Since fixations are replaced by smooth-pursuit eye movements, the reading of drifting text requires a close coordination of saccadic and smooth-pursuit eye movements. Traditionally, saccades and smooth pursuit have been considered to act on completely distinct input signals: Saccades compensate for retinal position errors and smooth pursuit compensates for retinal slip (Rashbass,
1961). However, in the last decade, converging evidence has supported the notion that the two types of eye movements are closely coupled. On the neurophysiological level, it has been shown that saccadic and smooth-pursuit eye movements are controlled by largely overlapping neural networks (for a review, see Krauzlis,
2004). Oculomotor research (for a review, see Orban de Xivry & Lefevre,
2007) has gathered evidence for velocity input to saccade control, affecting saccade amplitudes (Gellman & Carl,
1991; de Brouwer, Missal, & Lefevre,
2001) and saccade dynamics (de Brouwer, Missal, Barnes, & Lefevre,
2002; Guan, Eggert, Bayer, & Buttner,
2005). Conversely, studies have also provided evidence for position input to smooth-pursuit control (Segraves & Goldberg,
1994; Blohm, Missal, & Lefevre,
2005). In fact, both systems interact to optimize eye-movement behavior (Orban de Xivry, Bennett, Lefevre, & Barnes,
2006).