Víctor A. Villagrá (Ed.), Burkhard Stiller, Report on Akogrimo Training Activities (D8.1.1), Version: 1, 2006. (Technical Report)
|
|
Julian Gallop, Report on Standardisation Activities (D6.2.1), Version: 1, 2006. (Technical Report)
|
|
Joseph Hellerstein, Burkhard Stiller, Management of integrated end-to-end comunications and services, Vancouver, BBC, Canada, 2006. (Book/Research Monograph)
|
|
Martin Waldburger, Cristian Morariu, Peter Racz, Jürgen Jähnert, Stefan Wesner, Burkhard Stiller, Grids in a Mobile World: Akogrimos Network and Business Views, No. 0, Version: 1, 2006. (Technical Report)
|
|
Cristian Morariu, Martin Waldburger, Burkhard Stiller, An Accounting and Charging Architecture for Mobile Grids, No. 0, Version: 1, 2006. (Technical Report)
|
|
David Hausheer, PeerMart: Secure Decentralized Pricing and Accounting for Peer-to-Peer Systems, Shaker Verlag, ISBN 3-8322-4969-9, Aachen, Germany, 2006. (Book/Research Monograph)
|
|
Jürgen Jähnert, Detailed Overall Architecture (D3.1.2), Version: 1, 2006. (Technical Report)
|
|
Burkhard Stiller, Thomas Bocek, Cristian Morariu, Peter Racz, Martin Waldburger, Internet Economics II, No. 0, Version: 1, 2006. (Technical Report)
|
|
Thomas Bocek, David Hausheer, Reinhard Riedl, Burkhard Stiller, Introducing CPU Time as a Scarce Resource in P2P Systems, No. 0, Version: 1, 2006. (Technical Report)
|
|
C Liston, S Matalon, Todd Anthony Hare, M C Davidson, B J Casey, Anterior cingulate and posterior parietal cortices are sensitive to dissociable forms of conflict in a task-switching paradigm, Neuron, Vol. 50 (4), 2006. (Journal Article)
The conflict-monitoring hypothesis posits that anterior cingulate cortex (ACC) monitors conflict in information processing and recruits dorsolateral prefrontal cortex (DLPFC) to resolve competition as needed. We used fMRI to test this prediction directly in the context of a task-switching paradigm, in which subjects responded to the color or the motion of a visual stimulus. Conflict was indexed in terms of the product of activities in areas specialized for color or motion processing on a trial-by-trial basis. Here, we report that ACC and posterior parietal cortex (PPC) were sensitive to distinct forms of conflict, at the level of the response and the stimulus representation, respectively. Activity in PPC preceded increased activity in DLPFC and predicted enhanced behavioral performance on subsequent trials. These findings suggest that ACC and PPC may act in concert to detect dissociable forms of conflict and signal to DLPFC the need for increased control. |
|
Christian Ruff, J Driver, Attentional preparation for a lateralized visual distractor: behavioral and fMRI evidence, Journal of Cognitive Neuroscience, Vol. 18 (4), 2006. (Journal Article)
Attending to the location of an expected visual target can lead to anticipatory activations in spatiotopic occipital cortex, emerging before target onset. But less is known about how the brain may prepare for a distractor at a known location remote from the target. In a psychophysical experiment, we found that trial-to-trial advance knowledge about the presence of a distractor in the target-opposite hemifield significantly reduced its behavioral cost. In a subsequent functional magnetic resonance imaging experiment with similar task and stimuli, we found anticipatory activations in the occipital cortex contralateral to the expected distractor, but no additional target modulation, when participants were given advance information about a distractor's subsequent presence and location. Several attention-related control structures (frontal eye fields and superior parietal cortex) were active during attentional preparation for all trials, whereas the left superior prefrontal and right angular gyri were additionally activated when a distractor was anticipated. The right temporoparietal junction showed stronger functional coupling with occipital regions during preparation for trials with an isolated target than for trials with a distractor expected. These results show that anticipation of a visual distractor at a known location, remote from the target, can lead to (1) a reduction in the behavioral cost of that distractor, (2) preparatory modulation of the occipital cortex contralateral to the location of the expected distractor, and (3) anticipatory activation of distinct parietal and frontal brain structures. These findings indicate that specific components of preparatory visual attention may be devoted to minimizing the impact of distractors, not just to enhancements of target processing. |
