Eye and head movements in the acquisition of visual information

Much of the literature dealing with the measurement of eye movements is restricted to data collected under conditions where the head is "stabilized". Since in the "real world" such restrictions cannot readily be imposed on operators, we here review the literature dealing with the use of head movements in the acquisition and processing of visually presented information. Task difficulty is associated with an increase in the likelihood of using head movements to acquire visually presented information. Such head movements have been described with visual information subtending as little as 5-10 degrees of arc. The present study, requiring subjects to identify and process information presented at 10 degree eccentricity, determined that such head movements do occur. Head movements were not measured directly but inferred from differences in eye movement amplitude as measured with clectrooculography, and gaze shift measured with video camera technology. We raise the issue of generalizability of results obtained under conditions where the head is "stabilized" to conditions where it is free to move.