Patterns of phase-dependent spiral wave attenuation in excitable media.

We show that introducing periodic planar fronts with long excitation duration can lead to spiral attenuation. The attenuation occurs periodically over cycles of several planar fronts, forming a variety of complex spatiotemporal patterns. We find that these attenuation patterns occur only at specific phases of the descending fronts relative to the rotational phase of the spiral. These patterns fall into two general classes, each defined by a specific expression for the number of attenuated spirals per cycle of planar fronts, and represented by a structured diagram in parameter space. The spiral attenuation patterns we observe remain stable in time and do not change during the evolution of the system.

System-size resonance in a binary attractor neural network.

System size resonance (SSR) is a phenomenon in which the response of a system is optimal for a certain finite size, but poorer as the size goes to zero or infinity. In order to show SSR effects in binary attractor neural networks, we study the response of a network, in the ferromagnetic phase, to an external, time-dependent stimulus. Under the presence of such a stimulus, the network shows SSR, as is demonstrated by the measure of the signal amplification both analytically and by simulation.

Two-Hz wide EEG bands in Alzheimer's disease.

Twenty Alzheimer's Disease (AD) patients in a mild to moderate stage of the disease and 20 control subjects were compared in 17 2-Hz wide bands from the electrodes 01, 02, P3, P4, T5, T6, F3, F4, F7, F8, Fp1, and Fp2. Differences reached statistical significance for 0-2 and 4-6 Hz bands, where AD patients presented highest power values. The AD group was divided into two groups according to the stage of disease. Both groups of patients presented 0-2 Hz increase in frontal, right parieto-temporal, and occipital areas. The increase in 4-6 Hz band was mainly over frontal areas in both groups and over left parietal region in moderate AD patients. These results and those relative to dominant frequency and crossover frequency between groups are discussed according to previous results with conventional and 2-Hz wide bands in AD patients in a severe stage of the disease.

Topography of mobility and complexity parameters of the EEG in Alzheimer's disease.

Mobility and complexity, Hjorth's parameters of the electroencephalogram (EEG), were calculated in 16 electrodes in a group of 14 patients with probable Alzheimer's disease (AD) and 14 healthy controls. Mobility was decreased in the AD group, differences reaching high levels of significance over all the electrodes. Complexity was increased significantly in the AD group only in five electrodes. Sensitivity, specificity, positive predictive value, and negative predictive value were calculated for mobility at each electrode, considering only those AD patients with the lowest cognitive impairment. Sensitivity and negative predictive value were 100% at all the electrodes. Specificity and positive predictive value at several electrodes reached highly acceptable proportions, such as 71% or more for the former, and 64% or more for the latter. Implications of mobility in the early diagnosis of AD are discussed.

Topographic maps of brain electrical activity in primary degenerative dementia of the Alzheimer type and multiinfarct dementia.

The topography of the electroencephalographic (EEG) pattern of ten patients with primary degenerative dementia of the Alzheimer type, ten multiinfarct dementia patients, and ten age-matched controls was compared during three different behavioral conditions: resting condition with eyes open (EO), memorizing a list of words (M), and recalling the same list of words (R). Results indicate that the alpha frequency band does not show significant changes. On the other hand, the theta band could be considered an important factor in the differential diagnosis of the primary degenerative dementia of the Alzheimer type, showing a higher power over right posterior regions in this group of patients compared with the multiinfarct dementia patients under different behavioral conditions.

The Wisconsin Card Sorting Test and the assessment of frontal function: a validation study with event-related potentials.

The Wisconsin Card Sorting Test (WCST) is generally regarded as the prototype of abstract reasoning task and has been routinely used to assess frontal lobe function in a variety of clinical and research contexts. However, there are growing concerns that the WCST fails to discriminate frontal patients from those with lesions in other brain regions or from normals. Event-related potentials (ERP) from frontal, fronto-temporal, temporal, parietal and occipital areas were recorded during the performance of a computerized version of the WCST in order to explore frontal versus non-frontal ERP indexes during WCST activation. The task protocol was contrived to focus on the differences between early and late trials of each WCST series. Cognitive processes underlying these two task conditions have been described as extradimensional and intradimensional shifts in attention, respectively. Differences between early and late WCST trials appeared as soon as 120 msec poststimulus and were associated with a negative field potential centred at the fronto-temporal region of the left hemisphere. Significantly larger amplitudes of the posterior P3b wave for late as compared with early WCST trials also lent support to claims of a strong involvement of working memory mechanisms during WCST performance. Results are discussed in terms of the implications for the utility of ERP measures in clinical neuropsychology.

