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We examined the effects of drought on the water relations, osmotic adjustment and xylem vulnerability to embolism of olive (Olea europaea L.) varieties, ‘Meski’ and ‘Chemlali’. Two-year-old self-rooted cuttings growing in sand-filled pots in a greenhouse were subjected to water stress by withholding water for 60 days. Water relations and gas exchange measurements showed that ‘Chemlali’ was more drought resistant than ‘Meski’ and had a greater capacity for osmotic adjustment through solute accumulation. However, when water stress was acute, the effect of osmoregulation on leaf cell turgor was largely counteracted by xylem cavitation. Cavitation vulnerability curves showed that both varieties were highly resistant to embolism formation. The xylem water potential inducing 50% loss of stem conductivity approached −7 MPa in ‘Meski’ and only slightly less in ‘Chemlali’. Although the difference between varieties in susceptibility to xylem embolism was small, it appears to account in large part for the difference between them in the ability to tolerate severe drought. [ABSTRACT FROM PUBLISHER]

Motor-evoked potentials (MEPs) vary in size from one stimulus to the next. The objective of this study was to determine the cause and source of trial-to-trial MEP size variability. In two experiments involving 10 and 14 subjects, the variability of MEPs to cortical stimulation (cortical-MEPs) in abductor digiti minimi (ADM) and abductor hallucis (AH) was compared to those responses obtained using the triple stimulation technique (cortical-TST). The TST eliminates the effects of motor neuron (MN) response desynchronization and of repetitive MN discharges. Submaximal stimuli were used in both techniques. In six subjects, cortical-MEP variability was compared to that of brainstem-MEP and brainstem-TST. Variability was greater for MEPs than that for TST responses, by approximately one-third. The variability was the same for cortical- and brainstem-MEPs and was similar in ADM and AH. Variability concerned at least 10–15% of the MN pool innervating the target muscle. With the stimulation parameters used, repetitive MN discharges did not influence variability. For submaximal stimuli, approximately two-third of the observed MEP size variability is caused by the variable number of recruited alpha-MNs and approximately one-third by changing synchronization of MN discharges. The source of variability is most likely localized at the spinal segmental level. [ABSTRACT FROM AUTHOR]

Six experiments investigated how variability on irrelevant stimulus dimensions and variability on response dimensions contribute to spatial and nonspatial stimulus-response (S-R) correspondence effects. Experiments 1-3 showed that, when stimuli varied in location and number, S-R correspondence effects for location or numerosity occurred when responses varied on these dimensions but not when responses were invariant on these dimensions. These results are consistent with the response- discrimination account, according to which S-R correspondence effects should only arise for a dimension that is used for discriminating between responses in working memory. Experiments 4-6 showed that, when responses varied in location and number, both invariant and variable stimulus number produced correspondence effects in S-R numerosity. In summary, the present results indicate that the usefulness of a particular dimension for response discrimination can be sufficient for producing S-R correspondence effects, whereas variability of a stimulus dimension is not sufficient for producing such effects. [ABSTRACT FROM AUTHOR]

Background: Patients with attention-deficit/hyperactivity disorder (ADHD) typically show fronto-striatal abnormalities during functions of cognitive control. In this study we investigate whether medication-naïve children with ADHD are impaired in temporo-parietal neural networks that mediate purely perceptual attention allocation to a behaviorally neutral oddball task. Furthermore, we explore the relationship between the neural substrates of attention allocation and response variability, typically increased in patients. Method: Event-related functional magnetic resonance imaging was used to compare brain activation of 17 medication-naïve boys with ADHD with that of 18 handedness- and IQ-matched healthy comparison boys during a visual oddball task that required the same response to oddball and standard trials. Furthermore, to explore the relationship between behavioral dispersion and attention networks, regression analyses were conducted between response variability and brain activation networks. Results: Patients showed significantly reduced brain activation in left and right superior temporal lobes, basal ganglia, and posterior cingulate during the oddball versus standard contrast. The activation differences in superior temporal lobes correlated inversely with response variability in control subjects but not in patients with ADHD. Conclusions: Brain abnormalities in patients with ADHD are not confined to fronto-striatal networks mediating executive functions but are also observed in temporo-striatal and cingulate regions during perceptive visual attention processes. Furthermore, temporal lobe dysfunction in the context of perceptual attention might be related to their behavioral problems with response variability. [Copyright &y& Elsevier]

