Asymmetric interference between the perception of shape and the perception of surface properties
We previously showed that the processing of shape and the processing of surface properties linked to material properties engage different regions of the ventral stream (J. S. Cant & M. A. Goodale, 2007). Moreover, we recently used Garner's speeded-classification task to show that varying the surface (material) properties of objects does not interfere with shape judgments and vice versa (J. S. Cant, M. E. Large, L. McCall, & M. A. Goodale, 2008). In the present study, we looked at Garner interference when surface cues contributed to the perception of object shape and hypothesized that this would interfere with judgments about the width and the length of the objects. In contrast, we predicted that varying the width and the length of the objects would not interfere with surface-property judgments. This is precisely what we found. These results suggest that the shape and the surface properties of an object cannot be processed independently when both these sets of cues are linked to the perception of the object's overall shape. These observations, together with our previous findings, suggest that the surface cues that contribute to object shape are processed quite separately from the surface cues that are linked to an object's material properties.