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Abstract:
A state of successful episodic retrieval (ecphory) is always
embedded into the context of events that precede it. In a random
mixed design these events can be the encountering of repeated (old)
or newly presented (new) items. Here, we investigated the effects
of the sequence of such events on brain activity associated with
ecphory. Electric (32 electrodes) and magnetic (144 sensors)
DC-fields were simultaneously recorded while 10 healthy subjects
performed recognition judgments on test lists in which old words
were pseudo-randomly mixed with new words. Each test list was
preceded by a study list of nine words and contained nine old and
nine new words. Test words were presented one every two seconds
with a break of 4 seconds every three words. Source analysis of the
electromagnetic fields was performed by constraining localization
with individual structural MRI-scans using realistic head models.
Results: The electromagnetic signature of recognition preceded by a
new word differed from recognition preceded by an old word. This
difference appeared between 400 and 800 ms and was localized
bilaterally in the medial temporal lobes. Furthermore, three
successive recognition events were associated with a sustained (six
seconds) magnetic activity shift bilaterally over frontal and
temporo-polar brain regions when compared to three successive
correct rejections. In conclusion, the immediate context in which
ecphory occurs affects different brain areas at different time
scales. This indicates a variable involvement of frontal and
temporal brain areas during ecphory depending on the immediate
context in which it occurs.
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