Radiological characterisation of graphite components in Advanced Gas-cooled Reactor cores.

As the Advanced Gas-cooled Reactors approach the end of generation, research and characterisation are required to support the decommissioning strategy. Radiological data for AGR graphite are negligible and the radiological inventory of the AGR core and other graphite components rely on activation modelling. This is the first study of C-14 activity and its release behaviour in AGR core graphite and its associated carbonaceous deposits and provides valuable information that can support decommissioning activities. In combination with corresponding studies on Magnox core graphite, significant understanding is attained on the main C-14 precursors in the graphite and the deposits. In addition, this study reports C-14, H-3 and gamma spectrometry data on AGR graphite fuel sleeves. This is a waste stream that is currently stored in heavily engineered stores at a significant cost. The data indicate that alternative storage and disposal options with a lower environmental and financial impact are worth considering.

Exploiting the temporal patterning of transient VEP signals: a statistical single-trial methodology with implications to brain-computer interfaces (BCIs).

BACKGROUND: When visual evoked potentials (VEPs) are deployed in brain-computer interfaces (BCIs), the emphasis is put on stimulus design. In the case of transient VEPs (TVEPs) brain responses are never treated individually, i.e. on a single-trial (ST) basis, due to their poor signal quality. Therefore their main characteristic, which is the emergence during early latencies, remains unexplored. NEW METHOD: Following a pattern-analytic methodology, we investigated the possibility of using single-trial TVEP responses to differentiate between the different spatial locations where a particular visual stimulus appeared and decide whether it was attended or unattended by the subject. RESULTS: Covert spatial attention modulates the temporal patterning of TVEPs in such a way that a brief ST-segment, from a single synthesized sensor, is sufficient for a Mahalanobis-Taguchi (MT) system to decode subject's intention. COMPARISON WITH EXISTING METHOD(S): In contrast to previous VEP-based approaches, stimulus-related information and user's intention are being decoded from transient ST-signals via exploiting aspects of brain response in the temporal domain. CONCLUSIONS: We demonstrated that in the TVEP signals there is sufficient discriminative information, coming in the form of a temporal code. We were able to introduce an efficient scheme that can fully exploit this information for the benefit of online classification. The measured performance brings high expectations for incorporating these ideas in BCI-control.

On the quantization of time-varying phase synchrony patterns into distinct functional connectivity microstates (FCµstates) in a multi-trial visual ERP paradigm.

The analysis of functional brain connectivity has been supported by various techniques encompassing spatiotemporal interactions between distinct areas and enabling the description of network organization. Different brain states are known to be associated with specific connectivity patterns. We introduce here the concept of functional connectivity microstates (FCµstates) as short lasting connectivity patterns resulting from the discretization of temporal variations in connectivity and mediating a parsimonious representation of coordinated activity in the brain. Modifying a well-established framework for mining brain dynamics, we show that a small sized repertoire of FCµstates can be derived so as to encapsulate both the inter-subject and inter-trial response variability and further provide novel insights into cognition. The main practical advantage of our approach lies in the fact that time-varying connectivity analysis can be simplified significantly by considering each FCµstate as prototypical connectivity pattern, and this is achieved without sacrificing the temporal aspects of dynamics. Multi-trial datasets from a visual ERP experiment were employed so as to provide a proof of concept, while phase synchrony was emphasized in the description of connectivity structure. The power of FCµstates in knowledge discovery is demonstrated through the application of network topology descriptors. Their time-evolution and association with event-related responses is explored.

Analyzing functional brain connectivity by means of commute times: a new approach and its application to track event-related dynamics.

There is growing interest in studying the association of functional connectivity patterns with particular cognitive tasks. The ability of graphs to encapsulate relational data has been exploited in many related studies, where functional networks (sketched by different neural synchrony estimators) are characterized by a rich repertoire of graph-related metrics. We introduce commute times (CTs) as an alternative way to capture the true interplay between the nodes of a functional connectivity graph (FCG). CT is a measure of the time taken for a random walk to setout and return between a pair of nodes on a graph. Its computation is considered here as a robust and accurate integration, over the FCG, of the individual pairwise measurements of functional coupling. To demonstrate the benefits from our approach, we attempted the characterization of time evolving connectivity patterns derived from EEG signals recorded while the subject was engaged in an eye-movement task. With respect to standard ways, which are currently employed to characterize connectivity, an improved detection of event-related dynamical changes is noticeable. CTs appear to be a promising technique for deriving temporal fingerprints of the brain's dynamic functional organization.

