Biomedical subjects

S Pollmann

Publications and source records attributed to S Pollmann.

At least 19 recordsLinked to original sources

PubMed2006

Many roads lead to "auxin": of nitrilases, synthases, and amidases.

Recent progress in understanding the biosynthesis of the auxin, indole-3-acetic acid (IAA) in Arabidopsis thaliana is reviewed. The current situation is characterized by considerable progress in identifying, at the molecular level and in functional terms, individual reactions of several possible pathways. It is still too early to piece together a complete picture, but it becomes obvious that A. thaliana has multiple pathways of IAA biosynthesis, not all of which may operate at the same time and some only in particular physiological situations. There is growing evidence for the presence of an indoleacetamide pathway to IAA in A. thaliana, hitherto known only from certain plant-associated bacteria, among them the phytopathogen Agrobacterium tumefaciens.

Amidohydrolases↗

PubMed2006

ERP and fMRI correlates of endogenous and exogenous focusing of visual-spatial attention.

The aim of this study was to investigate the neural correlates of the functional distinction underlying attentional mechanisms of endogenous-sustained and exogenous-transient spatial selection. We recorded event related potentials (ERPs) and used functional magnetic resonance imaging (fMRI) in separate experiments while subjects performed a simple reaction time (RT) to the same visual stimulus displayed to one of several field locations. Endogenous-sustained or exogenous-transient focusing of attention onto target location were obtained by presenting the stimulus in blocks of same-point vs. randomised-point trials, respectively. Same-point stimuli yielded overall faster RT than randomised stimuli, indicating a facilitating effect of endogenous-sustained spatial attention on the perceptual processing of the impending stimulus. Moreover, same-point vs. randomised presentations revealed significant increases in the fMRI signal in the bilateral lingual and fusiform gyri as well as in the right calcarine sulcus, in conjunction with a larger amplitude of the posterior P1 component of ERPs, but no modulation of the amplitude of the N1 component. Rather, a larger amplitude of N1 was found in the reverse contrast, randomised minus same-point trials, which revealed increases in the fMRI signal along the posterior left superior frontal sulcus and bilaterally in the superior precuneus. These findings indicate that N1 indexes exogenous orienting of attention and is likely to represent the activity of frontal and parietal components of the attention network involved in eliciting attention changes. In contrast, the effects of those changes, resulting in a modulation of activation in visual occipital areas, are indexed by P1.

Adolescent↗

PubMed2004

Splenial lesions lead to supramodal target detection deficits.

Patients with lesions of the splenium showed higher validity effects of visuospatial cues than did patients with partial lesions of the corpus callosum anterior to the splenium and control participants. Many of the patients tested had also shown a left-ear suppression for consonant-vowel syllables in a previous dichotic listening study. The authors interpret these parallel findings as evidence for the disruption of signals that normally alert the individual to the presence of behaviorally relevant stimuli, possibly originating in the temporoparietal junction area. After splenial lesions, these signals may not reach the contralateral hemisphere, leading to supramodal deficits in target detection, especially under distracting conditions.

Adult↗

PubMed2003

Separating distractor rejection and target detection in posterior parietal cortex--an event-related fMRI study of visual marking.

Successful survival in a competitive world requires the employment of efficient procedures for selecting new in preference to old information. Recent behavioral studies have shown that efficient selection is dependent not only on properties of new stimuli but also on an intentional bias that we can introduce against old stimuli. Event-related analysis of functional magnetic resonance imaging data from a task involving visual search across time as well as space indicates that the superior parietal lobule is specifically involved in processes leading to the efficient segmentation of old from new items, whereas the temporoparietal junction area and the ascending limb of the right intraparietal sulcus are involved in the detection of salient new items and in response preparation. The study provides evidence for the functional segregration of brain regions within the posterior parietal lobe.

Adult↗

PubMed2002

Top-down controlled visual dimension weighting: an event-related fMRI study.

