PHYSIOLOGICAL WORKLOAD REACTIONS TO INCREASING LEVELS OF TASK-DIFFICULTY

The sensitivity of physiological measures to mental workload was inves tigated in a flight simulator. Twelve pilots had to fly through a tunn el with varying levels of difficulty. Additionally, they had to perfor m a memory task with four levels of difficulty. The easiest memory tas k was combined with the easiest tunnel task and the most difficult mem ory task with the most difficult tunnel task. Between the tunnel tasks . subjects had to Ay a pursuit task in which a target jet had to be fo llowed. Rest periods before and after the experiment were used as a ba seline for the physiological measures. Mental workload was measured wi th heart period, continuous blood pressure, respiration and eye blinks . Several respiratory parameters, heart rate variability, blood pressu re variability and the gain between systolic blood pressure and heart period (modulus) were scored. All measures showed differences between rest and flight, and between the pursuit and the tunnel task. Only hea rt period was sensitive to difficulty levels in the tunnel task. Heart rate variability increased when respiratory activity around 0.10 Hz i ncreased, which occurred often. The modulus was hardly influenced by r espiration and therefore appears to be a better measure than heart rat e variability. Among the respiratory parameters, the duration of a res piratory cycle was the most sensitive to changes in workload. The time in between two successive eye blinks (blink interval) increased and t he blink duration decreased as more visual information had to be proce ssed. Increasing the difficulty of the memory task led to a decrement in blink interval, probably caused by subvocal activity during rehears al of target letters. The data show that physiological measures are se nsitive to mental effort, whereas rating scales are sensitive to both mental effort and task difficulty.