There is growing evidence from functional imaging studies that distinct regions in the fusiform gyri are differentially sensitive to object category. In this paper, we investigate how the areas that are more sensitive to animals than tools respond to other visual and semantic variables. We illustrate that (1) category effects in the fusiform areas are stronger for pictures of objects than their written names; (2) retrieving information on the colour or size of objects activates a left lateralised fusiform area that lies anterior to the category-sensitive areas; and (3) both left and right category-sensitive areas respond strongly to visual feature detection on false fonts-meaningless visual stimuli with no semantic associations. These results dissociate the responses in two fusiform areas: The posterior category-sensitive areas are primarily modulated by visual input, whereas a more anterior polymodal region is involved in the retrieval of visual information. In addition, we demonstrate that the posterior areas which are more active for animals than tools are also more active for fruits than tools. Our data are therefore consistent with the proposal that activation in the lateral posterior fusiform gyri reflects the demands on structural differentiation. Since animals and fruits tend to have more structurally similar neighbours than man-made kinds of objects, category effects are likely to be observed during most picture identification tasks. In contrast, when the stimuli are written or auditory names, category effects may only be observed when the task requires access to fine spatial details in the objects' structures.