Persistence of internal representations of alternative voluntary actions.

We have investigated a situation in which externally available response alternatives and their internal representations could be dissociated, by suddenly removing some action alternatives from the response space during the interval between the free selection and the execution of a voluntary action. Choice reaction times in this situation were related to the number of initially available response alternatives, rather than to the number of alternatives available effectively available after the change in the external environment. The internal representations of response alternatives appeared to persist after external changes actually made the corresponding action unavailable. This suggests a surprising dynamics of voluntary action representations: counterfactual response alternatives persist, and may even be actively maintained, even when they are not available in reality. Our results highlight a representational basis for the counterfactual course of action. Such representations may play a key role in feelings of regret, disappointment, or frustration. These feelings all involve persistent representation of counterfactual response alternatives that may not actually be available in the environment

Volition and inhibition: Objective and subjective aspects of human volitional control

Action decisions can be directly driven by the current state of the external environment (instructed decisions); or they can be driven by internal mental states and goals, independently of the current environment (intentional decisions). Neural, behavioural and subjective data suggests that two separate neural systems may drive instructed and intentional action respectively. The distinction can be generalized beyond action itself to action inhibition, also classifiable as either instructed or intentional. However, the validity of the instructed/intentional distinction remains controversial. This thesis presents three linked sets of experiments that explored the validity and generality of the distinction, extending it in two key directions: action inhibition and subjective experience. The first group of experiments concerned decision making processes related to action and inhibition; the second focused on the period between decision making and action execution and the third on the subjective experience of intentional actions. Decision making processes were addressed by comparing electrophysiological and subjective measures prior to, during and after decisions to act or inhibit action. In the absence of external imperatives, intentional decisions may capitalize on spontaneous neural fluctuations, and show a weaker neural code than their instructed counterparts. To further explore the relative strength of intentional decisions, the period between decision making and action execution was addressed with EEG and behavioural methods. No evidence for unstable intentional decisions was found, during decision maintenance. Intentional action decisions may be strong, and persistent. Finally, two experiments directly compared the subjective experience of intentional and instructed actions. Neuroimaging results revealed possible mechanisms associated with the subjective experience of acting intentionally. Together, results support the broad distinction between instructed and intentional decisions. In particular, the coding of intentional actions may involve partial activations of alternative responses. Importantly, the thesis also demonstrates the feasibility of experimental studies addressing the subjective experience of intentional behaviour

Limited metacognitive access to one’s own facial expressions

As humans we communicate important information through fine nuances in our facial expressions, but because conscious motor representations are noisy, we might not be able to report these fine but meaningful movements. Here we measured how much explicit metacognitive information young adults have about their own facial expressions. Participants imitated pictures of themselves making facial expressions and triggered a camera to take a picture of them while doing so. They then rated confidence (how well they thought they imitated each expression). We defined metacognitive access to facial expressions as the relationship between objective performance (how well the two pictures matched) and subjective confidence ratings. Metacognitive access to facial expressions was very poor when we considered all face features indiscriminately. Instead, machine learning analyses revealed that participants rated confidence based on idiosyncratic subsets of features. We conclude that metacognitive access to own facial expressions is partial, and surprisingly limited

Negative motor phenomena in cortical stimulation: implications for inhibitory control of human action.

Electrical stimulation of the human cortex typically elicits positive sensorimotor effects. However, many neurosurgical studies have also reported negative motor areas (NMAs) in which stimulation produces inhibition of ongoing movement. The neurocognitive implications of these studies have not been systematically explored. Here we review the neurosurgical literature on NMAs and link this to cognitive mechanisms of inhibition and their role in voluntary control of action. In particular, we discuss the functional validity of NMAs. We contest the sceptical view that negative effects following stimulation merely reflect disruption of positive motor areas. Instead, we suggest that NMAs may produce an inhibitory mechanism under ecologically valid conditions

There is no free won't: antecedent brain activity predicts decisions to inhibit.

