We present a computational model of the processes involved in retrieving stored semantic and name information from objects, using a simple interactive activation and competition architecture. We simulate evidence showing a cross-over in normal reaction times to make semantic classification and identification responses to objects from categories with either structurally similar or structurally dissimilar exemplars, and that identification times to objects from these two different classes correlate differentially with measures of the structural similarity of objects within the category and the frequency of the object's name. Structural similarity exerts a negative effect on object decision as well as naming, though this effect is larger on naming. Also, on naming, structural similarity interacts with the effects of name frequency, captured in the model by varying the weight on connections from semantic to name units; frequency effects are larger with structurally dissimilar items. In addition, (1) the range of potential errors for objects from these two classes, when responses are elicited before activation reached a stable state, differ--a wider range of errors occur to objects from categories with structurally similar exemplars; and (2) simulated lesions to different locations within the model produce selective impairments to identification but not to semantic classification responses to objects from categories with structurally similar exemplars. We discuss the results in relation to data on visual object processing in both normality and pathology.

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This study investigates the development of an adaptive strategy for the estimation of numerosity from the theoretical perspective of "strategic change" (Lemaire & Siegler, 1995;Siegler & Shipley. 1995). A simple estimation task was used in which participants of three different age groups (20 university students. 20 sixth-graders and 10 second-graders) had to estimate 100 numerosities of (colored) blocks presented in a 1Ox1 0 rectangular grid. Generally speaking. this task allows for two distinct estimation procedures: either repeatedly adding estimations of groups of blocks (=addition procedure) or subtracting the estimated number of empty squares from the (estimated) total number ofsquares in the grid (=subtraction procedure). A rational task analysis indicates that the most efficient overall estimation strategy consists of the adaptive use of both procedures. depending on the ratio of the blocks to the empty squares. The first hypothesis was that there will be a developmental difference in the adaptive use of the two procedures. and according to the second hypothesis this adaptive use will result in better estimation accuracy. Converging evidence from different kinds ofdata (i.e.. response times. error rates. and retrospective reports) supported both hypotheses. From a methodological point of view, the study shows the potential of Beem :s (l995a. 1995b) "segmentation analysis" for unravelling subjects' adaptive choices between different procedures in cognitive tasks, and for examining the relationship between these adaptive choices and performance. Theoretical backgroundBased on a review of the literature on strategic change, Lemaire and Siegler (1995) The authors would like to thank Wim Fias for his assistance in writing the computer program for presenting the stimuli and reg .strating the data, as well as the two anonymous reviewers for their valuable comments on a previous version of this article.

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When a planar shape is viewed obliquely, it is deformed by a perspective deformation. If the visual system were to pick up geometrical invariants from such projections, these would necessarily be invariant under the wider class of projective transformations. Towhat extent can the visual system tell the difference between perspective and nonperspective but still projective deformations of shapes? To investigate this, observers were asked to indicate which of two test patterns most resembled a standard pattern. The test patterns were related to the standard pattern by a perspective or projective transformation, or they were completely unrelated, Performance was slightly better in a matching task with perspective and unrelated test patterns (92.6%) than in a projective-random matching task (88.8%). In a direct comparison, participants had a small preference (58.5%) for the perspectively related patterns over the projectively related ones. Preferences were based on the values of the transformation parameters (slant and shear). Hence, perspective and projective transformations yielded perceptual differences, but they were not treated in a categorically different manner by the human visual system. When do two shapes look alike? Obviously, when you take a shape and translate, reflect, or rotate it, it will still be perceived as the same shape (except perhaps for some highly familiar shapes, such as the outline of the United States when presented in a strange orientation; see Rock, 1973). Similarly, an object seen at different distances, although producing retinal images of different sizes, is perceived as having the same shape. Moreover, an object and its cast shadow or its projected image are usually considered shape equivalent.It is widely believed that shape constancy is based on the visual system's sensitivity to the geometric congruence under an increasingly larger set ofoperations in the above cases: from Euclidean, to similarity, to projective transformations. This kind ofinnate geometry can take the form of unconscious or automatic recovery of shape constancy based on implicit knowledge of the laws of geometry (e.g., Helmholtz, 1868Helmholtz, /1968Rock, 1983), or it can take the form of sensitivity to the invariants under each group of transformations (e.g., Cutting, 1986;Gibson, 1950).How large the set of operations can be for shapes before and after transformation to be considered perceptually equivalent is not clear yet. This may seem strange beThe work presented in this paper was supported by an Esprit Basic Research Action ("VIVA," or "Viewpoint-Invariant Visual Acquisition"), Grant F. (August 1995). The quality of the presentation has been improved thanks to helpful suggestions by James Cutting, Michael Kubovy, and two anonymous reviewers. Correspondence should be addressed to 1. Wagemans, Department of Psychology, University of Leuven, Tiensestraat 102, B-3000 Leuven, Belgium (e-mail: johan.wagemans@kuleuven.ac.be).cause psychological experiments could test specific predictions derived from a strong ma...