Markus Raabe

The Relationship between Brain Oscillations and BOLD Signal during Memory Formation: A Combined EEG–fMRI Study

Previous studies demonstrated that increases in the theta frequency band with concomitant decreases in the alpha/beta frequency band indicate successful memory formation. However, little is known about the brain regions and the cognitive processes that underlie these encoding-related oscillatory memory effects. We investigated this relationship using simultaneous EEG–fMRI recordings in humans during long-term memory encoding. In line with prior studies, we demonstrate that a decrease in beta power and an increase in theta power positively predict subsequent recall. In fMRI, stronger activity in the left inferior prefrontal cortex and the right parahippocampal gyrus correlated with successful memory formation. EEG source localization revealed that the subsequent memory effect in the beta band was localized in the left inferior prefrontal cortex, whereas the effect in the theta band was localized in medial temporal lobe regions. Trial-by-trial correlations between EEG and BOLD activity showed that beta power correlated negatively with left inferior prefrontal cortex activity. This correlation was more pronounced for items that could later be successfully recalled compared to items later forgotten. Based on these findings, we suggest that beta oscillations in the left inferior prefrontal cortex indicate semantic encoding processes, whereas theta oscillations in the medial temporal lobe reflect the binding of an item to its spatiotemporal context.

Prefrontally Driven Downregulation of Neural Synchrony Mediates Goal-Directed Forgetting

Neural synchronization between distant cell assemblies is crucial for the formation of new memories. To date, however, it remains unclear whether higher-order brain regions can adaptively regulate neural synchrony to control memory processing in humans. We explored this question in two experiments using a voluntary forgetting task. In the first experiment, we simultaneously recorded electroencephalography along with fMRI. The results show that a reduction in neural synchrony goes hand-in-hand with a BOLD signal increase in the left dorsolateral prefrontal cortex (dlPFC) when participants are cued to forget previously studied information. In the second experiment, we directly stimulated the left dlPFC with repetitive transcranial magnetic stimulation during the same task, and show that such stimulation specifically boosts the behavioral forgetting effect and induces a reduction in neural synchrony. These results suggest that prefrontally driven downregulation of long-range neural synchronization mediates goal-directed forgetting of long-term memories.

Evidence of fronto-temporal interactions for strategic inference processes during language comprehension.

We investigated how readers strategically infer context-appropriate information on the basis of the presented text and their world knowledge during passage reading. In the main experimental condition, participants were instructed to read short passages and to predict the development of the situation described in each passage during reading. To accomplish this task, we assumed that participants need to draw strategic inferences relevant to the contexts. Comparing this condition with a passage-reading condition without prediction, we found out that the left anterior prefrontal cortex (aPFC) in Brodmann area 9/10 and the left anterior ventral inferior frontal gyrus (vIFG) in Brodmann area 47 elicited increased hemodynamic responses. These two regions are probably critical in coherence evaluation and in drawing strategic inferences. Additionally, we used dynamic causal modelling (DCM) to investigate the fronto-temporal interactions induced by the experimental conditions. Ten models with different plausible ways to modulate the connections between frontal and temporal regions were compared. The DCM results showed a consistent conclusion: The connectivity between the left posterior superior temporal sulcus (pSTS) and the left dorsal lateral inferior frontal gyrus (dIFG) were enhanced when participants made inferential predictions during reading. The results support the role of top-down influences mediated by the neural pathways between dIFG and pSTS in retrieving strategic inferences. With these findings we discuss functional roles of aPFC, vIFG and dIFG-pSTS connections in drawing strategic inferences.

Neural indicators of inference processes in text comprehension: An event-related functional magnetic resonance imaging study

During language comprehension, readers or listeners routinely infer information that has not been stated literally in a given text or utterance in order to construct a coherent mental representation (situation model). We used a verification task in a behavioral study and in an event-related functional magnetic resonance imaging (fMRI) experiment to investigate the inference construction process. After having read sentences that mention the outcome of an event explicitly, implicitly, or not at all, participants verified the plausibility of short statements with respect to the context of the just read sentence. The results of the behavioral study established the verification task as a valid method for studying inferences. In the fMRI study, the dorsomedial prefrontal cortex was the most prominent area that was involved in the processing of inference statements. Regions in the left and right temporal lobes were associated with comparison processes that are based on the propositional representations of context sentence and test statements. The results are discussed with respect to levels of representations and the memory systems that underlie the verification process in the different sentence conditions.

