Benjamin Rahm

Response Properties of Human Amygdala Subregions: Evidence Based on Functional MRI Combined with Probabilistic Anatomical Maps

The human amygdala is thought to play a pivotal role in the processing of emotionally significant sensory information. The major subdivisions of the human amygdala—the laterobasal group (LB), the superficial group (SF), and the centromedial group (CM)—have been anatomically delineated, but the functional response properties of these amygdala subregions in humans are still unclear. We combined functional MRI with cyto-architectonically defined probabilistic maps to analyze the response characteristics of amygdala subregions in subjects presented with auditory stimuli. We found positive auditory stimulation-related signal changes predominantly in probabilistically defined LB, and negative responses predominantly in SF and CM. In the left amygdala, mean response magnitude in the core area of LB with 90–100% assignment probability was significantly larger than in the core areas of SF and CM. These differences were observed for pleasant and unpleasant stimuli. Our findings reveal that the probabilistically defined anatomical subregions of the human amygdala show distinctive fMRI response patterns. The stronger auditory responses in LB as compared with SF and CM may reflect a predominance of auditory inputs to human LB, similar to many animal species in which the majority of sensory, including auditory, afferents project to this subdivision of the amygdala. Our study indicates that the intrinsic functional differentiation of the human amygdala may be probed using fMRI combined with probabilistic anatomical maps.

Transfer entropy in magnetoencephalographic data: Quantifying information flow in cortical and cerebellar networks

The analysis of cortical and subcortical networks requires the identification of their nodes, and of the topology and dynamics of their interactions. Exploratory tools for the identification of nodes are available, e.g. magnetoencephalography (MEG) in combination with beamformer source analysis. Competing network topologies and interaction models can be investigated using dynamic causal modelling. However, we lack a method for the exploratory investigation of network topologies to choose from the very large number of possible network graphs. Ideally, this method should not require a pre-specified model of the interaction. Transfer entropy--an information theoretic implementation of Wiener-type causality--is a method for the investigation of causal interactions (or information flow) that is independent of a pre-specified interaction model. We analysed MEG data from an auditory short-term memory experiment to assess whether the reconfiguration of networks implied in this task can be detected using transfer entropy. Transfer entropy analysis of MEG source-level signals detected changes in the network between the different task types. These changes prominently involved the left temporal pole and cerebellum--structures that have previously been implied in auditory short-term or working memory. Thus, the analysis of information flow with transfer entropy at the source-level may be used to derive hypotheses for further model-based testing.

Dissociable Contributions of Left and Right Dorsolateral Prefrontal Cortex in Planning

It is well established that the mid-dorsolateral prefrontal cortex (dlPFC) plays a critical role in planning. Neuroimaging studies have yielded predominantly bilateral dlPFC activations, but the existence and nature of functionally specific contributions of left and right dlPFC have remained elusive. In recent experiments, 2 independent parameters have been identified which substantially determine planning: 1) the degree of interdependence between consecutive steps (search depth) and 2) the degree to which the configuration of the goal state renders the order of single steps either clearly evident or ambiguous (goal hierarchy). Thus, search depth affects the actual mental generation and evaluation of action sequences, whereas goal hierarchy reflects the extraction of goal information from an encountered problem. Here, both parameters were independently manipulated in an event-related functional magnetic resonance imaging study using the Tower of London task. Results revealed a double dissociation as indicated by a significant crossover interaction of hemisphere and task parameter: in left dlPFC, activations were stronger for higher demands on goal hierarchy than on search depth, whereas the reversed result emerged in right dlPFC. In conclusion, often observed bilateral patterns of dlPFC activation in complex tasks may reflect the concomitant operation of specific cognitive processes that show opposing lateralizations.

