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Abstract
The mechanisms underlying hemispheric specialization of memory are not completely understood. Functional magnetic resonance imaging (fMRI) can be used to develop and test models of hemispheric specialization. In particular for memory tasks however, the interpretation of fMRI results is often hampered by the low reliability of the data. In the present study we therefore analyzed the test-retest reliability of fMRI brain activation related to an implicit memory encoding task, with a particular focus on brain activity of the medial temporal lobe (MTL). Fifteen healthy subjects were scanned with fMRI on two sessions (average retest interval 35 days) using a commonly applied novelty encoding paradigm contrasting known and unknown stimuli. To assess brain lateralization, we used three different stimuli classes that differed in their verbalizability (words, scenes, fractals). Test-retest reliability of fMRI brain activation was assessed by an intraclass-correlation coefficient (ICC), describing the stability of inter-individual differences in the brain activation magnitude over time. We found as expected a left-lateralized brain activation network for the words paradigm, a bilateral network for the scenes paradigm, and predominantly right-hemispheric brain activation for the fractals paradigm. Although these networks were consistently activated in both sessions on the group level, across-subject reliabilities were only poor to fair (ICCs ≤ 0.45). Overall, the highest ICC values were obtained for the scenes paradigm, but only in strongly activated brain regions. In particular the reliability of brain activity of the MTL was poor for all paradigms. In conclusion, for novelty encoding paradigms the interpretation of fMRI results on a single subject level is hampered by its low reliability. More studies are needed to optimize the retest reliability of fMRI activation for memory tasks.
Highlights
Hemispheric specialization is a basic principle of human brain organization
We do not have precise models that explain which factors are responsible for hemispheric specialization, why the degree of lateralization varies from individual to individual, and how the brain integrates processes that are lateralized to opposite hemispheres
We found a left-lateralized brain activation network, with main activation centers located in the prefrontal cortex, the supplementary motor area, the inferior parietal cortex, the medial temporal lobe (MTL), and the cerebellum
Summary
Hemispheric specialization is a basic principle of human brain organization. We do not have precise models that explain which factors are responsible for hemispheric specialization, why the degree of lateralization varies from individual to individual, and how the brain integrates processes that are lateralized to opposite hemispheres. The investigation of brain lateralization is important from a neuroscientific perspective, but has clinical implications, for instance to better assess the long-term effects of a stroke or of a neurosurgical intervention. For instance the effects of damage to the left hemisphere on language performance might be less severe in individuals with bilateral or right-dominant language lateralization [2, 3]. In particular schizophrenia, have been associated with altered brain lateralization [4]. Any theory trying to describe the neural correlates of schizophrenia has to incorporate aspects of variability of hemispheric dominance
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