In 5 experiments, the author examined rats' sensitivity to the molar feedback function relating response rate to reinforcement rate on schedules of reinforcement. These studies demonstrated that, at lower rates of responding, rats' performance on variable ratio (VR), variable interval (VI), and variable interval with linear feedback loop (VI+) schedules was determined largely by reinforcement of interresponse times; response rates were faster on VR than on both VI and VI+ schedules. In contrast, when procedures were adopted to maintain high rates of response, rats showed sensitivity to the molar characteristics of the schedules; they responded as fast on a VI+ schedule as on a VR schedule and faster on both of these schedules than on a yoked VI schedule. When the variance of response rate was manipulated, this factor was noted as an important element in determining sensitivity to the molar characteristics of the schedule. [ABSTRACT FROM AUTHOR]

We have studied the contribution of feedback signals originating from one of the “form-processing” extrastriate cortical areas, area 21a (A21a), to orientation selectivity of single neurons in the ipsilateral area 17 (A17). Consistent with previous findings, reversible inactivation (cooling to 5–10°C) of area 21a resulted in a substantial reduction in the magnitude of the maximum response ( R max) of A17 cells accompanied by some changes in the half-width at half-height of the R max (HWHH). By fitting model functions to the neurons’ response profiles we found that in the vast majority of orientation-tuned A17 cells tested (30/39, 77%), inactivation of A21a resulted in a “flattening” of their orientation-tuning curves. It is characterised by a substantial reduction in the R max associated with either a broadening of the orientation-tuning curves (17 cells) or a relatively small reduction (12 cells) or no change (1 cell) in the HWHH. When the “flattening” effect was quantified using a simple ratio index or R/ W, defined as R max/HWHH, we found that R/ W was significantly reduced during inactivation of A21a. The change in R/ W is strongly correlated with the change in the maximum slope of the orientation-tuning curves. Furthermore, analysis of response variability indicates that “signal-to-noise” ratio of the responses of A17 neurons decreases during inactivation of A21a. Our results suggest that the predominately excitatory feedback signals originating from A21a play a role in enhancing orientation selectivity of A17 neurons and hence are likely to improve overall orientation discriminability. [ABSTRACT FROM AUTHOR]

Abstract: It is well known that expansion-based stochastic methods are approximate schemes, as they are based on a first or, at most, second order series expansion on the basic variable, e.g. displacement. Therefore, expansion-based stochastic analysis schemes are bound to show small response variability when compared with Monte Carlo simulation (MCS) results, and application of these schemes is limited to stochastic problems with relatively small variability. In order to overcome these general drawbacks of the expansion methods, we suggest a higher-order stochastic field function that can be employed in the expansion-based stochastic analysis scheme of the weighted integral method. We then propose a new weighted integral formulation using the higher-order stochastic field function. The new formulation is not only applicable to stochastic problems with a high degree of uncertainty but also can reproduce the phenomenon of accelerated increase in the response variability when the coefficient of variation of the stochastic field increases, as observed in the MCS. In order to show the validity of the proposed formulation, we provide two numerical examples and the results are discussed in detail. [Copyright &y& Elsevier]

Abstract: We previously showed that spike count response distributions in anterior cingulate neurons can be fitted by a mixture of a few Poisson distributions in our reward schedule task. Here we report that the neuronal responses in insular cortex, an area connected to anterior cingulate cortex, can also be nicely fitted. The ratio of Poisson distributions changed with schedule progress, suggesting that neuronal responses in these areas fall into discrete firing modes. More insular neurons show mode changes across the schedules. The selection of firing modes might be related to cognitive processes, but seems independent across the two areas. [Copyright &y& Elsevier]

