Activity Of Large Populations Research Articles

Optimization of population decoding with distance metrics

Recent advances in multi-electrode recording and imaging techniques have made it possible to observe the activity of large populations of neurons. However, to take full advantage of these techniques, new methods for the analysis of population responses must be developed. In this paper, we present an algorithm for optimizing population decoding with distance metrics. To demonstrate the utility of this algorithm under experimental conditions, we evaluate its performance in decoding both population spike trains and calcium signals with different correlation structures. Our results demonstrate that the optimized decoder outperforms other simple population decoders and suggest that optimization could serve as a tool for quantifying the potential contribution of individual cells to the population code.

Imaging Action Potentials with Calcium Indicators: Figure 1.

The understanding of neuronal circuits has been, and will continue to be, greatly advanced by the simultaneous imaging of action potentials in neuronal ensembles. This protocol describes "bulk" loading of brain slices with acetoxymethyl (AM) ester calcium indicators in order to monitor action potential activity in large populations of neurons simultaneously. The imaging of calcium influx into neurons provides an indirect, but accurate, measure of action potential generation in individual neurons. Single-cell resolution, and thus the easy identification of every active cell, is the key advantage of the technique.

Metaplasticity Governs Natural Experience-Driven Plasticity of Nascent Embryonic Brain Circuits

During embryogenesis, brain neurons receiving the same sensory input may undergo potentiation or depression. While the origin of variable plasticity in vivo is unknown, it plays a key role in shaping dynamic neural circuit refinement. Here, we investigate effects of natural visual stimuli on neuronal firing within the intact, awake, developing brain using calcium imaging of 100 s of central neurons in the Xenopus retinotectal system. We find that specific patterns of visual stimuli shift population responses toward either potentiation or depression in an N-methyl-D-aspartate receptor (NMDA-R)-dependent manner. In agreement with Bienenstock-Cooper-Munro metaplasticity, our results show that functional potentiation or depression can be predicted by individual neurons' specific receptive field properties and historic firing rates. Interestingly, this activity-dependent metaplasticity is itself NMDA-R dependent. Furthermore, network analysis reveals increased correlated firing of neurons that undergo potentiation. These findings implicate metaplasticity as a natural property regulating experience-dependent refinement of nascent embryonic brain circuits.

Optimal correlation codes in populations of noisy spiking neurons

In most areas of the brain, information is encoded in the correlated activity of large populations of neurons. We ask how neural responses should be coupled to best represent information about different ensembles of correlated stimuli. Three classical population coding strategies are independence, decorrelation and error correction. Here we demonstrate that balance between the intrinsic noise level and the statistics of the input ensemble induces smooth transitions between these three coding strategies in a network composed of pairwise-coupled neurons and tuned to maximize its information capacity.

Spatio-temporal spike dynamics: localization of single cell currents based on extracellular potentials patterns

Introduction Traditional current source density calculation (CSD) method allows calculation of transmembrane current source distribution on neurons from the extracellular potential patterns, thus provides important information for neurophysiology. The traditional CSD method is based on strong physical foundations, but in most cases it is applied onto one dimensional data, so derivatives according to the orthogonal dimensions are neglected. This one-dimensional method uses the implicit assumption that current source density changes can be neglected in two dimensions on the spatial scale of the electrode. In other words, it assumes a laminar source distribution, with infinitely large, homogeneous laminar sources. Considering the laminar organization of the cortex, this can be a good approximation in case of large population activities such as epilepsy or evoked potentials, but certainly not valid in case of single cells. Thus traditional onedimensional CSD method gives incorrect results for spatial potential patterns originated from a single neuron. To solve this problem, a new spike CSD (sCSD) method was designed, which fits more to the properties of individual cellular sources, making it able reconstruct the cellular transmembrane currents, based on extracellular potential measurements. This new method is based on the inverse solution of the Poisson-equation. Results The new method was tested on simulated data and its performance was compared to the traditional CSD method. It was shown, that the sCSD method was able to reconstruct the original source distribution with much higher accuracy than the traditional method and precisely determinate the cell-electrode distance as well. Our new method was applied to in vivo measured action potentials. These spikes were measured in cat primary auditory cortex with a sixteen-channel chronically implanted linear probe. Using our new method, many fine details of the spatio-temporal dynamics of spikes were uncovered. Dendritic back propagation was proven to be much more frequent than previously assumed; it was observable in every cell. The speed of back propagation was typically different in the apical and basal directions. In contrast to the literature, forward propagation preceding the spikes was also observable. In perspective, this new method raises the possibility of identifying synaptic inputs that cause a cell to fire.

