Physiological changes during computer tasks: responses to mental load or to motor demands?

Cardiac and electrodermal measures are regarded as indicators of user strain during computer work, not taking the possible influence of finger and hand movements on these measures into account. For the evaluation of such effects, motor demands and mental load were both varied as independent factors in two experiments. As a motor task, subjects had to produce compensatory keystrokes at different speed levels to keep a moving mark within a target area on the computer screen. Mental load was varied in experiment 1 by manipulation of keystroke synchronization accuracy, and in experiment 2 by manipulation of memory load. Physiological measures were affected by motor activity only at a very high typing speed. At lower levels of motor activity physiological effects were dependent exclusively on mental load. Frequency of skin conductance responses was the most sensitive indicator of the emotional consequences of mental load and mean heart period reflected its attentional aspects. The study supports the assumption that physiological effects of motor activity can be neglected during typical computer tasks, demanding keystrokes at intervals of 300 ms or longer.