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Visual object understanding

Key Points

  • Two relatively independent areas of visual cognition research examine important aspects of visual object understanding: Object Recognition and Perceptual Categorization. These areas have focused on different aspects of the same problems, with surprisingly little overlap. Nevertheless, they have ultimately arrived at complementary conclusions regarding the computational bases of visual object understanding.

  • Traditionally, computational models in Object Recognition provide a detailed description of the format of object representations, whereas Perceptual Categorization models emphasize how representations are used to make decisions. Both image-based theories and exemplar-based theories have articulated how the same representations can be used to recognize, identify and categorize objects.

  • Although intuition suggests that object recognition is effortless regardless of changes in viewpoint, and that knowledge about object categories is abstract, there is much evidence to the contrary. Just as recognizing an object is influenced by particular stored views, categorizing an object is influenced by particular stored exemplars. Image-based and exemplar-based models are supported by behavioural, neurophysiological and functional imaging results. There is also some renewed support for abstraction, and new hybrid models attempt to integrate structural descriptions with image-based representations and to integrate abstract category representations with exemplar-based representations.

  • Objects can be categorized at several levels of abstraction (for example, animal, mammal, cat, Abyssinian, Max). Some argue that basic-level categorization is the fundamental goal of vision, with identification relying on features other than object shape, whereas early tests of image-based theories emphasized discrimination at the subordinate level. Recently, image-based theorists have argued that categorization at all levels can be accomplished using image-based representations. Early work in Perceptual Categorization suggested that identification and categorization used distinct representations and processes, but recent evidence indicates that a common representational substrate can be used adaptively according to task demands.

  • Researchers in Object Recognition have traditionally discussed modularity of content: are there specific modules devoted to particular kinds of objects? The Perceptual Categorization literature focused on debates regarding the modularity of memory systems: are there specific modules devoted to particular tasks, irrespective of object category? In both fields, claims of modularity have been disputed, relying primarily on demonstrations that non-modular models can account for dissociations.

  • Traditionally, visual perception was thought to create the representational input to a conceptual system that identified or categorized objects in a linear fashion. Recently, more 'interactive' solutions have been proposed. The evidence indicates that there is an interaction between perception and conceptual knowledge, and that category learning can influence perceptual representations.

  • A new dynamic approach emphasizes the role of learning in most questions of interest in visual object understanding. Novices can demonstrate visual object understanding in qualitatively different ways than experts: for instance, people might initially categorize using rules but with experience start to retrieve exemplars from memory. Experience with certain categories leads to specialization in the visual system: for example, experts can process non-face objects such as cars, dogs, birds and novel objects in a manner similar to faces, using the same brain areas and with neural responses with the same latency.

  • Despite their historic differences, current theories of Object Recognition and Perceptual Categorization have begun to consider complementary problems and have converged on similar solutions. Ultimately, a complete understanding of visual object understanding will demand an integration of the best theoretical constructs from Object Recognition and Perceptual Categorization.

Abstract

Visual object understanding includes processes at the nexus of visual perception and visual cognition. A traditional approach separates questions that are more associated with perception — how are objects represented by high-level vision — from questions that are more associated with cognition — how are objects identified, categorized and remembered. However, to understand the bridge between perception and cognition, it is fruitful to abandon any sharp distinction between perceptual and cognitive aspects of visual object understanding. We provide a selective review of research from both the Object Recognition and Perceptual Categorization literatures, highlighting relevant behavioural, neuropsychological, neurophysiological and theoretical research into the representations and processes that underlie visual object understanding in humans and primates.

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Figure 1: Multidimensional representations of objects.
Figure 2: Neurophysiology of Perceptual Categorization.

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Acknowledgements

Supported by grants from the NSF, NIMH, NEI and James S. McDonnell Foundation. The authors wish to thank M. J. Tarr and the members of the Perceptual Expertise Network (funded by the James S. McDonnell Foundation) for helpful discussions. We also thank M. Graf for detailed comments on an earlier version of this paper.

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Glossary

IDENTIFICATION

A decision about an object's unique identity. Identification requires subjects to discriminate between similar objects and involves generalization across some shape changes as well as physical translation, rotation and so on.

CATEGORIZATION

A decision about an object's kind. Categorization requires generalization across members of a class of objects with different shapes.

RECOGNITION

A decision about whether an object has been seen before. We can recognize an object seen just moments before — as in many experiments from Object Recognition — or we can recognize an object seen on an earlier occasion — as in many experiments from Perceptual Categorization and the memory literature. Recognition involves generalization across size, location, viewpoint and illumination.

GEONS

(Geometric ions). Simple viewpoint-independent volumetric primitives that are the building blocks of object representation for recognition-by-components theory.

STRUCTURAL DESCRIPTION

A qualitative representation of an object in terms of its three-dimensional primitives (for example, 'geons') and their relative positions. Many structural descriptions are devoid of metric information regarding quantitative aspects of the primitives (specific shapes and sizes) and their positions (specific spatial locations).

IMAGE-BASED

A representation of an object that preserves much of the richness of the perceived two-dimensional image. It is viewpoint-specific, or represented in an egocentric frame of reference, and might contain information about illumination, colour and material (but is often proposed to be largely scale- and translation-invariant).

VIEWPOINT-INDEPENDENT (OR DEPENDENT) PERFORMANCE

Behavioural performance that is invariant of viewing position and independent of experience with particular views is said to be viewpoint-independent. By contrast, viewpoint-dependent performance depends systematically on experience with specific views of an object.

GREEBLES

Novel objects that, like faces, all share a common spatial configuration. Their features can be varied systematically to test aspects of object recognition and feature perception.

BASIC LEVEL

The level at which object descriptions (both functional and perceptual attributes) maximize a combination of informativeness and distinctiveness. Typically, the basic level is the entry level of recognition. Exceptions include atypical category members (such as penguin, palm tree).

ENTRY LEVEL

The first level of abstraction at which a perceived object triggers its representation in memory. Empirically it is the fastest level at which observers can verify that an object can be given a particular label at some level of the hierarchy (for example, canary, bird or animal).

CASCADE MODELS

Cascade models are those in which the later stages of information processing can begin before the completion of earlier stages, unlike discrete models in which computations at any given stage are completed before the subsequent step is engaged.

MODULARITY

A thesis concerning the structure of the mind that is based on special-purpose computational mechanisms termed 'modules'. Fodor8 proposed that modules are innate, that they perform their operations on a specific input or domain (for example, faces or speech) and that their operations are informationally encapsulated (not accessible to any other module).

PROSOPAGNOSIA

Originally defined as the inability to gain a sense of familiarity from known faces, prosopagnosia also now includes a deficit in the perception of faces. It typically occurs in the context of visual agnosia — a visual deficit in object recognition — and only a few cases have been suggested to present with a face-specific deficit.

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Palmeri, T., Gauthier, I. Visual object understanding. Nat Rev Neurosci 5, 291–303 (2004). https://doi.org/10.1038/nrn1364

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