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 manipulate info rmation in mind, is central to intelligent behavior. Here we take adva ntage of the high temporal resolution of electrophysiological measures to obtain a millisecond timescale view of the activity induced in dis tributed cortical networks by tasks that impose significant WM demands . We examined how these networks are affected by the type and amount o f information to be remembered, and by the amount of task practice. Ev oked potentials (EPs) were obtained from eight subjects performing spa tial and verbal versions of a visual n-back WM task (n = i, 2, 3) on e ach of three testing days. In well-trained subjects, WM tasks elicited transient responses reflecting different subcomponents of task proces sing, including transient (lasting 0.02-0.3 s) task-sensitive and load -sensitive EPs, as well as sustained responses (lasting 1-1.5 s), incl uding the prestimulus Contingent Negative Variation (CNV), and post-st imulus frontal and parietal Slow Waves. The transient responses, with the exception of the P300, differed between the verbal and spatial tas k versions, and between trials with different response requirements. T he P300 and the Slow Waves were not affected by task version but were affected by increased WM load. These results suggest that WM emerges f rom the formation of a dynamic cortical network linking task-specific processes with non-specific, capacity-limited, higher-order attentiona l processes. Practice effects on the EPs suggested that practice led t o the development of a more effective cognitive strategy for dealing w ith lower-order aspects of task processing, but did not diminish deman ds made on higher order processes. Thus a simple WM task is shown to b e composed of numerous elementary subsecond neural processes whose cha racteristics vary with type and amount of information being remembered , and amount of practice.