Abstract
In the natural world, objects are characterised by a variety of attributes, including colour and 3D shape. The contributions of these two attributes to object recognition are typically studied independently of each other, yet they are likely to interact in natural tasks. In this experiment, observers viewed a display of 16 real 3D objects, hemispheres sculpted from plaster of Paris and painted matte white. Distinct apparent surface colours were applied to the object surfaces and independently varied between trials. On each trial, any given shape might appear with any one of a fixed set of colours evenly distributed around the neutral chromaticity of the background. The observer's task was to select which of two indicated test objects was most similar to the designated reference object; both test and reference objects varied between trials. Each test object had either the same colour and different shape or the same shape and different colour relative to the reference object, and both shape and colour differences varied from sub- to supra-threshold values, as determined from independent single-variable shape- and colour-discrimination measurements with the same objects. The results demonstrate that 3D shape and colour interact significantly in determining object similarity: both shape-difference and colour-difference thresholds in the object similarity task were larger than in the single-variable discrimination measurements. Furthermore, threshold colour differences for object similarity increase as 3D shape differences decrease, and vice versa.