Abstract
fMRI-based retinotopic mapping was used to assess systematic variations in activated cortical surface area, amplitude, and coherence across sessions. Seven healthy subjects were scanned at 7 T in three separate sessions with intervals of 51.4 ± 5.4 days (Sessions 1 and 2) and 167.9 ± 24.4 days (Sessions 2 and 3). We found a reduction between Sessions 1 and 2 for activated cortical surface area, between Sessions 1 and 3 for amplitude, and between Sessions 1 and 2/3 for coherence. The results do not support head motion as a major cause of the observed effect seen in Session 1, suggesting that cognitive effects were the underlying cause of change. The phase correlations for both eccentricity and polar angle mapping were highly correlated between sessions, demonstrating the stability of the maps. Furthermore, the sensitivity in determining inter-session changes of cortical surface area, response amplitude, and coherence were, at a 5% significance level, estimated to be 1.5, 6, and 5%, respectively. Any future longitudinal fMRI study should carefully evaluate activation across sessions to determine the eligibility of inclusion of all time points. This experimental design provides guidance in methodological issues of clinical longitudinal fMRI-studies, specifically regarding effects of subject experience.
Highlights
Functional magnetic resonance imaging is a widely used method for investigating human brain functions
Two of the subjects involved were unfamiliar with the procedure, the other five were experienced Functional magnetic resonance imaging (fMRI) subjects including one who had participated in a different retinotopic mapping study 10 months prior to session 1 of this study
For a quantitative account of the effects we determined for each visual area the activated cortical surface area, i.e., the voxels with significant fMRI-responses (p < 0.05; Figure 2)
Summary
Functional magnetic resonance imaging (fMRI) is a widely used method for investigating human brain functions. Retinotopic mapping is the gold standard to define visual field locations in the occipital cortex. Recent advances in fMRI using retinotopic mapping stimuli enable the identification of individual cortical visual areas and the functional specialization of the visual system. Besides fMRI, positron emission tomography (PET), and event-related potentials (ERPs) can be used to identify visual topography or retinotopic organization in the human visual cortex (Woldorff et al, 1997). There are a number of factors that influence fMRI results; these factors can stem from the image signal-to-noise ratio, motion artifacts, magnetic field inhomogeneities, cognitive processes, cognitive strategies over time, or other causes (Krüger and Glover, 2001; Huettel et al, 2008; Bennett and Miller, 2010).
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