Functional differences in the semantic processing of concrete and abstract words.

There is considerable debate as to whether the semantic system is a unitary one in which meanings are available in a peculiar, perceptual-free format, or whether it is functionally segregated into anatomically discrete, modality-specific but semantic regions. In the former case, concrete and abstract words should not differ in the amount of activation of semantic areas. Neuroimaging studies in this field are, however, far from conclusive, and one reason for this may be that the degree of imageability of the stimuli - probably a crucial variable - has not been considered. Recognition Potential (RP) reflects semantic processing and appears to originate in basal extrastriate regions involved in semantic processing. In this study, we compared the RP of concrete and abstract words that actually differ in their degree of imageability. Results indicate that the semantic processing areas in which the RP originates display a higher activation for concrete (more imageable) material, but that abstract material also evokes a notably larger RP component compared with pseudowords or unpronounceable letter strings. Accordingly, the study appears to suggest that there is no full functional segregation of the semantic systems. Rather, our data support the existence of a semantic system that is specialised in concrete, imageable material, and that is also activated, though to a lower extent, by abstract material.

Attentional set shifting modulates the target P3b response in the Wisconsin card sorting test.

For years the Wisconsin card sorting test (WCST) has been used as a test of frontal lobe function. Recent event-related potential (ERP) research has shown large differences in the amplitude of P3b responses evoked by early and late trials within each WCST series ([8]: Barceló F., Sanz M., Molina V., Rubia FJ. The Wisconsin Card Sorting Test and the assessment of frontal function: A validation study with event-related potentials. Neuropsychologia 1997;35:399-408). In this study, 16 normal subjects performed a WCST adaptation to investigate the role of attentional set shifting in these WCST P3b effects. Two control tasks were designed to examine whether early-late WCST P3b changes reflect category selection (attention) or category storage (memory) operations. Results suggest both a sharp P3b attenuation during shift WCST trials, followed by a gradual P3b build-up during post-shift trials. This P3b modulation could not be attributed to selection or storage of simple sensory stimulus dimensions, nor was it observed when the new rule was externally prompted by the first card in the WCST series. Instead, WCST P3b changes seem related to the endogenously generated shift in the perceptual rule used to sort the cards (i.e., the shift in set). The gradual build-up in P3b amplitude paralleled a progressive improvement in sorting efficiency over several post-shift WCST trials. A model based on formal theories of visual attention and attentional set shifting is proposed to account for these effects. The model offers firm grounds for prediction and bridges the gap between related clinical and experimental evidence.

Distribution of neurons in the main cuneate nucleus projecting to the inferior olive in the cat. Evidence that they differ from those directly projecting to the cerebellum.

The distribution in the main cuneate nucleus of cells projecting to the inferior olive and the intermediate zone of the cerebellar anterior lobe were compared by means of double retrograde labeling methods in the cat. The tracer combinations were either Fast Blue and Diamidino Yellow Dihydrochloride; or horseradish peroxidase conjugated to wheat germ agglutinin and Diamidino Yellow Dihydrochloride. Neurons in the caudal, middle and rostral subdivisions of the main cuneate nucleus project to the inferior olive. Differences exist, however, in its number and location along the rostrocaudal extent of the nucleus. Cells projecting to the inferior olive predominate in the caudal and middle subdivisions, where they concentrate ventrally. No cells in the "clusters region" project to the inferior olive. Main cuneate nucleus neurons projecting to the cerebellum concentrate rostral to the obex, bordering the external cuneate nucleus and partially intermixing with the rostrally located cells projecting to the inferior olive. However, no double-labeled cells were found. The results indicate that the main cuneate nucleus projections to the inferior olive and cerebellar anterior lobe originate from different populations of neurons with high specific locations within the nucleus. This finding is in agreement with previous studies suggesting that each of the main cuneate nucleus targets receives its input from a distinct population of neurons within the nucleus.

Drug therapy and cerebral perfusion in obsessive-compulsive disorder.

