fMRI and Eye Tracking in Visual Search

This case study examines the critical role of eye tracking in a recent fMRI study titled “Fixating targets in visual search: The role of dorsal and ventral attention networks in the processing of relevance and rarity,” by Ischebeck et al. (2024). The research aimed to disentangle the neural processes associated with task relevance and item rarity during visual search, a common everyday activity from finding a specific book on a shelf to locating a friend in a crowd. By integrating eye tracking with fMRI, the researchers gained a nuanced understanding of brain activity precisely at the moment participants fixated on different types of visual stimuli.

The core objective of the study was to identify brain areas responsible for processing task relevance, distinct from item rarity, during serial visual search. Traditional fMRI studies often struggle to isolate brain responses to specific events within a continuous cognitive task like visual search. Ischebeck et al. addressed this challenge by adopting a fixation-based analysis approach. Participants engaged in a multiple-target search task, reporting the number of targets among common and rare distractors. Importantly, all items shared similar visual features and could only be identified by direct foveation, preventing peripheral vision from guiding the search.

Eye tracking was indispensable to this methodology. A long-range SR Research Eyelink 1000 eye tracking system recorded eye movements at 500 Hz while participants performed visual search tasks in the fMRI scanner. This allowed researchers to precisely determine the moment of fixation on each item in the visual search array. This real-time, high-resolution eye movement data was then synchronized with the fMRI recordings, enabling the researchers to link specific brain activations to individual fixations on targets, rare distractors, and common distractors.

fMRI and Eye Tracking Methodology for Visual Search

The integration of eye tracking provided several crucial insights:

• Precise Event Definition: Fixations on specific items served as the “events of interest” for the fMRI analysis. This allowed for a highly precise event-related fMRI analysis, moving beyond traditional block designs that average brain activity over longer periods.
• Disentangling Relevance and Rarity: By recording exact fixations, the researchers could compare brain activity when participants fixated on a task-relevant target versus a rare, but irrelevant, distractor. This distinction was central to their objective.
• Gaze Measures: Eye tracking data provided valuable metrics beyond manual response times, including fixation duration and saccade amplitudes. The study found that target fixations lasted longer than other fixation types, and rare distractor fixations also lasted longer than common distractor fixations. These eye movement patterns complemented the fMRI findings, indicating the cognitive resources allocated to processing different item types.
• Controlling for Confounds: The ability to match fixations based on their serial position (fixation rank) across different conditions was vital. This “fixation rank matching” procedure, facilitated by the detailed eye-tracking data, helped control for potential confounds that might arise from general differences between search conditions or variability in the search process.

Targets and Rare Distractors Activate the Visual Cortex and Dorsal Attention More Strongly than Common Distractors

The study revealed that while both targets and rare distractors activated the visual cortex and dorsal attention network more strongly than common distractors, the left intraparietal sulcus (IPS) and left insula showed stronger activation specifically for targets compared to rare distractors. Furthermore, multi-voxel pattern analysis (MVPA), which leverages the fine-grained spatial information of fMRI data, showed that activation patterns in the temporo-parietal junction (TPJ), bilaterally, distinguished between target and rare distractor fixations. These findings suggest that the IPS, insula, and TPJ play a crucial role in processing task relevance, beyond mere item rarity.

The success of this study highlights how eye tracking provides an invaluable bridge between observable behavior and underlying neural processes during complex cognitive tasks. This research underscores the power of combining advanced neuroimaging techniques with detailed oculomotor recordings to unravel the intricacies of human attention and cognition.

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