Abstract
Visual categorization appears both effortless and virtually instantaneous. The study by Thorpe et al. (1996) was the first to estimate the processing time necessary to perform fast visual categorization of animals in briefly flashed (20 ms) natural photographs. They observed a large differential EEG activity between target and distracter correct trials that developed from 150 ms after stimulus onset, a value that was later shown to be even shorter in monkeys! With such strong processing time constraints, it was difficult to escape the conclusion that rapid visual categorization was relying on massively parallel, essentially feed-forward processing of visual information. Since 1996, we have conducted a large number of studies to determine the characteristics and limits of fast visual categorization. The present chapter will review some of the main results obtained. I will argue that rapid object categorizations in natural scenes can be done without focused attention and are most likely based on coarse and unconscious visual representations activated with the first available (magnocellular) visual information. Fast visual processing proved efficient for the categorization of large superordinate object or scene categories, but shows its limits when more detailed basic representations are required. The representations for basic objects (dogs, cars) or scenes (mountain or sea landscapes) need additional processing time to be activated. This finding is at odds with the widely accepted idea that such basic representations are at the entry level of the system. Interestingly, focused attention is still not required to perform these time consuming basic categorizations. Finally we will show that object and context processing can interact very early in an ascending wave of visual information processing. We will discuss how such data could result from our experience with a highly structured and predictable surrounding world that shaped neuronal visual selectivity.
Highlights
How long does it take to process a natural scene? What kind of visual information can be extracted from a natural stimulus in the first few hundred milliseconds? How fast are the complex cognitive operations required for object categorization at different levels? Is there a hierarchical organization from detection to fine categorization? In the last two decades a large debate has engaged about such questions and is far from finding an end
We found no evidence that scene categorization could be completed faster than object categorization (Joubert et al, 2007), an extreme view would suggest that the visual system may be able to extract the global and ecologically relevant scene primitives needed to describe the spatial lay-out of a scene and conceptual information without the need to segment objects (Oliva and Torralba, 2006, 2007; Torralba et al, 2006; Greene and Oliva, 2009)
CONCLUDING COMMENTS For the last 15 years, fast go/no-go categorization tasks have been useful in describing the characteristics of very early visual processing
Summary
How long does it take to process a natural scene? What kind of visual information can be extracted from a natural stimulus in the first few hundred milliseconds? How fast are the complex cognitive operations required for object categorization at different levels? Is there a hierarchical organization from detection to fine categorization? In the last two decades a large debate has engaged about such questions and is far from finding an end.Visual categorization is a fascinating cognitive operation. All the results obtained in the above mentioned studies emphasize that a minimum processing time cannot be shortened in fast visual categorization tasks even when processing stimuli – such as human faces – that are claimed to have a very special status because they could depend upon a specific module of processing or when processing the gist of a scene that could rely on global statistics of the photographs (Oliva, 2005; Oliva and Torralba, 2006).
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