Experience dependent plasticity alters cortical synchronization

Theories of temporal coding by cortical neurons are supported by observations that individual neurons can respond to sensory stimulation with millisecond precision and that activity in large populations is often highly correlated. Synchronization is highest between neurons with overlapping receptive fields and modulated by both sensory stimulation and behavioral state. It is not yet clear whether cortical synchronization is an epiphenomenon or a critical component of efficient information transmission. Experimental manipulations that generate receptive field plasticity can be used to test the relationship between synchronization and receptive fields. Here we demonstrate that increasing receptive field size in primary auditory cortex by repeatedly pairing a train of tones with nucleus basalis (NB) stimulation increases synchronization, and decreasing receptive field size by pairing different tone frequencies with NB stimulation decreases synchronization. These observations seem to support the conclusion that neural synchronization is simply an artifact caused by common inputs. However, pairing tone trains of different carrier frequencies with NB stimulation increases receptive field size without increasing synchronization, and environmental enrichment increases synchronization without increasing receptive field size. The observation that receptive fields and synchronization can be manipulated independently suggests that common inputs are only one of many factors shaping the strength and temporal precision of cortical synchronization and supports the hypothesis that precise neural synchronization contributes to sensory information processing.

A cognitive model in which representations are images

Interpretations of images of the brain are starting to reveal the conceptual tasks in which the person was engaged at the time of imaging. Existing mathematical models can explain the patterns of activity observed in such images in terms of the coherent activity of large populations of neurons, but not in terms of cognition. This paper is an early investigation into how such patterns might provide the internal representations for a cognitive system. Probes, working memories and memories are all represented as images. The accompanying process model describes how attention is set according to the contents of working memory, how attention determines what parts of the probe are memorised, how memories are activated according to similarity to the probe in areas in attention, and how working memory is managed. The model is demonstrated on re-creations of classic simulations of recognition memory and categorisation.

MACRODYNAMICS OF ELECTRICAL ACTIVITY IN THE WHOLE BRAIN

The general problem of brain mechanisms involved in perception can now be studied directly by means of new analysis–methods for the activity of large population of neurons. These methods range from indirect means of measuring changes in cerebral blood flow in local regions of the human cortex (functional magnetic resonance imaging, fMRI), or changes in the electrical activity of the human brain with EEG-recording with topological distributed macroelectrodes, to the use of chronically implanted multiple microelectrodes in primates. f MRI has the disadvantage of low temporal resolution and with multiple microelectrodes long distance measurements cannot yet be properly performed. Accordingly, recording of macro-activity (EEG/ERP or MEG) with a time resolution of millisecond-range is the most possibly adequate method to measure the dynamic properties of memory and the integrative brain function.Since neuroscientists have come to the general conclusion that large numbers of different brain regions have to cooperate for any brain function, the analysis of relationships between different regions of the brain is becoming more and more important.Before new progresses and importance of EEG studies research became clear, scientists working with macrodynamics of the brain had a long way to go, in order to elucidate brain functioning. In this tutorial report we explain in a narrative way the developments leading to the concept of Macrodynamics in search of an integrative brain function. Moreover, elements of a brain theory, which we call neurons-brain theory describing the dynamics of electrical activity in the whole brain, are introduced. The concept of superbinding in integrative brain function, which emerged from experimental data, is a consequence of this theory.