The effect of transcranial magnetic stimulation on the latencies of vertical saccades.

In this study, we investigated the effect of transcranial magnetic stimulation (TMS) over the right posterior parietal cortex (PPC) on the latency of two different types of visually-guided vertical saccades: reflexive saccades triggered by the sudden onset of a target, and saccades towards target locations known in advance. For this reason, we used two oculomotor tasks: a gap and a delay task, respectively. Nine normal subjects performed vertical saccades at +/-7.5 and +/-15 degrees . TMS was applied at 80 and 100 ms after target onset in the gap task, and after fixation offset in the delay task. Without TMS, we confirmed a latency asymmetry in the gap task favouring upward saccades at the lower eccentricity (7.5 degrees ), and a latency symmetry in the delay task. TMS increased the latencies of all saccades in the delay task, when delivered at 100 ms. This effect was mostly pronounced for downward saccades at 7.5 degrees . As a result, saccade latencies showed an asymmetry in this condition, similar to the one observed in the gap task without TMS. The gap task with TMS resulted in a variable latency distribution and no significant overall effect on saccade latency. Our results indicate that the right PPC is involved in the initiation of vertical saccades in the delay task, and that this involvement appears to be enhanced for downward saccades. A conclusion for the involvement of this area in the gap task could not be drawn from this study.

Wavelet transform of the EEG reveals differences in low and high gamma responses to elementary visual stimuli.

Multiunit electrophysiological studies indicate that oscillatory activity is common in the awake mammalian central nervous system. Synchronous 20-80 Hz oscillations, so called gamma rhythms, have been proposed as a possible fundamental physiological mechanism of binding neuronal activity underlying object recognition. The purpose of this study was to determine whether or not gamma band oscillatory activity in the human brain is modulated by attributes of elementary visual stimulation. The experiment was performed on 7 normal subjects. Sinusoidal gratings were presented over a range of spatial frequencies. Evoked potentials were recorded over 5 surface electrodes placed in a horizontal occipital chain across the back of the head. Discrete wavelet transform was performed on the first 200 msec following stimulus onset on the average data of 256 sweeps. Power was analyzed with ANOVA across conditions. In our previous studies we have separated a "low" (14-28 Hz) and "high" (28-55 Hz) gamma band. The current results indicate that both gamma bands to full-field stimulation have the highest power at the midline (inion) electrode to a spatial frequency of 5.5 cpd, which is the peak spatial frequency from foveal psychophysical data. However, the spatial frequency bandwidth is considerably narrower in the HG than in the LG band. Occipital spatial frequency tuning of the massed high gamma response is narrower than the tuning of individual cortical neurons. The bandwidth difference between low and high gamma band suggests that different frequency gamma range oscillations may represent not only different functional properties of visual processing, but may also reflect underlying differences in excitatory and postsynaptic inhibitory circuits shaping the contrast sensitivity of the human observer. Our study emphasizes the importance of elementary visual filter properties for gamma responses and the need to subdivide gamma frequency ranges according to functional properties.

Early (N70m) neuromagnetic signal topography and striate and extrastriate generators following pattern onset quadrant stimulation.

The MEG signal generated by sinusoidal grating pattern onset at 1 and 3 cpd, presented randomly to the four quadrants, was analyzed in terms of gross signal properties and current dipole modeling and for a subset of subjects with magnetic field tomography (MFT). In all subjects a prominent wave was identified with a peak latency around 70 ms (N70m), modulated by spatial frequency and varying systematically with the stimulation quadrant. Sensors over occipital areas recorded stronger responses with lower field quadrants, while the signal for sensors a few centimeters more superior was stronger with upper quadrant stimuli. A strong signal in inferior occipitotemporal areas was less sensitive to upper and lower field stimulation and was stronger in the left hemisphere with contralateral (right) visual field stimulation. For lower visual field stimulation a good fit to the average data was obtained with a single dipole for 3 cpd, but was less consistent across run repetitions for 1 cpd. Neither the single-dipole model nor the two-dipole model produced a good fit across runs with the upper field stimuli. MFT solutions identified overlapping activity in striate and extrastriate areas in all conditions. The MFT solutions in the V1/V2 at the N70m were highly reproducible across run repetitions for 1 and 3 cpd, and consistent with the cruciform model, even though they were often weaker than simultaneously activated extrastriate generators. Extrastriate generators in V5 and the human homologue of V6, which were variable across run repetitions at N70m, settled to highly reproducible activations between 100 and 200 ms.

Functional properties of sub-bands of oscillatory brain waves to pattern visual stimulation in man.