In this paper the need for a rational treatment of uncertainties in structural mechanics and analysis is reasoned. It is shown that the traditional deterministic conception can be easily extended by applying statistical and probabilistic concepts. The so-called Monte Carlo simulation procedure is the key for those developments, as it allows the straightforward use of the currently used deterministic analysis procedures. A numerical example exemplifies the methodology. It is concluded that uncertainty analysis may ensure robust predictions of variability, model verification, safety assessment, etc. [Copyright &y& Elsevier]

Background: The present study explores the relationships between several information processing capacities and internalizing and externalizing behavior problems in a general population sample of 10- to 12-year olds ( N = 2,037 51% girls). Methods: Parent-reported behavior problems as assessed by the Child Behavior Checklist were used to form four groups of children with 1) neither internalizing nor externalizing problems ( n = 1,470), 2) only internalizing problems ( n = 237), 3) only externalizing problems ( n = 182), and 4) both internalizing and externalizing problems ( n = 148). These groups were compared on measures of speed and accuracy from the Amsterdam Neuropsychological Tasks program reflecting the efficiency of several input-, central cognitive-, and output-related information processing capacities. Results: Children with both internalizing and externalizing problems demonstrated the least efficient performance, followed by children with only externalizing problems, whereas children with only internalizing problems did not differ from children without problems. More specifically, response variability and the ability to maintain and quickly compare information in working memory were found to be related to the severity of problem behavior. The ability to inhibit prepotent responses was related to the type of problem behavior, discriminating between the children with only internalizing problems and only externalizing problems. However, this latter capacity no longer differentiated when controlling for IQ. No differences were found between boys and girls. Conclusions: The results suggest that, in general, cognitive processing deficits are more strongly related to the degree than to the type of maladaptive behavior. Furthermore, response variability and working memory may serve as potential markers for identifying high-risk children and response inhibition as an indicator of the type of maladaptive behavior. [ABSTRACT FROM AUTHOR]

Studies have been carried out on satisfaction and international pleasure travel. Some have related satisfaction and past visits to a particular destination with intentions to recommend it, to revisit it. and to visit other destinations in its area. This study arose from reconsideration of research on satisfaction and stated intentions of travelers from Britain and Germany to Mallorca and Turkey. Analysis confirmed that satisfaction is a significant predictor of stated intentions and that contingent factors moderate the influence of satisfaction on intentions. Explaining limited variance led to the logical examination of why the explanation was not better. It is concluded that variance explanation was low because segments had particular satisfaction-likelihood relations and because satisfaction and likelihood responses were ambiguous. Sources of variation that can be controlled are identified. They include some first-time visitors not being likely to return regardless of satisfaction: personal satisfaction responses not reflecting what the party will do: and likelihood responses not having a clear meaning. Practical and research implications are presented. [ABSTRACT FROM AUTHOR]

Abstract: Instrumental learning involves both variation and selection: variation of what the animal does, and selection by reward from among the variation. Four experiments with rats suggested a rule about how variation is controlled by recent events. Experiment 1 used the peak procedure. Measurements of bar-press durations showed a sharp increase in mean duration after the time that food was sometimes given. The increase was triggered by the omission of expected food. Our first explanation of the increase was that it was a frustration effect. Experiment 2 tested this explanation with a procedure in which the first response of a trial usually produced food, ending the trial. In Experiment 2, unlike Experiment 1, omission of expected food did not produce a large increase in bar-press duration, which cast doubt on the frustration explanation. Experiments 3 and 4 tested an alternative explanation: a decrease in expectation of reward increases variation. Both used two signals associated with different probabilities of reward. Bar presses were more variable in duration during the signal with the lower probability of reward, supporting this alternative. These experiments show how variation can be studied with ordinary equipment and responses. [Copyright &y& Elsevier]

Abstract: In the majority of research works on stochastic finite element analysis, the main concern is put on the effect of uncertain elastic modulus on the response variability. This is mainly due to the importance of this parameter in the stochastic response of structures but also to the difficulties in introducing other material and/or geometrical uncertain parameters in the stochastic formulation. In this paper, a formulation is suggested in order to determine the response variability of in-plane and plate structures with multiple uncertain material parameters, i.e., elastic modulus and Poisson’s ratio. To accomplish this, use is made of the general equation for nth joint moment and a polynomial expansion on the coefficients of constitutive matrix. For computational purposes, the infinite series of stochastic sub-stiffness matrices is truncated preserving sufficient accuracy. To demonstrate the validity of the proposed formulation, some numerical examples are provided and the results are compared with those obtained by means of classical Monte Carlo simulation combined with the local average scheme. [Copyright &y& Elsevier]

