Spatio-temporal dynamics of attentional selection stages during multiple object tracking
Introduction
Visual attention can be based on features (e.g.: color, motion, orientation) (, , ), space (e.g.: like a spotlight) (, , ) or entire objects (as integrated ensembles of features) (, ). A puzzling finding in this framework is the ability to simultaneously track the trajectories of several randomly moving objects (Pylyshyn and Storm 1988), which can not be fully explained by classic attention models (Pylyshyn 1989). However as during object tracking all items must be visually indistinguishable, their maintenance is believed to be an active task in nature (, ). Ideas of pre-attentive indexing of the tracked items as a possible mechanism have been experimentally disproved (Pylyshyn 2004). A dynamic assignment of attentional resources to multiple items, was suggested to underlie multiple object tracking, which was supported by findings of decreased tracking performance with higher attentional demands (more objects, higher speed, closer proximity) (, , ). Another idea (Yantis, 1992) was that subjects build an illusory figure by connecting the target items via imaginary lines and thereby track one object instead of multiple targets. Tracking performance improved indeed when subjects were instructed to perform in this way. In a previous study we could confirm these previous behavioral findings. Subjects who employed the illusory figure strategy elicited an early enhanced response in visual area LO to luminance changes of all tracked items compared to a partial set of targets and non targets at identical locations (Merkel et al., 2014). The lateral occipital complex is hereby involved in shape extraction (, , ).
Here we investigate the nature of this effect by examining the spatiotemporal dynamics of attentional modulations during multiple objects tracking. While subjects tracked four out of eight moving items a small stationary task-irrelevant probe-stimulus was briefly presented in the right upper visual field. Depending of the trajectories of the moving items the probe was either within the figure formed by the tracked target items, on its boundary or outside of it. Event-related potentials elicited by the probe were compared to reveal the spatial-temporal pattern of neural activity indexing the attentional gradient at the location of the probe. The prediction is that the event-related potentials to probes located within and on the boundary of the illusory figure will be different from those outside of it. Furthermore, we expect that the modulations occur within different timeframes corresponding to the spatiotemporal attentional distribution patterns during multiple objects tracking.
Section snippets
Subjects
31 paid volunteers participated in the current study approved by the local ethics committee after giving written informed consent. The mean age of all subjects was 30.9 years (SD=3.3) and all subjects had normal or corrected-to normal vision and were right handed. All subjects participated in at least one similar object-tracking task before, thus were experienced in tracking multiple objects.
Stimuli
Participants were seated inside a shielded room at a viewing distance of 100 cm in front of a video
Influence of the probes on task performance
In the debriefing after the experiment the majority of the subjects (26 out of 31) reported that they were not aware of the probe stimulus presented during the motion phase. Results of a separate analysis only including those 26 subjects revealed no differences from the current results (n=31). More importantly none of the subjects reported being in any way aware of the probes relative spatial relation to the targets. Furthermore, no subject reported explicitly the use of any specific tracking
Discussion
Previous work suggested that the tracking of multiple moving objects might be accomplished by building an illusory shape based on the spatial positions of the tracked targets (Yantis 1992). Recently, behavioral as well as neurophysiological evidence was presented to support this idea. In a task in which subjects indicated the full congruence of a probe with all tracked targets their behavior naturally divided them into a group that used the aforementioned strategy and another group that did not
Author contributions
C.M. designed and conducted the experiment, analyzed the data and wrote the initial draft of the manuscript. J-M.H. provided expertise and feedback. M.A.S. conceptualized the work, designed the experiment, supervised the investigation and wrote the final draft of the manuscript.
Conflict of interest
The authors declare no competing financial interests.
Acknowledgements
This work was supported by the Deutsche Forschungsgemeinschaft Grant SFB779/TPA1.
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