Target detection in visual singleton feature search is slowed when consecutive targets are defined in different visual dimensions. Behavioral data provide evidence that attentional weight needs to be shifted between dimension-specific processing modules. We found similar dimension-specific change effects in a conjunction search task, in which observers searched for an odd-one-out target defined by a unique combination of size and color or, respectively, size and motion direction. Changes of the secondary target dimension (color or motion) across trials, but not target feature changes within a dimension, increased the time required to detect the target. Dimensional change costs were greatly increased for singleton conjunction search compared to singleton feature search. This suggests involvement of top-down control processes in dimensional change in conjunction search, in contrast to stimulus-driven dimensional change in singleton feature search. The functional anatomical correlates of top-down controlled visual dimension changes were investigated in two event-related functional magnetic resonance imaging (fMRI) experiments. In Experiment 1, dimensional change in singleton conjunction search was accompanied by transient activations in a fronto-posterior network of brain areas that was largely non-overlapping with the general network activated during visual search. Experiment 2, which contrasted singleton feature and conjunction search within the same session, revealed a double dissociation in anterior prefrontal cortex: left frontopolar cortex was selectively involved in stimulus-driven dimension changes but not in top-down controlled dimension changes, whereas the reverse was observed in frontomedian cortex.

Adult↗

PubMed2001

Switching between dimensions, locations, and responses: the role of the left frontopolar cortex.

We review event-related fMRI data regarding the role of the left lateral frontopolar cortex (LFPC) in attentional switching processes. We found LFPC activation when subjects had to reallocate attentional resources, either between visual dimensions (color and motion) or between locations. However, LFPC activation during these dimension or location switches was observed only when subjects had to counteract stimulus-driven attention to an invalid dimension or location. LFPC was not activated following changes of stimulus-response associations. Further experiments will have to show whether LFPC is actively involved in the reallocation of attentional resources or whether it rather has a monitoring function.

Attention↗

PubMed2000

Dissociation of memory retrieval and search processes: an event-related fMRI study.

A cognitive task can often be subdivided into several subprocesses, which follow a specific temporal order. Here, we report an event-related functional magnetic resonance imaging (fMRI) experiment on memory search, in which the temporal onset of search in primary memory was varied relative to retrieval from secondary memory. Furthermore, previous behavioral studies demonstrated that search times in primary memory depend on the number of items in a memory set, whereas retrieval from secondary memory is a set-size independent process. We analyzed the dependency of the blood oxygenation level dependent (BOLD)-response on the temporal onset of memory search on the one hand and on memory set size on the other hand to differentiate the contribution of retrieval from secondary memory, maintenance in primary memory, item search in primary memory, and response-related processes. The timing of activation followed cue presentation bilaterally in the middle frontal gyri (Brodmann area (BA) 9,46) and the inferior parts of the precentral gyri (BA6). In all other regions of interest (ROI), supplementary motor area (SMA), posterior parietal cortex, antero-superior insula, and primary motor cortex, the onset of activation was delayed with delayed probe presentation, ruling out participation in retrieval from secondary memory. The amplitude of the BOLD-response increased with increasing memory set size in all ROI except primary motor cortex and left posterior parietal cortex. All areas with cue-associated BOLD onset, suggesting involvement in retrieval, showed prolonged BOLD activation, suggesting that they also support maintenance of the retrieved information.

Adult↗

PubMed2000

Extinction-like effects in normals: independence of localization and response selection.

An extinction-like effect in normal subjects was previously elicited when a low-salience target in the left was simultaneously presented with a highly salient distractor in the right visual hemifield, but not vice versa (Pollmann, 1996). We investigated in four experiments whether this extinction-like effect depends on (a) explicit localization and (b) response competition. It was found that the extinction-like effect could be replicated in the absence of both. In contradistinction to our previous results, low-salience distractors had no effect on pop-out target search. This showed that explicit spatial localization demands lead to low-salience distractor interference on pop-out search.

Adult↗

PubMed2000

Object working memory and visuospatial processing: functional neuroanatomy analyzed by event-related fMRI.

We report an event-related functional magnetic-resonance-imaging (fMRI) experiment that investigates the relationship of transient visual object memory, visuospatial orienting, and object recognition. Delayed object matching and visuospatial orienting involved a highly overlapping network of brain areas. Common areas were the frontal eye fields (FEF), the pre-supplementary motor area (pre-SMA)/SMA complex, the precentral gyri, and the horizontal and descending branches of the intraparietal sulcus (IPS). Selective delay activation was observed anterior to the FEF and in the ascending part of the IPS. Right dorsolateral prefrontal cortex was involved in goal-directed visual search, but showed no delay activity.

