MeV photoelectron spectrometer for ultraintense laser interactions with atoms and molecules.

Traditional laser-matter spectroscopy techniques fail to accurately analyze photoelectrons and ions from ultrahigh intensity studies with terawatt and petawatt laser systems. We present a magnetic deflection, photoelectron spectrometer for ultrahigh intensity laser interactions with atoms and molecules in the single atom/molecule limit. Spectrometer fabrication and calibration, and noise background are presented as well as example photoelectron spectra for argon and chloromethane over an energy range from 20 keV to 2 MeV.

Limits of Strong Field Rescattering in the Relativistic Regime.

Recollision for a laser driven atomic system is investigated in the relativistic regime via a strong field quantum description and Monte Carlo semiclassical approach. We find the relativistic recollision energy cutoff is independent of the ponderomotive potential U_{p}, in contrast to the well-known 3.2U_{p} scaling. The relativistic recollision energy cutoff is determined by the ionization potential of the atomic system and achievable with non-negligible recollision flux before entering a "rescattering free" interaction. The ultimate energy cutoff is limited by the available intensities of short wavelength lasers and cannot exceed a few thousand Hartree, setting a boundary for recollision based attosecond physics.

Electron shell ionization of atoms with classical, relativistic scattering.

We investigate forward scattering of ionization from neon, argon, and xenon in ultrahigh intensities of 2 × 10(19) W/cm(2). Comparisons between the gases reveal the energy of the outgoing photoelectron determines its momentum, which can be scattered as far forward as 45° from the laser wave vector k(laser) for energies greater than 1 MeV. The shell structure in the atom manifests itself as modulations in the photoelectron yield and the width of the angular distributions. We arrive at an agreement with theory by using an independent electron model for the atom, a dipole approximation for the bound state interaction, and a relativistic, three-dimensional, classical radiation field including the laser magnetic field. The studies provide the atomic physics within plasmas, radiation, and particle acceleration in ultrastrong fields.

Cognitive functioning in sporadic amyotrophic lateral sclerosis: a six month longitudinal study.

OBJECTIVE: To observe changes in cognition over six months in subjects with recently diagnosed sporadic amyotrophic lateral sclerosis (ALS). METHODS: The study used a between-group and within-group longitudinal design. Nineteen ALS subjects and eight matched caregivers were recruited to participate in baseline neuropsychological assessments that were repeated six months later. Between group comparisons for these variables were undertaken at baseline and six months later. Within group/across time comparisons for these variables were carried out for both groups. Individual analyses for the neuropsychological variables using z scores were done for the ALS subjects using their baseline performance as the basis for comparison with their six month performance. RESULTS: The between-group and within-group comparisons did not show significant differences in cognitive function over time. In individual analyses, however, seven of 19 ALS subjects (36.84%) developed abnormal neuropsychological performance over six months. CONCLUSIONS: Early in the disease course, over one third of the ALS subjects developed cognitive deficits over six months. These findings support the hypothesis that cognitive deficits in ALS become more prominent over time.

Comparison of reading and spelling in patients with probable Alzheimer's disease.

Patients with probable Alzheimer's disease (AD) are reported to show mild, but reliable, difficulties reading aloud and spelling to dictation exception words, which have unusual or unpredictable correspondence between their spelling and pronunciation (e.g., touch). To understand the cognitive dysfunction responsible for these impairments, 21 patients and 27 age-and education-matched controls completed specially designed tests of single-word oral reading and spelling to dictation. AD patients performed slightly below controls on all tasks and showed mildly exaggerated regularity effects (i.e., the difference in response accuracy between words with regular spellings minus exception words) in reading and spelling. Qualitative analyses, however, did not demonstrate response patterns consistent with impairment in central lexical orthographic processing. The authors conclude that the mild alexia and agraphia in AD reflect semantic deficits and nonlinguistic impairments rather than a specific disturbance in lexical orthographic processing.

Impaired spelling in Alzheimer's disease: a linguistic deficit?

