A group of 32 healthy adult volunteers completed
three blocks of a reaction time task that varied in the
degree of controlled processing load. A rest period preceded
each of the task blocks. The task blocks were presented
in the order of either increasing or decreasing cognitive
load. For each of the six periods, mean values and spectral
measures of heart rate and respiration variability were
calculated. The spectral measures were obtained for three
different frequency bands. Differences between the cardiac
measures of the task and preceding rest periods were compared
with respect to differences in task load and the order
of task presentation. All comparisons were carried out
while adjusting for respiratory variability in the corresponding
frequency band. The frequency band in which task load-related
changes in heart rate variability became manifest appeared
to be dependent on the individual's breathing pattern.