Christine Chiarello - US grants

The major objective of this research is to investigate, in the normal brain, the hemispheric for semantic functions which may be impaired in thought disorders. The proposed studies will use visual half-field stimulation to investigate semantic facilitation and inhibition processes in the left and right hemispheres of normal individuals. The primary hypothesis is that the left hemisphere processes meaning in a focal manner, by facilitating relevant meanings and inhibiting those deemed irrelevant, while the right hemisphere maintains facilitation for a set of semantically related meanings without inhibition. The role of attentional selectivity in semantic processing in each cerebral hemisphere will be examined in three series of experiments. In Series 1, lexical priming paradigms will be used to investigate the role of semantic expectancies in producing semantic inhibition and facilitation. In Series 2, the "negative priming" paradigm will be used to investigate facilitatory and inhibitory processes produced when one of two simultaneously presented words is the focus of visual attention. In Series 3, semantic facilitation and inhibition are examined in higher order cognitive tasks which required judgments of meaning. It is predicted that semantic facilitation will occur in both cerebral hemispheres, but that inhibitory processes, which depend upon attentional selection, will be restricted to the left hemisphere. It is important to understand the neurological basis of these functions, since breakdowns in the normal interplay of semantic facilitation and inhibition could produce some schizophrenic symptoms (e.g., loose associations).

The cortex of the human brain is subdivided into two halves, the left and right hemispheres, which process information in characteristically different ways. Although it is acknowledged that nearly all mental activities require some contribution from each cerebral hemisphere, understanding when and how each system participates in a variety of cognitive tasks remains a significant scientific challenge. The long-term goal of this research is to determine the precise contributions of the left and right hemispheres when people comprehend words. The experiments will test the hypothesis that the relative participation of each hemisphere in word recognition is determined by the linguistic context in which a word is experienced, rather than by intrinsic characteristics of the word itself. Words will be presented in the left or right visual fields, thereby selectively stimulating the right or left hemisphere. Two important word properties will be investigated: grammatical class (noun vs verb) and concreteness of meaning (concrete vs abstract). It is predicted that the right cerebral hemisphere will make a greater contribution to recognizing nouns and concrete words in the context of verbs or abstract words, than in more homogeneous contexts. Such findings would suggest that cerebral asymmetries for word recognition vary dynamically in concert with variation in the stimulus environment, rather than reflecting static (and hence unmodifiable) processing differences. This research will indicate the extent to which context modulates the brain systems used to recognize words, thereby enhancing our understanding of the neural bases for language comprehension.

DESCRIPTION (provided by applicant): The long-term objective of the proposed research is to understand the biological mechanisms underlying individual differences in cerebral lateralization for language. Understanding the factors that determine the variability inherent in normal brain organization is essential in order to effectively treat individuals with unilateral brain injuries. It is proposed that individuals vary both in the degree and consistency of left hemisphere predominance and that these variations have behavioral significance for language processing. A large sample of normal persons will be tested in a series of linguistic tasks in which stimuli are presented to the left and right hemispheres. The tasks measure both basic word recognition, and semantic retrieval, processes. Structural magnetic resonance imaging (MRI) will be performed to assess asymmetries in posterior (planum temporale, planum parietale, Heschl's gyrus) and anterior (pars triangulars, cingulate and paracingulate sulcus) brain regions. It is hypothesized that (1) individuals differ in the extent to which critical cortical regions are asymmetrical and that this structural variability is linked to the inter-hemispheric organization of language function; (2) individuals differ in the consistency of their asymmetries such that some have a consistent pattern of moderate asymmetries across tasks and brain regions, while others show behavioral and anatomical asymmetries that depart from the population mean randomly; and (3) individuals with consistent, typical asymmetries will demonstrate greater speed and accuracy across a variety of nonlateralized lexical tasks. Although treatment for developmental and acquired brain dysfunction is administered individually, the understanding of normal function is based primarily on group averages. Hence, the study of individual differences in uninjured populations can provide a critical bridge between the basic science of cerebral lateralization and its clinical application.