Summary: In the ordinary structural materials, one of the parameters that can be assumed to have spatial uncertainty is Poisson’s ratio. Therefore the independent evaluation of the effects of this parameter on the response variability is of importance. The difficulties in obtaining the response variability due to randomness in Poisson’s ratio lie in the fact that the Poisson’s ratio enters the stiffness matrix as a non-linear parameter. In this paper, a formulation to determine the response variability in plane strain and plane stress states due to the randomness in the Poisson’s ratio is given. The formulation is accomplished by means of the stochastic decomposition of the constitutive matrix into several sub-matrices taking into consideration of the polynomial expansion on the coefficients of constitutive relation. To demonstrate the validity of the proposed formulation, some example structures are chosen and the results are compared with those obtained by means of Monte Carlo simulation. Through the formulation proposed in this study, it becomes possible for the weighted integral stochastic finite element analysis to consider all the uncertain material parameters in its application. [Copyright &y& Elsevier]

Abstract: Introduction: The aim of this study was to investigate the relationship between working memory and inhibition in children with attention deficit hyperactivity disorder (ADHD), high-functioning autism (HFA), and Tourette syndrome (TS), compared to normally developing children. Furthermore, the contribution of variation in processing speed on working memory and inhibition was investigated in these childhood psychopathologies. Method: Four groups of children are reported in this study: 65 children with ADHD, 66 children with HFA, 24 children with TS, and 82 normal control children. All children were in the age range of 6–13 years. Results: The relationship between working memory and inhibition was similar in children with ADHD, HFA, TS, and normally developing children. The relationship between both domains did not alter significantly for any of the groups, when variation in processing speed was taken into account. More symptoms of hyperactivity/impulsivity are related to a poorer inhibitory process and greater response variability. More symptoms of autism are related to a poorer working memory process. Conclusion: The current study showed that working memory, inhibition, and response variability, are distinct, but related cognitive domains in children with developmental psychopathologies. Research with experimental manipulations is needed to tackle the exact relationship between these cognitive domains. [Copyright &y& Elsevier]

Abstract: This study asked whether the concurrent reinforcement of behavioral variability facilitates learning to emit a difficult target response. Sixty students repeatedly pressed sequences of keys, with an originally infrequently occurring target sequence consistently being followed by positive feedback. Three conditions differed in the feedback given to non-target sequences: concurrent positive feedback presented contingent on response variability, positive feedback presented non-contingently, or no reinforcement for any non-target responses (control condition). Contrary to the result of analogous rat studies, if anything, the participants in the control condition more readily learned to emit the target sequence than did the subjects in each of the other two conditions. It is argued that these contradictory findings are primarily caused by procedural differences, such as differences in the density of the reinforcement schedule applied to non-target behavior, rather than reflecting a true species difference. [Copyright &y& Elsevier]

This article analyzes the effects of value-driven ambivalence and group attachment on response variability in public attitudes toward campaign finance reform. The analysis demonstrates that group attachment, when activated by affective cues, moderates the effects of ambivalence on response variability. By tipping the balance of considerations in one direction or the other, group attachments make it easier for ambivalent respondents to make tradeoffs between competing values during policy choices and, as a result, dampen response variability. Methodologically, the results offer an important cautionary note about the use of linear ambivalence scales by calling into question the assumption that indifference is an intermediate state between preference and ambivalence. [ABSTRACT FROM AUTHOR]

Abstract: Electrophysiology is a chronometric tool to describe the time course of mental operations: this technique is unsurpassed when precise accounts of the covert activity of the brain are required. The scope of investigation in psychiatric research ranges from genes to cognition, from neurology to mind. This contribution reviews the efficacy of electrophysiology in delineating causal mechanisms, taking attention deficit hyperactivity disorder (ADHD) as an example. We review key electrophysiological findings in the study of ADHD, which contribute to the aetiological framework. Further, it is suggested that electrophysiological studies that build on cognitive-experimental findings may direct future lines of investigation, especially in combination with genetic methodologies. [Copyright &y& Elsevier]

