Browsing by Author "Krebs, Ruth M"
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Item Restricted High-field FMRI reveals brain activation patterns underlying saccade execution in the human superior colliculus.(PloS one, 2010) Krebs, Ruth M; Woldorff, Marty G; Tempelmann, Claus; Bodammer, Nils; Noesselt, Toemme; Boehler, Carsten N; Scheich, Henning; Hopf, Jens-Max; Duzel, Emrah; Heinze, Hans-Jochen; Schoenfeld, Mircea ABACKGROUND: The superior colliculus (SC) has been shown to play a crucial role in the initiation and coordination of eye- and head-movements. The knowledge about the function of this structure is mainly based on single-unit recordings in animals with relatively few neuroimaging studies investigating eye-movement related brain activity in humans. METHODOLOGY/PRINCIPAL FINDINGS: The present study employed high-field (7 Tesla) functional magnetic resonance imaging (fMRI) to investigate SC responses during endogenously cued saccades in humans. In response to centrally presented instructional cues, subjects either performed saccades away from (centrifugal) or towards (centripetal) the center of straight gaze or maintained fixation at the center position. Compared to central fixation, the execution of saccades elicited hemodynamic activity within a network of cortical and subcortical areas that included the SC, lateral geniculate nucleus (LGN), occipital cortex, striatum, and the pulvinar. CONCLUSIONS/SIGNIFICANCE: Activity in the SC was enhanced contralateral to the direction of the saccade (i.e., greater activity in the right as compared to left SC during leftward saccades and vice versa) during both centrifugal and centripetal saccades, thereby demonstrating that the contralateral predominance for saccade execution that has been shown to exist in animals is also present in the human SC. In addition, centrifugal saccades elicited greater activity in the SC than did centripetal saccades, while also being accompanied by an enhanced deactivation within the prefrontal default-mode network. This pattern of brain activity might reflect the reduced processing effort required to move the eyes toward as compared to away from the center of straight gaze, a position that might serve as a spatial baseline in which the retinotopic and craniotopic reference frames are aligned.Item Open Access Reward associations reduce behavioral interference by changing the temporal dynamics of conflict processing.(PLoS One, 2013) Krebs, Ruth M; Boehler, Carsten N; Appelbaum, Lawrence G; Woldorff, Marty GAssociating stimuli with the prospect of reward typically facilitates responses to those stimuli due to an enhancement of attentional and cognitive-control processes. Such reward-induced facilitation might be especially helpful when cognitive-control mechanisms are challenged, as when one must overcome interference from irrelevant inputs. Here, we investigated the neural dynamics of reward effects in a color-naming Stroop task by employing event-related potentials (ERPs). We found that behavioral facilitation in potential-reward trials, as compared to no-reward trials, was paralleled by early ERP modulations likely indexing increased attention to the reward-predictive stimulus. Moreover, reward changed the temporal dynamics of conflict-related ERP components, which may be a consequence of an early access to the various stimulus features and their relationships. Finally, although word meanings referring to potential-reward colors were always task-irrelevant, they caused greater interference compared to words referring to no-reward colors, an effect that was accompanied by a relatively early fronto-central ERP modulation. This latter observation suggests that task-irrelevant reward information can undermine goal-directed behavior at an early processing stage, presumably reflecting priming of a goal-incompatible response. Yet, these detrimental effects of incongruent reward-related words were absent in potential-reward trials, apparently due to the prioritized processing of task-relevant reward information. Taken together, the present data demonstrate that reward associations can influence conflict processing by changing the temporal dynamics of stimulus processing and subsequent cognitive-control mechanisms.Item Open Access The influence of different Stop-signal response time estimation procedures on behavior-behavior and brain-behavior correlations.(Behav Brain Res, 2012-04-01) Boehler, C Nicolas; Appelbaum, L Gregory; Krebs, Ruth M; Hopf, Jens-Max; Woldorff, Marty GThe fundamental cognitive-control function of inhibitory control over motor behavior has been extensively investigated using the Stop-signal task. The critical behavioral parameter describing stopping efficacy is the Stop-signal response time (SSRT), and correlations with estimates of this parameter are commonly used to establish that other variables (e.g., other behavioral measures or brain activity measures) are closely related to inhibitory motor control. Recently, however, it has been argued that SSRT estimates can be strongly distorted if participants strategically slow down their responses over the course of the experiment, resulting in the SSRT no longer reliably representing response-inhibition efficacy. Here, we performed new analyses on behavioral and functional data from an fMRI version of the Stop-signal task to gauge the consequences of using different SSRT estimation approaches that are differentially prone to the influence of strategic response slowing. The results indicate that the SSRT estimation approach can dramatically change behavior-behavior correlations. Specifically, a correlation between the SSRT and Go-trial accuracy that was highly significant with one estimation approach, virtually disappeared for the other. Additional analyses indeed supported that this effect was related to strategic response slowing. Concerning brain-behavior correlations, only the left anterior insula was found to be significantly correlated with the SSRT within the set of areas tested here. Interestingly, this brain-behavior correlation differed little for the different SSRT-estimation procedures. In sum, the current results highlight that different SSRT-estimation procedures can strongly influence the distribution of SSRT values across subjects, which in turn can ramify into correlational analyses with other parameters.Item Open Access The role of stimulus salience and attentional capture across the neural hierarchy in a stop-signal task.(PLoS One, 2011) Boehler, Carsten N; Appelbaum, Lawrence G; Krebs, Ruth M; Chen, Ling-Chia; Woldorff, Marty GInhibitory motor control is a core function of cognitive control. Evidence from diverse experimental approaches has linked this function to a mostly right-lateralized network of cortical and subcortical areas, wherein a signal from the frontal cortex to the basal ganglia is believed to trigger motor-response cancellation. Recently, however, it has been recognized that in the context of typical motor-control paradigms those processes related to actual response inhibition and those related to the attentional processing of the relevant stimuli are highly interrelated and thus difficult to distinguish. Here, we used fMRI and a modified Stop-signal task to specifically examine the role of perceptual and attentional processes triggered by the different stimuli in such tasks, thus seeking to further distinguish other cognitive processes that may precede or otherwise accompany the implementation of response inhibition. In order to establish which brain areas respond to sensory stimulation differences by rare Stop-stimuli, as well as to the associated attentional capture that these may trigger irrespective of their task-relevance, we compared brain activity evoked by Stop-trials to that evoked by Go-trials in task blocks where Stop-stimuli were to be ignored. In addition, region-of-interest analyses comparing the responses to these task-irrelevant Stop-trials, with those to typical relevant Stop-trials, identified separable activity profiles as a function of the task-relevance of the Stop-signal. While occipital areas were mostly blind to the task-relevance of Stop-stimuli, activity in temporo-parietal areas dissociated between task-irrelevant and task-relevant ones. Activity profiles in frontal areas, in turn, were activated mainly by task-relevant Stop-trials, presumably reflecting a combination of triggered top-down attentional influences and inhibitory motor-control processes.