The effects of semantic category and knowledge type on lexical-semantic access: A PET study

Neuropsychological studies of patients with category-specific recognition d isorders, as well as PET investigations of semantic category effects in vis ual recognition tasks, have led some authors to the hypothesis that visual- perceptual knowledge plays a crucial role in the recognition of natural ite ms, such as animals, while functional-associative information is more impor tant for the recognition of man-made tools. To study the cerebral correlate s of the retrieval of different types of semantic knowledge about living an d nonliving entities, we performed a PET experiment in which normal subject s were required to access visual- and functional-associative information re lated to visually presented words corresponding to animals and tools. The e xperimental conditions were the following: (1) Rest. (2) Baseline: letter d etection in pseudo-words. (3) Animal, visual knowledge: decide whether the animal has a long or short tail with respect to the body. (4) Animal, assoc iative knowledge: decide whether the animal is typically found in Italy. (5 ) Tool, visual knowledge: decide whether the object is longer than wider or vice versa. (6) Tool, functional knowledge: decide whether the object is t ypically used as a kitchen tool. Lexical-semantic access tall lexical condi tions pooled) activated the prefrontal cortex on the left and the parietal- occipital junction and posterior cingulate cortex bilaterally. An analysis of the individual experimental conditions in comparison with the nonword ba seline showed that accessing visual versus associative knowledge was associ ated with different activation patterns: predominantly frontal in the case of visual features, temporoparietal for associative knowledge. While the ac tivation patterns involved similar areas for living and nonliving entities, in the case of the latter they were restricted to the left hemisphere. The analysis of main effects confirmed these findings: there mere several sign ificant differences in the visual-associative comparison, while category-re lated differences were less prominent. These findings indicate that the ret rieval of different types of knowledge is associated with distinct patterns of brain activation; on the other hand, category-related differences were less evident than in picture matching and naming tasks. (C) 1998 Academic P ress.