Biomedical subjects

Alexander Maier

Publications and source records attributed to Alexander Maier.

7 recordsLinked to original sources

PubMed2006

Single-trial evoked potential estimation using wavelets.

In this paper we present conventional and translation-invariant (TI) wavelet-based approaches for single-trial evoked potential estimation based on intracortical recordings. We demonstrate that the wavelet-based approaches outperform several existing methods including the Wiener filter, least mean square (LMS), and recursive least squares (RLS), and that the TI wavelet-based estimates have higher SNR and lower RMSE than the conventional wavelet-based estimates. We also show that multichannel averaging significantly improves the evoked potential estimation, especially for the wavelet-based approaches. The excellent performances of the wavelet-based approaches for extracting evoked potentials are demonstrated via examples using simulated and experimental data.

Algorithms↗

PubMed2006

Neuroimaging: perception at the brain's core.

Two new functional imaging studies have shown that activity in the lateral geniculate nucleus of the thalamus strongly reflects perceptual dominance during binocular rivalry, raising new questions about how subjective percepts arise in the brain.

Dominance, Ocular↗

PubMed2006

Relaxation-based multichannel signal combination (RELAX-MUSIC) for ROC analysis of percept-related neuronal activity.

In this letter, we consider how to combine neuronal signals from multiple electrodes to optimally predict behavioral choices from observed neural activity. The predictability is often quantified by the area under the receiver operating characteristic (ROC) curve, also called choice probability (CP) in neurophysiology. We exploit a distribution-free relaxation based multichannel signal combination (RELAX-MUSIC) approach that requires only simple pairwise combination and recursive implementation for optimizing the area under the ROC curve. A permutation test is employed to assess the statistical significance of the derived CP. We demonstrate that the RELAX-MUSIC approach outperforms the commonly used response pooling and Fisher linear discriminant (FLD) methods. The excellent performances of the RELAX-MUSIC approach for predicting perceptual decisions from neural activity are demonstrated via examples using simulated and experimental data.

Action Potentials↗

PubMed2005

Global competition dictates local suppression in pattern rivalry.

Certain pairs of visual patterns, when superimposed as if transparent, elicit a wavering percept in which one or the other pattern can enjoy temporary periods of exclusive visibility. This multistable perceptual phenomenon is called monocular rivalry or pattern rivalry (PR) and is most pronounced when the component patterns are faint and devoid of detail. The principal mechanisms that give rise to PR continue to be a topic of speculation. In the present study, we examine the determinants of exclusive dominance during PR using a novel stimulus in which a central portion is free of conflict. By observing the properties of suppression in this so-called rivalry-free region, we demonstrate that perception is driven largely by the global and holistic interpretation of the patterns, rather than by the need to resolve local spatial conflict. The suppression of this central region was often complete and varied as a function of the parameters of the global stimulus, including the size of the surround region, its ocular configuration, and stereoscopic depth ordering. Suppression also varied as a function of pattern continuity across the central region as well as with the temporal offset of the overlapping components. These findings demonstrate that the visibility or invisibility of a pattern is not fundamentally a product of local processing, but is instead shaped by the brain's global interpretive assumptions regarding the composition of the stimulus.

Adolescent↗

PubMed2003

Perception of temporally interleaved ambiguous patterns.

BACKGROUND: Continuous viewing of ambiguous patterns is characterized by wavering perception that alternates between two or more equally valid visual solutions. However, when such patterns are viewed intermittently, either by repetitive presentation or by periodic closing of the eyes, perception can become locked or "frozen" in one configuration for several minutes at a time. One aspect of this stabilization is the possible existence of a perceptual memory that persists during periods in which the ambiguous stimulus is absent. Here, we use a novel paradigm of temporally interleaved ambiguous stimuli to explore the nature of this memory, with particular regard to its potential impact on perceptual organization. RESULTS: We found that the persistence of a perceptual configuration was robust to interposed visual patterns, and, further, that at least three ambiguous patterns, when interleaved in time, could undergo parallel, stable time courses. Then, using an interleaved presentation paradigm, we established that the occasional reversal in one pattern could be coupled with that of its interleaved counterpart, and that this coupling was a function of the structural similarity between the patterns. CONCLUSIONS: We postulate that the stabilization observed with repetitive presentation of ambiguous patterns can be at least partially accounted for by processes that retain a recent perceptual interpretation, and we speculate that such memory may be important in natural vision. We further propose that the interleaved paradigm introduced here may be of great value to gauge aspects of stimulus similarity that appeal to particular mechanisms of perceptual organization.

Adolescent↗

PubMed2003

Depletion of GIM5 causes cellular fragility, a decreased glycosome number, and reduced levels of ether-linked phospholipids in trypanosomes.

Microbody division in mammalian cells, trypanosomes, and yeast depends on the PEX11 microbody membrane proteins. The function of PEX11 is not understood, and the suggestion that it affects microbody (peroxisome) numbers in mammals and yeast, because it plays a role in beta-oxidation of fatty acids, is controversial. PEX11 and two PEX11-related proteins, GIM5A and GIM5B, are the predominant membrane proteins of the microbodies (glycosomes) of Trypanosoma brucei. The compartmentation of glycosomal enzymes is essential in trypanosomes. Deletion of the GIM5A gene from the form of the parasite that lives in the mammalian blood has no effect on trypanosome growth, but depletion of GIM5B on a gim5a null background causes death. We show here that procyclic trypanosomes, adapted for life in the Tsetse fly vector, survive without GIM5A and with very low levels of GIM5B. The depleted cells have fewer glycosomes than usual and are osmotically fragile, which is a novel observation for a microbody defect. Thus trypanosomes require both GIM5B and PEX11 for the maintenance of normal glycosome numbers. Procyclic cells lacking GIM5A, like mouse cells partially defective in PEX11, have fewer ether-linked phospholipids, even when GIM5B levels are not reduced. Metabolite measurements on GIM5A/B-depleted bloodstream form trypanosomes suggested a change in the flux through the glycolytic pathway. We conclude that PEX11 family proteins play important roles in determining microbody membrane structure, with secondary effects on a subset of microbody metabolic pathways.

Amino Acid Sequence↗

PubMed2002

Stable perception of visually ambiguous patterns.

During the viewing of certain patterns, widely known as ambiguous or puzzle figures, perception lapses into a sequence of spontaneous alternations, switching every few seconds between two or more visual interpretations of the stimulus. Although their nature and origin remain topics of debate, these stochastic switches are generally thought to be the automatic and inevitable consequence of viewing a pattern without a unique solution. We report here that in humans such perceptual alternations can be slowed, and even brought to a standstill, if the visual stimulus is periodically removed from view. We also show, with a visual illusion, that this stabilizing effect hinges on perceptual disappearance rather than on actual removal of the stimulus. These findings indicate that uninterrupted subjective perception of an ambiguous pattern is required for the initiation of the brain-state changes underlying multistable vision.

Adolescent↗