What “Works” in Working Memory? Separate Systems for Selection and Updating of Critical Information

Cognition depends critically on working memory, the active representation of a limited number of items over short periods of time. In addition to the maintenance of information during the course of cognitive processing, many tasks require that some of the items in working memory become transiently more important than others. Based on cognitive models of working memory, we hypothesized two complementary essential cognitive operations to achieve this: a selection operation that retrieves the most relevant item, and an updating operation that changes the focus of attention onto it. Using functional magnetic resonance imaging, high-resolution oculometry, and behavioral analysis, we demonstrate that these two operations are functionally and neuroanatomically dissociated. Updating the attentional focus elicited transient activation in the caudal superior frontal sulcus and posterior parietal cortex. In contrast, increasing demands on selection selectively modulated activation in rostral superior frontal sulcus and posterior cingulate/precuneus. We conclude that prioritizing one memory item over others invokes independent mechanisms of mnemonic retrieval and attentional focusing, each with its distinct neuroanatomical basis within frontal and parietal regions. These support the developing understanding of working memory as emerging from the interaction between memory and attentional systems.

Reviewing the impact of problem structure on planning: a software tool for analyzing tower tasks.

Cognitive, clinical, and neuroimaging studies on planning abilities most frequently implement the Tower of London task or one of its variants. Yet, cumulating evidence from a series of experiments suggests that the commonly used approximation of problem difficulty in terms of the minimum number of moves for goal attainment is too coarse a measure for the underlying cognitive operations, and in some cases may be even misleading. Rather, problem difficulty can be more specifically characterized by a set of structural task parameters such as the number and nature of optimal and suboptimal solution paths, the required search depths, the patterns of intermediate and goal moves, goal hierarchies and the associated degree of ambiguity in the sequential ordering of goal moves. First applications in developmental and patient studies have proven fruitful in targeting fundamental alterations of planning abilities in healthy and clinical conditions. In addition, recent evidence from neuroimaging shows that manipulations of problem structure relate to separate cognitive and neural processes and are accompanied by dissociable brain activation patterns. Here, we briefly review these structural problem parameters and the concepts behind. As controlling for task parameters and selecting a balanced problem set is a complex and error-prone endeavor, we further present TowerTool, a software solution that allows easy access to in-depth analysis of the problem structure of widely used planning tasks like the Tower of London, the Tower of Hanoi, and their variants. Thereby, we hope to encourage and facilitate the implementation of structurally balanced task sets in future studies on planning and to promote transfer between the cognitive, developmental, and clinical neurosciences.

Human gamma-band activity and behavior

Human gamma-band activity (GBA) has been related to a variety of functions ranging from perception and attention to memory and consciousness. Indeed fast spectral activity derived during numerous experimental paradigms has been interpreted as providing support for the functional importance of these signals. The present review provides an overview of findings demonstrating direct or indirect associations between GBA and behavioral measures of task performance or perceptual experience in humans. While the majority of papers have focused on perception and awareness, relationships between GBA and behavior have also been observed during insightful problem solving, short- or long-term memory, and motor tasks. In these studies, GBA was reported to predict behavioral measures such as correct response rates or reaction times both in healthy subjects and patients with neuropsychiatric disorders. The review demonstrates that there is increasingly strong evidence for a close association between GBA during both perceptual and cognitive tasks and behavioral outcome measures. With this in mind, the investigation of GBA might help elucidate the mechanisms underlying deficient functioning in patients with neuropsychiatric disorders.

Eye movements and visuospatial problem solving: Identifying separable phases of complex cognition

Identifying overtly observable indicators of cognitive processes should provide a promising basis for a more precise tracking of the associated cognitive and neural events. In the current study we used recordings of eye movements to gain deeper insight into the time course of visuospatial problem solving as measured by the Tower of London. Single-trial, saccade-locked analyses revealed that, despite the complexity of the implemented task, gaze alternations between start and goal state followed a highly regular pattern. Consistent with the buildup of an internal representation, the first two fixations were of constant duration and unaffected by experimental variations of planning demands. Instead, planning manipulations exclusively influenced the duration of the very last fixation before problem execution. Our results demonstrate that different phases of complex cognition can be identified on a single-trial level using eye movement analyses.

