Effects of Fixation-Spot on Fine-Scaled Functional Connectivity Within and Beyond the Visual Cortex
Poster No:
2081
Submission Type:
Abstract Submission
Authors:
Janine Mendola1, Austin Cooper2, Amir Shmuel1
Institutions:
1McGill University, Montreal, Quebec, 2McGill University, Montréal, Quebec
First Author:
Co-Author(s):
Introduction:
Movie viewing during fMRI elicits structured, rich, and reliable global brain dynamics in the visual cortex and beyond (Meer et al., 2020), and can decrease head motion (Cantlon & Li, 2013; Vanderwal et al., 2015). In addition, the primary visual cortex (V1) engages in non-random spontaneous activity during rest (K. Wang et al., 2008), particularly eccentricity-specific structured activity (Dawson et al., 2018). To date, experiments assessing movie functional magnetic resonance imaging (fMRI) scans have primarily focused on global brain activity (Demirtaş et al., 2019), meaning that the fine-scaled functional connectivity patterns in areas V1-V3 associated with movie-viewing are not well understood. One aspect that may be of interest to visual neuroscientists is the impact that a fixation spot may have on connectivity within the visual cortex during movie viewing. This is of interest since movie viewing paradigms generally allow participants to freely direct their gaze.
Methods:
To fill this gap, a total of 168 individuals from the HCP database, with both RS and free-viewing movie-viewing fMRI scans have been analysed, along with locally obtained RS and fixation-based movie-viewing fMRI of 39 subjects (same movie content throughout). An atlas that predicts V1-V3 retinotopy (Benson et al. 2012) was applied and used to create eccentricity-specific regions of interest (ROIs). Each eccentricity-based ROI was ensured to have the same number of voxels and the maximum eccentricity used was approximately the size of the movie screen leading to eccentricity intervals of (0.1-1.06°), (1.06-2.84°), (2.84-5.71°), (5.72-8.25°), and (8.25-13.96°). Additionally, a probabilistic atlas including many higher tier visual areas (L. Wang et al., 2015) was used to assess downstream effects beyond V1-V3. Functional connectivity between all ROIs was computed with Pearson's correlation.
To account for the fact that our local movies contained a fixation point, while HCP movie did not, a double subtraction was performed. Specifically, each dataset's RS correlation matrix was subtracted from its own respective movie viewing correlation matrix, and secondly the fixation-based movie-viewing was subtracted from the free-viewing condition. This double subtraction was performed since the scans were not collected at the same site and did not have the same field strength, thus the RS connectivity matrices effectively served as a normalization between the movie datasets.
To account for the fact that our local movies contained a fixation point, while HCP movie did not, a double subtraction was performed. Specifically, each dataset's RS correlation matrix was subtracted from its own respective movie viewing correlation matrix, and secondly the fixation-based movie-viewing was subtracted from the free-viewing condition. This double subtraction was performed since the scans were not collected at the same site and did not have the same field strength, thus the RS connectivity matrices effectively served as a normalization between the movie datasets.
Results:
Within the fine-scale eccentricity-based ROIs across V1-V3, the presence of a fixation point selectively decreased functional connectivity only between the smallest eccentricity bins (particularly 0.1-1.06° and 1.06-2.84°). This decreased connectivity strength was observed within and between lower-tier visual areas V1-V3. Effects of the fixation spot could also be observed extending into higher tier visual areas, where the smallest eccentricity representations consistently yielded less connectivity strengths with nearly all higher-order visual ROIs, including the frontal eye field. We attribute this effect to the reduction in visibility of movie content occluded by the fixation point, and the possible reduction in salience of the central-most region of the movie screen.
·Figure 1. Functional Connectivity Results
Conclusions:
fMRI is capable of detecting the presence of a fixation spot in movie-viewing data, showing reduced foveal-centric functional connectivity. Including a fixation point in movie stimuli is important in certain applications including clinical studies of ophthalmology patients with known (or hypothesized) visual field deficits. In addition, the use of a fixation point provides a very simple 'task' that can help to maximize the similarity of visual input to subjects and reduce the attentional modulation (e.g., Ki et al., 2016; Masuda et al., 2010). These findings should be considered for correct interpretation of data from healthy subjects and, for example, patients with glaucoma that are being currently studied.
Perception, Attention and Motor Behavior:
Attention: Visual 2
Perception: Visual 1
Keywords:
Cortex
Design and Analysis
FUNCTIONAL MRI
Perception
Vision
Other - functional connectivity
1|2Indicates the priority used for review
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Healthy subjects only or patients (note that patient studies may also involve healthy subjects):
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