Wednesday, 13 April 2005

Seeing what things are

As soon as we know something is there, we already know what category of thing it is. That's according to Kalanit Grill-Spector and Nancy Kanwisher at Stanford University and MIT.

They showed people images for between 17 to 167ms, some of which contained an object - a jeep or a German shepherd dog, for example - some of which just showed an incoherent scramble of images. Sometimes participants had to say as fast as they could whether an object was present or not ('detection'), other times they had to indicate what category the object belonged to (e.g. a car or a dog), other times they had to identify the object specifically (e.g. to indicate as fast as possible whether or not the object was a German shepherd dog, for example).

People took no longer to categorise an object than they took to detect whether an object was present or not. Moreover, on trials when categorisation performance failed, detection performance was no better than chance, and vice versa, thus suggesting the processes were mutually dependent on each other. This has important implications for theories of visual object recognition. It suggests that segmentation of a visual scene into background and foreground, needed to detect objects, occurs at the same time as object categorisation. Either that, or conscious awareness only kicks in after the categorisation stage.

However, identifying an object took longer by about 65ms, suggesting object identification occurs only after its category has been determined.

Grill-Spector, K. & Kanwisher, N. (2005). Visual recognition. As soon as you know it is there, you know what it is. Psychological Science, 16, 152-159.

Post written by Christian Jarrett (@psych_writer) for the BPS Research Digest.

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