The Organization of Visually Mediated Actions in a Subject without Eye Movements

Abstract

We investigated the visual strategy of a subject without eye movements (AI), comparing her with normal subjects on the ‘real-life’ task of making a cup of tea. Differences in overall performance were surprisingly few. She took no more time than the controls to complete the tea-making task and the division of the task into object-related actions was essentially similar. However, the way AI look in visual information was very different from the normal subjects who used a typical ‘saccade and fixate’ strategy when moving between and scrutinizing objects. AI made saccades with the head, which were on average 1.5 times larger than the eye-head saccades of the controls and lasted four times as long, meaning that AI would have had impaired vision for more of the time than the controls. She also made only approximately one-third as many saccades as normals during the same task. However, she had another strategy, ‘slow drift’, in which she allowed her eyes to move smoothly across the scene at speeds of up to 30°/s. Such movements were never seen in the controls, and we assume that AI used them to offset the cost in time of the slow head saccades, even though they had their own cost in terms of reduced resolution. We demonstrate that these differences have a minimal effect on the timings of events during an object-related action. We discuss supervisory checking operations within actions, and consider what information is needed for appropriate gaze control during object-related actions.