Cerebral perfusion in a previously untreated patient with obsessive-compulsive disorder was studied qualitatively and semi-quantitatively with SPECT before, and during and 6 wk after treatment with clomipramine. The patient's symptoms disappeared while on medication and relapsed after drug withdrawal. At baseline, there was an increased perfusion ratio in the bilateral orbitofrontal, anterior cingular, frontotemporal and right caudate regions. These alterations disappeared during drug therapy. After treatment discontinuation and symptomatic relapse, the same pattern of hyperactivity was found. Semiquantitative measurements after treatment withdrawal showed a return to perfusion values similar to those observed before treatment in subcortical structures. In cortical areas, this level was not completely achieved. Subtraction SPECT images showed perfusion changes at the orbitofrontal, caudate and thalamic levels.

Technetium-99m-HMPAO in Tourette's syndrome on neuroleptic therapy and after withdrawal.

UNLABELLED: Both decreased and increased perfusion and metabolism have been described with PET and SPECT in different areas of the brain in patients with Gilles de la Tourette's syndrome. The aim of this study was to define the regional cerebral perfusion pattern in drug-free patients and the changes in perfusion with the usual neuroleptic treatment. METHODS: A group of 13 normal control subjects and 15 unmedicated Gilles de la Tourette's syndrome patients were studied with 99mTc-HMPAO brain SPECT. Thirteen of the initial group of patients were retested on neuroleptic treatment. A semiquantitative analysis of the images was performed. RESULTS: Decreased perfusion in orbital and anterior medial regions of both frontal lobes as well as in both temporal lobes was observed in the nontreated group compared with control subjects. With treatment, a perfusion increase in these frontal regions and in the left medial temporal cortex was observed. CONCLUSION: Neuroleptic treatment could decrease the hyperactivity of the dopaminergic system leading to improvement of the clinical symptoms and reperfusion of some previously hypoperfused regions.

Non-frontal P3b-like activity evoked by the Wisconsin Card Sorting Test.

Event related potentials (ERP) were recorded from 29 electrode positions in 10 normal subjects while they performed a simplified version of the Wisconsin Card Sorting Test (WCST). The design focused on ERP differences between early and late trials within each WCST series. Topographic and dipole analyses confirmed the reliability of two ERP signs: one conspicuous mid-parietal P3b wave and one asymmetrical frontal-temporal component. A three-dipole model accounted for these ERP signs with > 90% accuracy even in individual subjects, and suggests a sub-second activation of temporal-parietal and medial temporal association areas during card sorting. The WCST-related P3b wave is proposed to reflect working memory operations such as template matching and template formation during card sorting.

Common basal extrastriate areas for the semantic processing of words and pictures.

OBJECTIVE: The recognition potential (RP) is an electrophysiological brain response which is sensitive to the semantic processing of meaningful stimuli. In this study we attempt to elucidate the topography and neural origin of the RP evoked by pictures and to compare it with the RP evoked by words. METHODS: Words, pictures, Chinese characters and control stimuli were presented to 20 subjects following the rapid stream stimulation procedure. The activity was recorded using 60 cephalic electrodes. RESULTS: We found a RP displaying its maximal amplitude at the left inferior parieto-occipital electrode (PO7) for words and at the right homologue electrode (PO8) for pictures and Chinese characters. Both the amplitude and the latency of the RP were larger in the case of words. A profile analysis indicated that the neural generators of the RP were common regardless of the type of stimulus, and a dipole analysis placed them about the lingual gyrus. CONCLUSIONS: Words and pictures share the same neural generators for the RP despite of subtle differences in lateralization. This is interpreted as an index of a multimodal semantic processing in basal extrastriate areas.

Auditory P300 event related potential and serotonin reuptake inhibitor treatment in obsessive-compulsive disorder patients.

Neuropsychological findings in obsessive-compulsive disorder (OCD) have been explained in terms of reduced cognitive shifting ability as a result of low levels of frontal inhibitory activity. This deficit could be reflected in an abnormal P300 component of the event-related potential. The improvement in cognitive processing due to pharmacological treatment would modify the P300 component, bringing it close to that of normal controls. Nineteen patients suffering from OCD and 19 normal controls were recorded. We used a computerized version of the auditory 'odd-ball paradigm' to obtain the P300 component at the Pz electrode. Patients were tested twice, drug-free and under treatment with clomipramine in 250-300 mg doses. We observed the P300 component to have lower amplitude and longer latency in drug-free OCD patients when compared with controls. P300 amplitude in OCD increased after treatment, although this was supported only by a statistical trend. There was no modification in P300 latency after treatment. It is possible that inhibitory activity improves with treatment and allows patients to answer with more confidence, which results in an increase in P300 amplitude. This study suggests that cognitive dysfunction in OCD fluctuates with changes in the clinical associated with treatment, probably in relationship to central serotoninergic transmission.