|
Christian Ruff, F Blankenburg, O Bjoertomt, S Bestmann, E Freeman, J-D Haynes, G Rees, O Josephs, R Deichmann, J Driver, Concurrent TMS-fMRI and psychophysics reveal frontal influences on human retinotopic visual cortex, Current Biology, Vol. 16 (15), 2006. (Journal Article)
Our results provide causal evidence that circuits originating in the human FEF can modulate activity in retinotopic visual cortex, in a manner that differentiates the central and peripheral visual field, with functional consequences for perception. More generally, our study illustrates how the new approach of concurrent TMS-fMRI can now reveal causal interactions between remote but interconnected areas of the human brain. |
|
A Galvan, Todd Anthony Hare, C E Parra, J Penn, H Voss, G Glover, B J Casey, Earlier development of the accumbens relative to orbitofrontal cortex might underlie risk-taking behavior in adolescents, Journal of Neuroscience, Vol. 26 (25), 2006. (Journal Article)
Adolescence has been characterized by risk-taking behaviors that can lead to fatal outcomes. This study examined the neurobiological development of neural systems implicated in reward-seeking behaviors. Thirty-seven participants (7-29 years of age) were scanned using event-related functional magnetic resonance imaging and a paradigm that parametrically manipulated reward values. The results show exaggerated accumbens activity, relative to prefrontal activity in adolescents, compared with children and adults, which appeared to be driven by different time courses of development for these regions. Accumbens activity in adolescents looked like that of adults in both extent of activity and sensitivity to reward values, although the magnitude of activity was exaggerated. In contrast, the extent of orbital frontal cortex activity in adolescents looked more like that of children than adults, with less focal patterns of activity. These findings suggest that maturing subcortical systems become disproportionately activated relative to later maturing top-down control systems, biasing the adolescent's action toward immediate over long-term gains. |
|
T Fangmeier, M Knauff, Christian Ruff, V Sloutsky, FMRI evidence for a three-stage model of deductive reasoning, Journal of Cognitive Neuroscience, Vol. 18 (3), 2006. (Journal Article)
Deductive reasoning is fundamental to science, human culture, and the solution of problems in daily life. It starts with premises and yields a logically necessary conclusion that is not explicit in the premises. Here we investigated the neurocognitive processes underlying logical thinking with event-related functional magnetic resonance imaging. We specifically focused on three temporally separable phases: (1) the premise processing phase, (2) the premise integration phase, and (3) the validation phase in which reasoners decide whether a conclusion logically follows from the premises. We found distinct patterns of cortical activity during these phases, with initial temporo-occipital activation shifting to the prefrontal cortex and then to the parietal cortex during the reasoning process. Activity in these latter regions was specific to reasoning, as it was significantly decreased during matched working memory problems with identical premises and equal working memory load. |
|
T S Woodward, T A Cairo, Christian Ruff, Y Takane, M A Hunter, E T C Ngan, Functional connectivity reveals load dependent neural systems underlying encoding and maintenance in verbal working memory, Neuroscience, Vol. 139 (1), 2006. (Journal Article)
One of the main challenges in working memory research has been to understand the degree of separation and overlap between the neural systems involved in encoding and maintenance. In the current study we used a variable load version of the Sternberg item recognition test (two, four, six, or eight letters) and a functional connectivity method based on constrained principal component analysis to extract load-dependent neural systems underlying encoding and maintenance, and to characterize their anatomical overlap and functional interaction. Based on the pattern of functional connectivity, constrained principal component analysis identified a load-dependent encoding system comprising bilateral occipital (Brodmann's area (BA) 17, 18), bilateral superior parietal (BA 7), bilateral dorsolateral prefrontal (BA 46), and dorsal anterior cingulate (BA 24, 