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1.
J Neurophysiol ; 114(1): 80-98, 2015 Jul.
Article in English | MEDLINE | ID: mdl-25948867

ABSTRACT

The evolution of a visually guided perceptual decision results from multiple neural processes, and recent work suggests that signals with different neural origins are reflected in separate frequency bands of the cortical local field potential (LFP). Spike activity and LFPs in the middle temporal area (MT) have a functional link with the perception of motion stimuli (referred to as neural-behavioral correlation). To cast light on the different neural origins that underlie this functional link, we compared the temporal dynamics of the neural-behavioral correlations of MT spikes and LFPs. Wide-band activity was simultaneously recorded from two locations of MT from monkeys performing a threshold, two-stimuli, motion pulse detection task. Shortly after the motion pulse occurred, we found that high-gamma (100-200 Hz) LFPs had a fast, positive correlation with detection performance that was similar to that of the spike response. Beta (10-30 Hz) LFPs were negatively correlated with detection performance, but their dynamics were much slower, peaked late, and did not depend on stimulus configuration or reaction time. A late change in the correlation of all LFPs across the two recording electrodes suggests that a common input arrived at both MT locations prior to the behavioral response. Our results support a framework in which early high-gamma LFPs likely reflected fast, bottom-up, sensory processing that was causally linked to perception of the motion pulse. In comparison, late-arriving beta and high-gamma LFPs likely reflected slower, top-down, sources of neural-behavioral correlation that originated after the perception of the motion pulse.


Subject(s)
Motion Perception/physiology , Visual Cortex/physiology , Action Potentials , Animals , Beta Rhythm/physiology , Gamma Rhythm/physiology , Macaca mulatta , Male , Neurons/physiology , Neuropsychological Tests , Photic Stimulation , Signal Processing, Computer-Assisted
2.
Neural Comput ; 24(12): 3126-44, 2012 Dec.
Article in English | MEDLINE | ID: mdl-22970871

ABSTRACT

Dendrites carry signals between synapses and the soma and play a central role in neural computation. Although they contain many nonlinear ion channels, their signal-transfer properties are linear under some experimental conditions. In experiments with continuous-time inputs, a resonant linear two-port model has been shown to provide a near-perfect fit to the dendrite-to-soma input-output relationship. In this study, we focused on this linear aspect of signal transfer using impedance functions that replace biophysical channel models in order to describe the electrical properties of the dendritic membrane. The membrane impedance model of dendrites preserves the accuracy of the two-port model with minimal computational complexity. Using this approach, we demonstrate two membrane impedance profiles of dendrites that reproduced the experimentally observed two-port results. These impedance profiles demonstrate that the two-port results are compatible with different computational schemes. In addition, our model highlights how dendritic resonance can minimize the location-dependent attenuation of signals at the resonant frequency. Thus, in this model, dendrites function as linear-resonant filters that carry signals between nonlinear computational units.


Subject(s)
Computer Simulation , Dendrites/physiology , Models, Neurological , Pyramidal Cells/physiology , Animals , CA1 Region, Hippocampal/physiology , Humans
3.
Proteomics ; 12(8): 1122-33, 2012 Apr.
Article in English | MEDLINE | ID: mdl-22577013

ABSTRACT

Large-scale proteomics applications using SRM analysis on triple quadrupole mass spectrometers present new challenges to LC-MS/MS experimental design. Despite the automation of building large-scale LC-SRM methods, the increased numbers of targeted peptides can compromise the balance between sensitivity and selectivity. To facilitate large target numbers, time-scheduled SRM transition acquisition is performed. Previously published results have demonstrated incorporation of a well-characterized set of synthetic peptides enabled chromatographic characterization of the elution profile for most endogenous peptides. We have extended this application of peptide trainer kits to not only build SRM methods but to facilitate real-time elution profile characterization that enables automated adjustment of the scheduled detection windows. Incorporation of dynamic retention time adjustments better facilitate targeted assays lasting several days without the need for constant supervision. This paper provides an overview of how the dynamic retention correction approach identifies and corrects for commonly observed LC variations. This adjustment dramatically improves robustness in targeted discovery experiments as well as routine quantification experiments.


Subject(s)
Chromatography, Liquid/methods , Mass Spectrometry/methods , Peptides/analysis , Proteomics/methods , Amino Acid Sequence , Calibration , Cell Extracts , Chromatography, Liquid/instrumentation , Chromatography, Liquid/standards , Humans , Mass Spectrometry/instrumentation , Mass Spectrometry/standards , Molecular Sequence Data , Peptides/chemical synthesis , Proteomics/instrumentation , Proteomics/standards , Reference Standards , Reproducibility of Results , Saccharomyces cerevisiae/chemistry , Sensitivity and Specificity , Time Factors
4.
Interface Focus ; 2(5): 658-68, 2012 Oct 06.
Article in English | MEDLINE | ID: mdl-24098851

ABSTRACT

In recent decades, there has been an explosion in the number and variety of embedded triply-periodic minimal surfaces (TPMS) identified by mathematicians and materials scientists. Only the rare examples of low genus, however, are commonly invoked as shape templates in scientific applications. Exact analytic solutions are now known for many of the low genus examples. The more complex surfaces are readily defined with numerical tools such as Surface Evolver software or the Landau-Ginzburg model. Even though table-top versions of several TPMS have been placed within easy reach by rapid prototyping methods, the inherent complexity of many of these surfaces makes it challenging to grasp their structure. The problem of distinguishing TPMS, which is now acute because of the proliferation of examples, has been addressed by Lord & Mackay (Lord & Mackay 2003 Curr. Sci. 85, 346-362).

