Is reality decision a semantic or pre-semantic task?

In reality decision tasks, subjects have to decide whether a picture represents a real or an unreal object: the perceptual representation of the stimulus is compared with the representations of objects that have been seen previously and are stored in the so called Structural Description System (SDS); when matching occurs, the corresponding conceptual knowledge is activated. One critical issue in cognitive neuropsychology is the relationship between the SDS and semantic memory. Several clinical cases show double-dissociations, but evidence suggesting an interaction between the two systems has also been gathered. In the present study we aimed at investigating whether semantic and perceptual variables play a role at the level of the structural description system. We created a set of 260 black-and-white line drawings of objects, half real and half unreal. In the unreal group we manipulated the visual and semantic characteristics of the stimuli so as to obtain five categories of false objects: Kroll & Potter-like pictures; scrambled pictures; pictures with metrical alterations; non semantically-related chimeras and semantically-related chimeras. Twenty normal subjects performed the reality decision task. Reaction times and accuracy were recorded. Longer reaction times and poorer accuracy were found for the semantically-related chimeras, with respect to all the other categories. Our findings suggest that semantic knowledge supports visual object recognition and thus that the semantic memory store and the structural description system are highly interactive. Next step will be to verify the involvement of the semantic system in real-unreal judgments in patients with semantic memory impairment.