Biomedical subjects

Tobias Sommer

Publications and source records attributed to Tobias Sommer.

14 recordsLinked to original sources

PubMed2007

Associative symmetry versus independent associations in the memory for object-location associations.

The formation of associations between objects and locations is a vital aspect of episodic memory. More specifically, remembering the location where one experienced an object and, vice versa, the object one encountered at a specific location are both important elements for the memory of an event. Whether episodic associations are holistic representations of individual components or whether there are unidirectional, separately modifiable connections between them has been investigated nearly exclusively using verbal stimuli. A preliminary conclusion concerning this controversy is that verbal associations are, at least, highly correlated (M. J. Kahana, 2002). This theoretical debate, which in the past has undergone a major empirical effort, is still of relevance for the concurrent global matching models of associative memory (S. E. Clark & S. D. Gronlund, 1996). The authors used variations of a novel object-location learning paradigm to complement the accumulated evidence regarding the nature of episodic associations.

Adolescent↗

PubMed2006

Neural correlates of memory confidence.

The present study aimed to shed light on the neural underpinnings of high vs. low memory confidence. To dissociate memory confidence from accuracy, the Deese-Roediger McDermott (DRM) paradigm was employed, which - compared to other memory paradigms - elicits a rather evenly distributed number of high-confident responses across all possible combinations of memory response types (i.e., hits, false alarms, correct rejections, and misses). In the standard DRM procedure, subjects are first presented with thematically interrelated word lists at encoding, which at recognition are intermixed with related and unrelated distractor items. The signature of a false memory or DRM effect is an increased number of high-confident false memories, particularly for strongly related lure items. For the present study, 17 female subjects were administered a verbal DRM task, whereas neural activation was indexed by fMRI. The behavioral analyses confirmed the expected false memory effect: subjects made more high-confident old responses (both hits and false alarms) the closer the items were related to the central list theme. Across all four memory response types, an increase in confidence at recognition was associated with bilateral activation in the anterior and posterior cingulate cortex along with medial temporal regions. In contrast, increments in doubt were solely related to activation in the superior posterior parietal cortex. To conclude, the study provides some evidence for dissociable systems for confidence and doubt.

Adult↗

PubMed2006

Dissociable systems for gain- and loss-related value predictions and errors of prediction in the human brain.

Midbrain dopaminergic neurons projecting to the ventral striatum code for reward magnitude and probability during reward anticipation and then indicate the difference between actual and predicted outcome. It has been questioned whether such a common system for the prediction and evaluation of reward exists in humans. Using functional magnetic resonance imaging and a guessing task in two large cohorts, we are able to confirm ventral striatal responses coding both reward probability and magnitude during anticipation, permitting the local computation of expected value (EV). However, the ventral striatum only represented the gain-related part of EV (EV+). At reward delivery, the same area shows a reward probability and magnitude-dependent prediction error signal, best modeled as the difference between actual outcome and EV+. In contrast, loss-related expected value (EV-) and the associated prediction error was represented in the amygdala. Thus, the ventral striatum and the amygdala distinctively process the value of a prediction and subsequently compute a prediction error for gains and losses, respectively. Therefore, a homeostatic balance of both systems might be important for generating adequate expectations under uncertainty. Prevalence of either part might render expectations more positive or negative, which could contribute to the pathophysiology of mood disorders like major depression.

Adult↗

PubMed2006

Hippocampal-prefrontal encoding activation predicts whether words can be successfully recalled or only recognized.

The main goal of the present fMRI-study was to identify the neural correlates underlying the successful encoding of words which can subsequently be freely recalled or recognized but not recalled. We were particularly interested in common as well as distinct neural substrates of both retrieval modes. To assess qualitatively differently activated brain areas, categorical subsequent memory analyses were applied. In addition, we used linear parametric modulation to detect brain regions associated with "memory-strength". Our findings suggest that the successful verbal encoding of words, which were recognized but not recalled relies on a subset of the regions engaged during successful encoding of freely recalled words. Furthermore, it seems to be dependent on the magnitude of relational binding in a prefrontal-hippocampal circuit whether a word can subsequently be recalled freely or only recognized.

Adult↗

PubMed2006

Dissociable parietal systems for primacy and subsequent memory effects.

The frequently observed superior memory for the first items on a list is referred to as primacy. The aetiology of this effect in terms of cognitive processes and their neural substrate is subject to an ongoing debate. However, the brain areas generally involved in successful encoding are well described by subsequent memory studies in which activity during encoding is correlated with memory performance. We employed an object-location association paradigm to differentiate the neural correlate of the primacy from the subsequent memory effect. Activity in the intraparietal sulcus predicted memory performance across all encoding positions. Increased activity in the inferior parietal lobe and angular gyrus resulted exclusively in a more efficient encoding of the first item presented. These areas are part of the ventral frontoparietal network involved in stimulus driven attention. Our results implicate the relatively elevated attention to the first item probably due to its contextual distinctiveness, as a major contributor to the primacy effect.

Adult↗

PubMed2005

Integration of local features to a global percept by neural coupling.

