A pet study of visual and semantic knowledge about objects.

The distinctiveness of temporal lobe regions activated during the retrieval of knowledge regarding structural, colour and associative (encyclopaedic) aspects of familiar objects was investigated using PET. These three types of knowledge were contrasted using well matched tasks requiring the detection, in a series of coloured-in line drawings, of occasional anomalous objects (in the three conditions: structurally incorrect chimeras composed of parts of real objects; inappropriately coloured objects; familiar objects that do not exist in the modern world). Relative to a resting baseline condition, all semantic retrieval tasks yielded extensive bilateral activations in occipital and temporal areas, extending anteriorly on the ventral surface of the brain, plus an area in the right superior parietal lobe. In direct semantic-task comparisons focussing on the temporal lobe: (i) structural relative to associative decisions activated the right posterior middle/inferior temporal gyrus; (ii) colour decisions relative to structural judgements were associated with a region in the right superior temporal gyrus; (iii) the associative decision task selectively activated the left anterior middle/superior temporal gyrus and temporal pole relative to both object structure and colour, and also the homologous right temporal pole relative to colour only. These results indicate that each type of stored knowledge involves at least partially distinct cortical areas, and suggest that both anterior/posterior and left/right temporal regions have specialised roles.