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Based on an artistic analysis of the construction of "impossible figures," their structure and the origin of the perceptual effects they induce are discussed, including the use of a basic indeterminate space of intersection, penetration of indeterminate solids, and symmetry. These and other points are illustrated by 17 progressively more complex figures constructed by the author.
When the origin of magnification-minification of an outline rectangle had a horizontal locus which exceeded one-fourth of the rectangle's horizontal dimension, 16 observers of 21 reported apparent depth characteristic of looming and recession.
Information about letters and the physical structure of language printed in Roman characters was given to children beginning to read. Experimental investigations coupled three alternative graphic modes of printing upper- and lower-case letters with an instructional intervention termed "Alpha-Beta" which provides practice in letter sorting, matching of letters, associative matching, and memory matching. In respect to graphics, Mode A letters were in standard alphabet form. Mode B provided standard letters with each backed by a unique half-tone (Visually Stippled Alphabet); Mode C provided standard letters with each backed by a unique visual texture (Visually Patterned Alphabet). Pre-posttest change in reading readiness was measured using the Metropolitan Readiness Test. In the first study 224 English-speaking 5- to 6-yr.-old children were tested. In the second there were 158 Spanish-speaking girls and boys 6 to 7 years old. It was predicted that Alpha-Beta intervention involving visually patterned alphabet would lead to the greatest increases in readiness scores. This is confirmed in both studies for children low in reading readiness preexperiment. Children high in reading readiness are less affected. The second experiment involved Spanish-speaking children and investigated intervention by Alpha-Beta against a no-intervention control. This confirms the value of Alpha-Beta per se. Possible explanations for the improvements are identified.
An experiment was conducted to investigate the effect of order of report and stimulus type on multidimensional stimulus-identification performance. Analysis showed that the proper order of reporting dimensional attributes depended on stimulus type. If the dimensions were independent heterogeneous attributes, the effect of order of report was not significant. If homogeneous attributes were involved, they should be reported consecutively. The order in which the color attributes should be reported depended on the characteristics of the stimulus dimensions. If there was a natural language-appropriate order of reporting the dimensional attributes, the color attribute should be used as an adjective and reported first. Otherwise, reporting color attribute later seemed to have an advantage because color is less vulnerable to memory deterioration.
Following F. Zaal and R. J. Bootsma (1995), the authors studied whether the decelerative phase of a reaching movement could be modeled as a constant tau-dot strategy resulting in a soft collision with the object. Specifically, they investigated whether that strategy is sustained over different viewing conditions. Participants (N = 11) were required to reach for 15- and 50-mm objects at 2 different distances under 3 conditions in which visual availability of the immediate environment and of the reaching hand were varied. Tau-dot estimates and goodness-of-fit were highly similar across the 3 conditions. Only within-participant variability of tau-dot estimates was increased when environmental cues were removed. That finding suggests that the motor system uses a tau-dot strategy involving the intermodal (i.e., visual, proprioceptive, or both) specification of information to regulate the decelerative phase of reaching under restricted viewing conditions. The authors provide recommendations for improving the derivation of tau;(x) estimates and stress the need for further research on how time-to-contact information is used in the regulation of the dynamics of actions such as reaching.
We partially replicate and extend Shepard, Hovland, and Jenkins's (1961) classic study of task difficulty for learning six fundamental types of rule-based categorization problems. Our main results mirrored those of Shepard et al., with the ordering of task difficulty being the same as in the original study. A much richer data set was collected, however, which enabled the generation of block-by-block learning curves suitable for quantitative fitting. Four current computational models of classification learning were fitted to the learning data: ALCOVE (Kruschke, 1992), the rational model (Anderson, 1991), the configural-cue model (Gluck & Bower, 1988b), and an extended version of the configural-cue model with dimensionalized, adaptive learning rate mechanisms. Although all of the models captured important qualitative aspects of the learning data, ALCOVE provided the best overall quantitative fit. The results suggest the need to incorporate some form of selective attention to dimensions in category-learning models based on stimulus generalization and cue conditioning.
