The Ability of the Saccadic System to Change Motor Plans in Scanning Letter Strings

Article excerpt

Evidence from recent studies bolsters the idea of preestablished motor plans in scanning isolated items. Thus, refixation saccades are preplanned at the same time as the primary saccade directed to a peripheral item and are completed with fixed amplitudes irrespective of the first fixation position in the item. In order to examine the saccadic system's ability to correct the motor plan during its execution on the basis of new visual information, an experiment was conducted in which 11-letter strings were changed to two 5-letter strings at different times after the primary saccade was directed to the stimulus. The results demonstrate that the saccadic system is able to cancel the preplanned refixation saccade and plan a saccade directed to the next item only when the new visual information is available at least 220 msec before the execution of the saccade.

It is well known that low-level visual information, such as the spaces between words and the length of a to-befixated word, is the major determinant of where the eyes initially land in a word (McConkie, Kerr, Reddix, & Zola, 1988; O'Regan, 1979; Rayner, 1979). Low-level information also influences whether a refixation saccade is made on the word1 (McConkie, Kerr, Reddix, Zola, & Jacobs, 1989; O'Regan, Lévy-Schoen, Pynte, & Brugaillère, 1984) and where the saccade lands (Beauvillain, Dukic, & Vergilino, 2000). Recently, Reichle, Pollatsek, Fisher, and Rayner (1998) have proposed that a refixation saccade is automatically planned after the initial saccade is completed. However, studies in which isolated words were read have shown that the saccadic system can acquire length information in periphery in order to plan more than one saccade at a time. A preestablished organized plan has been proposed for the refixation saccade that could constitute a unit of motor action encoded with the primary saccade and memorized before the execution of the last one (Beauvillain et al., 2000; Vergilino & Beauvillain, 2000, 2001). However, the initial representation of the refixation saccade may be canceled during the first fixation on the word. How much time the saccadic system needs to cancel the initial refixation plan and to redirect the second saccade toward the next word is still unknown.

In the present experiment, we investigated the scanning of nonsense strings rather than words in order to eliminate possible influences of nonperceptual factors such as meaning or familiarity. We used an eye-movement-contingent display change in which a single long letter string was segmented into two short letter strings at different times during the initial fixation on the item. Thus, observers had to cancel an initial plan to refixate a long letter string and program a saccade directed to the second, short item. In this situation, the initial spatial representation used for planning saccadic eye movement had to be completely transformed. Indeed, we previously showed a different coding of the interword and refixation saccades in reading isolated words (Vergilino & Beauvillain, 2001). When the second saccade is directed within the same word, the linear regression of the second saccade position on the first position has a slope close to 1. This indicates that such a saccade is coded as a fixed-size motor vector determined by word length, irrespective of the initial landing position in the item. Clearly, such a saccade does not aim for a precise target position in the word, whereas the saccade directed toward a new word selected in periphery tends to aim for a target position left of the center of the selected word. If the eyes always land precisely at the center of the second word regardless of the position of the current fixation, we predict a slope of O between the two successive interword saccades. However, replicating data observed previously in reading text (McConkie et al., 1988), we found a slope near 0.5. This reflects the fact that the interword saccade, influenced by its launch site, lands on the center of gravity of the configuration between the current fixation and the end boundary of the selected word. …