The integration of different visual attributes into the percept of a single global shape is a central aspect of object processing. In hierarchically organized stimuli with local and global levels, the attentional focus largely determines which level is processed. Here we tested the hypothesis that object processing during attention to the global aspect of the stimulus is characterized by an increased neural coupling between visual areas reflecting the integration of local features. In the present experiment, we used global letters that were constructed by smaller local letters, and a cue signaled which spatial level should be identified. On the local level, only 1 relevant letter was presented laterally in 1 visual hemifield. In contrast, the global letter extended into both hemifields, and the integration of information from both hemispheres was necessary to identify the global stimulus. Therefore, we expected an increased functional coupling between hemispheres during global processing. This hypothesis was investigated using electroencephalographic recordings and an analysis of phase locking and coherence. The results show that stimulus-locked neural coupling within the gamma band (30-40 Hz) across hemispheres in visual cortex increased for global processing after stimulus presentation and could therefore reflect the integration of local visual information.

Adult↗

PubMed2005

Dissociable contributions within the medial temporal lobe to encoding of object-location associations.

The crucial role of the medial temporal lobe (MTL) in episodic memory is well established. Although there is little doubt that its anatomical subregions-the hippocampus, peri-, entorhinal and parahippocampal cortex (PHC)-contribute differentially to mnemonic processes, their specific functions in episodic memory are under debate. Data from animal, human lesion, and neuroimaging studies suggest somewhat contradictory perspectives on this functional specialization: a general participation in declarative memory, an exclusive involvement in associative mnemonic processes, and a specific contribution to spatial memory are reported for the hippocampus, adjacent cortices, and the PHC. A functional lateralization in humans dependent on the verbalizability of the material is also discussed herein. To further elucidate the differential contributions of the various MTL subregions to encoding, we employed an object-location association memory paradigm. The memory for each of the studied associations was tested twice: by the object, and by the location serving as retrieval cue. The memory accuracy in response to both cue types was also assessed parametrically. Brain activity during encoding which leads to different degrees of subsequent memory accuracy under the two retrieval conditions was compared. We found the bilateral posterior PHC to participate in encoding of both the object associated with a location and the location associated with an object. In contrast, activity in an area in the left anterior PHC and the right anterior MTL was only correlated with the memory for the location associated with an object.

Adult↗

PubMed2005

Contributions of occipital, parietal and parahippocampal cortex to encoding of object-location associations.

Events are stored in our episodic memory in varying degrees of accessibility for conscious retrieval and combined with varying amounts of associated information. A crucial aspect of episodic memory is to bind information together, e.g. linking an object to a certain location. Spontaneous or experimenter-induced variance in the allocation of attentional resources during encoding critically determines the strength of the resulting memory trace especially for associative information as measured in subsequent memory tests. The neural correlates of associative memory encoding were investigated by lesion, PET- and fMRI-studies. So far no study aimed to assess the memory for associative information in a non-categorical way. Such a parametric assessment opens the possibility for a closer inspection of the relationship between brain activity and memory trace formation. We established a novel associative object-location memory paradigm, together with a parametric assessment of subsequent memory performance for spatial location which allows: (i) to determine if associative information is retrieved in a continuous or discontinuous manner during recognition and (ii) to investigate the relationship between activity during encoding and the resulting memory trace. Such correlations were observed in the occipital, parietal and the parahippocampal cortex, areas known to be involved in processing either objects and/or locations. The analysis of response functions revealed for the majority of areas involved in encoding a continuous relationship between brain activity during encoding and the confidence in a subsequent memory test.

Adult↗

PubMed2004

The functional and temporal characteristics of top-down modulation in visual selection.

Perceptual load of an attended task influences the processing of irrelevant background stimuli. In a series of behavioral, functional magnetic resonance (fMRI) and electroencephalography (EEG) experiments we examined the influence of working memory (WM) load related to a relevant visual stimulus on the processing of irrelevant backgrounds. We further addressed two open questions about the mechanism of load-dependent modulation: (i) is this modulation dependent on regional activity (i.e. phasic)? (ii) At what processing stage does this modulation take place? Load was manipulated by a WM task and concurrently the processing of irrelevant visual objects was assessed with fMRI and EEG. To examine the dependency of this modulation on intrinsic activity, we varied the activity level of visual areas by presenting objects with different levels of degradation. Activity in the lateral occipital complex (LOC) increased with object visibility and was phasically modulated by WM load. Event related potentials revealed that this phasic modulation occurred approximately 170 ms after stimulus onset, indicative of an early selection under high load. The results indicate a phasic modulatory effect of WM load on visual object processing in the LOC that is comparable to the effects found for perceptual load manipulations.

Adult↗

PubMed2004

Evidence of developmental differences in implicit sequence learning: an fMRI study of children and adults.