32) regions. For maintenance, in contrast, constrained principal component analysis identified a system that was characterized by both load-dependent increases and decreases in activation. The structures in this system jointly activated by maintenance load involved left posterior parietal (BA 40), left inferior prefrontal (BA 44), left premotor and supplementary motor areas (BA 6), and dorsal cingulate regions (BA 24, 32), while the regions displaying maintenance-load-dependent activity decreases involved bilateral occipital (BA 17, 18), posterior cingulate (BA 23) and rostral anterior cingulate/orbitofrontal (BA 10, 11, 32) regions. The correlation between the encoding and maintenance systems was strong and negative (Pearson's r = -.55), indicting that some regions important for visual processing during encoding displayed reduced activity during maintenance, while subvocal rehearsal and phonological storage regions important for maintenance showed a reduction in activity during encoding. In summary, our analyses suggest that separable and complementary subsystems underlie encoding and maintenance in verbal working memory, and they demonstrate how constrained principal component analysis can be employed to characterize neuronal systems and their functional contributions to higher-level cognition. |
|
Philippe Tobler, J P O'Doherty, R J Dolan, W Schultz, Human neural learning depends on reward prediction errors in the blocking paradigm, Journal of Neurophysiology, Vol. 95 (1), 2006. (Journal Article)
Learning occurs when an outcome deviates from expectation (prediction error). According to formal learning theory, the defining paradigm demonstrating the role of prediction errors in learning is the blocking test. Here, a novel stimulus is blocked from learning when it is associated with a fully predicted outcome, presumably because the occurrence of the outcome fails to produce a prediction error. We investigated the role of prediction errors in human reward-directed learning using a blocking paradigm and measured brain activation with functional magnetic resonance imaging. Participants showed blocking of behavioral learning with juice rewards as predicted by learning theory. The medial orbitofrontal cortex and the ventral putamen showed significantly lower responses to blocked, compared with nonblocked, reward-predicting stimuli. In reward-predicting control situations, deactivation in orbitofrontal cortex and ventral putamen occurred at the time of unpredicted reward omissions. Responses in discrete parts of orbitofrontal cortex correlated with the degree of behavioral learning during, and after, the learning phase. These data suggest that learning in primary reward structures in the human brain correlates with prediction errors in a manner that complies with principles of formal learning theory. |
|
B Pleger, Christian Ruff, F Blankenburg, S Bestmann, K Wiech, Klaas Enno Stephan, A Capilla, K J Friston, R J Dolan, Neural coding of tactile decisions in the human prefrontal cortex, Journal of Neuroscience, Vol. 26 (48), 2006. (Journal Article)
The neural processes underlying tactile decisions in the human brain remain elusive. We addressed this question in a functional magnetic resonance imaging study using a somatosensory discrimination task, requiring participants to compare the frequency of two successive tactile stimuli. Tactile stimuli per se engaged somatosensory, parietal, and frontal cortical regions. Using a statistical model that accounted for the relative difference in frequencies (i.e., Weber fraction) and discrimination accuracy (i.e., correct or incorrect), we show that trial-by-trial relative frequency difference is represented linearly by activity changes in the left dorsolateral prefrontal cortex (DLPFC), the dorsal anterior cingulate cortex, and bilateral anterior insular cortices. However, a circumscribed region within the left DLPFC showed a different response pattern expressed as activity changes that were monotonically related to relative stimulation difference only for correct but not for incorrect trials. Our findings suggest that activity in the left DLPFC encodes stimulus representations that underlie veridical tactile decisions in humans. |
|
J J Geng, E Eger, Christian Ruff, A Kristjánsson, P Rotshtein, J Driver, On-line attentional selection from competing stimuli in opposite visual fields: effects on human visual cortex and control processes, Journal of Neurophysiology, Vol. 96 (5), 2006. (Journal Article)
