Biomedical subjects

Torsten Schubert

Publications and source records attributed to Torsten Schubert.

7 recordsLinked to original sources

PubMed2005

Veratrol-O-demethylase of Acetobacterium dehalogenans: ATP-dependent reduction of the corrinoid protein.

The anaerobic veratrol O-demethylase mediates the transfer of the methyl group of the phenyl methyl ether veratrol to tetrahydrofolate. The primary methyl group acceptor is the cobalt of a corrinoid protein, which has to be in the +1 oxidation state to bind the methyl group. Due to the negative redox potential of the cob(II)/cob(I)alamin couple, autoxidation of the cobalt may accidentally occur. In this study, the reduction of the corrinoid to the superreduced [Co(I)] state was investigated. The ATP-dependent reduction of the corrinoid protein of the veratrol O-demethylase was shown to be dependent on titanium(III) citrate as electron donor and on an activating enzyme. In the presence of ATP, activating enzyme, and Ti(III), the redox potential versus the standard hydrogen electrode (E (SHE)) of the cob(II)alamin/cob(I)alamin couple in the corrinoid protein was determined to be -290 mV (pH 7.5), whereas E (SHE) at pH 7.5 was lower than -450 mV in the absence of either activating enzyme or ATP. ADP, AMP, or GTP could not replace ATP in the activation reaction. The ATP analogue adenosine-5'-(beta,gamma-imido)triphosphate (AMP-PNP, 2-4 mM) completely inhibited the corrinoid reduction in the presence of ATP (2 mM).

Acetobacterium↗

PubMed2005

The neural effect of stimulus-response modality compatibility on dual-task performance: an fMRI study.

Recent fMRI studies suggest that the inferior frontal sulcus (IFS) is involved in the coordination of interfering processes in dual-task situations. The present study aims to further specify this assumption by investigating whether the compatibility between stimulus and response modalities modulates dual-task-related activity along the IFS. It has been shown behaviorally that the degree of interference, as measured by dual-task costs, increases in modality-incompatible conditions (e.g. visual-vocal tasks combined with auditory-manual tasks) as compared to modality-compatible conditions (e.g. visual-manual tasks combined with auditory-vocal tasks). Using fMRI, we measured IFS activity when participants performed modality-compatible and modality-incompatible single and dual tasks. Behaviorally, we replicated the finding of higher dual-task costs for modality-incompatible tasks compared to modality-compatible tasks. The fMRI data revealed higher activity along the IFS in modality-incompatible dual tasks compared with modality-compatible dual tasks when inter-individual variability in functional brain organization is taken into account. We argue that in addition to temporal order coordination (Szameitat et al., 2002), the IFS is involved in the coordination of cognitive processes associated with the concurrent mapping of sensory information onto corresponding motor responses in dual-task situations.

Association Learning↗

PubMed2005

Task-order coordination in dual-task performance and the lateral prefrontal cortex: an event-related fMRI study.

A crucial demand in dual tasks suffering from a capacity limited processing mechanism is task-order scheduling, i.e. the control of the order in which the two component tasks are processed by this limited processing mechanism. The present study aims to test whether the lateral prefrontal cortex (LPFC) is associated with this demand. For this, 15 participants performed a psychological refractory paradigm (PRP) type dual task in an event-related functional magnetic resonance (fMRI) experiment. In detail, two choice reaction tasks, a visual (response with right hand) and an auditory (response with left hand), were presented with a temporal offset of 200 ms, while the participants were required to respond to the tasks in the order of their presentation. Importantly, the presentation order of the tasks changed randomly. Based on previous evidence, we argue that trials in which the present task order changed as compared to the previous trial (different-order trials) impose higher demands on task coordination than same-order trials do. The analyses showed that cortical areas along the posterior part of the left inferior frontal sulcus as well as the right posterior middle frontal gyrus were more strongly activated in different-order than in same-order trials, thus supporting the conclusion that one function of the LPFC for dual-task performance is the temporal coordination of two tasks. Furthermore, it is discussed that the present findings favour the active scheduling over the passive queuing hypothesis of dual-task processing.

Adult↗

PubMed2004

Aging and input processing in dual-task situations.

