Distributed semantic representations of object shape and size

Under sensorimotor models of semantic memory, object knowledge should be stored in a distributed pattern amongst the sensorimotor processing regions involved in acquisition of the knowledge (e.g., knowledge of a "telephone" may be stored in or near cortical regions responsible for visual, tactile, auditory, and action processing). For "visual" attributes, this is typically thought to be in or near visual cortex. However, given that certain visual attributes, such as object shape and size, may be learned through other types of processing such as tactile exploration and through forming the correct grip aperture to grasp and act on an object, regions of parietal cortex involved in these processes may be involved in the representation of these attributes. This idea is explored via two fMRI studies testing whether variability in the amount of prior tactile and action experience with objects can predict the degree of involvement of parietal regions in shape retrieval from memory. Both experiments draw upon semantic memory for shape (and size in Experiment 2) knowledge in that subjects answer questions about this attribute given only object names. These experiments identify portions of the left inferior parietal lobe (IPL) and the right postcentral gyrus (BA2) for their involvement in shape retrieval and reveal greater involvement of these regions during retrieval of shape knowledge for objects with a strong history of tactile and action processing. The two studies offer complementary approaches to testing the distributed representation of object shape: whereas Experiment 1 assesses the effects of prior tactile and action experience on existing representations of familiar objects, Experiment 2 manipulates the effect of sensorimotor experience directly by training subjects on a set of novel objects. Together the results suggest that the left IPL and right postcentral gyrus are involved in the shape representation of objects experienced tactilely and/or as the targets of actions. The results support sensorimotor models of memory (e.g., Allport, 1985) which propose that semantic knowledge for objects is stored according to the sensorimotor regions involved in knowledge acquisition and suggests that distributed representation can be observed even within the representation of a single object attribute (i.e., shape).