Lesion studies indicate that the lateral and inferior temporal cortex is a critical area of semantic memory storage, but little is known about the cortical organization of semantics within this area. One proposition has been that dominant physical characteristics of objects (structure, motility) are determining factors. A positron emission tomography experiment using the H 2 15 O bolus method was performed to test this hypothesis by contrasting activation for concrete and abstract concepts. Unlike previous studies that considered this question, the task required explicit word meaning judgments, and blocks of trials were designed to be of equal difficulty for the two word classes. The task required elderly participants to read aloud the pair of words that was closer in meaning (e.g., spade–shovel vs. spade–carpet). Subtraction analyses that compared the semantic judgment tasks with a baseline condition indicated that both abstract and concrete concepts activated the left lateral temporal cortex. A direct comparison of abstract versus concrete scans indicated differences in the lateralization of fusiform activation. We conclude that although concreteness might be a critical factor in the fusiform cortex, it is not dominant in the lateral temporal cortex. A multistudy overview suggests that tasks that focus on one concept per trial activate areas posterior to y = −40, whereas those that invoke several concepts, as in the present study, activate areas anterior to this. Increased processing complexity may proceed in a posterior– anterior direction in the lateral temporal cortex.

A PET study of 10 normal males was carried out using the bolus H 2 15 O intravenous injection technique to examine the effects of picture naming and semantic judgment on blood flow. In a series of conditions, subjects (1) passively viewed flashing plus signs, (2) noted the occurrence of abstract patterns, (3) named animal pictures, or (4) carried out a semantic judgment on animal pictures. Anticipatory scans were carried out after the subjects were presented with the instructions but before they began the cognitive task, as they were passively viewing plus signs. Our results serve to clarify a number of current controversies regarding the neural substrate of picture naming. The results indicate that the fusiform gyrus is unlikely to be the region where low-level perceptual processing such as shape analysis is undertaken. In fact, our evidence suggests that activation of the fusiform gyrus is most likely related to visual perceptual semantic processing. In addition, the inferior/middle frontal lobe activity observed while performing the picture naming and semantic judgment tasks does not appear to be due to the effects of anticipation or preparation. Furthermore, there appears to be a set of regions (a semantic network) that becomes activated regardless of whether the subjects perform a picture naming or semantic judgment task. Finally, picture naming of animals did not activate either parietal regions or anterior inferior left temporal regions, regardless of what subtraction baseline was used.