Adult↗

PubMed2000

Prefrontal cortex activation in task switching: an event-related fMRI study.

When a switch between two tasks has to be carried out, performance is slower than in trials where the same task is performed repeatedly. This finding has been attributed to time-consuming control processes required for task switching. Previous results of other paradigms investigating cognitive control processes suggested that prefrontal cortex is involved in executive control. We used event-related fMRI to investigate prefrontal cortex involvement in task switching. Regions in the lateral prefrontal and premotor cortex bilaterally, the anterior insula bilaterally, the left intraparietal sulcus, the SMA/pre-SMA region and the cuneus/precuneus were activated by the task repetition condition and showed additional activation in the task switch condition. This confirmed the hypothesis that lateral prefrontal cortex is involved in task switching. However, the results also showed that this region is neither the only region involved in task switching nor a region specifically involved in task switching.

Adult↗

PubMed2000

A fronto-posterior network involved in visual dimension changes.

Objects characterized by a unique visual feature may pop out of their environment. When participants have to search for such "odd-one-out" targets, detection is facilitated when targets are consistently defined within the same feature dimension (e.g., color) compared with when the target dimension is uncertain (e.g., color or motion). Further, with dimensional uncertainty, there is a cost when a given target is defined in a different dimension to the preceding target, relative to when the critical dimension remains the same. Behavioral evidence suggests that a target dimension change involves a shift of attention to the new dimension. The present fMRI study revealed increased activation in the left frontopolar cortex, as well as in posterior visual areas of the dorsal and ventral streams, specific to changes in the target dimension. In contrast, activation in the striate cortex was decreased. This pattern suggests control of cross-dimensional attention shifts by the frontopolar cortex, modulating visual cortical processing by increased activation in higher-tier visual areas and suppression of activation in lower-tier areas.

Adolescent↗

PubMed2000

Event-related fMRI: comparison of conditions with varying BOLD overlap.

Recently, event-related fMRI-experiments have been reported in which subsequent trials were separated by only 2 sec or less. Because the BOLD response needs 10 sec and longer to return to baseline, the event-related signal in these experiments has to be extracted from the overlapping responses elicited by successive trials. Usually it is assumed that this convolved signal is a summation of the overlapping BOLD responses. We tested this assumption by comparing event-related signals in conditions with little and with substantial BOLD overlap in two fMRI experiments of a task-switching paradigm. We analyzed the difference in the activational time course elicited by a critical task and a baseline task when trials of both tasks were separated by intertrial intervals of 15 sec or when the critical trials were embedded in a stream of baseline trials with ISI = 1 sec. The change of the BOLD signal elicited by the critical trials showed a high correspondence between both experiments in five out of six cortical ROI. Our data support the view that BOLD overlap leads to largely linear signal changes. In the present study, task-related increases in the BOLD response were detected equally well with substantial BOLD overlap as with mostly nonoverlapping BOLD responses.

Adult↗

PubMed1999

Application of double voxel functional spectroscopy to event-related cognitive experiments.

The hemodynamic response to functional activation can be regarded as the convolution of the neuronal response with an unknown kernel. As such, it introduces an intrinsic blurring that limits the attainable temporal resolution of functional magnetic resonance (fMR) techniques. This study demonstrates that by measurement of displacements in activation onsets between different types of trial, it is nevertheless possible to obtain a subsecond temporal accuracy in fMR. A single trial stimulation paradigm was adopted: a simple search task embedded in a longer period of visual flicker stimulation that produced reliable activations in the primary visual cortex and supplementary motor area. Data were acquired from both of these regions using double voxel functional spectroscopy.

Evoked Potentials, Visual↗

PubMed1999

Redundancy gains for visual search after complete commissurotomy.

Redundant-targets effects (RTE) for visual search were investigated in 2 commissurotomy patients (L.B., N.G.). L.B., who showed no evidence of visual interhemispheric transfer, exhibited a paradoxical enhancement of the redundancy gain in the bilateral compared with the within-hemifield redundant-targets conditions, whereas N.G., who showed evidence of interhemispheric transfer of visual information, exhibited no enhancement of the bilateral redundancy gain. When only uncrossed responses were considered, both bilateral and within-field RTE were evident only when attentional demands were high. Bilateral redundant targets led to stronger gains, some indicative of coactivation, in the slower response hand. The authors suggest that the enhancement of the bilateral RTE comes about by neural coactivation, which is especially pronounced when the slower hemisphere elicits the response.

