The vocabulary of shape: principal shapes for probing perception and neural response

Abstract

Humans perceive shape rapidly and effortlessly but have great difficulties describing what they perceive. This suggests that the representation of shape in the brain is abstract and very unlike that used in conscious thought. Here we explore the proposal that this representation is matched to the statistical properties of objects in the environment. From an ensemble of several hundred laser-scanned three-dimensional (3D) human heads we extract the principal components which provide a compact basis for head shape. We show that, with good accuracy, a given head can be represented by linear combinations of a few dozen primary shapes just as colours can be synthesized by combining the three principal colours. We suggest new perceptual adaptation experiments for testing the brain's shape representation system. The principal head shapes can also be used to probe response properties of ‘face-cells’ in the inferior temporal cortex.