Research reportERP time course of perceptual and post-perceptual mechanisms of spatial selection
Introduction
Many studies have examined event-related potentials (ERP) to reveal the time course of visual attention. The tasks used have primarily been spatial cueing tasks in which attended and unattended stimuli are compared, or visual search tasks in which the target is presented in a visual array with one or more distractors. It is fairly well accepted that the mechanisms of attention for selection of spatial location are distinct from the mechanisms for selection of other cues such as colour, form, orientation, spatial frequency, and feature conjunctions [15], [26]. A robust finding is that there are timing differences between spatial and non-spatial selection. Spatial selection appears to have precedence in processing over attentional selection based on non-spatial cues [11], [17]. One way to characterize these differences is that spatial cues benefit from early attentional selection mechanisms and are more influenced by bottom-up exogenous input, whereas non-spatial cues rely on later mechanisms and are linked with top-down endogenous processes.
Although there is some evidence to suggest that it is possible to reverse the order of selection (e.g., colour prior to location) [17], the mechanisms which determine early or late selection of spatial location per se are not yet clear. This paper examines the nature of early perceptual and attentional processing of object information under conditions that make the spatial location of the target easy or difficult to extract from the visual display.
In Experiment 1, observers made simple decisions about the spatial location of object parts embedded in random-dot autostereograms. Extracting information about an object embedded in a random-dot stereogram requires processing that is additional to that needed to recognize a 2D line drawing of the same object. Form perception in random-dot stereograms is not available in the 2D representation of the image, but is based on the correlation between the information presented to each eye. Thus, the cyclopean object contours are defined by retinal disparity, and corresponding monocular dot patterns must be fused before the depth object can be identified [20].
Recent single cell work from monkeys suggests that, although V1 neurons respond to stereoscopic surfaces related to receptive field location [4], [40], stereoscopic edges and edge orientation are explicitly represented in area V2 [40]. This is interesting in light of the evidence that V2 cells are also important in the response to illusory contours [30], for example, the contours of a Kanizsa triangle (but see [9]). Essentially, the contours of the cyclopean object embedded in a random-dot stereogram are illusory in the same sense that the contours of a Kanizsa triangle are illusory. In such cases, the second-order features depend on the configuration of first-order features. The first-order features of random-dot stereograms are the dots, and the second-order features are the contours of the object perceived once the correct correspondence has been attained between the dot patterns represented on the left and right retinae.
The results of Experiment 1 suggested that simple selection of spatial location is delayed in time when the stimulus is defined by second-order features. The results may provide insight into the interpretation of ERP components for which the generator sites are not easily localized. Note that cells in V2 are selective for complex shapes [13], and could be important to timing differences for object analysis.
Section snippets
Participants
Twelve volunteers participated in a 2-h session. All had participated in previous experiments and training sessions that used the same stimuli [1] and were skilled at diverging their eyes to fuse the autostereograms. Two volunteers were eliminated from the final analysis due to excessive eye movements. An additional volunteer was eliminated from the ERP analysis due to excessive blinking which required discarding 40% of trials.
Stimuli
The stimuli used in Experiment 1 were single-image random-dot
Experiment 2a (random-dot pattern detection) and 2b (2D target localization)
One question we addressed in Experiment 2 asked whether the early N115 component in Experiment 1 was related to disparity processing or whether it was a function of the high frequency and high contrast display of random dots. The latter hypothesis is consistent with a study by Zani and Proverbio [42], who showed a similar early component in response to high frequency high contrast checks. If so, then it is possible that the N115 is an index of an attentional filtering process. Recent work by
Experiment 3: Target onset salience
The task in Experiment 3 was identical to the task in Experiments 1 and 2b in that a response was required to indicate the location of a single target. We used the same stimuli as we did in Experiment 2b, with only one procedural difference. The line drawing remained visible for the entire trial rather than a duration of 100 ms, and was immediately replaced by a new random-dot background and a new line drawing at the beginning of the next trial.
The result of the extended duration was to produce
General discussion
The results reported in this paper distinguish between two different kinds of visuospatial selection. We presented simple stimuli that required simple location detection of a single target, under conditions that required different degrees of visual analysis. Experiment 2a showed that the N115 observed in Experiment 1 was not related to spatial selection. When the targets appeared with abrupt onset, as they did in Experiment 2b and the Target Onset condition in Experiment 3, early spatial
Human subject ethics statement
All participants were volunteers and all experimental procedures and protocols were approved by the Human Ethics Board of McMaster University. Experiments were undertaken with the understanding and written consent of each volunteer.
Acknowledgements
We would like to thank Stephanie Hevenor, Shanty Ratnasingam, and Amanda Reid for assistance in data collection. We appreciate the comments of an anonymous reviewer on an earlier version of the manuscript. This research was supported by a Natural Sciences and Engineering Research Council of Canada Grant #OGP0170353 to Judith M. Shedden. Correspondence should be directed to Judith M. Shedden, 406 Psychology, McMaster University, Hamilton, Ontario, L8S 4K1. Electronic mail may be directed to
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