The n-back task was hypothesized to be a dual task,
permitting the imposition of parametrically increasing attentional
and working memory demands, while keeping constant the demands
of an embedded matching subtask. Visual targets were presented
for 200 ms every 2.2 s at pseudorandomly varying positions on
a computer screen. Participants were required to remember the
most recent 0, 1, 2, or 3 positions and responded with a choice
button push to whether the current target position matched the
position presented n items previously. P300 peak latency
was constant across n-back tasks, reflecting constant
perceptual and cognitive demands of the matching subtask. P300
peak amplitude decreased with increasing memory load, reflecting
reallocation of attention and processing capacity away from
the matching subtask to working memory activity. These data
support a dual-task nature of the n-back, which should
be considered when employing this paradigm.