The scalp recorded transient visual evoked potential (VEP) represents the massed activity of a large number of neurons of the human visual cortex. Animal studies show that intracerebrally-recorded high frequency electrical activity represents binding between neurons participating in a cooperative response. We evaluated the relationship between scalp recorded high frequency activity and transient VEPs elicited by a repetitive (grating) pattern. Stimuli were 1 and 4 cycles/degree sinusoidal gratings, presented in an on/off mode. Following conventional averaging, the discrete wavelet transform (DWT) was applied. Multi-resolution decomposition was used to divide the responses into 6 orthogonal frequency bands. The results show that high frequency oscillatory activity in the beta and gamma frequency range is closely related in time to the N70 peak of the simultaneous VEP. Power in both bands is modulated by spatial frequency. Beta range response to hemifield stimulation recorded over a chain of electrodes over the occipital area lateralizes in the same manner as N70, while gamma range activity is insensitive to lateralization and is more closely linked to foveal stimulation. This dissociation between beta and gamma range activity suggests that different bands of high frequency oscillatory activity in humans, linked to visual stimulation, may represent different aspects of visual processing.

The push-pull action of dopamine on spatial tuning of the monkey retina: the effects of dopaminergic deficiency and selective D1 and D2 receptor ligands on the pattern electroretinogram.

Retinal dopamine depletion in monkeys using either systemic MPTP or 6-OHDA results in attenuated electroretinographic (ERG) responses to peak spatial frequency stimuli. Diverse dopamine receptors have been identified in the primate retina. ERG studies performed using Haloperidol (a mixed antagonist), L-Sulpiride (D2 antagonist) and CY 208-243 (a D1 agonist) cause spatial frequency dependent diverse effects. 'Tuning' of the normal spatial contrast response PERG, was quantified by dividing the amplitude of the response at the peak spatial frequency with the amplitude to the low spatial frequency response yielding a number greater than one. Tuning for the pharmacological experiments was defined by dividing the actual amplitude obtained at the normal peak response with the actual amplitude at the low spatial frequency response. The PERG spatial contrast response function is discussed as the envelope output of retinal ganglion cells or the average or 'equivalent' retinal ganglion cell. However, we postulate the existence of two dopamine sensitive pathways with different weights for two classes of ganglion cells. It is inferred that D1 receptors are primarily affecting the 'surround' organization of ganglion cells with large centers, while D2 post-synaptic receptors contribute to 'center' response amplification of ganglion cells with smaller centers. These inferences are consistent with some lower vertebrate data. It is also inferred that low affinity D2 autoreceptors may be involved in the D1 'surround' pathway. An understanding of the logic performed by retinal D1 and D2 receptors may be useful to discern the functional role of diverse dopamine receptors in DA circuits elsewhere in the CNS.

Electrophysiological evidence for visuocognitive dysfunction in younger non Caucasian patients with Parkinson's disease.

A study of "primary" (VEPs) and "cognitive" (ERPs) visual evoked potentials was carried out in a group of non-demented Afro-American Parkinson's disease (PD) patients. Current studies suggest that differences exist in the clinical manifestations of PD in Caucasian and non-Caucasian populations. Two horizontal sinusoidal gratings differing in spatial frequency, i.e., 1 and 4 cycles per degree (cpd), were presented in an "odd-ball" paradigm to 17 patients with PD and 17 age-matched control subjects. While the 1 cpd stimulus, is not expected to reveal retinal dopaminergic deficency, but only visuocognitive deficits, the 4cpd may give direct information of both "retinal" and "cognitive" visual deficits. We measured the latencies and amplitudes of N70, P100 and P300 components, and derived the "normalized" measures of P300-N70 latency difference (Central Processing Time-CPT70), the P300-P100 latency difference (CPT100) and the P300 amplitude responses normalized to either N70 and P100 amplitude (Amplitude Ratios AR70 and AR100). Our results do show that cognitive electrophysiological deficits in younger PD patients exist in non-Caucasians, perhaps to an even greater degree than in Caucasians, and confirm that absolute and normalized ERP amplitude and latency abnormalities are a distinguishing feature of younger PD patients from controls. In particular P300 measures are abnormal for 1 cpd pattern. A negative correlation exists between P300 amplitude and the motor score. By comparing the results for 1 and 4cpd stimuli it can be concluded that "primary" and "cognitive" visual abnormalities are independently affected in PD, implying that visuo-cognitive abnormalities are not passively determined by retinal dopaminergic deficiency.