ReviewEye movement trajectories and what they tell us
Introduction
In everyday life, we are continuously faced with complex visual scenes that might contain important information. As visual acuity is best only in a small part of our retina, the fovea, we typically make rapid eye movements called saccades to examine different locations in our environment. It is commonly assumed that saccades are ballistic movements: once launched, their trajectories are fixed as that of a bullet. However, eye movement research has suggested that this is not the case (Robinson, 1975, Becker, 1989). A saccade can initially be executed to one location but mid-flight turn around and land on a second location (Van Gisbergen et al., 1987, Amador et al., 1998). This finding has been taken as evidence that saccades are not pre-programmed movements, but are dynamic in nature.
Also in a second way saccades are not comparable to bullets going straight to their target. Instead, eye movements can better be compared to the flight of an airplane. The trajectory of an airplane from the start location to its destination is rarely if ever straight, but deviates from a straight line under influence of a multitude of factors like airstreams, fixed air traffic corridors and the other airplane traffic. When looking at the trajectory of saccadic eye movements, one will also observe that the eyes are almost never moved in a straight line.
One of the first to report this finding was Yarbus (1967), who wrote that “saccades performed at an angle … are most frequently recorded as curved lines” (pp. 140). After this observation, many other researchers investigated this phenomenon and its origins (e.g. Viviani et al., 1977, Minken et al., 1993, Erkelens and Sloot, 1995). They found that there is substantial between-subjects variability, but the within-subject variability in the curvature in the trajectory of a simple eye movement is limited (Bahill and Stark, 1975). This led Smit and Van Gisbergen (1990) to describe dynamic properties of saccade curvature as a ‘signature’ (pp. 341): when saccading to the same location, idiosyncratic eye movement trajectories can be observed for each participant (Fig. 1). Although the exact cause of this phenomenon is not yet known, it has been suggested that saccade curvature is determined by mechanisms located in the final pathway of the eye movement production system (Smit and Van Gisbergen, 1990).
In addition to individual variation in trajectories, recent studies have revealed that environmental determinants can modify the idiosyncratic saccade trajectories. For instance, recent studies reported effects of the allocation of attention and the presence of an irrelevant distractor on saccade trajectories (i.e. Sheliga et al., 1994, Doyle and Walker, 2001, Van der Stigchel and Theeuwes, in press, Walker et al., in press). The current paper reviews these studies. We will claim that modifications observed in saccade trajectories are a measure of visual processing, and that they can inform us about the underlying mechanisms that control saccadic eye movements.
Two terms have been used to denote the environmental modifications of the baseline eye movement trajectory, namely ‘curvature’ and ‘deviation’. Although this description does not fit for all studies, ‘curvature’ is generally used to describe differences in trajectories from saccadic fixation to the saccade endpoint (i.e. whether the saccade was a straight line or a curved one), whereas ‘deviation’ measures mainly compare the saccade trajectory with a straight line from saccadic fixation to the designated target position. This last measure includes possible changes in saccade endpoints relative to the target location (see Appendix for an overview of the different trajectory measures). In this review, we consistently will use the term ‘deviation,’ because our overview concerns the influence of environmental factors on the total change of the trajectory of the saccade, including the saccade endpoint.
The studies we will review typically use a paradigm in which a central fixation cross is presented at the start of the trial. Participants are required to saccade to the location of a target that appears abruptly in the visual display (‘an abrupt onset’). The trajectory of this eye movement is the measure central to this review. Other important measures are saccade latencies (the time between target presentation and saccade initiation) and the correctness of the saccade. The experimental variations used to manipulate trajectories include the addition of distracting elements in the visual fields (‘distractors’) and manipulations of attentional allocation, expectancy, memory and inhibition of return. In these experiments participants are typically either humans or rhesus monkeys.
Saccade deviations can be divided into deviations towards or away from locations in the visual scene other than the target location. First, we will discuss conditions in which there is a deviation of the eye movement towards an element in the visual scene that is not the target. How these saccades come about is relatively well understood, and we will review both theoretical accounts of deviation towards and physiological evidence. In a later section, we review deviations away from non-target locations. It is much less clear how and why saccades deviate away from locations. We will review existing theories and discuss physiological evidence in line with each theory.
Section snippets
Deviation towards
In three situations deviation towards are observed: (1) in double step paradigms; (2) paradigms investigating the global effect and (3) in visual search paradigms. We discuss each in turn.
Deviation away
Paradigms in which deviation away occurs can be subdivided into two rough categories: those in which eye movements deviate away from irrelevant distractors and conditions in which saccades deviate away from a location to which attention is voluntary allocated. We will now discuss each in turn.
When do saccades curve towards and when away?
It may seem inconsistent that eye movements sometimes deviate towards, but in other cases deviate away from a location. For instance, what determines whether saccade trajectories deviate towards a distractor, as in visual search, or away from it as in many other reported studies? We have suggested that deviation towards results from the averaging of populations coding the target and the distractor, but that deviation away occurs when a distractor is either inhibited or when a compensating
Conclusion
Because vision is impaired during an eye movement, it is important that the flight is executed with the greatest possible speed to minimize the period of poor vision. The exact trajectory is therefore irrelevant to the observer, provided it is fast enough. Indeed, trajectories are seldom straight. On top of idiosyncratic, individually set deviations in normal saccade trajectories, trajectories have been found to deviate away or towards locations depending on the behavioral paradigm. Here, we
Acknowledgements
This research was funded by a grant from NWO (Netherlands organization for Scientific Research), grant 402-01-630-PROG to Jan Theeuwes.
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