Subjects viewed 3 X 3 grids in which different subsets of the nine squares were designated as "figure," either by physical shading of those squares or by a verbal instruction to imagine those squares as shaded. The time taken by participants to respond "on" or "off" the figure was measured for single or multiple probe dots, which all appeared on or off the figural subset together, and which had already been shown to be equally detectable against shaded or unshaded squares and in all nine locations within the grid. In contrast to the set-size effect generally found in experiments on memory scanning, reaction time did not necessarily increase with the number of squares in the figural subset. Instead, the critical variable, which in previous research may often have adventitiously covaried with set size, was the spatial compactness of the subsets (as indexed by square-root-area over perimeter): Probes of less compact figures required more time to classify correctly. Subjects were evidently more successful in confining their attention to sets of mutually proximal items. Reasons are given for believing that this principle may also apply in the more abstract representational or semantic spaces that determine reaction times and errors in various other cognitive tasks.
Interobject visual similarity is often measured by the time that subjects require to say that two objects are not identical, with long RTs indicating high similarity. In Experiment 1, using a complete set of uppercase alphabetic stimuli, we show: (1) RTs correlate strongly with direct ratings of visual similarity. (2) Stimuli that are similar as measured by RT or by direct judgment have the same profile of similarity to all other stimuli in the set as measured by correlational analysis. (3) The order in which the stimuli are compared has only a small effect on measured similarity. All of these results indicate desirable properties of any measure of similarity. Intraobject similarity is a concept pertinent to the relation of an object to itself. In Experiments 1 and 2 we show: (1) All letters are not equally similar to themselves, since RT for the "same" response is different for different letters. (2) The relation between RT and intraobject similarity is opposite to that for interobject similarity, with short RTs indicating high intraobject similarity. (3) Even though intraobject similarities differ, each letter is more similar to itself than to any other letter in the set. We discuss the implications of these results in terms of assumed proximity constraints underlying similarity data.
Subjects determined whether probe dots appearing in component squares of a 5 X 5 grid fell on a figure that, depending on the condition, was (a) visually presented as a pattern of darkened squares, (b) only remembered from a preceding presentation of such a pattern, or (c) imaginally generated from a verbal code. The speed and accuracy of the responses to the probes as well as the functional dependencies of the reaction times on structural variables were essentially the same whether the figural pattern was imagined, remembered, or actually seen. Reaction times averaged between 400 and 500 msec and were consistently faster (a) for on-figure than for off-figure responses, (b) for simpler figures that had fewer squares, (c) for off-figure probes that were more distant from the figure, and (d) for on-figure probes that consisted either of more dots on the figure or of a dot at the intersection of a horizontal and a vertical bar. The reults were consistent with a model in which a subject's perceptual or imaginal representations of these forms contain barlike units that respond independently to the probes.
Subjects judged whether two visual patterns had the same shape under conditions in which the patterns could vary orthogonally in size and orientation. Analyses of reaction times and errors indicated that, before judging the correspondence between shapes, subjects normalized orientational variation by means of a continuous mental rotation. However, an analogous normalization for variation in size was not detected, and thus no evidence for an internal operation of size scaling was obtained in this experiment. The latter result is at variance with previous findings, and we discuss two possible explanations for this lack of effect. When two patterns differ in ways that could be eliminated by performing a rigid physical transformation, such as rotation or translation, we willingly say that the patterns are the same. In order to reach this decision, perceivers must establish that such physical transformations would in fact bring the patterns into spatial congruence. A recent and influential view is that perceivers perform mental operations that are in many ways analogous to physical transformations and that they decide, by means of these internal operations, whether patterns are identical according to a criterion of congruence (Bundesen &
Two experiments were carried out using a same-different task with sets of four stimuli varying orthogonally in three dimensions. Sameness was defined by each of the three dimensions in turn, as well as by physical identity. Two types of dimensions, physical and cognitive, were studied. In Experiment 1, the numerals 6, 10, VI, and X, which vary in Length, System, and Name, were used. With simultaneous presentation, order of difficulty was from the physical dimension of Length to the cognitive dimension of Name. While overall difficulty was related to dimensional discriminability, internal evidence suggested that Name was not a dimension in the same sense as Length is, and that some stimulus pairs were simply easier to process than others, regardless of the response required (e.g., short stimulus pairs and Arabic numerals). With sequential presentation, Name was processed as fast as System, due largely to the fact that much faster responding occurred when an Arabic numeral was the second stimulus. Thus, with sequential presentation, Name provides a processing mechanism not provided by physical dimensions. In Experiment 2, the numerals 3, 4, 6, and 7, which vary in Magnitude, Oddness, and Curvilinearity, were used. The cognitive dimension of Magnitude was processed most rapidly, and numerical distance between pairs of numbers dominated the results for "same" responses, regardless of the sameness rule used. Again, evidence was found for fast processing of some stimulus pairs (e.g., 3 4), regardless of the response required. Overall, these experiments are interpreted as indicating that cognitive factors such as stimulus familiarity may override as~ts of physical discriminability with many dimensions and stimuli, that, even though a cognitive dimension can be used to generate a logically proper set of stimuli, it does not necessarily act as other, more physical dimensions do, and that clarification of the functional role of a dimension is more important than attempts to locate stages.Stimulus comparison techniques such as the samedifferent task have been used quite commonly to establish levels or hierarchies of processing, starting with the Posner and Mitchell (1967) research, with more recent research well reviewed in Posner (1978). The most frequently used stimulus materials have been visual letters, with the rule for sameness varied to manipulate the presumptive level of processing. To illustrate, in the Posner and Mitchell (1967) experiments, upper-and lowercase letters were used,
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