Three experiments were performed to investigate hemispheric differences in susceptibility to the Oppel-Kundt illusion presented tachistoscopically to the two visual hemifields. Experiment 1 used a successive comparison mode, in which 16 undergraduate students indicated whether the second of two successive extents looked shorter or longer than the first. Experiment 2 (20 under-graduates) and Experiment 3 (1 commissurotomy patient) required judgments of two extents presented simultaneously. The first experiment found no significant visual field differences, although females were more susceptible than males. In the second experiment, the illusion magnitude was greater in the left visual field, and in the third experiment, it was greater in the right visual field. Post hoc analyses resolve the conflicting results and show that in all three experiments, the susceptibility of the right hemisphere declined more than that of the left hemisphere during illusion processing. An interpretation is offered in terms of two parallel processes, one that is fast, uses feature extraction, and is performed more effectively in the left hemisphere, and one that is slow, uses visuospatial analysis to compute distances between parts of the illusion figure, and is performed more effectively in the right hemisphere.
It has been shown that there is a transition from a global to a local advantage in reaction time as visual angle increases (Kinchla & Wolfe, 1979), and it has been assumed that this transition reflects lower level (e.g., retinal) processes. In three experiments, we examined whether higher level (e.g., attentional) processes play a role in this transition. In each experiment, subjects received a different stimulus set in each of two blocks of trials. In Experiment 1, stimuli subtending 1.5 degrees, 3 degrees, 4.5 degrees, or 6 degrees of visual angle vertically (small-stimuli set) were randomly presented in one block, while the other block consisted of random presentations of 3 degrees, 6 degrees, 9 degrees, or 12 degrees stimuli (large-stimuli set). The subjects' task was to identify targets that appeared randomly at either the local or the global level. It was found that the transition from a global to a local reaction-time advantage took place at a larger visual angle for the large-stimuli set than for the small-stimuli set. The same effects of stimulus set were found in Experiment 2, in which the small-stimuli set included 1.5 degrees, 3 degrees, or 6 degrees stimuli while the large-stimuli set included 3 degrees, 6 degrees, or 9 degrees stimuli. In Experiment 3, eye position was monitored to rule out the possibility that subjects adopted different fixation strategies depending on which stimulus set was being presented. The findings suggest that attention plays a major role in determining the relative speed of processing of local-and global-level information.
Students participated in 3 experiments investigating the use of environment- and action-centered reference frames in selective reaching. They pointed to a green target appearing either with or without a red distractor. Target-distractor distance was manipulated, and distractor interference (difference between distractor trials and no-distractor trials) was measured in reaction time, movement time, and movement endpoint. Target-distractor distance determined the dominant frame of reference. Small distances evoked an environment-centered framework that encoded targets within an external context. Large distances evoked an action-centered framework that encoded targets relative to the start position of the hand. Results support the hypothesis that the brain represents spatial information in multiple frames of reference, with the dominant frame of reference being dependent on the task demands.
A combined influence of stimulus orientation and structure on the judgement of length was tested in psychophysical experiments. The subjects adjusted the test part of a stimulus to be equal in length to the reference part. The V-shaped stimuli (three dots, or the Oppel-Kundt figure, or one dot and two Müller-Layer wings) were generated on the monitor. In the Oppel-Kundt and Müller-Layer figures, the filled part was considered as a reference and the empty part as a test. In session of the experiments, values of errors measured as functions of orientation of the parts of the stimuli. We assume that experiments with the three-dot stimuli yielded pure characteristics of visual field anisotropy, while those with the Oppel-Kundt and Müller-Layer figures showed a combined effect of both anisotropy and illusions. The data demonstrated that illusions and anisotropy are to be interpreted as independent factors, which converge to an algebraic summation in a simultaneous manifestation.
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The assumption that the Ebbinghaus/Titchener illusion deceives perception but not grasping, which would confirm the two-visual-systems hypothesis (TVSH) as proposed by Milner and Goodale (The visual brain in action, 1995), has recently been challenged. Franz et al. (Exp Brain Res 149:470-477, 2003) found that the illusion affects both perception and grasping, and showed that the effect of the illusion on the peak grip aperture (PGA) cannot be accounted for by different sizes of the gap that separates the central target disk from the surrounding flankers. However, it is not yet clear if the presence of flankers per se influences grasping. We therefore compared kinematic parameters of prehension, using the Ebbinghaus illusion, and a neutral control condition where normal subjects grasped a disk without any flankers. In accordance with the well-known effects of the illusion on perceived size, the PGA was smaller when the target disk was surrounded by large flankers, and larger when it was encircled by small flankers. However, the largest PGA values were reached in the neutral control condition. Hence the presence of flankers leads to a general reduction of the PGA, possibly because the flankers are regarded as obstacles. This 'reduction effect' casts doubts on how appropriate it is to directly compare perceptual measures and PGA values when using the Ebbinghaus illusion. Even smaller effects of the illusion on the PGA compared to larger perceptual effects cannot be unequivocally interpreted.