Temporal dynamics of stimulus-specific gamma-band activity components during auditory short-term memory

Recently we have demonstrated that during auditory short-term memory maintenance, gamma-band activity (GBA) components can be identified which are specific to the retained stimulus. These activations peaked in the middle of the delay phase between sample and test stimuli, and their magnitude during the final part of this period correlated with performance. However, using a constant delay duration did not allow to answer the question whether stimulus-specific GBA components represented responses to sample sounds or anticipatory activations preceding test stimuli. Here we addressed this unresolved issue by investigating the temporal dynamics of stimulus-specific GBA during two delay durations. Magnetoencephalogram was recorded in 18 adults during an auditory spatial short-term memory task involving lateralized sample stimuli presented with two different interaural time delays. Subjects had to decide whether test stimuli presented after retention phases of 800 or 1200 ms had the same lateralization as sample sounds. Statistical probability mapping served to identify oscillatory activations differentiating between the two sample sounds. We found stimulus-specific GBA components over posterior cortex peaking about 400 ms prior to the onset of test stimuli regardless of delay duration. Their magnitude correlated with task performance. In summary, stimulus-specific GBA components with a predictive power for short-term memory performance were observed in anticipation of test stimuli. They may reflect the preparatory activation of memory representations or the shifting of attention to the specific expected location of the test stimulus.

Common capacity-limited neural mechanisms of selective attention and spatial working memory encoding.

One characteristic feature of visual working memory (WM) is its limited capacity, and selective attention has been implicated as limiting factor. A possible reason why attention constrains the number of items that can be encoded into WM is that the two processes share limited neural resources. Functional magnetic resonance imaging (fMRI) studies have indeed demonstrated commonalities between the neural substrates of WM and attention. Here we investigated whether such overlapping activations reflect interacting neural mechanisms that could result in capacity limitations. To independently manipulate the demands on attention and WM encoding within one single task, we combined visual search and delayed discrimination of spatial locations. Participants were presented with a search array and performed easy or difficult visual search in order to encode one, three or five positions of target items into WM. Our fMRI data revealed colocalised activation for attention‐demanding visual search and WM encoding in distributed posterior and frontal regions. However, further analysis yielded two patterns of results. Activity in prefrontal regions increased additively with increased demands on WM and attention, indicating regional overlap without functional interaction. Conversely, the WM load‐dependent activation in visual, parietal and premotor regions was severely reduced during high attentional demand. We interpret this interaction as indicating the sites of shared capacity‐limited neural resources. Our findings point to differential contributions of prefrontal and posterior regions to the common neural mechanisms that support spatial WM encoding and attention, providing new imaging evidence for attention‐based models of WM encoding.

Differential Impact of Continuous Theta-Burst Stimulation Over Left and Right DLPFC on Planning

Most neuroimaging studies on planning report bilateral activations of the dorsolateral prefrontal cortex (dlPFC). Recently, these concurrent activations of left and right dlPFC have been shown to double dissociate with different cognitive demands imposed by the planning task: Higher demands on the extraction of task‐relevant information led to stronger activation in left dlPFC, whereas higher demands on the integration of interdependent information into a coherent action sequence entailed stronger activation of right dlPFC. Here, we used continuous theta‐burst stimulation (cTBS) to investigate the supposed causal structure‐function mapping underlying this double dissociation. Two groups of healthy subjects (left‐lateralized stimulation, n = 26; right‐lateralized stimulation, n = 26) were tested within‐subject on a variant of the Tower of London task following either real cTBS over dlPFC or sham stimulation over posterior parietal cortex. Results revealed that, irrespective of specific task demands, cTBS over left and right dlPFC was associated with a global decrease and increase, respectively, in initial planning times compared to sham stimulation. Moreover, no interaction between task demands and stimulation type (real vs. sham) and/or stimulation side (left vs. right hemisphere) were found. Together, against expectations from previous neuroimaging data, lateralized cTBS did not lead to planning‐parameter specific changes in performance, but instead revealed a global asymmetric pattern of faster versus slower task processing after left versus right cTBS. This global asymmetry in the absence of any task‐parameter specific impact of cTBS suggests that different levels of information processing may span colocalized, but independent axes of functional lateralization in the dlPFC. Hum Brain Mapp, 2013.