Physiological workload reactions to increasing levels of task difficulty

The sensitivity of physiological measures to mental workload has been investigated in a flight simulator. Twelve pilots had to fly through a tunnel with different levels of difficulty. Additionally, they had to perform a memory task with four levels of difficulty. The easiest memory task was combined with the easiest tunnel task and the most difficult memory task with the most difficult tunnel task. Between the tunnel tasks, subjects had to fly a pursuit task in which a target jet had to be followed. Rest periods before and after the experiment were used as a baseline for the physiolog-ical measures. Mental workload was measured with heart period, continuous blood pressure, respiration and eye blinks. Heart rate variability, blood pressure variability and the gain between systolic blood pressure and heart period (modulus) was derived from the measures. All measures showed differences between rest and flight, and between the pursuit and the tunnel task. Only heart period was sensitive to the different levels in the tunnel task. Since heart rate variability was affected by respiration, it could only be interpreted together with the respiratory data. The modulus, how-ever, was hardly influenced by respiration and therefore, appears te be a better measure than heart rate variability. From the respiratory parameters, the duration of a respiratory cycle was the most sensitive to changes in workload. The time in between two successive eye blinks (blink interval) increased and the blink duration decreased as more visual information had to be processed. Increas-ing the difficulty of the memory task leaded to a decrement in blink interval, probably caused by subvocal activity during the rehearsal of the letters. Blink duration was not influenced by memory load.