Academic journal article Perception and Psychophysics

The Control of Saccade Trajectories: Direction of Curvature Depends on Prior Knowledge of Target Location and Saccade Latency

Academic journal article Perception and Psychophysics

The Control of Saccade Trajectories: Direction of Curvature Depends on Prior Knowledge of Target Location and Saccade Latency

Article excerpt

Recent reports have shown that saccades can deviate either toward or away from distractors. However, the specific conditions responsible for the change in initial saccade direction are not known. One possibility, examined here, is that the direction of curvature (toward or away from distractors) reflects preparatory tuning of the oculomotor system when the location of the target and distractor are known in advance. This was investigated by examining saccade trajectories under predictable and unpredictable target conditions. In Experiment 1, the targets and the distractors appeared unpredictably, whereas in Experiment 2 an arrow cue presented at fixation indicated the location of the forthcoming target prior to stimulus onset. Saccades were made to targets on the horizontal, vertical, and principal oblique axis, and distractors appeared simultaneously at an adjacent location (a separation of ±45° of visual angle). On average, saccade trajectories curved toward distractors when target locations were unpredictable and curved away from distractors when target locations were known in advance. There was no overall difference in mean saccade latencies between the two experiments. The magnitude of the distractor modulation of saccade trajectory (either toward or away from) was comparable across the different saccade directions (horizontal, vertical, and oblique). These results are interpreted in terms of the time course of competitive interactions operating in the neural structures involved in the suppression of distractors and the selection of a saccade target. A relatively slow mechanism that inhibits movements to distractors produces curvature away from the distractor. This mechanism has more time to operate when target location is predictable, increasing the likelihood that the saccade trajectory will deviate away from the distractor.

The paths of saccadic eye movements are often found to be curved (Dodge, 1917; Helmholtz, 1962; Yarbus, 1967). This natural tendency for curved trajectories is greater for oblique than for horizontal or vertical saccades (Viviani & Swensson, 1982). Furthermore, the direction and magnitude of saccade curvature can be modulated by a range of different factors. For example, in the so-called double-step task, saccades are directed initially toward the first target step but, on occasions, show strong curvature toward the second target (Findlay & Harris, 1984; Van Gisbergen, Van Opstal, & Roebroek, 1987). Saccades in visual search tasks have been shown to deviate toward the location of competing distractors (McPeek, Han, & Keller, 2003; McPeek & Keller, 2001), and after an erroneous saccade has been made toward a distractor, a subsequent secondary corrective saccade may be made that deviates back toward the target (McPeek & Keller, 2001). By contrast, in other situations, saccade trajectories have been shown to deviate away from attended distractor locations. Sheliga and colleagues (Sheliga, Craighero, Riggio, & Rizzolatti, 1997; Sheliga, Riggio, Craighero, & Rizzolatti, 1995; Sheliga, Riggio, & Rizzolatti, 1995) showed that saccades deviated away from a previously attended location and that the magnitude of curvature was greatest when the saccade target was in the attended hemifield. This finding was attributed to a specific process of inhibition operating within spatial attention. However, Doyle and Walker (2001) showed that prior attentional orienting is not necessary for the effect; saccades also deviated away from the location of a task-irrelevant distractor onset presented simultaneously with the saccade target (see also Tipper, Howard, & Paul, 2001).

The distractor modulation of saccadic, and manualreach, trajectories (Sheliga et al., 1997; Tipper et al., 2001) has been interpreted in terms of competitive processes operating within neural structures involved in the encoding of potential targets (Doyle & Walker, 2001, 2002; McPeek & Keller, 2001; McSorley, Haggard, & Walker, 2004; Sheliga, Riggio, & Rizzolatti, 1995; Tipper et al. …

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