Research ReportAttention and selection for predictive smooth pursuit eye movements
Introduction
Humans typically cannot produce smooth eye movements without a moving target to follow. However, if a moving stimulus is presented repeatedly, humans are able to make predictive eye movements, i.e., begin to move their eyes at an appropriate velocity prior to the appearance of the moving target [5]. Indeed, humans are able to make predictive eye movements even if they fixated during the initial presentation and did not actively follow the moving target [7]. This has been attributed to a short-term velocity store [48]. In the current work, we investigated a more complex situation in which two moving targets are presented.
Many authors have emphasized the role of selection and attention in smooth pursuit [3], [4], [22], [29], [31], [35]. For example, Atkin [1] states that “…conflicting and redundant velocity information available to the pursuit control system can be utilized in a highly selective manner,” and it has been demonstrated that participants can track a moving target against stationary and moving backgrounds [29], [50]. More recent studies of elicited smooth pursuit in both humans and monkeys have shown that participants can selectively track one of two moving objects, when the selected object is either cued by the experimenter [15], [16], [32] or chosen by the participant [33]. In the latter case, monkeys typically make an initial eye movement that is the average of the two target directions, before making a saccade to one or other target, and exhibiting the velocity of the selected target post-saccade [17]. Furthermore, it was recently shown that when a saccade was produced by microstimulation which brought the monkey's eye to one of two moving targets, the monkey took on the velocity of the target at the end point of this evoked saccade [18]. This suggests that the velocities of both targets were represented prior to a decision being made about which target to follow [43].
Previous studies of selection and smooth pursuit have investigated how people select which object to track, from simultaneously presented moving objects. Here, we investigated the selective reproduction of previously viewed velocities. In an initial study, we developed a task in which participants fixated during the presentation of two targets moving at different velocities, before tracking one of the targets during a subsequent presentation (Fig. 1; [36]). We demonstrated that participants were able to make a predictive eye movement of an appropriate velocity, for one of two previously viewed targets. They were able to do so, despite being uncertain which of the two targets they would subsequently be required to follow. These results indicate that participants were able to exert cognitive control and select which previously viewed velocity to reproduce. Hereafter, the velocity of the target presented in the tracking presentation is referred to as the cued velocity, and the velocity of the target presented only during fixation is referred to as the uncued velocity.
This initial study threw up several questions. How does performance with two targets compare to performance with a single target? In other words, is there a cost to the dividing of resources between the 2 targets (henceforth referred to as the Divided condition)? And would there be any benefit in knowing the identity of the cued target in advance? Research into motion perception has shown that people can selectively attend to motion [10] or a particular direction of motion [2], [39], [44]. Third, what are the effects of the uncued velocity, i.e., the velocity of the target which is viewed but never tracked? In Experiment 1 of the current study, we compared the Divided condition to a Selective condition, in which the same target (upper or lower) was cued throughout a block. Both these conditions were compared to a control Single condition in which a single target was presented during fixation. In Experiment 2, we explored competing influences on the predictive eye movement in a particular trial, by systematically investigating the effects of the uncued velocity, the previously cued velocity and the previously uncued velocity.
Section snippets
Experiment 1
Each trial of the task involved 2 presentations: participants fixated during an initial presentation of moving stimuli and then tracked a single moving target on the next presentation. In the Divided condition, 2 targets were presented in the fixation condition, and a cue was presented before the tracking presentation to indicate which target they would be required to follow. In the Selective condition, the stimuli were the same as the Divided condition, but the same target was cued throughout
Experiment 2
There were two main aims of this experiment. First, to examine the effects of cued and uncued velocity in the Divided condition controlling for previously cued velocity and for switching between targets. Second, to directly examine the effect of the cued or uncued velocities presented in the previous trial and whether this effect is modulated by switching or remaining tracking the same target. This was accomplished using an interleaved trial design and only Divided and Single conditions; the
General discussion
We have demonstrated that participants are able to extract velocity information from two simultaneously moving targets and produce a subsequent predictive eye movement for just one of these targets (see also [36]). In the current study, the target always moved in a rightward direction, but we can conclude that the eye movements were predictive as they were at an appropriate velocity for the cued target; if participants were making a generic rightward movement, eye velocity would be at an
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