Abstract: In this paper, we use single-trial analysis of electroencephalography (EEG) to ascertain the cortical origins of response time variability in a rapid serial visual presentation (RSVP) task. We extract spatial components that maximally discriminate between target and distractor stimulus conditions over specific time windows between stimulus onset and the time of a motor response. We then compute the peak latency of this differential activity on a trial-by-trial basis, and correlate this with response time. We find, for our nine participants, that the majority of the latency is introduced by component activity which begins far-frontally 200 ms prior to the response and proceeds to become parietally distributed near the time of response. This activity is consistent with the hypothesis that cortical networks involved in generating the late positive complexes may be the origins of the observed response time variability in rapid discrimination of visual objects. [Copyright &y& Elsevier]

Abstract: The Mindlin assumption in the plate bending allows the shear deformation together with the bending one. The constitutive relation for shear behavior, however, has different order of contribution from the plate thickness t than that in bending. Therefore, when we have concern on the stochastic behavior of Mindlin plate taking into account the uncertain plate thickness, the bending and shear parts have to be handled in different ways. Furthermore, if the concern is put on the multiple uncertain parameters such as between elastic modulus and plate thickness, the influence of correlation between these parameters also has to be resolved. In this study, a formulation for stochastic analysis is given in the context of the weighted integral method for Mindlin plate with uncertainties in elastic modulus and plate thickness. The effects of correlation between these parameters on the structural behavior are examined. In addition, the stability of response variability in terms of the aspect ratio of a plate is investigated. Several square plates with various boundary conditions subjected to uniformly distributed or concentrated loads are taken as examples, and the results are compared with those of precedent works as well as those of classical Monte Carlo simulation. [Copyright &y& Elsevier]

The recorded responses of single neurons often vary considerably in the numbers of spikes emitted across repeats of a single experimental condition. Because of this irregularity and for theoretical convenience the responses are often approximated using a Poisson process. However, it has been frequently pointed out that many details of the responses, including the distribution of spike counts across similar trials, are not consistent with a Poisson process, even an inhomogeneous one. Wiener and Richmond (2003, J Neurosci 23:2394-2406) showed that the spike count distributions could usually be fitted nicely by mixtures of a few (1-3) Poisson distributions, a step they regarded as a computational convenience. Now, we find that a substantial proportion (47%) of the neuronal responses from anterior cingulate cortex, which we conceptualize as part of a system related to the balance between work and reward, have responses with multimodal firing rate distributions. When these distributions are modeled as mixtures of Poisson distributions, the proportions of the different Poisson distributions are related to behavioral state, and might be related to cognitive factors. This suggests that the neurons undergo behaviorally-related mode changes. [ABSTRACT FROM AUTHOR]

Objective: This study was designed to clarify whether poor performance of children with attention-deficit/hyperactivity disorder (ADHD) on motor timing tasks reflects a true deficit in the temporal organization of motor output or is due to a lack of intrinsic motivation. Method: Eighteen children with ADHD (age 8-12) were compared with 18 age- and gender-matched normal controls with respect to timing precision, timing variability, and the frequency of extreme under- and overestimations during a 1-second interval production task. Monetary reward, response cost, and no reward were implemented to manipulate motivation. Results: Children with ADHD produced significantly more inaccurate and more variable time intervals and exhibited a larger number of extreme over- and underestimations than control children. Although all children performed significantly better when monetary incentives were applied, group differences were not eliminated. Conclusions: In this study, no evidence was found for a motivational deficit as an explanation for impaired performance on a time production task in ADHD. Rather, results provide clear support for a generic motor timing deficit, probably due to a dysfunctional frontostriatocerebellar network involved in temporal aspects of motor preparation. [ABSTRACT FROM AUTHOR]

