Dynamic cortical networks of verbal and spatial working memory: effects of memory load and task practice

Working memory (WM), the ability to briefly retain and manipulateinformation in mind, is central to intelligent behavior. Here we takeadvantage of the high temporal resolution of electrophysiological measuresto obtain a millisecond timescale view of the activity induced indistributed cortical networks by tasks that impose significant WM demands.We examined how these networks are affected by the type and amount ofinformation to be remembered, and by the amount of task practice. Evokedpotentials (EPs) were obtained from eight subjects performing spatial andverbal versions of a visual n-back WM task (n = 1, 2, 3) on each of threetesting days. In well-trained subjects, WM tasks elicited transientresponses reflecting different subcomponents of task processing, includingtransient (lasting 0.02-0.3 s) task- sensitive and load-sensitive EPs, aswell as sustained responses (lasting 1-1.5 s), including the prestimulusContingent Negative Variation (CNV), and post-stimulus frontal and parietalSlow Waves. The transient responses, with the exception of the P300,differed between the verbal and spatial task versions, and between trialswith different response requirements. The P300 and the Slow Waves were notaffected by task version but were affected by increased WM load. Theseresults suggest that WM emerges from the formation of a dynamic corticalnetwork linking task-specific processes with non-specific, capacity-limited, higher-order attentional processes. Practice effects on the EPssuggested that practice led to the development of a more effectivecognitive strategy for dealing with lower-order aspects of task processing,but did not diminish demands made on higher order processes. Thus a simpleWM task is shown to be composed of numerous elementary subsecond neuralprocesses whose characteristics vary with type and amount of informationbeing remembered, and amount of practice.