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Biomedical subjects

G Gratton

Publications and source records attributed to G Gratton.

31 records · Page 2Linked to original sources
PubMed1994

Feasibility of intracranial near-infrared optical scanning.

Light injected at a point on a surface of a scattering medium is emitted at the surface after traveling a quasisemicircular path deep into the medium. This phenomenon can be exploited to detect objects immersed in the medium from time-resolved measurements of light intensity at the surface. Our experiments on model systems demonstrate that absorbing objects, surrounded by bone and other scattering material, can be detected. The technique yields surface images of absorbing objects submerged in a scattering medium. Images of the same phantoms inside the cavity of a skull can be obtained by the same technique.

Brain↗
PubMed1992

Functional correlates of a three-component spatial model of the alpha rhythm.

We applied a three-component (temporal, occipital and parietal) spatial model to EEG data obtained from 46 young and 11 elderly subjects. With closed eyes the mean alpha frequency was 9.51 Hz for the temporal, 9.88 Hz for the occipital and 10.14 Hz for the parietal component. The power of the occipital component was larger than that of the others. With open eyes it was reduced to the same level as the temporal and parietal components. The power of the occipital component decreased significantly with age.

Adult↗
PubMed1992

Optimizing the use of information: strategic control of activation of responses.

Recent studies indicate that subjects may respond to visual information during either an early parallel phase or a later focused phase and that the selection of the relevant phase is data driven. Using the noise-compatibility paradigm, we tested the hypothesis that this selection may also be strategic and context driven. At least part of the interference effect observed in this paradigm is due to response activation during the parallel-processing phase. We manipulated subjects' expectancies for compatible and incompatible noise in 4 experiments and effectively modulated the interference effect. The results suggest that expectancies about the relative utility of the information extracted during the parallel and focused phases determine which phase is used to activate responses.

Adult↗
PubMed1990

Event-related brain potentials as indices of information extraction and response priming.

Measures of overt response and of the event-related brain potential (ERP) were used to investigate the processing of a priming stimulus varying in its information content. Subjects were shown sequences of 2 letters that served as a priming and an imperative stimulus. In 3 randomly interspersed conditions the imperative stimulus had a 0.80, 0.50, or 0.20 probability of physically matching the priming letter. The different probability conditions were signaled by the position of a dot flanking the priming letter. Reaction time and accuracy data indicated that the subjects primed their responses as a function of the information conveyed by the priming stimulus. The amplitude and latency of the P300 to the priming stimulus were sensitive to the amount of information conveyed by the priming stimulus and the duration of the processing required. The readiness potential in the foreperiod was lateralized as a function of the priming stimulus. Furthermore, the larger the amplitude of the P300 to the priming stimulus, the larger the lateralization of the readiness potential, indicating that information extraction, indexed by the P300, was related to response priming, indexed by the readiness potential. The results indicate that ERP measures make manifest covert aspects of the priming process occurring in the foreperiod.

Adolescent↗
PubMed1989

A procedure for using multi-electrode information in the analysis of components of the event-related potential: vector filter.

This paper describes a procedure, Vector filter, based on a multiple regression model, which decomposes the event-related brain potential into components on the basis of scalp distribution. It is assumed that the voltage values observed at several electrode sites at any point in time are given by the linear combination of a set of components and background noise and that the scalp distribution of each component is invariant and known. Each component's scalp distribution is expressed by a set of weights, one for each electrode. The amplitude of the component, at any point in time, is then derived using a least squares criterion. Unlike other component decomposition procedures, Vector filter can be applied when the latency of a component varies as a function of trial, condition, or subject population. We review two problems in the use of the procedure: the selection of the set of components, and the derivation of the scalp distribution of each of the components. Two applications of the procedure are discussed: identification of the component structure of an event-related potential waveform, and filtering of a waveform for a particular component.

Arousal↗
PubMed1989

Simulation studies of latency measures of components of the event-related brain potential.

We compared the accuracy of P300 latency estimates obtained with different procedures under several simulated signal and noise conditions. Both preparatory and signal detection techniques were used. Preparatory techniques included frequency filters and spatial filters (single electrode selection and Vector filter). Signal detection techniques included peak-picking, cross-correlation, and Woody filter. Accuracy in the latency estimation increased exponentially as a function of the signal-to-noise ratio. Both Woody filter and cross-correlation provided better estimates than peak-picking, although this advantage was reduced by frequency filtering. For all signal detection techniques, Vector filter provided better estimates than single electrode selection. Large component overlap impaired the accuracy of the estimates obtained with both single electrode selection and Vector filter, but with Vector filter impairment occurred only when the overlapping component had a scalp distribution that was similar to the scalp distribution of the signal component. The effects of varying noise characteristics, P300 duration and latency, and the parameters of Vector filter were also investigated.

Arousal↗
PubMed1988

Detecting early communication: using measures of movement-related potentials to illuminate human information processing.

In this paper we review evidence that suggests that the stimulus evaluation system can pass information to the response activation system before evaluation is completed ("early communication"). This evidence is derived from measures of the lateralized readiness potential, which have been related in previous research to the preparation for movement. Early communication is evident in conflict and congruence paradigms. In both paradigms, a single stimulus, or two different stimuli, deliver two aspects of information. In the conflict paradigm, the first aspect of information (derived from preliminary evaluation) primes the incorrect response, while the second primes the correct response. In the congruence paradigm, information derived from preliminary and complete evaluation is congruent. In both paradigms, lateralized readiness potential measures suggest that preliminary evaluation is able to prime the response system, although the overt motor response may not be released until evaluation is completed. This demonstration of early communication has both theoretical and practical implications. First, it does not support single-decision models of information processing. Second, it suggests that the lateralized readiness potential, a continuous, analog measure of the activity of the response system, can be used to make inferences about the nature of the evaluation process, and to localize the effects of various manipulations on the information processing system.