Differential cortical activation during saccadic adaptation

The human saccadic system can dynamically adjust its gain if errors occur after saccade execution. Although this ability has long been studied, the underlying neural mechanisms and its functional purpose remain as of yet unclear. Using functional magnetic resonance imaging coupled with gaze-contingent visual stimulation, we compared brain activation before and after subjects adapted to a gaze-contingent shift in the target location (inward step). This comparison suggests the existence of a predictive signal related to the gain adjustment of upcoming saccades to decrease saccadic gain. Contrary to previous studies, we were able to identify activation differences in the supplementary eye fields that vary with the amount of saccadic gain decrease. In addition to signal amplitude differences in saccade-related eye fields, we also found active cortical regions in the temporal lobe and the posterior insula, which have been functionally related to vestibular processing and to the representation of head position and head motion. The results might point to new directions for research on saccadic adaptation pointing to the functional role of this mechanism.

Neural correlates of spatial working memory load in a delayed match-to-sample saccade task

We propose a new way to identify the neural correlates of memory load in a delayed match-to-sample saccade task with a constant perceptual load. Two conditions were compared with low and high memory loads. In the low-load condition, a rectangular shaped probe defined by its color and orientation was presented centrally. After a delay period, four stimuli were presented peripherally, one in each quadrant. The participants were instructed to saccade to the stimulus that matched the previously viewed sample on both color and orientation. In the high-load condition, the order of stimulus presentation was reversed: first four eccentric stimuli were presented and after a delay the central probe. In the high-load condition, the participant executed a saccade to the remembered location of the stimulus that matched the central probe in color and orientation. The behavioral results indicate that greater working memory load is associated with prolonged saccadic reaction times. A general linear model revealed regions in prefrontal cortex (left anterior insula, right superior and middle frontal gyrus, anterior medial cingulum), and bilaterally along the intraparietal sulcus extending into surrounding areas (precuneus, superior and inferior parietal lobe) that were more activated when participants had to conjointly remember the locations, colors and orientations of four objects (load-4) compared to when they only had to remember the features of a single object (load-1). Specific responses for greater working memory load are focused on regions responsible for feature binding (occipital-temporal cortex) and allocation of attention (anterior insular cortex). Multivariate pattern analysis during the retrieval period of a trial revealed voxel clusters in the ventral visual pathway and the frontal eye fields that correctly classify the target location during the retrieval period of both tasks.

Design of a new fMRI compatible haptic interface

In this paper, we present the design of a new fMRI compatible haptic interface with 3DOFs, based on electrical DC actuation, for the study of brain mechanisms of human motor control. In order to evaluate the validity of the proposed solution, we performed some preliminary experiments with a single degree of freedom device to test the compatibility with the fMRI environment. The 1DOF design was extended to the implementation of a 3DOFs parallel manipulator with 3UPU kinematics. Due to the dimensional constraints imposed by the fMRI environment, the choice of the dimensions and of the adopted mechanical solution was a result of an optimization process, that is presented in this work. Reachable workspace, kinematic isotropy, end-point stiffness, minimum force and translational clearance were studied in different configurations to select the stroke of the prismatic joint and the radius difference between the moving platform and the base. A further optimization of the mechanical design was then conducted in order to reduce the torque requested to the actuators for gravity compensation and consequently improve the performance of the manipulator. The final design resulted in a system capable of satisfying all the environment and user requirements.

Motor imagery of voluntary coughing: a functional MRI study using a support vector machine

Investigating respiratory acts using motor imagery has the advantage that motion artifacts are much less likely to occur. To test whether motor imagery of voluntary coughing shows similar spatiotemporal activity patterns as compared to overt coughing, 12 participants underwent functional MRI scanning performing both tasks. We analyzed the data using a pattern classifier, that is, a support vector machine. Results showed that during imagined coughing, a number of brain areas reported previously to be involved in respiration showed more similarity in their spatiotemporal activity patterns with overt coughing than with a resting baseline. We conclude that motor imagery can be a suitable paradigm to investigate respiration, and that support vector machine analysis is potentially more sensitive and specific than a standard univariate analysis.