The psychological refractory period (PRP) paradigm was used to test whether older participants suffer from input interference in dual-task situations. Young (24 years) and older (57 years) adults gave speeded responses to 2 successively presented stimuli. The results showed increased susceptibility of older participants to input interference. Further experiments revealed that this input interference is related to the salience of the 2nd stimulus and that it is specific to older participants. Our findings indicate that parallel processing at the input stages of dual-task performance requires cognitive control. An age-related decline in the control of input processes should be considered as one source of age effects in dual-task performance.

Adolescent↗

PubMed2003

Functional neuroanatomy of interference in overlapping dual tasks: an fMRI study.

A basic characteristic of the human action and cognition system is the occurrence of interference when participants attempt to perform two tasks at the same time. Such interference has been studied for a long time with so-called overlapping dual tasks, where two stimuli presented in rapid succession require separate responses. As an indicator of interference, reaction times on the second stimulus increase the smaller the interval between both tasks. While most behavioral studies investigated the temporal dynamics of the interference, we focused on the functional neuroanatomy of overlapping dual-task performance by using functional magnetic resonance imaging (fMRI). Participants were asked to perform two choice reaction tasks concurrently [Pashler, Psychol. Bull., 116 (1994) 220-244]. When activation in this overlapping dual-task situation was compared with the summed activation of the single component tasks, activation in the prefrontal, temporal, parietal, and occipital cortices was detected. These data suggest that the processing of the overlapping dual tasks requires an extensive and distributed network of processing centers. However, the main focus of the dual-task-related activation was located in regions surrounding the left inferior frontal sulcus. Based on our findings and on findings of other recent neuroimaging studies, we argue that activation of the left inferior frontal sulcus reflects increased synaptic activity related to the need to manage interfering information in order to determine the appropriate action.

Adult↗

PubMed2002

Localization of executive functions in dual-task performance with fMRI.

We report a study that investigated the neuroanatomical correlates of executive functions in dual-task performance with functional magnetic resonance imaging. Participants performed an auditory and a visual three-choice reaction task either separately as single tasks or concurrently as dual tasks. In the dual-task condition, two stimuli were presented in rapid succession to ensure interference between the component tasks (psychological refractory period). The behavioral data showed considerable performance decrements in the dual-task compared to the single-task condition. Dual-task-related activation was detected with two different neuroimaging methods. First, we determined dual-task-related activation according to the method of cognitive subtraction. For that purpose, activation in the dual-task was compared directly with activation in the single-task conditions. This analysis revealed that cortical areas along the inferior frontal sulcus (IFS), the middle frontal gyrus (MFG), and the intraparietal sulcus (IPS) are involved in dual-task performance. The results of the subtraction method were validated with the method of parametric manipulation. For this purpose, a second dual-task condition was introduced, where the difficulty of the dual-task coordination was increased compared with the first dual-task condition. As expected, behavioral dual-task performance decreased with increased dual-task difficulty. Furthermore, the increased dual-task difficulty led to an increase of activation in those cortical regions that proved to be dual-task related with the subtraction method, that is, the IFS, the MFG, and the IPS. These results support the conclusion that dorsolateral prefrontal and superior parietal cortices are involved in the coordination of concurrent and interfering task processing.

Acoustic Stimulation↗

PubMed2002

A non-dechlorinating strain of Dehalospirillum multivorans: evidence for a key role of the corrinoid cofactor in the synthesis of an active tetrachloroethene dehalogenase.

A strain of Dehalosprillum multivorans, designated strain N, was isolated from the same source as the formerly described tetrachloroethene (PCE)-dechlorinating D. multivorans, herein after referred to as strain K. Neither growing cells nor cell extracts of strain N were able to dechlorinate PCE. The pceA and pceB genes encoding for the PCE-reductive dehalogenase were detected in cells of strain N; and they were 100% homologous to the corresponding genes of strain K. Since the PCE dehalogenase of D. multivorans strain K contains a corrinoid cofactor, the corrinoids of strain N cells were extracted. Analysis of the corrinoids revealed the absence of the specific corrinoid, which is the cofactor of the PCE dehalogenase of strain K cells. RT-PCR of mRNA indicated that the pceA gene was transcribed in strain N cells to a far lower extent than the pceA gene of strain K under the same experimental conditions. Western blot analysis of crude extracts of strain N showed that, if at all, an insignificant amount of the apoprotein of the PCE dehalogenase was present. The results indicate that the inability of strain N to dechlorinate is due to the absence of the corrinoid cofactor of the enzyme mediating PCE dechlorination.

Chromatography, High Pressure Liquid↗