![]() Thus, as before, activation was compared between a difficult semantic task (abstract/concrete classifications) and an easy nonsemantic task (uppercase/lowercase classifications). In an attempt to disambiguate these two potential bases of left prefrontal activations, one fMRI study ( 8) added a third condition with minimal semantic processing but even more extended processing than the semantic task. Thus, left prefrontal activation occurs in a range of semantic tasks. The fact that activations occurred in the same left prefrontal region for verb generation and for two-choice judgments involving mostly nouns indicates that the activations reflect the semantic requirements common across these diverse tasks rather than generation of verbs per se. Similarly, a PET study found greater left prefrontal activations when participants decided whether words referred to living or nonliving entities than when the words contained a particular letter ( 7). The perceptual input (visual words), output (two-choice response), and stimuli (words) were constant across the two encoding conditions, so the left prefrontal activation reflected only the difference between the semantic and nonsemantic requirements of the two conditions. For example, one fMRI study found greater left prefrontal activations when participants made semantic judgments about whether words referred to concrete (“table”) or abstract (“truth”) entities than when they made nonsemantic judgments about whether words appeared in uppercase letters (“CHAIR”) or lowercase letters (“love”) ( 6). Subsequent PET and functional magnetic resonance imaging (fMRI) studies have found similar left prefrontal activations associated with semantic relative to nonsemantic analysis of words. ![]() ![]() ![]() It is hypothesized that activations in left inferior prefrontal cortex reflect a domain-specific semantic working memory capacity that is invoked more for semantic than nonsemantic analyses regardless of stimulus modality, more for initial than for repeated semantic analysis of a word or picture, more when a response must be selected from among many than few legitimate alternatives, and that yields superior later explicit memory for experiences. New findings are reported showing that patients with global amnesia show deactivations in the same region associated with repetition priming, that activation in this region reflects selection of a response from among numerous relative to few alternatives, and that activations in a portion of this region are associated specifically with semantic relative to phonological processing. In this region, activations are associated with meaningful encoding that leads to superior explicit memory for stimuli and deactivations with implicit semantic memory (repetition priming) for words and pictures. This activation appears in the right prefrontal cortex of people known to be atypically right-hemisphere dominant for language. Activations in this region occur during semantic, relative to nonsemantic, tasks for the generation of words to semantic cues or the classification of words or pictures into semantic categories. ![]() This article reviews attempts to characterize the mental operations mediated by left inferior prefrontal cortex, especially the anterior and inferior portion of the gyrus, with the functional neuroimaging techniques of positron emission tomography and functional magnetic resonance imaging. ![]()
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