Different kinds of real words and pronounceable pseudowords (PWs) were presented for writing to dictation to patients with the diagnosis of probable Alzheimer's disease (AD) and to age- and education-matched healthy controls. Though spelling less accurately on all tasks, AD patients responded in a manner generally qualitatively similar to controls. Except for a slightly enhanced effect of spelling regularity in real word writing accuracy, AD patients showed the same sensitivity to various lexical, orthographic and phonological variables as controls. Both groups showed no difference in spelling accuracy for words and PWs with regular vs ambiguous spelling patterns, and groups also showed similar orthographic preferences when spelling PWs having several different acceptable pronunciations. Finally, AD patients and controls produced similar types of errors when spelling real words. Dementia severity was related to the overall accuracy, but not to the pattern, of spelling responses. It is suggested that the decline in response accuracy in cognitively demanding writing tasks in patients with more advanced dementia is most likely due to semantic impairment and impairments of nonlinguistic functions of attention, executive control and praxis, rather than to a disturbance within language specific processes.

Cognitive mechanisms for processing nonwords: evidence from Alzheimer's disease.

Repetition and reading of various types of pronounceable nonwords (pseudowords) was examined in patients with probable Alzheimer's disease (AD) and healthy elderly controls. Overall accuracy of performance was lower in AD patients compared to controls, but the two groups showed qualitatively similar response patterns when reading different kinds of pseudowords aloud and when repeating pseudowords composed of familiar phonological forms, analogous to those in real English words. AD patients diverged in performance from controls, however, when repeating pseudowords composed of phonologically unusual forms. These results support two conclusions: (1) Aspects of phonological processing may become disrupted in AD patients in association with increasing dementia severity, while orthographic processing remains comparatively less impaired. (2) The results are consistent with the view that the processing of pseudowords is achieved through the same system as real words, and further show that the influence of prior language experience on the processing of novel linguistic forms occurs primarily at the level of phonological, rather than orthographic processing.

Lexical semantic and associative priming in Alzheimer's disease.

Semantic memory impairment was investigated in patients with probable Alzheimer's disease (AD) using a threshold oral word reading task to assess priming of different lexical relationships. Healthy elderly controls showed significant priming for associatively related nouns (tempest-teapot) and also for nouns semantically related either because both designate basic-level exemplars of a common superordinate category (cousin-nephew) or because the target names the superordinate category of the prime (daughter-relative). AD patients, in contrast, showed preserved priming of lexical associates but impaired priming of certain semantic relationships. They showed no priming between words designating coordinate exemplars within a category, despite preserved priming of the superordinate category label. Findings are consistent with the view that at least part of the semantic deficit in AD is due to disruption of semantic knowledge that affects relationships among basic-level concepts, more than the relationships between these concepts and their corresponding superordinate category of membership.

Semantic memory impairment does not impact on phonological and orthographic processing in a case of developmental hyperlexia.

Recent evidence from patients with progressive language disorders and dementia has been used to suggest that phonological and orthographic processing depend on intact semantic memory. These claims challenge the traditional view that there are functionally separate modules in the language system. The effect of a severe, but nonprogressive, semantic impairment on phonological and orthographic processing was evaluated in LA, a mentally retarded child with hyperlexia. Knowledge of a word's meaning did not affect LA's word repetition, a measure of phonological processing, or his acquisition and retention of orthographic patterns for writing to dictation low-frequency words with exceptional spellings. These findings support the assertion that both orthographic and phonological whole-word representations can be acquired, stored, and retrieved in the absence of a functional link to semantic memory.

Repetition of single words and nonwords in Alzheimer's disease.

Repetition of single words and pronounceable nonwords (pseudowords) was assessed in Alzheimer's Disease (AD) patients to evaluate how lexical phonological processing might be accomplished when semantic and conceptual knowledge is impaired. AD patients performed significantly worse than healthy elderly controls on all repetition tasks. However, repetition abilities and dementia severity were not correlated, and AD patients produced the same distribution of error types as controls. Furthermore, despite their semantic problems, AD patients, like controls, showed a significant advantage for repeating real words compared to pseudowords, even when repeating low frequency phonologically complex words whose meaning is not likely to have been retained. The results support the postulated existence of a lexical phonological system that is used to repeat both known and novel words and that processes linguistic information independent of its meaning.