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Abstract:
Abstract: Performing cognitive tasks generally demands a
combination of "state" and "item" processes (demands that span
across trials and occur on individual trials, respectively). We
present a method for separating out these processes. The method
relies on the assumption that state and item related processes
differ, in that the timecourse of their associated neural
correlates should be either sustained or transient in nature
respectively. A mixed "blocked and event-related" experimental
design was employed; 9 subjects alternated between blocks of
fixation and recognition memory (discriminating studied from
unstudied words). Within the recognition blocks standard
event-related procedures were embedded, using rapidly presented
trials (~2.5 sec apart) and intermixed brief fixation trials. Data
were analyzed using the general linear model to estimate the
independent contributions of transient (responses to items
presented during recognition memory) and sustained (differences
between fixation and recognition memory) activity. Analysis of
transient responses revealed that medial parietal, lateral parietal
and anterior left frontal cortex were more active for hits than
correct rejections. This converges with Sanders et al. (abstract)
and Henson et al. (1999 J. Neurosci.), suggesting brain areas that
are modulated by successful retrieval. Moreover, left middle
frontal gyrus, bilateral frontal operculum and medial frontal gyrus
exhibited both transient and sustained responses. Activity in these
latter regions reflects a combination of state and item related
processes.
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