Attentional shifts towards an expected visual target alter the level of alpha-band oscillatory activity in the human calcarine cortex

Neuronal operations associated with the top-down control process of shifting attention from one locus to another involve a network of cortical regions, and their influence is deemed fundamental to visual perception. However, the extent and nature of these operations within primary visual areas are unknown. In this paper, we used magnetoencephalography (MEG) in combination with magnetic resonance imaging (MRI) to determine whether, prior to the onset of a visual stimulus, neuronal activity within early visual cortex is affected by covert attentional shifts. Time/frequency analyses were used to identify the nature of this activity. Our results show that shifting attention towards an expected visual target results in a late-onset (600 ms postcue onset) depression of alpha activity which persists until the appearance of the target. Independent component analysis (ICA) and dipolar source modeling confirmed that the neuronal changes we observed originated from within the calcarine cortex. Our results further show that the amplitude changes in alpha activity were induced not evoked (i.e., not phase-locked to the cued attentional task). We argue that the decrease in alpha prior to the onset of the target may serve to prime the early visual cortex for incoming sensory information. We conclude that attentional shifts affect activity within the human calcarine cortex by altering the amplitude of spontaneous alpha rhythms and that subsequent modulation of visual input with attentional engagement follows as a consequence of these localized changes in oscillatory activity. © 2005 Elsevier B.V. All rights reserved.

Attentional modulation of oscillatory activity in human visual cortex

The effects of attentional modulation on activity within the human visual cortex were investigated using magnetoencephalography. Chromatic sinusoidal stimuli were used to evoke activity from the occipital cortex, with attention directed either toward or away from the stimulus using a bar-orientation judgment task. For five observers, global magnetic field power was plotted as a function of time from stimulus onset. The major peak of each function occurred at about 120 ms latency and was well modeled by a current dipole near the calcarine sulcus. Independent component analysis (ICA) on the non-averaged data for each observer also revealed one component of calcarine origin, the location of which matched that of the dipolar source determined from the averaged data. For two observers, ICA revealed a second component near the parieto-occipital sulcus. Although no effects of attention were evident using standard averaging procedures, time-varying spectral analyses of single trials revealed that the main effect of attention was to alter the level of oscillatory activity. Most notably, a sustained increase in alpha-band (7-12 Hz) activity of both calcarine and parieto-occipital origin was evident. In addition, calcarine activity in the range of 13-21 Hz was enhanced, while calcarine activity in the range of 5-6 Hz was reduced. Our results are consistent with the hypothesis that attentional modulation affects neural processing within the calcarine and parieto-occipital cortex by altering the amplitude of alpha-band activity and other natural brain rhythms. © 2003 Elsevier Inc. All rights reserved.

Pharmacologically induced and stimulus evoked rhythmic neuronal oscillatory activity in the primary motor cortex in vitro

Parkinson's disease (PD) is associated with enhanced synchronization of neuronal network activity in the beta (15-30 Hz) frequency band across several nuclei of the basal ganglia (BG). Deep brain stimulation of the subthalamic nucleus (STN) appears to reduce this pathological oscillation, thereby alleviating PD symptoms. However, direct stimulation of primary motor cortex (M1) has recently been shown to be effective in reducing symptoms in PD, suggesting a role for cortex in patterning pathological rhythms. Here, we examine the properties of M1 network oscillations in coronal slices taken from rat brain. Oscillations in the high beta frequency range (layer 5, 27.8 +/- 1.1 Hz, n=6) were elicited by co-application of the glutamate receptor agonist kainic acid (400 nM) and muscarinic receptor agonist carbachol (50 mu M). Dual extracellular recordings, local application of tetrodotoxin and recordings in M1 micro-sections indicate that the activity originates within deep layers V/VI. Beta oscillations were unaffected by specific AMPA receptor blockade, abolished by the GABA type A receptor (GABAAR) antagonist picrotoxin and the gap-junction blocker carbenoxolone, and modulated by pentobarbital and zolpidem indicating dependence on networks of GABAergic interneurons and electrical coupling. High frequency stimulation (HFS) at 125 Hz in superficial layers, designed to mimic transdural/transcranial stimulation, generated gamma oscillations in layers 11 and V (incidence 95%, 69.2 +/- 7.3 Hz, n=17) with very fast oscillatory components (VFO; 100-250 Hz). Stimulation at 4 Hz, however, preferentially promoted theta activity (incidence 62.5%, 5.1 +/- 0.6 Hz, n=15) that effected strong amplitude modulation of ongoing beta activity. Stimulation at 20 Hz evoked mixed theta and gamma responses. These data suggest that within M1, evoked theta, gamma and fast oscillations may coexist with and in some cases modulate pharmacologically induced beta oscillations.

Gap junctions and emergent rhythms

Gap junction coupling is ubiquitous in the brain, particularly between the dendritic trees of inhibitory interneurons. Such direct non-synaptic interaction allows for direct electrical communication between cells. Unlike spike-time driven synaptic neural network models, which are event based, any model with gap junctions must necessarily involve a single neuron model that can represent the shape of an action potential. Indeed, not only do neurons communicating via gaps feel super-threshold spikes, but they also experience, and respond to, sub-threshold voltage signals. In this chapter we show that the so-called absolute integrate-and-fire model is ideally suited to such studies. At the single neuron level voltage traces for the model may be obtained in closed form, and are shown to mimic those of fast-spiking inhibitory neurons. Interestingly in the presence of a slow spike adaptation current the model is shown to support periodic bursting oscillations. For both tonic and bursting modes the phase response curve can be calculated in closed form. At the network level we focus on global gap junction coupling and show how to analyze the asynchronous firing state in large networks. Importantly, we are able to determine the emergence of non-trivial network rhythms due to strong coupling instabilities. To illustrate the use of our theoretical techniques (particularly the phase-density formalism used to determine stability) we focus on a spike adaptation induced transition from asynchronous tonic activity to synchronous bursting in a gap-junction coupled network.