Prevailing theories of implicit or unaware learning propose a developmental invariance model, with implicit function maturing early in infancy or childhood despite prolonged improvements in explicit or intentional learning and memory systems across childhood. Neuroimaging studies of adult visuomotor sequence learning have associated fronto-striatal brain regions with implicit learning of spatial sequences. Given evidence of continued development in these brain regions during childhood, we compare implicit sequence learning in adults and 7- to 11-year-old children to examine potential developmental differences in the recruitment of fronto-striatal circuitry during implicit learning. Participants performed a standard serial reaction time task. Stimuli alternately followed a fixed 10-step sequence of locations or were presented in a pseudorandom order of locations. Adults outperformed children, achieving a significantly larger sequence learning effect and showing learning more quickly than children. Age-related differences in activity were observed in the premotor cortex, putamen, hippocampus, inferotemporal cortex, and parietal cortex. We observed differential recruitment of cortical and subcortical motor systems between groups, presumably reflecting age differences in motor response execution. Adults showed greater hippocampal activity for sequence trials, whereas children demonstrated greater signal during random trials. Activity in the right caudate correlated significantly with behavioral measures of implicit learning for both age groups, although adults showed greater signal change than children overall, as would be expected given developmental differences in sequence learning magnitude. These results challenge the idea of developmental invariance in implicit learning and instead support a view of parallel developments in implicit and explicit learning systems.

Adult↗

PubMed2003

Mapping the genetic variation of executive attention onto brain activity.

Brain imaging data have repeatedly shown that the anterior cingulate cortex is an important node in the brain network mediating conflict. We previously reported that polymorphisms in dopamine receptor (DRD4) and monoamine oxidase A (MAOA) genes showed significant associations with efficiency of handling conflict as measured by reaction time differences in the Attention Network Test (ANT). To examine whether this genetic variation might contribute to differences in brain activation within the anterior cingulate cortex, we genotyped 16 subjects for the DRD4 and MAOA genes who had been scanned during the ANT. In each of the two genes previously associated with more efficient handling of conflict in reaction time experiments, we found a polymorphism in which persons with the allele associated with better behavioral performance showed significantly more activation in the anterior cingulate while performing the ANT than those with the allele associated with worse performance. The results demonstrate how genetic differences among individuals can be linked to individual differences in neuromodulators and in the efficiency of the operation of an appropriate attentional network.

Adolescent↗

PubMed2003

Synaptogenesis and heritable aspects of executive attention.

In humans, changes in brain structure and function can be measured non-invasively during postnatal development. In animals, advanced optical imaging measures can track the formation of synapses during learning and behavior. With the recent progress in these technologies, it is appropriate to begin to assess how the physiological processes of synapse, circuit, and neural network formation relate to the process of cognitive development. Of particular interest is the development of executive function, which develops more gradually in humans. One approach that has shown promise is molecular genetics. The completion of the human genome project and the human genome diversity project make it straightforward to ask whether variation in a particular gene correlates with variation in behavior, brain structure, brain activity, or all of the above. Strategies that unify the wealth of biochemical knowledge pertaining to synapse formation with the functional measures of brain structure and activity may lead to new insights in developmental cognitive psychology.

Alleles↗

PubMed2002

Assessing the molecular genetics of attention networks.

BACKGROUND: Current efforts to study the genetic underpinnings of higher brain functions have been lacking appropriate phenotypes to describe cognition. One of the problems is that many cognitive concepts for which there is a single word (e.g. attention) have been shown to be related to several anatomical networks. Recently, we have developed an Attention Network Test (ANT) that provides a separate measure for each of three anatomically defined attention networks. RESULTS: In this study we have measured the efficiency of neural networks related to aspects of attention using the ANT in a population of 200 adult subjects. We then examined genetic polymorphisms in four candidate genes (DRD4, DAT, COMT and MAOA) that have been shown to contribute to the risk of developing various psychiatric disorders where attention is disrupted. We find modest associations of several polymorphisms with the efficiency of executive attention but not with overall performance measures such as reaction time. CONCLUSIONS: These results suggest that genetic variation may underlie inter-subject variation in the efficiency of executive attention. This study also shows that genetic influences on executive attention may be specific to certain anatomical networks rather than affecting performance in a global or non-specific manner. Lastly, this study further validates the ANT as an endophenotypic assay suitable for assessing how genes influence certain anatomical networks that may be disrupted in various psychiatric disorders.

Adolescent↗

PubMed2002

Testing the efficiency and independence of attentional networks.

In recent years, three attentional networks have been defined in anatomical and functional terms. These functions involve alerting, orienting, and executive attention. Reaction time measures can be used to quantify the processing efficiency within each of these three networks. The Attention Network Test (ANT) is designed to evaluate alerting, orienting, and executive attention within a single 30-min testing session that can be easily performed by children, patients, and monkeys. A study with 40 normal adult subjects indicates that the ANT produces reliable single subject estimates of alerting, orienting, and executive function, and further suggests that the efficiencies of these three networks are uncorrelated. There are, however, some interactions in which alerting and orienting can modulate the degree of interference from flankers. This procedure may prove to be convenient and useful in evaluating attentional abnormalities associated with cases of brain injury, stroke, schizophrenia, and attention-deficit disorder. The ANT may also serve as an activation task for neuroimaging studies and as a phenotype for the study of the influence of genes on attentional networks.

Adult↗