Adult↗

PubMed1998

Cortical areas and the control of self-determined finger movements: an fMRI study.

We investigated cortical areas involved in the control of self-determined finger movements. In a tapping task, subjects tapped with different movement frequencies in two different movement conditions (predetermined vs self-determined). fMRI provided evidence for the involvement of the horizontal and ascending parts of the intraparietal sulcus (IPS), the left superior frontal gyrus and the posterior cingulate gyrus in the control of self-determined finger movements. Higher movement frequency increased the extent of activated area only in the horizontal part of IPS. The results suggest a major role of the IPS in controlling sequences of finger movements. This area probably serves as a region for integration of motor, sensory and sensorimotor feedback information used for movement control.

Fingers↗

PubMed1998

D1- versus D2-receptor modulation of visuospatial working memory in humans.

The effects of pergolide, a mixed D1/D2 receptor agonist, and bromocriptine, a selective D2 receptor agonist, were assessed in a visual delay task to further investigate the "dopamine link" of working memory in humans and to look for differential D1 versus D2 receptor contributions. Two groups of 32 healthy young adults (16 female) received either 0.1 mg of pergolide or 2.5 mg of bromocriptine in a placebo-controlled cross-over design. A pretreatment with domperidone, a peripherally active D2 antagonist, was performed in both groups to reduce side effects. Interindividual differences in pharmacokinetics were controlled by the time course of serum prolactin inhibition. The working memory paradigm was a visuospatial delayed matching task; the location of a randomly generated seven-point pattern had to be memorized and compared after 2, 8, or 16 sec with a second pattern that was either identical or slightly shifted within a reference frame. The task was designed with the intention to present unique stimuli at each trial and to require minimal motor demands. Practice effects between the two pharmacological test days were minimized by training sessions that preceded the tests. The paradigm showed significant error and reaction time increases with longer delays. After comparable doses, only pergolide, but not bromocriptine, facilitated visuospatial working memory performance as demonstrated by a significant drug-by-delay interaction. These findings are in accordance with the monkey literature as well as with neuroanatomical findings, and they confirm a preferential role of prefrontal D1 receptors for working memory modulation in humans.

Adult↗

PubMed1998

Use of short intertrial intervals in single-trial experiments: a 3T fMRI-study.

We investigated the detectability of task-related changes in the fMRI-signal in an averaged single trial design under systematic variation of intertrial intervals (ITI) in the range between 4 and 12 s. Investigation of the signal timecourses showed a shortening of the baseline period and subsequently a reduction in signal amplitude with decreasing ITI. The main finding is that effect size, i.e., the ratio of task-related signal changes and error variance remained approximately constant from ITI of 12 s down to 6 s. At ITI = 4 s, the effect size was reduced by about 50%. The effects of ITI reduction were comparable in all six cortical ROI which were analyzed. In two subcortical ROI, effect size was already reduced at longer ITI. At ITI = 4 s, the rising flank of the BOLD response was delayed compared to longer ITI. When the data were corrected for the temporal overlap of successive BOLD-responses, the signal amplitudes at ITI = 4 s were comparable to the amplitudes measured at an interval of 12 s. This indicated that the amplitude reduction was mainly due to a linear superposition of the contiguous BOLD-responses.

Adult↗

PubMed1998

The role of the corpus callosum in visual orienting: importance of interhemispheric visual transfer.

In a tachistoscopic visual search task, the effects of ipsi- and contralateral distractors on target search were investigated in two complete commissurotomy patients. Pop-out distractors slowed the search for contralateral targets in both patients, i.e. search was not independent in both hemifields. In normals, we previously observed an extinction-like asymmetry in that distractors in the right visual hemifield interfered with target search in the left visual hemifield, but not vice versa. This pattern was also found in one of our patients, N.G., whereas the other, L.B., showed a reversed laterality effect. While N.G. is able to transfer visual shape information between hemispheres, L.B. is not. The data suggest that the reversal of the contralateral distractor asymmetry in L.B. is due to the disruption of ipsilateral visual input to the right hemisphere.

Corpus Callosum↗