Neural mechanisms of rapid natural scene categorization in human visual cortex

Abstract

People are remarkably adept at quickly detecting the presence of items of interest in their field of view. A functional magnetic resonance imaging study of healthy volunteers presented with a series of photographs, and asked to spot either people or cars in them, shows how this is achieved by the visual system. The brain rapidly determines whether there are objects relevant to whatever task it is attempting anywhere in the field of view even if they are not in regions that are under direct scrutiny. Intriguingly, the evidence shows that, contrary to our subjective experience of a complete internal representation of the external world, the neural representation of real-world scenes is limited to those objects that are directly relevant for ongoing behaviour. Human beings are able to rapidly detect the presence of object categories such as animals or vehicles, even when a scene is presented very briefly. The use of functional magnetic resonance imaging during an object categorization task now indicates that the rapid detection of categorical information in natural scenes is mediated by a category-specific biasing mechanism in object-selective cortex that operates across the visual field. The visual system has an extraordinary capability to extract categorical information from complex natural scenes. For example, subjects are able to rapidly detect the presence of object categories such as animals or vehicles in new scenes that are presented very briefly1,2. This is even true when subjects do not pay attention to the scenes and simultaneously perform an unrelated attentionally demanding task3, a stark contrast to the capacity limitations predicted by most theories of visual attention4,5. Here we show a neural basis for rapid natural scene categorization in the visual cortex, using functional magnetic resonance imaging and an object categorization task in which subjects detected the presence of people or cars in briefly presented natural scenes. The multi-voxel pattern of neural activity in the object-selective cortex evoked by the natural scenes contained information about the presence of the target category, even when the scenes were task-irrelevant and presented outside the focus of spatial attention. These findings indicate that the rapid detection of categorical information in natural scenes is mediated by a category-specific biasing mechanism in object-selective cortex that operates in parallel across the visual field, and biases information processing in favour of objects belonging to the target object category.