Inhibition of prepotent action is an important aspect of self-control, particularly in social contexts. Action inhibition and its neural bases have been extensively studied. However, the neural precursors of free decisions to inhibit have hardly been studied. We asked participants to freely choose to either make a rapid key press in response to a visual cue, or to transiently inhibit action, and briefly delay responding. The task required a behavioural response on each trial, so trials involving inhibition could be distinguished from those without inhibition as those showing slower reaction times. We used this criterion to classify free-choice trials as either rapid or inhibited/delayed. For 13 participants, we measured the mean amplitude of the ERP activity at electrode Cz in three subsequent 50 ms time windows prior to the onset of the signal that either instructed to respond or inhibit, or gave participants a free choice. In two of these 50 ms time windows (-150 to -100, and -100 to -50 ms relative to action onset), the amplitude of prestimulus ERP differed between trials where participants "freely" chose whether to inhibit or to respond rapidly. Larger prestimulus ERP amplitudes were associated with trials in which participants decided to act rapidly as compared to trials in which they decided to delay their responses. Last-moment decisions to inhibit or delay may depend on unconscious preparatory neural activity

Application of extremely compact capillary discharge soft x-ray lasers to dense plasma diagnostics

Includes bibliographical references.Table-top capillary discharge soft x-ray lasers combine the advantages of a small size and a high repetition rate with an extremely high brightness similar to that of their laboratory-size predecessors. When utilized to probe high density plasmas their short wavelength results in a higher critical density, reduced refraction, decreased free-electron absorption, and higher resolution as compared to optical probes. These characteristics allow the design of experiments capable of measuring the evolution of plasmas with density-scale length products that are outside the reach of optical lasers. This paper reviews the use of a 46.9 nm wavelength Ne-like Ar capillary discharge table-top laser in dense plasma diagnostics, and reports soft x-ray laser interferometry results of spot-focus Nd:YAG laser plasmas created at moderate irradiation intensity (~7×1012 W cm-2) with ; ~13 ns pulse width duration laser pulses. The measurements produced electron density maps with densities up to 0.9×1021 cm-3 that show the development of a concave electron density profile that differ significantly from those of a classical expansion. This two-dimensional behavior, that was recently also observed in line-focus plasmas, is analyzed here for the case of spot-focus plasmas with the assistance of hydrodynamic model simulations. The results demonstrate the use of a table-top soft x-ray laser interferometer as a new high resolution tool for the study of high density plasma phenomena and the validation of hydrodynamic codes.This work was supported by the U.S. Department of Energy Grant No. DE-FG03-02NA00062 and by the National Science Foundation. Part of this work was performed under the auspices of the U.S. Department of Energy by the University of California, Lawrence Livermore National Laboratory through the Institute of Laser Science and Application, under Contract No. W-7405-Eng-48

Two-dimensional effects in laser-created plasmas measured with soft-x-ray laser interferometry

Includes bibliographical references (pages 056409-6).Soft-x-ray laser interferograms of laser-created plasmas generated at moderate irradiation intensities (1×1011-7×1012 W cm-2) with λ = 1.06 μm light pulses of ~13-ns-FWHM (full width at half maximum) duration and narrow focus (~30 μm) reveal the unexpected formation of an inverted density profile with a density minimum on axis and distinct plasma sidelobes. Model simulations show that this strong two dimensional hydrodynamic behavior is essentially a universal phenomena that is the result of plasma radiation induced mass ablation and cooling in the areas surrounding the focal spot

Extremely compact soft X-ray lasers based on capillary discharges

Includes bibliographical references (page 522).Extremely compact high repetition rate soft X-ray lasers based on capillary discharge excitation have demonstrated average powers of a few milliWatt at 46.9 nm, milli-Joule-level pulse energy, peak spectral brightness several orders of magnitude larger than third-generation synchrotron beam lines, and excellent spatial coherence. Examples of the use of a capillary discharge soft X-ray laser in dense plasma diagnostics and laser ablation of materials are summarized

Opacity calculation for target physics using the ABAKO/RAPCAL code

Radiative properties of hot dense plasmas remain a subject of current interest since they play an important role in inertial confinement fusion (ICF) research, as well as in studies on stellar physics. In particular, the understanding of ICF plasmas requires emissivities and opacities for both hydro-simulations and diagnostics. Nevertheless, the accurate calculation of these properties is still an open question and continuous efforts are being made to develop new models and numerical codes that can facilitate the evaluation of such properties. In this work the set of atomic models ABAKO/RAPCAL is presented, as well as a series of results for carbon and aluminum to show its capability for modeling the population kinetics of plasmas in both LTE and NLTE regimes. Also, the spectroscopic diagnostics of a laser-produced aluminum plasma using ABAKO/RAPCAL is discussed. Additionally, as an interesting application of these codes, fitting analytical formulas for Rosseland and Planck mean opacities for carbon plasmas are reported. These formulas are useful as input data in hydrodynamic simulation of targets where the computation task is so hard that in line computation with sophisticated opacity codes is prohibitive