Self-ordered search: A novel fMRI task to study working memory in children with catastrophic disease

Children treated for brain tumors are at increased risk for developing cognitive deficits. The self-ordered search (SOS) is a computerized neuropsychological test used to investigate working memory, a cognitive system whose function is integral to many high level cognitive processes. Functional MRI (fMRI) provides important opportunities to characterize neural correlates of SOS performance non-invasively. Implementation of the SOS task presents challenges in the unique environment of the MRI scanner. First, SOS requires participants to select a single stimulus from a set. Second, SOS is a behaviorally driven task that entails variable event timing among participants which complicates group analysis of fMRI data. The work presented here consists of the implementation, validation and application of the SOS for fMRI and associated analysis techniques. Eye-tracking with a MRI-safe response device was used as an interface for the fMRI task, allowing the participant to select an individual stimulus from a two-dimensional array. Performance information was used to generate individual subject design matrices for fMRI analysis, preserving important behavioral measures (time to completion). Healthy volunteers and patients treated for childhood brain tumors performed the SOS task and N-back task, a commonly used working memory task for fMRI. The eye-tracking interface performed well after initial problems with equipment and calibration routine were solved. Activation patterns identified by general linear model (GLM) analysis were similar between the SOS and N-back tasks and included dorsolateral prefrontal cortex, ventral prefrontal cortex, dorsal cingulate, bilateral premotor, and parietal areas.;Independent component analysis identified task-correlated components that were consistent with the GLM. Increasing activation across the general network was associated with fewer errors during the N-back task. Differences in activation between the patient group and healthy group were identified in the parietal and retrosplenial cortex. Analysis of the performance data suggests differences between the healthy and patient groups. Our novel eye-tracking interface provides a natural interface that controls for movement and motor planning associated complex response devices. The SOS for fMRI provides a new tool that will allow us to investigate deficits of working memory in children treated for brain tumors.