The Effect of Distractor Processing on the Distribution of Visual Attention

W. Ryan McGarry

Advisor: Craig McDonald, PhD, Department of Psychology

Committee Members: Pamela Greenwood, James Thompson

David J. King Hall, #2027
November 13, 2023, 01:00 PM to 03:00 PM


The distribution of visuospatial attention is integral for performing everyday tasks such as driving safely, finding an object on a cluttered desk, or identifying a friend in a crowd. There has been extensive research using search tasks to investigate the properties of visuospatial attention relating to phenomena such as scaling attention and surround suppression, although our understanding of how attention is distributed in visual search is still incomplete. A greater understanding of the enhancements and distortions imposed by visuospatial attention on target processing could be used to improve human performance on attentionally demanding tasks. To this end, we examined across three studies how various parameters that were designed to alter task difficulty modulate the distribution of visual attention and the suppression of distractor information surrounding a target of search. In Study 1, we investigated whether healthy older individuals would show a predicted heightened surround suppression compared to young individuals during search for a target in a closely spaced two-dimensional array of distractors. A probe was flashed at varying distances from the target to map the attentional distribution. For target sensitivity, a marginal two-way interaction between age group and probe-target distance was found. Exploratory post-hoc pairwise comparisons showed (weak) evidence for stronger surround suppression two degrees from the target location in the old group, with enhanced attention at the target and rebound of attention outside of the suppressed region. The young group showed a more shallow distribution, with attention focused at the target and decreasing attention as the probe appeared farther from the target. Little is known about the effects of aging on surround suppression, and this work sheds light on how age-related perceptual declines may be associated with an increased reliance on spatial cues. In Study 2, we investigated the impact of task difficulty on surround suppression by manipulating the shape of distractors to be more similar to the target. Using a similar search task to Study 1, we predicted that greater discrimination difficulty would elicit heightened surround suppression. A significant two-way interaction between difficulty and probe-target distance was found. The results showed stronger surround suppression between one to two degrees from the target when distractors were the most similar to the target (i.e. most difficult condition). Together, Studies 1 and 2 show suppression in the presence of greater task difficulty, either through age-related perceptual declines or target-distractor similarity, and at the same approximate distance between one to two degrees from the target. However, there still remains the question of how distractors with a more or less similar orientation to that of the target during visual search modulates surround suppression.  In Study 3, we investigated how task difficulty in the context of feature-based attention modulates surround suppression. We used a different search task that featured a radial array and manipulated the orientation of a single distractor near the target to be more or less similar in orientation to the target. We predicted a nearby distractor oriented similarly to the target would enhance suppression around the target. A significant two-way interaction between difficulty and probe-target distance was found. The results showed that when a nearby distractor was less similar in orientation to the target, there was a distribution of attention that resembled surround suppression. When the adjacent distractor was more similar in orientation to the target, attention was tightly focused over the target. These results contradicted our hypothesis. This could be due to a need to include both the target and distractor in the focus of attention in order to discriminate the target. That we observed less attention directed to the less similar distractor could be due to its greater similarity to the other distractors in the array. In sum, each of these studies revealed  dynamic changes in the distribution of visuospatial attention in response to varying requirements for suppressing distractors. Studies 1 and 2 offered distinct lines of evidence supporting the role of task difficulty in eliciting surround suppression, while Study 3 provided insights into contextual effects on the magnitude of surround suppression, dependent on the local nature of the effect of distractor properties in forming an environment for the target. These studies collectively highlight the dynamic nature of attentional distribution in response to the need to discriminate the target and suppress distractors.