We used fMRI to investigate competition and on-line attentional selection between targets and distractors in opposite visual hemifields. Displays comprised a high-contrast square-wave grating, defined as target by its orientation, presented alone (unilateral) or with a similar distractor of orthogonal orientation in the opposite hemifield (bilateral displays). The target appeared unpredictably on the left or right, precluding anticipatory attention to one side. We found greater activation in target-contralateral superior occipital gyrus for unilateral than for bilateral displays, indicating suppression of the target's visual representation by distractor presence despite the competing distractor projecting to a different occipital hemisphere. Several frontal and parietal regions showed greater activation for bilateral than unilateral trials, suggesting involvement in on-line attentional selection. This was particularly pronounced for regions in bilateral intraparietal sulcus (IPS), which also showed greater functional coupling with occipital cortex specifically on bilateral trials that required selection plus some repetition-suppression effects when target side was repeated, but again only on bilateral trials requiring selection. Our results indicate that competition between visual stimuli in opposite hemifields can influence occipital cortex, and implicate IPS in resolution of this competition by selection. |
|
B Pleger, F Blankenburg, S Bestmann, Christian Ruff, K Wiech, Klaas Enno Stephan, K J Friston, R J Dolan, Repetitive transcranial magnetic stimulation-induced changes in sensorimotor coupling parallel improvements of somatosensation in humans, Journal of Neuroscience, Vol. 26 (7), 2006. (Journal Article)
Repetitive transcranial magnetic stimulation (rTMS) is an established technique for non-invasive stimulation of human cortex. Although studies have shown an influence of rTMS on single cortical regions and on simple behavioral response patterns, its influences on the dynamics of task-related activity in cortical networks have not been characterized. We provide such a characterization by showing that 5 Hz rTMS over primary somatosensory cortex (SI) induces a reconfiguration of activity patterns in a sensorimotor network, comprising the stimulated region and ipsilateral primary motor cortex (MI). These plastic changes endure for up to 120 min and are correlated with behavioral improvement in discrimination. Dynamic causal modeling showed that this reconfiguration could be explained by an rTMS-induced increase in SI excitability (self-connection) and an increase in the effective connectivity from SI to MI. Thus, our data demonstrate that rTMS can temporarily induce behaviorally relevant reorganization within a complex cortical network underlying human somatosensory experience. |
|
T S Woodward, Christian Ruff, E T C Ngan, Short- and long-term changes in anterior cingulate activation during resolution of task-set competition, Brain Research, Vol. 1068 (1), 2006. (Journal Article)
Alternating between task sets involves detection that the current task set is unfavorable, initiation of a change in set, and application of the new task set while fine-tuning to optimally adjust to the demands of the environment. Functional magnetic resonance imaging (fMRI) studies of cognitive flexibility consistently report activation of the anterior cingulate cortex and/or adjacent pre-supplementary motor regions (ACC/pre-SMA, medial Brodmann's areas 24/32/6), suggesting that these cortical regions are involved in switching task set. In the current study, our objective was to probe whether ACC/pre-SMA activation would decrease for a number of trials following a switch in task set, implying longer-term involvement in fine-tuning adjustments. By measuring activation when switching between word reading and color naming in response to Stroop stimuli, ACC/pre-SMA activation was observed when actively countering the influence of the irrelevant task set, and this activation decreased as a function of the number of trials since a task switch. Basal ganglia and thalamic regions also displayed a decreased response over successive trials after task switches. These findings suggest that the ACC/pre-SMA are not only involved in generating a new course of action, but are also involved (along with subcortical regions) in fine-tuning operations that resolve competition between task sets over subsequent repetitions of the same task. |
|