Up to now, the Young''s modulus is mainly dealt within the analysis of response variability. However, since the Poisson''s ratio is the other material constant which influences the behavior of structures, the independent evaluation of the effects of this parameter on the response variability is of importance. In this paper, a formulation to determine the response variability in plate structure due to the randomness of Poisson''s ratio is given. To filter out the independent contributions of randomness in Poisson''s ratio to the response variability, the constitutive matrix has to be decomposed into several sub-matrices. In order to include the Poisson''s ratio in the constitutive relation as a non-linear parameter, a polynomial expansion of Poisson''s ratio is introduced. To demonstrate the validity of the proposed formulation, an example is chosen and the results are compared with those obtained by means of Monte Carlo simulation. Through the formulation proposed in this paper, it becomes possible for the non-statistical weighted integral stochastic approach to deal with all the uncertain material parameters in its application. [Copyright &y& Elsevier]

Measured responses from visual cortical neurons show that spike times tend to be correlated rather than exactly Poisson distributed. Fano factors vary and are usually greater than 1, indicating a tendency toward spikes being clustered. We show that this behavior emerges naturally in a balanced cortical network model with random connectivity and conductance-based synapses. We employ mean-field theory with correctly colored noise to describe temporal correlations in the neuronal activity. Our results illuminate the connection between two independent experimental findings: high-conductance states of cortical neurons in their natural environment, and variable non-Poissonian spike statistics with Fano factors greater than 1. [Copyright &y& Elsevier]

In this article we describe a signal-processing framework for mining information from event-related recordings. Pattern-analytic tools are combined with graph-theoretic techniques and signal understanding methodologies in a user-friendly environment with the scope of learning, parameterization, and representation of the ST data manifold. Through the first part, we provide a general outline of our methodological approach while trying to demonstrate all the different stages, where DM tools can be applied. In the second part, we provide a more detailed demonstration, give a synopsis of the obtained results and take the opportunity to underline the merits of the adopted algorithmic procedures. To enable the full justification of our framework, instead of just including a technical demonstration of some of the incorporated DM and KDD tools, we address the problem of response variability: an issue of great neuroscientific importance and the subject of continuous debate. The major question in all the previous studies was the validity of "signal plus noise" model, i.e., whether a stereotyped evoked response is linearly superimposed on the ongoing brain activity after every stimulus presentation, a prerequisite for the validity of ensemble-averaging. Using data from a simple visual experiment targeting at the early neuromagnetic response known as N70m, we try to bridge the gap between the "conservative-party" that suggests heavy averaging as the only way to study the brain's response and the "neurodynamics-party" that claims the averaged-signal has very little to say about how the real-time processing of an input from a sensory pathway is actually performed in the cortex. [ABSTRACT FROM PUBLISHER]

Two experiments were carried out to test the differential effects of hunger and thirst on memory performance. In Experiment 1, two groups of rats were exposed to an original radial-maze task and then to a 30-min retention-memory task. The food-deprived group completed the original task more quickly than the water-deprived group, but the thirsty group mastered the memory task more quickly than the hungry group ( p < 0.01). In Experiment 2, deprivation conditions were changed from the original to the memory task. The food-water group completed the memory task more rapidly than the water-food group ( p < 0.05). Thirst proved to constitute a more favorable condition for retention-memory learning. The applicability of several theories is discussed. [ABSTRACT FROM AUTHOR]

We investigated individual differences in second language (L2) proficiency by looking at the efficiency or automaticity of semantic priming using behavioural and event-related brain potential (ERP) measures. In Experiment 1, 37 first language (L1) English speakers varying in second language (L2, French) proficiency made living/non-living judgments to English and French nouns in lists blocked by language. Sixty critical words were each presented twice, once primed by a semantic associate in the preceding trial (e.g. ADULT, CHILD) and once unprimed (e.g. RABBIT, CHILD). Measures of response time (RT) and intra-individual variability in response time (coefficient of variation, CV) were obtained. The CV provided an index of processing efficiency that has been related to automaticity. Participants performed faster and with lower CVs (i.e. with greater efficiency) in L1 than L2, and the more highly proficient bilinguals had lower CVs than the less proficient bilinguals. Experiment 2 replicated these results with 29 participants and provided an electrical brain activity measure of processing efficiency using the N400 ERP. The similar pattern of results obtained between the behavioural and N400 ERP CV measures supported the idea that the CV measure of electrical brain activity can provide useful information about the automaticity or efficiency of cognitive processing. [Copyright &y& Elsevier]