Outbreaks of Hantavirus induced by seasonality.

Using a model for rodent population dynamics, we study outbreaks of Hantavirus infection induced by the alternation of seasons. Neither season by itself satisfies the environmental requirements for propagation of the disease. This result can be explained in terms of the seasonal interruption of the relaxation process of the mouse population toward equilibrium, and may shed light on the reported connection between climate variations and outbreaks of the disease.

Extinction in population dynamics.

We study a generic reaction-diffusion model for single-species population dynamics that includes reproduction, death, and competition. The population is assumed to be confined in a refuge beyond which conditions are so harsh that they lead to certain extinction. Standard continuum mean field models in one dimension yield a critical refuge length L(c) such that a population in a refuge larger than this is assured survival. Herein we extend the model to take into account the discreteness and finiteness of the population, which leads us to a stochastic description. We present a particular critical criterion for likely extinction, namely, that the standard deviation of the population be equal to the mean. According to this criterion, we find that while survival can no longer be guaranteed for any refuge size, for sufficiently weak competition one can make the refuge large enough (certainly larger than L(c)) to cause extinction to be unlikely. However, beyond a certain value of the competition rate parameter it is no longer possible to escape a likelihood of extinction even in an infinite refuge. These unavoidable fluctuations therefore have a severe impact on refuge design issues.

Random Ginzburg-Landau model revisited: reentrant phase transitions.

We analyze the phase diagram of the random Ginzburg-Landau model, where a quenched dichotomous noise affects the control parameter. We show that the system exhibits two types of counterintuitive reentrant second-order phase transitions. In the first case, increasing the coupling drives the system from a disordered to an ordered state and then back to a disordered state. In the second case, increasing the intensity of the quenched noise, the system goes from an ordered phase to a disordered phase and back to an ordered state. We discuss the general mechanism that produces these reentrant phase transitions, showing that it may appear in other physical systems, such as a modification of the spin-1 Blume-Capel model proposed to describe the critical behavior of helium mixtures in a random medium.

Coherence enhancement in nonlinear systems subject to multiplicative Ornstein-Uhlenbeck noise.

We show that for two biologically relevant models with self-sustained oscillations under the action of a multiplicative Ornstein-Uhlenbeck process, their coherence response behaves nonmonotonically with the process correlation time. There is a correlation time for which the quality factor is optimized. This phenomenon is a consequence of the interplay between the correlation time and the system's periodicity. This relation is evidenced through a power law relation with an exponent close to -1 / 2.

Effects of internal fluctuations on the spreading of Hantavirus.

We study the spread of Hantavirus over a host population of deer mice using a population dynamics model. We show that taking into account the internal fluctuations in the mouse population due to its discrete character strongly alters the behavior of the system. In addition to the familiar transition present in the deterministic model, the inclusion of internal fluctuations leads to the emergence of an additional deterministically hidden transition. We determine parameter values that lead to maximal propagation of the disease and discuss some implications for disease prevention policies.

Brain mapping in dysphonemic dyslexia: in resting and phonemic discrimination conditions.

Topographic mapping of EEG bands and alpha asymmetry were analyzed during resting conditions and an auditory phonemic discrimination task (APD) in carefully screened samples of 9.0- to 12-year-old control and dysphonemic-sequencing dyslexic children. The EEG was recorded over the entire scalp, with a linked ears reference. We found: (1) no differences between groups during resting conditions in any frequency band analyzed; (2) a left hemisphere alpha responsiveness in dyslexic children during the APD condition; and (3) a beta 2 decrease in the left posterior quadrant in dyslexics during the APD condition. We conclude that the lower alpha and beta 2 responsiveness may be due to a diminished cortical reactivity and may be related to intrinsic disorganization of the neural processors associated directly with the APD task demands, the alpha activity reflecting poor attention to an external stimuli source and the beta 2 decrease reflecting an unusual processing associated with the linguistic cognitive requirements.