Reproducibility and relative validity of the short questionnaire to assess health-enhancing physical activity

Objective The purpose of this study is to determine reproducibility and relative validity of the Short QUestionnaire to ASsess Health-enhancing physical activity (SQUASH). Methods Participants (36 men and 14 women, aged 27–58) were asked to complete the SQUASH twice with an inbetween period of approximately 5 weeks. In addition, participants wore the Computer Science and Applications (CSA) Activity Monitor for a 2-week period following the first questionnaire. Results The Spearman correlation for overall reproducibility of the SQUASH was 0.58 (95%-CI 0.36–0.74). Correlations for the reproducibility of the separate questions varied between 0.44 and 0.96. Spearman's correlation coefficient between CSA readings and the total activity score was 0.45 (95%-CI 0.17–0.66). Conclusion In conclusion, the SQUASH is a fairly reliable and reasonably valid questionnaire and may be used to order subjects according to their level of physical activity in an adult population. Because the SQUASH is a short and simple questionnaire, it may proof to be a very useful tool for the evaluation of health enhancing physical activity in large populations.

Assessment of physical activity level in relation to obesity: current evidence and research issues.

Validations of methods for the assessment of physical activity and studies on the relation between energy expenditure of activity and obesity were reviewed, with suggestions for further research. Validation studies of field methods for the assessment of physical activity against doubly labeled water were evaluated, studies on the relation between doubly labeled water assessed energy expenditure of activity and obesity are discussed. Three field methods for the assessment of physical activity, validated with doubly labeled water as a criterion method, were included: activity questionnaires, heart rate monitoring, and motion sensors. The triaxial accelerometer came out as the best field method for the assessment of physical activity, followed by the Baecke activity questionnaire. The majority of obese subjects are moderately active, and an increase in the activity level of obese subjects is limited by the ability to perform exercise of higher intensity. Accelerometers are an objective tool for the assessment of physical activity in large populations over periods long enough to be representative for normal daily life and with minimal discomfort to the subjects. The accelerometer can be used to distinguish differences in activity levels between individuals and to assess the effect of interventions on physical activity within individuals.

Physical activity assessment with accelerometers.

Evaluation of motion sensors, specifically accelerometers, as an objective tool for the assessment of physical activity in large populations, over periods long enough to be representative of normal daily life and with minimal discomfort to the subjects. Review of validation studies of accelerometers with indirect calorimetry as a reference method. Accelerometers were commercially available one-axial accelerometers: Caltrac, Computer Science Application (CSA) accelerometer, Mini Motionlogger Actigraph; the tri-axial accelerometer Tritrac-R3 D; and an tri-axial accelerometer for movement registration (Tracmor) from our laboratory. There is no clear difference for correspondence between indirect calorimetry and accelerometer counts during level walking, whether one-axial or tri-axial and placed at the wrist, hip or low back. Sedentary activities are better reflected with a tri-axial accelerometer than with a one-axial accelerometer. Two accelerometers were validated in free living conditions with doubly labeled water. The highest correlation between accelerometer output and activity induced energy expenditure was found for Tracmor. From all accelerometers tested, the tri-axial accelerometer for movement registration is an objective method that can be used to distinguish differences in activity levels between individuals and assess the effect of interventions on physical activity within individuals.

Theory in motion

Modeling studies are now a significant part of mainstream research in motor control. Novel and classical modeling techniques used in recent work on small and large motor systems illustrate the different roles that models play in furthering our understanding of motor systems. The models presented reveal single neuron short-term memory, unexpected effects of reciprocal inhibition and methods for decoding activity in large populations of neurons.

Imaging of cerebellar surface activation In vivo using voltage sensitive dyes

The understanding of the information processing performed by complex neuronal networks in the central nervous system will require techniques permitting the simultaneous monitoring of the electrical activity of neuronal ensembles. Voltage sensitive dyes offer the potential for non-invasive optical monitoring of the activity in large populations of neurons. In this report we describe the use of voltage sensitive dyes and image processing techniques to monitor in vivo the activation of parallel fibers and associated neuronal events produced by stimulation of the cerebellar cortex in the rat. Despite the temporal limitations of video processing a relatively brief set of neuronal events was successfully imaged. Using this methodology we demonstrate that the detected fluorescent light changes were highly correlated with the evoked extracellular field potentials. Graded surface stimulation produced graded spatial patterns consistent with known parallel fiber anatomy and physiology. The optical signals were dependent on the presence of the voltage sensitive dyes and were abolished by topical application of a local anesthetic agent. In essence, activation of a parallel fiber beam and associated activity were imaged at relatively high resolution.