Cognition↗
PubMed1988

Pre- and poststimulus activation of response channels: a psychophysiological analysis.

To examine mechanisms of response activation, we asked subjects to respond differentially to the central letter of one of four arrays--HHHHH, SSHSS, SSSSS, and HHSHH--and measured event-related brain potentials (ERPs) and electromyographic activity (EMG). For very fast responses, accuracy was at chance level for all arrays, suggesting that subjects were guessing. For intermediate latency responses, accuracy was above chance if the noise was compatible with the targets and below chance if it was incompatible, suggesting that these responses were based on partial stimulus analysis. For slow responses, accuracy was above chance for all arrays, suggesting that these responses were based on complete stimulus analysis. The occurrence and accuracy of fast responses could be predicted by examining motor potentials preceding the presentation of the array. Measures of the motor potentials in the period following the presentation of the array suggested that partial analysis of stimulus information could activate responses and that the level of response activation at the time of the EMG response was constant for trials with different response latencies. The data are discussed in terms of a response channel conception.

Adult↗
PubMed1985

A psychophysiological investigation of the continuous flow model of human information processing.

Twelve subjects responded to target letters "H" or "S" by squeezing dynamometers with the left or right hand. Targets could be surrounded by compatible (e.g., HHHHH) or incompatible noise (SSHSS) letters. Measures of the P300 component of the event-related brain potential and of correct and incorrect electromyographic and squeeze activity were used to study stimulus evaluation and response-related processes. When incorrect squeeze activity was present, execution of the correct response was prolonged, indicating a process of response competition. This process occurred more often under incompatible noise conditions, which were also associated with a delayed P300. Thus, the noise/compatibility manipulation influenced both stimulus evaluation and response competition processes. In contrast, a warning tone that preceded array presentation on half the trials, increased response speed without influencing evaluation time. The data suggest that the latency and accuracy of overt behavioral responses are a function of (a) a response activation process controlled by an evaluation process that accumulates evidence gradually, (b) a response priming process that is independent of stimulus evaluation, and (c) a response competition process.

Adolescent↗
PubMed1985

Enantioselectivity of the binding of (S)- and (R)-7-chloro-1,3-dihydro-3-methyl-5-phenyl-2H-1,4-benzodiazepines to human serum albumin.

By combining the gel filtration and circular dichroism (CD) methods in studying the binding of chiral (S)/(R)-(I) to human serum albumin (HSA) the following results were obtained: HSA affinity for (S)-(I) is about 17 times higher than for (R)-(I); there exist two independent and nonequivalent binding sites for (S)-(I), and one site of lower affinity for (R)-(I); at equimolar concentrations of (I) and HSA, (S)-enantiomer is bound up to 45%, but (R)-enantiomer binds up to 22% only; differential CD-spectra at various concentrations, in the presence of HSA at 1.45 X 10(-5) M concentration, reveals distortions of the chromophoric system i.e. of the conformation of (S)-(I). This effect, and the low affinity of both enantiomers for HSA, allows only a qualitative interpretation of CD-data.

Benzodiazepines↗
PubMed1983

A new method for off-line removal of ocular artifact.

A new off-line procedure for dealing with ocular artifacts in ERP recording is described. The procedure (EMCP) uses EOG and EEG records for individual trials in an experimental session to estimate a propagation factor which describes the relationship between the EOG and EEG traces. The propagation factor is computed after stimulus-linked variability in both traces has been removed. Different propagation factors are computed for blinks and eye movements. Tests are presented which demonstrate the validity and reliability of the procedure. ERPs derived from trials corrected by EMCP are more similar to a 'true' ERP than are ERPs derived from either uncorrected or randomly corrected trials. The procedure also reduces the difference between ERPs which are based on trials with different degrees of EOG variance. Furthermore, variability at each time point, across trials, is reduced following correction. The propagation factor decreases from frontal to parietal electrodes, and is larger for saccades than blinks. It is more consistent within experimental sessions than between sessions. The major advantage of the procedure is that it permits retention of all trials in an ERP experiment, irrespective of ocular artifact. Thus, studies of populations characterized by a high degree of artifact, and those requiring eye movements as part of the experimental task, are made possible. Furthermore, there is no need to require subjects to restrict eye movement activity. In comparison to procedures suggested by others, EMCP also has the advantage that separate correction factors are computed for blinks and movements and that these factors are based on data from the experimental session itself rather than from a separate calibration session.

Blinking↗
PubMed1982

Synthesis and properties of some new 11-acyl-5,11-dihydro-6H-pyrido [2,3-b] [1,4]-benzodiazepin-6-ones.

The synthesis of 11-acyl-5,11-dihydro-6H-pyrido [2,3-b]-[1,4] benzodiazepin-6-ones (III), (V-IX), is described as well as their spectroscopic characteristics, dipole moments and partition coefficients. The same properties are determined for their N-oxide analogues (X) and (XI), while pharmacological data for some of the above compounds are compared with those of pirenzepin (I), a well known antiulcer drug.

Acetylcholine↗