Scale Ambiguities in Material Recognition

Many natural materials have complex, multi-scale structures. Consequently, the apparent identity of a surface can vary with the assumed spatial scale of the scene: a plowed field seen from afar can resemble corduroy seen up close. We investigated this ‘material-scale ambiguity’ using 87 photographs of diverse materials (e.g., water, sand, stone, metal, wood). Across two experiments, separate groups of participants (N = 72 adults) provided judgements of the material depicted in each image, either with or without manipulations of apparent distance (by verbal instructions, or adding objects of familiar size). Our results demonstrate that these manipulations can cause identical images to appear to belong to completely different material categories, depending on the perceived scale. Under challenging conditions, therefore, the perception of materials is susceptible to simple manipulations of apparent distance, revealing a striking example of top-down effects in the interpretation of image features.

Do We See Scale?

The visual system is supposed to extract distance information from the environment in order to scale the size and distance of objects in the visual scene. The purpose of this article is to challenge this account in three stages: First, I identify three shortcomings of the literature on vergence as our primary cue to near distances. Second, I present the results from two experiments that control for these shortcomings, but at the cost of eradicating vergence and accommodation as effective distance cues (average gain of y = 0.161x + 38.64). Third, I argue that if all our cues to distance are either (a) ineffective (vergence; accommodation; motion parallax), (b) merely relative (angular size; diplopia), or (c) merely cognitive (familiar size; vertical disparity), then the visual system does not appear to extract absolute distance information, and we should be open to the possibility that vision functions without scale.