Separating Processes within a Trial in Event-Related Functional MRI

Department of Radiology, Washington University, St. Louis, Missouri, 63110, USA.
NeuroImage (Impact Factor: 6.36). 02/2001; 13(1):210-7. DOI: 10.1006/nimg.2000.0710
Source: PubMed


Many behavioral paradigms involve temporally overlapping sensory, cognitive, and motor components within a single trial. The complex interplay among these factors makes it desirable to separate the components of the total response without assumptions about shape of the underlying hemodynamic response. We present a method that does this. Four conditions were studied in four subjects to validate the method. Two conditions involved rapid event-related studies, one with a low-contrast (5%) flickering checkerboard and another with a high-contrast (95%) checkerboard. In the third condition, the same high-contrast checkerboard was presented with widely spaced trials. Finally, multicomponent trials were formed from temporally adjacent low-contrast and high-contrast stimuli. These trials were presented as a rapid event-related study. Low-contrast stimuli presented in isolation (partial trials) made it possible to uniquely estimate both the low-contrast and high-contrast responses. These estimated responses matched those measured in the first three conditions, thereby validating the method. Nonlinear interactions between adjacent low-contrast and high-contrast responses were shown to be significant but weak in two of the four subjects.

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Available from: Maurizio Corbetta, Dec 06, 2014
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    • "The event-related design runs were analyzed using a separate GLM that independently modeled cue-only trials and cue + stimulus trials, as shown in Fig. 1B. Each trial type was modeled with 12 regressors in a finite impulse response model (Ollinger et al., 2001). The peak magnitude timepoint for all effects were modeled using the FIR model was defined as the timepoint with the greatest mean absolute difference from timethat -point zero (effect onset) across the four different task conditions as previously described (Elkhetali et al., 2015). "
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    ABSTRACT: Task sets are task-specific configurations of cognitive processes that facilitate task-appropriate reactions to stimuli. While it is established that the trial-by-trial deployment of visual attention to expected stimuli influences neural responses in primary visual cortex (V1) in a retinotopically specific manner, it is not clear whether the mechanisms that help maintain a task set over many trials also operate with similar retinotopic specificity. Here, we address this question by using BOLD fMRI to characterize how portions of V1 that are specialized for different eccentricities respond during distinct components of an attention-demanding discrimination task: cue-driven preparation for a trial, trial-driven processing, task-initiation at the beginning of a block of trials, and task-maintenance throughout a block of trials. Tasks required either unimodal attention to an auditory or a visual stimulus or selective intermodal attention to the visual or auditory component of simultaneously presented visual and auditory stimuli. We found that while the retinotopic patterns of trial-driven and cue-driven activity depended on the attended stimulus, the retinotopic patterns of task-initiation and task-maintenance activity did not. Further, only the retinotopic patterns of trial-driven activity were found to depend on the presence of inter-modal distraction. Participants who performed well on the intermodal selective attention tasks showed strong task-specific modulations of both trial-driven and task-maintenance activity. Importantly, task-related modulations of trial-driven and task-maintenance activity were in opposite directions. Together, these results confirm that there are (at least) two different processes for top-down control of V1: One, working trial-by-trial, differently modulates activity across different eccentricity sectors-portions of V1 corresponding to different visual eccentricities. The second process works across longer epochs of task performance, and does not differ among eccentricity sectors. These results are discussed in the context of previous literature examining top-down control of visual cortical areas. Copyright © 2015. Published by Elsevier Inc.
    Full-text · Article · Oct 2015 · NeuroImage
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    • "Compound trials required a yes-no response. In addition, participants were presented with 20 partial trials (Ollinger et al., 2001a,b) and 44 randomized short periods of rest, each equal in length to one trial. Partial trials presented only the classifier followed by a blank screen in the noun time window, and did not require a grammaticality judgment on part of the participants. "
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    Full-text · Article · Aug 2015 · Frontiers in Psychology
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    • "Of interest was whether the shape of the BOLD response to these relatively long video clips differed from the canonical HRF typically implemented in SPM. The shape of the BOLD response was estimated for each participant by modeling a finite impulse response function (Ollinger et al., 2001). Each trial was represented by a sequence of 12 consecutive TRs, beginning at the onset of each video clip. "
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