Visual recognition begins with categorization

Feb 13 2005

Take a moment and look at a picture near you. What did you see? How long did it take you to understand what was in the image, meaning how long did it take you to realize the green blob was a tree? Or that the orange circle was a piece of fruit? Most likely you assume that it took you no time at all, you just knew.

Psychologists who study how we perceive images used to think that, before the process of object recognition and categorization could begin, the brain must first separate the figure in the image—such as a tree, or a piece of fruit—from its background. However, new research shows we actually categorize objects before we identify them. It means that, by the time your brain even realizes you are looking at something, you already know what that thing is.

The new research was conducted by Kalanit Grill-Spector of Stanford University and Nancy Kanwisher of the Massachusetts Institute of Technology. Their article, "Visual Recognition: As Soon as You Know It’s There, You Know What It Is," will appear in the February 2005 issue of Psychological Science, a journal of the American Psychological Society.

In their research, Grill-Spector and Kanwisher tested three types of visual recognition by quickly flashing images before the eyes of research participants. The first type, object detection, was tested by showing images that may or may not have contained figures. Participants had to quickly judge whether or not there was a figure present against the background. The second grouping tested categorization, where participants were shown images of figures and had to state what type of figure they saw, such as bird, car, or food. In the last section of the test, more specific images were shown in order to test identification. Participants had to identify the figures within categories such as parrot or pigeon, versus just the category of "bird." It turned out the participants were as fast and accurate in saying what category an object belonged to as they were at saying whether or not they had seen an object at all. The ability for the subjects to process the images in such a short time proved that by the time they knew an image contained some sort of object, they already knew its general category.

"There are two main processing stages in object recognition: categorization and identification, with identification following categorization," the authors wrote. "Overall, these findings provide important constraints for theories of object recognition."

This built-in human process of rapid categorization before identification restricts the brain’s search for a match between the visual input (the picture you looked at) and an internal representation to category-relevant representations (stored images of other objects you have seen and identified prior to today).

"Future research building on these psychophysical techniques and multimodal imaging techniques will enhance our knowledge about the processes and representations that enable rapid and efficient visual perception," Grill-Spector said. "Rapid categorization obviously facilitates our survival and interaction with the environment on an everyday level."