Episodic encoding and recognition of pictures and words: role of the humanmedial temporal lobes

In the present PET study, we examined brain activity related to processing of pictures and printed words in episodic memory. Our goal was to determine how the perceptual format of objects (verbal versus pictorial) is reflecte d in the neural organization of episodic memory for common objects. We inve stigated this issue in relation to encoding and recognition with a particul ar focus on medial temporal-lobe (MTL) structures. At encoding, participant s saw pictures of objects or their written names and were asked to make sem antic judgments. At recognition, participants made yes-no recognition judgm ents in four different conditions. In two conditions, target items were pic tures of objects; these objects had originally been encoded either in pictu re or in word format. In two other conditions, target items were words; the y also denoted objects originally encoded either as pictures or as words. O ur data show that right MTL structures are differentially involved in pictu re processing during encoding and recognition. A posterior MTL region showe d higher activation in response to the presentation of pictures than of wor ds across all conditions. During encoding, this region may be involved in s etting up a representation of the perceptual information that comprises the picture. At recognition, it may play a role in guiding retrieval processes based on the perceptual input, i.e. the retrieval cue. Another more anteri or right MTL region was found to be differentially involved in recognition of objects that had been encoded as pictures, irrespective of whether the r etrieval cue provided was pictorial or verbal in nature; this region may be involved in accessing stored pictorial representations. Our results sugges t that left MTL structures contribute to picture processing only during enc oding. Some regions in the left MTL showed an involvement in semantic encod ing that was picture specific; others showed a task-specific involvement ac ross pictures and words. Together, our results provide evidence that the in volvement of some but not all MTL regions in episodic encoding and recognit ion is format specific. (C) 2000 Elsevier Science B.V. All rights reserved. PsycINFO classification: 2343: 2540.