and was asked to judge whether the regions were centered on an occlusion point. This task is easy if junctions are visible, but I found performance to be poor for small regions, not approaching ceiling levels until observers were given fairly large (∼ 50 pixels in diameter) regions over which to make the judgment. Control experiments ruled out the possibility that the effects are just due to junctions at multiple scales. Experiments reported here suggest that, although some junctions in real images are locally defined and can be detected with simple mechanisms, a substantial fraction necessitate the use of more complex and global processes. This raises the possibility that junctions in such cases may not be detected prior to scene interpretation.

Earlier psychophysical and physiological studies, obtained mostly with two-dimensional (2-D) stimuli, provided evidence for the hypothesis that the processing of faces differs from that of scenes. We report on our experiments, employing realistic three-dimensional (3-D) stimuli of a hollow mask and a scene, that offer further evidence for this hypothesis. The stimuli used for both faces and scenes were bistable, namely they could elicit either the veridical or an illusory volumetric percept. Our results indicate that the illusion is weakened when the stimuli are inverted, suggesting the involvement of top-down processes. This inversion effect is statistically significant for the facial stimulus, but the trend did not reach statistical significance for the scene stimulus. These results support the hypothesis that configural processing is stronger for the 3-D perception of faces than it is for scenes, and extend the conclusions of earlier studies on 2-D stimuli.

In some cases, perceptual learning is task-specific. However, task-dependent effects of perceptual learning on psychophysical motion-tuning functions have yet to be clarified. In the present study, subjects performed motion detection or discrimination of the same stimulus over the course of four sessions held on separate days. Subjects who performed motion detection showed the most highly improved performance on the trained motion directions. However, after discrimination training, the highest improvement was not observed at the trained directions but shifted away from them. These results can be explained by lateral inhibition. Task demands may differentially modulate excitatory and inhibitory signals to directions in the vicinity of the trained directions.