Cerebral Cortex, Vol 8, 563-574, Copyright © 1998 by Oxford University Press
LK McEvoy, ME Smith and A Gevins
Working memory (WM), the ability to briefly retain and manipulate
information in mind, is central to intelligent behavior. Here we take
advantage of the high temporal resolution of electrophysiological measures
to obtain a millisecond timescale view of the activity induced in
distributed cortical networks by tasks that impose significant WM demands.
We examined how these networks are affected by the type and amount of
information to be remembered, and by the amount of task practice. Evoked
potentials (EPs) were obtained from eight subjects performing spatial and
verbal versions of a visual n-back WM task (n = 1, 2, 3) on each of three
testing days. In well-trained subjects, WM tasks elicited transient
responses reflecting different subcomponents of task processing, including
transient (lasting 0.02-0.3 s) task- sensitive and load-sensitive EPs, as
well as sustained responses (lasting 1-1.5 s), including the prestimulus
Contingent Negative Variation (CNV), and post-stimulus frontal and parietal
Slow Waves. The transient responses, with the exception of the P300,
differed between the verbal and spatial task versions, and between trials
with different response requirements. The P300 and the Slow Waves were not
affected by task version but were affected by increased WM load. These
results suggest that WM emerges from the formation of a dynamic cortical
network linking task-specific processes with non-specific, capacity-
limited, higher-order attentional processes. Practice effects on the EPs
suggested that practice led to the development of a more effective
cognitive strategy for dealing with lower-order aspects of task processing,
but did not diminish demands made on higher order processes. Thus a simple
WM task is shown to be composed of numerous elementary subsecond neural
processes whose characteristics vary with type and amount of information
being remembered, and amount of practice.
ARTICLES
Dynamic cortical networks of verbal and spatial working memory: effects of memory load and task practice
EEG Systems Laboratory, San Francisco, CA 94105, USA.
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