Delays in activity based neural networks

In this paper we study the effect of two distinct discrete delays on the dynamics of a Wilson-Cowan neural network. This activity based model describes the dynamics of synaptically interacting excitatory and inhibitory neuronal populations. We discuss the interpretation of the delays in the language of neurobiology and show how they can contribute to the generation of network rhythms. First we focus on the use of linear stability theory to show how to destabilise a fixed point, leading to the onset of oscillatory behaviour. Next we show for the choice of a Heaviside nonlinearity for the firing rate that such emergent oscillations can be either synchronous or anti-synchronous depending on whether inhibition or excitation dominates the network architecture. To probe the behaviour of smooth (sigmoidal) nonlinear firing rates we use a mixture of numerical bifurcation analysis and direct simulations, and uncover parameter windows that support chaotic behaviour. Finally we comment on the role of delays in the generation of bursting oscillations, and discuss natural extensions of the work in this paper.

A field study to describe diel, tidal and semilunar rhythms of larval release in an assemblage of tropical rocky shore crabs

Available information on the larval release rhythms of brachyurans is biased to temperate estuarine species and outcomes resulting from some sort of artificial manipulation of ovigerous females. In this study we applied field methods to describe the larval release rhythms of an assemblage of tropical rocky shore crabs. Sampling the broods of ovigerous females of Pachygrapsus transversus at two different shores indicated a spatially consistent semilunar pattern, with larval release maxima around the full and new moon. Yet, synchronism between populations varied considerably, with the pattern obtained at the site exposed to a lower wave action far more apparent. Breeding cohorts at one of the sampled shores apparently belonged to actual age groups composing the ovigerous population. The data suggest that these breeding groups release their larvae in alternate syzygy periods, responding to a lunar cycle instead of the semilunar pattern observed for the whole population. For the description of shorter-term rhythms, temporal series at hour intervals were obtained by sampling the plankton and confinement boxes where ovigerous females were held. Unexpectedly, diurnal release activity prevailed over nocturnal hatching. Yet, only grapsids living higher on the shore exhibited strong preferences over the diel cycle, with P. transversus releasing their larvae during the day and Geograpsus lividus during the night. The pea crab Dissodactylus crinitichelis, the spider crab Epialtus brasiliensis and a suite of xanthoids undertook considerable releasing activity in both periods. Apart from the commensal pea crab D. crinitichelis, all other taxa revealed tide-related rhythms of larval release, with average estimates of the time of maximum hatching always around the time of high tides; usually during the flooding and slack, rather than the ebbing tide. Data obtained for P. transversus females held in confinement boxes indicated that early larval release is mostly due to nocturnal hatching, while zoeal release in diurnal groups took place at the time of high tide. Since nocturnal high tides at the study area occurred late, sometimes close to dusk, early release would allow more time for offshore transport of larvae when the action of potential predators is reduced.

Depolarization induced suppression of excitation and the emergence of ultra-slow rhythms in neural networks

Ultra-slow fluctuations (0.01-0.1 Hz) are a feature of intrinsic brain activity of as yet unclear origin. We propose a candidate mechanism based on retrograde endocannabinoid signaling in a synaptically coupled network of excitatory neurons. This is known to cause depolarization-induced suppression of excitation (DISE), which we model phenomenologically. We construct emergent network oscillations in a globally coupled network and show that for strong synaptic coupling DISE can lead to a synchronized population burst at the frequencies of resting brain rhythms.

Improvements in fundamental movement skill competency mediate the effect of the SCORES intervention on physical activity and cardiorespiratory fitness in children

Numerous studies have identified a positive association between fundamental movement skill (FMS) competency and physical activity in children; however, the causal pathways have not been established. The aim of this study is to determine if changes in FMS competency mediated the effect of the Supporting Children's Outcomes using Rewards, Exercise and Skills (SCORES) intervention on physical activity and cardiorespiratory fitness in children. Eight primary schools (25 classes) and 460 children (aged 8.5 ± 0.6, 54% girls) were randomised to the SCORES intervention or control group for the 12-month study. The outcomes were accelerometer-determined moderate-to-vigorous physical activity (MVPA) and cardiorespiratory fitness. The hypothesised mediators were actual FMS competency and perceived sport competence. Mediation analyses were conducted using multilevel linear analysis in MPlus. From the original sample, 138 (30.0%) and 370 (80.4%) children provided useable physical activity and cardiorespiratory fitness data at post-test assessments. There were significant treatment effects for locomotor skills and overall FMSs. Changes in MVPA were associated with changes in object-control skills, overall FMSs and perceived competence. The overall FMSs had a significant mediating effect on MVPA (AB = 2.09, CI = 0.01-4.55). Overall FMSs (AB = 1.19, CI = 0.002-2.79) and locomotor skills (AB = 0.74, CI = 0.01-1.69) had a significant mediating effect on cardiorespiratory fitness. The results of this study conclude that actual but not perceived movement skill competency mediated the effect of the SCORES intervention on physical activity and cardiorespiratory fitness.