Where is my head? Knowing head orientation in space is necessary to estimate the extent of the visual field in tasks requiring visual feedback such as driving or manual materials handling. Visually guided tasks are generally dependent on head and eye movements for visual acquisition of the target, and head movements are of significant importance when target eccentricity from the neutral reference point is large. The aim of the present work was to investigate head orientation in space in hand pointing tasks and to model the head response.
Standing subjects were required to direct their gaze at one of three targets, equally distributed (vertically) in the sagittal plane. The task was performed while standing a) with the arms next to the body, b) holding a load in a static condition, c) aiming at targets with a heavy or light load held in the hands. Movements of the head and the body segments were recorded by the motion capture systems. Spatial head orientation was determined as a function of target locations.
When the subjects looked at the upper and lower targets the head orientation was shifted up by 2.16° and 1.84° respectively, in the loaded condition when compared to the reference condition. Similarly, when the subject aimed at the targets with the hand loaded, the head orientation was shifted up by 2.32° and 1.22° for the upper and lower targets, respectively, in the heavy load condition when compared to the light load condition.
Three major indications can be derived from the above results: 1) head movements contribute significantly to target acquisition, 2) head orientation away from the straight ahead position will be accompanied by a shift of the visual field that must be taken into account for workplace design, and 3) when a load is carried in the hand, head movements may be used to compensate shifts in the center of mass and are part of the associated reorganization of posture; hence systematic head orientation changes can be predicted.