In this work, an extended stochastic formulation of the triangular composite facet shell element TRIC is presented for the case of combined uncertain material (Young’s modulus, Poisson’s ratio) and geometric (thickness) properties. These properties are assumed to be described by uncorrelated two-dimensional homogeneous stochastic fields. The stochastic finite element analysis of shell structures is performed using the spectral representation method for the description of the random fields in conjunction with Monte Carlo simulation (MCS) for the computation of the response variability. Useful conclusions regarding the influence of each one of the structural parameters on the response variability are derived from the numerical tests examined. [Copyright &y& Elsevier]

We use mean field theory to study the response properties of a simple randomly connected model cortical network of leaky integrate-and-fire neurons with balanced excitation and inhibition. The formulation permits arbitrary temporal variation of the input to the network and takes exact account of temporal firing correlations. We find that neuronal firing statistics depend sensitively on the firing threshold. In particular, spike count variances can be either significantly greater than or significantly less than those for a Poisson process. These findings may help in understanding the variability observed experimentally in cortical neuronal responses. [Copyright &y& Elsevier]

Reinforcement was presented contingent upon human subjects simultaneously varying three dimensions of an operant response. The response was drawing rectangles on a computer screen. The dimensions were area of the rectangle, its location on the screen, and its shape. In Experiment 1, an experimental group was reinforced for satisfying the three-part variability contingency. A control group was equally reinforced for drawing rectangles but independently of levels of variability. Results showed that the experimental group varied significantly more along each of the dimensions than did the control group. In Experiment 2, another group of subjects was reinforced for repeating instances along one of the dimensions, e.g. repeatedly draw a rectangle in approximately the same location, while simultaneously varying along the other two dimensions. The subjects learned to satisfy these contingencies as well. These results show reinforcement simultaneously and independently controls the variability of three orthogonal dimensions of a response. [Copyright &y& Elsevier]

It is well-known that humans show a wide range of variation in skin reactivity to irritant chemicals. This has been established through population studies, through the examination of inter-subject variability, and (to a limited extent) through studies of skin site variation in response within subjects. However, simple response variability within individual test subjects has not been examined as carefully, and this has implications for our ability to predict irritant reactivity. Some key questions are: (i) how consistently do human beings respond, even within a given study, to different equally irritating chemicals, or to the same chemical when comparing different concentrations or durations of exposure, and, (ii) Do individual test subjects’ responses to one chemical (or exposure scenario) correlate with their responses to another? To examine these questions in some detail, we reexamined individual study subjects’ responses from earlier published studies involving both acute and cumulative irritation patch test protocols. Acute irritation responses were compared across chemicals with similar irritation profiles. Cumulative irritation responses were compared across different concentrations of the anionic surfactant, sodium dodecyl sulfate (SDS). Acute (high concentration) and cumulative (low concentration) patch test responses to SDS were also compared. The analysis showed that, as might be expected, response correlations were greatest within test types, either when comparing chemicals of similar overall irritancy, or when comparing similar concentrations of a single chemical. However, individually divergent responses were also frequent, reinforcing the conclusion that a given individual’s response to one chemical or exposure condition does not always predict their response to another. This has important ramifications for other questions related to population differences in skin reactivity. [ABSTRACT FROM AUTHOR]

Behavior is controlled by neural activity in the brain. The final outcome of this neural control may critically depend on the firing reliability of individual neurons. A nearly constant, proportional relationship is usually found between the response mean and response variance. Here we asked whether lateral interactions within striate cortex that modulate response magnitude also proportionately modify the response variance of cortical neurons. In many cases, response variability depended on stimulus organization: discrete flankers colinearly placed well outside the neuron's receptive field increased response magnitude without a proportional increase in variance, thus improving the neuron's response reliability. Since colinear flanker facilitation is often seen near the neuron's firing threshold, increased response reliability for weak stimuli may contribute to enhancing perceptual saliency. [ABSTRACT FROM AUTHOR]