Nonaccidental properties underlie human categorization of complex natural scenes

Humans can categorize complex natural scenes quickly and accurately. Which scene properties enable people to do this with such apparent ease? We extracted structural properties of contours (orientation, length, curvature) and contour junctions (types and angles) from line drawings of natural scenes. All of these properties contain information about scene categories that can be exploited computationally. However, when we compared error patterns from computational scene categorization with those from a six-alternative forced-choice scene-categorization experiment, we found that only junctions and curvature made significant contributions to human behavior. To further test the critical role of these properties, we perturbed junctions in line drawings by randomly shifting contours and found a significant decrease in human categorization accuracy. We conclude that scene categorization by humans relies on curvature as well as the same nonaccidental junction properties used for object recognition. These properties correspond to the visual features represented in area V2.