EYE-MOVEMENT CONTROL DURING THE INSPECTION OF WORDS UNDER CONDITIONS OF PULSATING ILLUMINATION

Previous work has suggested that eye movement control in reading is di sturbed by pulsating illumination such as that characteristic of visua l display units, which are periodically refreshed. The average size of the first saccade directed towards a word (the ''entry saccade'') var ies as a function of screen refresh rate. We report the results of an experiment which attempted to discern whether this arises through a di rect effect of pulsation on saccade control or as a result of a strate gic adaptation to phenomenal image degradation which might be associat ed with refreshed displays. Subjects examined a prompt word followed b y two target words. Depending on the prompt, a decision was required a s to whether the two target words were physically identical or had the same meaning. The task was conducted in four conditions of screen pul sation (50, 75, 100 and 125 Hz) and in both positive (i.e. black-on-wh ite) and negative (white-on-black) polarity. Since launch position int o the first target word was tightly controlled, saccade extent into th is should reflect only direct effects of screen pulsation. In contrast , consideration of saccade extent into the second target word, taken t ogether with launch position in the first, allowed us to distinguish b etween a direct influence on saccade extent and indirect effects arisi ng from refixations. The results support the contention that pulsation has a direct influence on saccade control. Two kinds of effect can be observed. Following steady fixation of the prompt word, screen pulsat ion modulates saccade extent towards the first target word primarily i n negative polarity, where it is the displayed ''figure'', rather than the background, which pulsates. The saccade from the first to the sec ond target word (when it may be argued more normal reading dynamics ar e engaged), is affected to a greater extent in positive polarity. In t his case, the eyes must traverse a pulsating field when moving from wo rd to word.