One of the unsolved problems in the field of human concept learning concerns the factors that determine the subjective difficulty of concepts: why are some concepts psychologically simple and easy to learn, while others seem difficult, complex or incoherent? This question was much studied in the 1960s but was never answered, and more recent characterizations of concepts as prototypes rather than logical rules leave it unsolved. Here I investigate this question in the domain of Boolean concepts (categories defined by logical rules). A series of experiments measured the subjective difficulty of a wide range of logical varieties of concepts (41 mathematically distinct types in six families--a far wider range than has been tested previously). The data reveal a surprisingly simple empirical 'law': the subjective difficulty of a concept is directly proportional to its Boolean complexity (the length of the shortest logically equivalent propositional formula)--that is, to its logical incompressibility.

This article proposes a new model of human concept learning that provides a rational analysis of learning feature-based concepts. This model is built upon Bayesian inference for a grammatically structured hypothesis space-a concept language of logical rules. This article compares the model predictions to human generalization judgments in several well-known category learning experiments, and finds good agreement for both average and individual participant generalizations. This article further investigates judgments for a broad set of 7-feature concepts-a more natural setting in several ways-and again finds that the model explains human performance.

The notion that visual attention can operate over visual objects in addition to spatial locations has recently received much empirical support, but there has been relatively little empirical consideration of what can count as an`object' in the ®rst place. We have investigated this question in the context of the multiple object tracking paradigm, in which subjects must track a number of independently and unpredictably moving identical items in a ®eld of identical distractors. What types of feature clusters can be tracked in this manner? In other words, what counts as an`object' in this task? We investigated this question with a technique we call target merging: we alter tracking displays so that distinct target and distractor locations appear perceptually to be parts of the same object by merging pairs of items (one target with one distractor) in various ways ± for example, by connecting item locations with a simple line segment, by drawing the convex hull of the two items, and so forth. The data show that target merging makes the tracking task far more dif®cult to varying degrees depending on exactly how the items are merged. The effect is perceptually salient, involving in some conditions a total destruction of subjects' capacity to track multiple items. These studies provide strong evidence for the object-based nature of tracking, con®rming that in some contexts attention must be allocated to objects rather than arbitrary collections of features. In addition, the results begin to reveal the types of spatially organized scene components that can be independently attended as a function of properties such as connectedness, part structure, and other types of perceptual grouping. q 2001 Elsevier Science B.V. All rights reserved.

Abstract. We demonstrate that a single moving object can create the subjective impression that it is alive, based solely on its pattern of movement. Our displays differ from conventional biological motion displays (which normally involve multiple moving points, usually integrated to suggest a human form) in that they contain only a single rigid object moving across a uniform field. We focus on motion paths in which the speed and direction of the target object change simultaneously. Naive subjects' ratings of animacy were significantly influenced by (i) the magnitude of the speed change, (ii) the angular magnitude of the direction change, (iii) the shape of the object, and (iv) the alignment between the principal axis of the object and its direction of motion. These findings are consistent with the hypothesis that observers classify as animate only those objects whose motion trajectories are otherwise unlikely to occur in the observed setting.

Our first review paper on the occasion of the centennial anniversary of Gestalt psychology focused on perceptual grouping and figure-ground organization. It concluded that further progress requires a reconsideration of the conceptual and theoretical foundations of the Gestalt approach, which is provided here. In particular, we review contemporary formulations of holism within an information-processing framework, allowing for operational definitions (e.g., integral dimensions, emergent features, configural superiority, global precedence, primacy of holistic/configural properties) and a refined understanding of its psychological implications (e.g., at the level of attention, perception, and decision). We also review four lines of theoretical progress regarding the law of Prägnanz—the brain’s tendency of being attracted towards states corresponding to the simplest possible organization, given the available stimulation. The first considers the brain as a complex adaptive system and explains how self-organization solves the conundrum of trading between robustness and flexibility of perceptual states. The second specifies the economy principle in terms of optimization of neural resources, showing that elementary sensors working independently to minimize uncertainty can respond optimally at the system level. The third considers how Gestalt percepts (e.g., groups, objects) are optimal given the available stimulation, with optimality specified in Bayesian terms. Fourth, Structural Information Theory explains how a Gestaltist visual system that focuses on internal coding efficiency yields external veridicality as a side-effect. To answer the fundamental question of why things look as they do, a further synthesis of these complementary perspectives is required.