ABSTRACT
PURPOSE: To compare 2D and 3D echo-planar imaging (EPI) in a higher cognitive level fMRI paradigm. In particular, to study the link between the presence of task-correlated physiological fluctuations and motion and the fMRI contrast estimates from either 2D EPI or 3D EPI datasets, with and without adding nuisance regressors to the model. A signal model in the presence of partly task-correlated fluctuations is derived, and predictions for contrast estimates with and without nuisance regressors are made. MATERIALS AND METHODS: Thirty-one healthy volunteers were scanned using 2D EPI and 3D EPI during a virtual environmental learning paradigm. In a subgroup of 7 subjects, heart rate and respiration were logged, and the correlation with the paradigm was evaluated. FMRI analysis was performed using models with and without nuisance regressors. Differences in the mean contrast estimates were investigated by analysis-of-variance using Subject, Sequence, Day, and Run as factors. The distributions of group level contrast estimates were compared. RESULTS: Partially task-correlated fluctuations in respiration, heart rate and motion were observed. Statistically significant differences were found in the mean contrast estimates between the 2D EPI and 3D EPI when using a model without nuisance regressors. The inclusion of nuisance regressors for cardiorespiratory effects and motion reduced the difference to a statistically non-significant level. Furthermore, the contrast estimate values shifted more when including nuisance regressors for 3D EPI compared to 2D EPI. CONCLUSION: The results are consistent with 3D EPI having a higher sensitivity to fluctuations compared to 2D EPI. In the presence partially task-correlated physiological fluctuations or motion, proper correction is necessary to get expectation correct contrast estimates when using 3D EPI. As such task-correlated physiological fluctuations or motion is difficult to avoid in paradigms exploring higher cognitive functions, 2D EPI seems to be the preferred choice for higher cognitive level fMRI paradigms.
ABSTRACT
In rodents representations of environmental positions follow a granularity gradient along the hippocampal and entorhinal anterior-posterior axis; with fine-grained representations most posteriorly. To investigate if such a gradient exists in humans, functional magnetic resonance imaging data were acquired during virtual environmental learning of the objects' positions and the association between the objects and room geometry. The Objects-room geometry binding led to increased activation throughout the hippocampus and in the posterior entorhinal cortex. Within subject comparisons related specifically to the level of spatial granularity of the object position encoding showed that activation in the posterior and intermediate hippocampus was highest for fine-grained and medium-grained representations, respectively. In addition, the level of fine granularity in the objects' positions encoded between subjects correlated with posterior hippocampal activation. For the anterior hippocampus increased activation was observed for coarse-grained representations as compared to failed encoding. Activation in anterior hippocampus correlated with the number of environments in which the objects positions were remembered when permitting a coarse representation of positions. In the entorhinal cortex, activation in the posterior part correlated with level of fine granularity for the objects' positions encoded between subjects, and activation in the posterior and intermediate entorhinal cortex increased for medium-grained representations. This demonstrates directly that positional granularity is represented in a graded manner along the anterior-posterior axis of the human hippocampus, and to some extent entorhinal cortex, with most fine-grained positional representations posteriorly.