5.
Mol Cell Proteomics ; 10(2): M110.002931, 2011 Feb.
Article in English | MEDLINE | ID: mdl-20664071

ABSTRACT

Proteomics is gradually complementing large shotgun qualitative studies with hypothesis-driven quantitative experiments. Targeted analyses performed on triple quadrupole instruments in selected reaction monitoring mode are characterized by a high degree of selectivity and low limit of detection; however, the concurrent analysis of multiple analytes occurs at the expense of sensitivity because of reduced dwell time and/or selectivity due to limitation to a few transitions. A new data acquisition paradigm is presented in which selected reaction monitoring is performed in two ways to simultaneously quantify and confirm the identity of the targeted peptides. A first set of primary transitions is continuously monitored during a predetermined elution time window to precisely quantify each peptide. In addition, a set of six to eight transitions is acquired in a data-dependent event, triggered when all the primary transitions exceed a preset threshold. These additional transitions are used to generate composite tandem mass spectra to formally confirm the identity of the targeted peptides. This technique was applied to analyze the tryptic digest of a yeast lysate to demonstrate the performance of the technique. We showed a limit of detection down to tens of attomoles injected and a throughput exceeding 6000 transitions in one 60-min experiment. The technique was integrated into a linear work flow, including experimental design, data acquisition, and data evaluation, enabling large scale proteomic studies.


Subject(s)
Proteomics/methods , Biomarkers/chemistry , Chromatography, High Pressure Liquid/methods , Chromatography, Liquid/methods , Fungal Proteins/chemistry , Mass Spectrometry/methods , Models, Statistical , Peptides/chemistry , Reproducibility of Results , Saccharomyces cerevisiae/metabolism , Software , Systems Biology
6.
Rapid Commun Mass Spectrom ; 17(6): 561-8, 2003.
Article in English | MEDLINE | ID: mdl-12621618

ABSTRACT

In this study, accurate mass measurements were made by electrospray ionization (ESI) on a triple quadrupole mass spectrometer operating in enhanced mass-resolution mode (peak width = 0.1 u FWMH), to give qualitative information relating to the pharmaceutical, cabergoline. Accurate mass determinations by ESI-MS were performed on a protonated impurity formed during cabergoline storage. The accurate mass measurement resulted in only one proposed elemental composition for the impurity, using reasonable elemental limits and mass tolerance for the calculation. This information was sufficient to propose a structure for the impurity where ESI-MS/MS proved consistent. The difference between the accurate mass measurement and the exact mass calculated for the proposed structure was 0.8 mmu, with a standard deviation of 0.7 mmu for replicate accurate mass determinations. Accurate mass determinations in ESI-MS/MS provided information on cabergoline fragment ions formed through collisionally-induced dissociation. Since the potential formation of isobaric ions exists for two major cabergoline fragment ions, accurate mass measurement allowed for the determination of the most probable fragment ion structures. The differences between the accurate mass measurements and exact masses calculated for the proposed fragment ions were 1.9 and 2.1 mmu, with standard deviations of 0.4 and 0.8 mmu, respectively, for replicate determinations.


Subject(s)
Ergolines/chemistry , Ergolines/metabolism , Spectrometry, Mass, Electrospray Ionization/methods , Cabergoline , Drug Contamination , Ions/chemistry , Molecular Structure , Pharmaceutical Preparations/chemistry , Pharmaceutical Preparations/metabolism
7.
Rapid Commun Mass Spectrom ; 16(21): 2060-6, 2002.
Article in English | MEDLINE | ID: mdl-12391581

ABSTRACT

Triple quadrupole mass spectrometers, when operated in multiple reaction monitoring (MRM) mode, offer a unique combination of sensitivity, specificity, and dynamic range. Consequently, the triple quadrupole is the workhorse for high-throughput quantitation within the pharmaceutical industry. However, in the past, the unit mass resolution of quadrupole instruments has been a limitation when interference from matrix or metabolites cannot be eliminated. With recent advances in instrument design, triple quadrupole instruments now afford mass resolution of less than 0.1 Dalton (Da) full width at half maximum (FWHM). This paper describes the evaluation of an enhanced resolution triple quadrupole mass spectrometer for high-throughput bioanalysis with emphasis on comparison of selectivity, sensitivity, dynamic range, precision, accuracy, and stability under both unit mass (1 Da FWHM) and enhanced (

Subject(s)
Chromatography, High Pressure Liquid/methods , Spectrometry, Mass, Electrospray Ionization/methods , Calibration , Loratadine/analysis , Mometasone Furoate , Pharmaceutical Preparations/analysis , Piperidines/analysis , Pregnadienediols/analysis , Pyridines/analysis , Sensitivity and Specificity
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