Human performance in dual task situations - an experimental study of the concept of primary and secondary tasks.
thesisposted on 19.11.2015, 08:57 by J. M. Rolfe
Measuring human performance in terms of variables relating directly to the task under evaluation does not always provide indications of changes in the operator's response. In consequence techniques have been developed which attempt to assess the load imposed by the task in question on the subject in terms of his response on an additional, secondary, task. The literature on the use of secondary tasks is examined and the techniques' successes and shortcomings are assessed. A series of experiments are described in which a secondary task was used to obtain information about the use of a purely numerical display, instead of one embodying a scale and pointer indication, as part of a psycho-motor task. Secondary task measures were able to indicate that parity of performance on the two displays appeared to have been achieved only at the expense of increased effort on the part of the subjects when using the purely numerical display. It was found that the presence of the secondary task impaired performance on the primary task. This deterioration took place in spite of instructions to the subjects regarding which task was to be considered the primary task. Further experiments were undertaken in an attempt to discover if the source of the interference between the two tasks could be located. It was found that the greatest interference occurred during those phases of the primary task in which decision making and an ensuing action were required. Presenting the secondary task via the same and different sensory pathways to that used for the primary task had no influence upon the patterns of interference obtained. This result was interpreted as indicating that the source of the interference was at a central level in the operator rather than at a peripheral sensory level. This finding is attributed to the limitations in the central information handling processes of the human operator. Physiological measures recorded during two of the experiments showed that the addition of a secondary task raised the level of the subject's responses. Patterns of response variation, particularly in terms of heart rate, could be closely related to the decision making and action components of the primary task.