Differential brain mechanisms for processing distracting information in task-relevant and -irrelevant dimensions in visual search

Abstract

A crucial function of our goal-directed behavior is to select task-relevant targets among distractor stimuli, some of which may share properties with the target and thus compete for attentional selection. Here, by applying functional magnetic resonance imaging (fMRI) to a visual search task in which a target was embedded in an array of distractors that were homogeneous or heterogeneous along the task-relevant (orientation or form) and/or task-irrelevant (color) dimensions, we demonstrate that for both (orientation) feature search and (form) conjunction search, the fusiform gyrus is involved in processing the task-irrelevant color information, while the bilateral frontal eye fields (FEF), the cortex along the left intraparietal sulcus (IPS), and the left junction of intraparietal and transverse occipital sulci (IPTO) are involved in processing task-relevant distracting information, especially for target-absent trials. Moreover, in conjunction (but not in feature) search, activity in these frontoparietal regions is affected by stimulus heterogeneity along the task-irrelevant dimension: heterogeneity of the task-irrelevant information increases the activity in these regions only when the task-relevant information is homogeneous, not when it is heterogeneous. These findings suggest that differential neural mechanisms are involved in processing task-relevant and task-irrelevant dimensions of the searched-for objects. In addition, they show that the top-down task set plays a dominant role in determining whether or not task-irrelevant information can affect the processing of the task-relevant dimension in the frontoparietal regions.

Keywords: conjunction search; fMRI; feature search; task-irrelevant; task-relevant; visual search.

Figure 1

(a) Example of trial sequence and exemplar display with target‐present in Experiment 1 or Experiment 2. Stars (not shown in the real search display) in the display examples here are the remaining four positions after eight were randomly selected from the total 12 possible positions for search items to be displayed. The four experimental conditions of target‐present trials in Experiment 1 (b) and in Experiment 2 (c) in terms of variations along the task‐relevant and task‐irrelevant conditions. Re_hom = relevant dimension has homogeneous distractors, Re_het = relevant dimension has heterogeneous distractors; Ir_hom = irrelevant dimension has homogeneous colors; and Ir_het = irrelevant dimension has heterogeneous colors [Color figure can be viewed at wileyonlinelibrary.com]

Figure 2

Behavioral results. RTs (msec) with standard errors in terms of the experimental conditions in Experiments 1 and 2. See abbreviations in the legends of Figure 1

Figure 3

The brain activations related to processing task‐irrelevant heterogeneous information (ir_het vs. ir_hom) in Experiments 1 and 2, and the extracted beta values from these two regions [Color figure can be viewed at wileyonlinelibrary.com]

Figure 4

The activated regions (bilateral FEF, left IPS, and left IPTO) involved in the conjunction analysis of re_het versus re_hom across two experiments. The extracted beta values from these regions are reported in terms of the experimental conditions in both experiments [Color figure can be viewed at wileyonlinelibrary.com]

Figure 5

Correlation between beta values in frontoparietal brain regions (left IPTO in Experiment 1, and left FEF, left IPS, and left IPTO in Experiment 2) and behavioral RTs (standardized residuals, after controlling for variations along the task‐irrelevant dimension, the task‐relevant dimension, and target‐presence)