Intracranial Recordings of Occipital Cortex Responses to Illusory Visual Events

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Within-subject variation in BOLD-fMRI signal changes across repeated measurements: Quantification and implications for sample size

Item does not contain fulltextFunctional magnetic resonance imaging (fMRI) can be used to detect experimental effects on brain activity across measurements. The success of such studies depends on the size of the experimental effect, the reliability of the measurements, and the number of subjects. Here, we report on the stability of fMRI measurements and provide sample size estimations needed for repeated measurement studies. Stability was quantified in terms of the within-subject standard deviation (σw) of BOLD signal changes across measurements. In contrast to correlation measures of stability, this statistic does not depend on the between-subjects variance in the sampled group. Sample sizes required for repeated measurements of the same subjects were calculated using this σw. Ten healthy subjects performed a motor task on three occasions, separated by one week, while being scanned. In order to exclude training effects on fMRI stability, all subjects were trained extensively on the task. Task performance, spatial activation pattern, and group-wise BOLD signal changes were highly stable over sessions. In contrast, we found substantial fluctuations (up to half the size of the group mean activation level) in individual activation levels, both in ROIs and in voxels. Given this large degree of instability over sessions, and the fact that the amount of within-subject variation plays a crucial role in determining the success of an fMRI study with repeated measurements, improving stability is essential. In order to guide future studies, sample sizes are provided for a range of experimental effects and levels of stability. Obtaining estimates of these latter two variables is essential for selecting an appropriate number of subjects.11 p

The impact of etiology in lesion-symptom mapping: A direct comparison between tumor and stroke

Introduction: Lesion-symptom mapping is a key tool in understanding the relationship between brain structures and behavior. However, the behavioral consequences of lesions from different etiologies may vary because of how they affect brain tissue and how they are distributed. The inclusion of different etiologies would increase the statistical power but has been critically debated. Meanwhile, findings from lesion studies are a valuable resource for clinicians and used across different etiologies. Therefore, the main objective of the present study was to directly compare lesion-symptom maps for memory and language functions from two populations, a tumor versus a stroke population. Methods: Data from two different studies were combined. Both the brain tumor (N = 196) and stroke (N = 147) patient populations underwent neuropsychological testing and an MRI, pre-operatively for the tumor population and within three months after stroke. For this study, we selected two internationally widely used standardized cognitive tasks, the Rey Auditory Verbal Learning Test and the Verbal Fluency Test. We used a state-of-the-art machine learning-based, multivariate voxel-wise approach to produce lesion-symptom maps for these cognitive tasks for both populations separately and combined. Results: Our lesion-symptom mapping results for the separate patient populations largely followed the expected neuroanatomical pattern based on previous literature. Substantial differences in lesion distribution hindered direct comparison. Still, in brain areas with adequate coverage in both groups, considerable LSM differences between the two populations were present for both memory and fluency tasks. Post-hoc analyses of these locations confirmed that the cognitive consequences of focal brain damage varied between etiologies. Conclusion: The differences in the lesion-symptom maps between the stroke and tumor population could partly be explained by differences in lesion volume and topography. Despite these methodological limitations, both the lesion-symptom mapping results and the post-hoc analyses confirmed that etiology matters when investigating the cognitive consequences of lesions with lesion-symptom mapping. Therefore, caution is advised with generalizing lesion-symptom results across etiologies

Widespread fMRI activity differences between perceptual states in visual rivalry are correlated with differences inobserver biases

When observing bistable stimuli, the percept can change in the absence of changes in the stimulus itself. When intermittently presenting a bistable stimulus, the number of perceptual alternations can increase or decrease, depending on the duration of the period that the stimulus is removed from screen between stimulus presentations (off-period). Longer off-periods lead to stabilization of the percept, while short off-periods produce perceptual alternations. Here we compare fMRI brain activation across percept repetitions and alternations when observing an intermittently presented ambiguously rotating structure from motion sphere. In the first experimental session, subjects were requested to voluntarily control the percept into either a repeating or an alternating perceptual regime at a single off-period. In a consecutive session, subjects observed the sphere uninstructed, and reported alternations and repetitions. The behavioral data showed that there were marked individual biases for observing the sphere as either repeating or alternating. The fMRI data showed activation differences between alternating and repeating perceptual regimes in an extensive network that included parietal cortex, dorsal premotor area, dorsolateral prefrontal cortex, supplementary motor area, insula, and cerebellum. However, these activation differences could all be explained by intersubject differences in the bias for one of the two perceptual regimes. The stronger the bias was for a particular perceptual regime, the less activation and vice versa. We conclude that widespread activation differences between perceptual regimes can be accounted for by differences in the perceptual bias for one of the two regime