Biomedical subjects

Marcus Kaiser

Publications and source records attributed to Marcus Kaiser.

7 recordsLinked to original sources

PubMed2006

The fate of object features during perisaccadic mislocalization.

Visual objects flashed before a saccade appear compressed toward the saccade target. Simultaneously flashed objects merge perceptually into one. To better understand cortical interactions in perisaccadic processing, we study the perception of features of mislocalized objects. We report four new findings: First, when multiple objects of different colors are compressed onto a single position, their color attributes remain distinguishable. Second, color attributes of objects compressed onto the same position compete for access to visual awareness. Third, objects presaccadically mislocalized onto a static background of identical color and luminance appear visible on top of that background. Object shape can be determined. Fourth, objects flashed during a saccade become invisible when a larger object is present at the mislocalized position. Thus, perisaccadic mislocalization affects the position of objects but retains other object features. Mislocalization must either occur in parallel to color and shape processing or at late stages of the visual pathway.

Adult↗

PubMed2006

Nonoptimal component placement, but short processing paths, due to long-distance projections in neural systems.

It has been suggested that neural systems across several scales of organization show optimal component placement, in which any spatial rearrangement of the components would lead to an increase of total wiring. Using extensive connectivity datasets for diverse neural networks combined with spatial coordinates for network nodes, we applied an optimization algorithm to the network layouts, in order to search for wire-saving component rearrangements. We found that optimized component rearrangements could substantially reduce total wiring length in all tested neural networks. Specifically, total wiring among 95 primate (Macaque) cortical areas could be decreased by 32%, and wiring of neuronal networks in the nematode Caenorhabditis elegans could be reduced by 48% on the global level, and by 49% for neurons within frontal ganglia. Wiring length reductions were possible due to the existence of long-distance projections in neural networks. We explored the role of these projections by comparing the original networks with minimally rewired networks of the same size, which possessed only the shortest possible connections. In the minimally rewired networks, the number of processing steps along the shortest paths between components was significantly increased compared to the original networks. Additional benchmark comparisons also indicated that neural networks are more similar to network layouts that minimize the length of processing paths, rather than wiring length. These findings suggest that neural systems are not exclusively optimized for minimal global wiring, but for a variety of factors including the minimization of processing steps.

Animals↗

PubMed2004

Edge vulnerability in neural and metabolic networks.

Biological networks, such as cellular metabolic pathways or networks of corticocortical connections in the brain, are intricately organized, yet remarkably robust toward structural damage. Whereas many studies have investigated specific aspects of robustness, such as molecular mechanisms of repair, this article focuses more generally on how local structural features in networks may give rise to their global stability. In many networks the failure of single connections may be more likely than the extinction of entire nodes, yet no analysis of edge importance (edge vulnerability) has been provided so far for biological networks. We tested several measures for identifying vulnerable edges and compared their prediction performance in biological and artificial networks. Among the tested measures, edge frequency in all shortest paths of a network yielded a particularly high correlation with vulnerability and identified intercluster connections in biological but not in random and scale-free benchmark networks. We discuss different local and global network patterns and the edge vulnerability resulting from them.

Animals↗

PubMed2004

Spatial growth of real-world networks.

Many real-world networks have properties of small-world networks, with clustered local neighborhoods and low average-shortest path. They may also show a scale-free degree distribution, which can be generated by growth and preferential attachment to highly connected nodes, or hubs. However, many real-world networks consist of multiple, interconnected clusters not normally seen in systems grown by preferential attachment, and there also exist real-world networks with a scale-free degree distribution that do not contain highly connected hubs. We describe spatial-growth mechanisms, not using preferential attachment, that address both aspects.

Journal Article↗

PubMed2004

Perisaccadic mislocalization orthogonal to saccade direction.

Saccadic eye movements transiently distort perceptual space. Visual objects flashed shortly before or during a saccade are mislocalized along the saccade direction, resembling a compression of space around the saccade target. These mislocalizations reflect transient errors of processes that construct spatial stability across eye movements. They may arise from errors of reference signals associated with saccade direction and amplitude or from visual or visuomotor remapping processes focused on the saccade target's position. The second case would predict apparent position shifts toward the target also in directions orthogonal to the saccade. We report that such orthogonal mislocalization indeed occurs. Surprisingly, however, the orthogonal mislocalization is restricted to only part of the visual field. This part comprises distant positions in saccade direction but does not depend on the target's position. Our findings can be explained by a combination of directional and positional reference signals that varies in time course across the visual field.

Adult↗

PubMed2004

Organization, development and function of complex brain networks.

Recent research has revealed general principles in the structural and functional organization of complex networks which are shared by various natural, social and technological systems. This review examines these principles as applied to the organization, development and function of complex brain networks. Specifically, we examine the structural properties of large-scale anatomical and functional brain networks and discuss how they might arise in the course of network growth and rewiring. Moreover, we examine the relationship between the structural substrate of neuroanatomy and more dynamic functional and effective connectivity patterns that underlie human cognition. We suggest that network analysis offers new fundamental insights into global and integrative aspects of brain function, including the origin of flexible and coherent cognitive states within the neural architecture.

Brain↗

PubMed2004

Clustered organization of cortical connectivity.

Long-range corticocortical connectivity in mammalian brains possesses an intricate, nonrandom organization. Specifically, projections are arranged in "small-world" networks, forming clusters of cortical areas, which are closely linked among each other, but less frequently with areas in other clusters. In order to delineate the structure of cortical clusters and identify their members, we developed a computational approach based on evolutionary optimization. In different compilations of connectivity data for the cat and macaque monkey brain, the algorithm identified a small number of clusters that broadly agreed with functional cortical subdivisions. We propose a simple spatial growth model for evolving clustered connectivity, and discuss structural and functional implications of the clustered, small-world organization of cortical networks.

Animals↗