THE EFFECTS OF INTERMITTENT ILLUMINATION ON A VISUAL INSPECTION TASK

Two experiments are described in which eye movements were monitored as subjects performed a simple target-spotting task under conditions of intermittent illumination produced by varying the display-screen frame rate on a computer VDU. In Experiment 1, subjects executed a saccade from a fixation point to a target which appeared randomly at a fixed e ccentricity of 14 character positions to the left or right. Saccade la tency did not differ reliably as a function of screen refresh rate, bu t average saccade extent at 70 Hz and 110 Hz was reliably shorter than at 90 Hz and 100 Hz. Experiment 2 examined the same task using a rang e of target eccentricities (7, 14, and 28 character positions to the l eft and right) and across a wider range of screen refresh rates. The r esults confirmed the curvilinear relationship obtained in Experiment 1 , with average saccade extent reliably shorter at refresh rates of 50 Hz and 125 Hz than at 75 Hz and 100 Hz. While the effect was greater f or remote targets, analyses of the proportional target error failed to show a reliable interaction between target eccentricity and display r efresh rate. In contrast to Experiment 1, there was a pronounced effec t of refresh rate on saccade latency (corrected for time to write the screen frame), with shorter latencies at higher refresh rates. It may be concluded that pulsation at frequencies above fusion disrupts sacca de control. However; the curvilinear functional relationship between s creen refresh rate and saccade extent obtained in these studies differ s from previously reported effects of intermittent illumination on the average size of ''entry saccades'' (the first saccade to enter a give n word) in a task involving word identification (Kennedy & Murray, 199 3a, 1996). This conflict of data may arise in part because within-word adjustments in viewing position, which are typical of normal reading, influence measures of average saccade extent.