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Biomedical subjects

A Achim

Publications and source records attributed to A Achim.

At least 19 recordsLinked to original sources
PubMed2001

Statistical detection of between-group differences in event-related potentials.

OBJECTIVE: Many event-related potential (ERP) situations do not fulfill the multivariate statistics requirement of more cases than measurements. Five simulation studies were carried to select a sensitive test for comparing two independent groups of ERPs. METHODS: Simulated signal and noise waveforms were generated using different combinations of parameters: cases or replications per group, correlation between data points, number of points, signal-to-noise ratio (S/N), etc. The false alarm (FA) rate of each method was assessed and their sensitivity compared over sets of 2000 simulated experiments per condition. RESULTS: Study 1 identified the 'Projection onto Centroids Difference Vectors' (PCDV) method of Haig and Gordon (Brain Topogr 1995;8:67) as very good, but its FA rate was erratic under several conditions. The following studies served to shape the implementation parameters of a version of PCDV, termed PCDVp, that assesses significance through random permutations of the case labels. The final form is very sensitive. For instance, with two groups of 48 trials of 30-point EEG-like waveforms, its power for alpha=0.05 is about 50% at S/N 1.0 and 90% at S/N 1.5 (amplitude). CONCLUSION: PCDVp requires no a priori knowledge and is sensitive to detect differences between independent sets of waveforms, topographies or spatio-temporal data.

Event-Related Potentials, P300↗
PubMed2000

Emotional distress among couples involved in first-trimester induced abortions.

OBJECTIVE: To establish the prevalence of clinically significant psychological distress in women and men involved in first-trimester abortions and to identify related risk factors. DESIGN: Prospective cohort study. SETTING: A downtown Montreal public abortion clinic and the Montreal metropolitan area. PARTICIPANTS: We recruited 197 women and 113 men involved in first-trimester abortions and compared them with control groups composed of 728 women and 630 men 15 to 35 years old who had taken part in a previous public health survey (Enquête Santé Québec 1987). One hundred twenty-seven women and 69 men completed the follow-up questionnaire. MAIN OUTCOME MEASURES: Level of distress as measured by the Ilfeld Psychological Symptom Index. RESULTS: Before the abortion, 56.9% of women and 39.6% of men were much more distressed than their respective controls. Three weeks after the abortion, 41.7% of women and 30.9% of men were still highly distressed. Predictors of distress for women were fear of negative effects on the relationship, unsatisfactory relationships, relationships of less than 1 year, ambivalence about the decision to abort, not having a previous child, and suicidal ideation (this association was weaker than in controls). Predictors for men were fear of negative effects on the relationship, relationships of less than 1 year, preoccupation with the abortion and anxiety about its accompanying pain, negative perceptions of their own health, suicidal gestures in the past, and suicidal ideation in the past year (only the association with suicidal gestures was marginally stronger than in controls). CONCLUSION: Being involved in a first-trimester abortion can be highly distressing for both women and men.

Abortion, Legal↗
PubMed1999

Topography of averaged electrical brain activity relating to interhemispheric dynamics in normal humans: where does the critical relay take place?

Interhemispheric relay time (IHRT) was investigated using a 16-electrode montage in 10 normal subjects during a simple visual reaction time task (Poffenberger paradigm). The P1 latency was the dependent measure. Response-locked components were separated from the stimulus-locked components based on the reaction times using a Woldorff-like filter. The issues of theoretical interest were the following: (1) does the responding hand modulate IHRT estimates as a function of electrode site, or stimulated field?; (2) what are the respective advantages of various pairs of symmetrical across-midline electrode sites for tracking IHRT?; and (3) what are the respective advantages of symmetrical vs. asymmetrical across-midline electrode site-pair configurations for tracking IHRT? There were significant lags attributable to IHRT in both evoked potential P1 latencies and in reaction times--but these were uncorrelated. The responding hand was not found to significantly modulate IHRT estimates derived from the P1 peak latency of response-corrected ERPs. Anterior sites yielded more precocious components and briefer estimates of IHRT which were closer to those observed in reaction time estimates. However, posterior leads yielded more reliable estimates. Various analyses suggested that IHRT can be tracked, very vaguely, with asymmetric configurations as well as symmetric ones, each for different reasons.

Adult↗
PubMed1999

Separate memory-related processing for auditory frequency and patterns.

Detecting deviant, and potentially meaningful, auditory events depends on transient representations of preceding stimuli. Here, we examined whether the neural circuitry underlying deviance detection system varied as a function of deviance type. In different blocks of trials, participants were presented with a sequence that included standard and deviant tones differing in frequency or a sequence of tones that alternated regularly in frequency with occasional deviant repetitions. Both frequency- and pattern-deviant stimuli elicited a mismatch negativity (MMN) peaking between 120 and 175 ms poststimulus. The MMN amplitude distribution was more frontal for frequency-deviant than for pattern-deviant stimuli. There are two possible explanations for these results. Both frequency- and pattern-deviation MMNs might arise in the same set of generators whose relative strength of activation varies. Alternatively, frequency- and pattern-deviation MMNs could originate in different generators. These alternatives were investigated using principal component analysis and signal identification methods. These methods revealed that no common signal space could account for both of the MMNs, indicating different generator sources for the analysis of frequency and pattern deviance. The results suggest separate memory-related processing for auditory frequency and patterns and indicate that the neural circuit of deviance detection varies as a function of the perceptual context.

Adult↗
PubMed1998

Dissociation of hemispheric exploitation of rods and cones for simple detection.

Sergent (1982a, 1982b, 1982c) proposed that stimuli carrying high luminous energy will be better detected in the right field (left hemisphere) and stimuli carrying low energy in the left visual field (right hemisphere). A photoreceptor-based model could explain the same effects as well as several others (eccentricity effects, retinal adaptation effects). Braun, Mailloux, and Dufresne (1995) suggested that cones might favor right field stimuli and rods might favor left field stimuli. We implemented a series of near-threshold simple detection experiments comprising stimuli varying along four dimensions (color, eccentricity, duration, and size) in dark-adapted subjects. Consistent and significant right field advantages accrued for near-meridianal foveal red stimuli, but there were no field effects for eccentric blue stimuli. An alternative to Sergent's stimulus energy model and the photoreceptor-based model, namely a parvocellular-magnocellular model, is proposed.

Adult↗
PubMed1997

Toward a dynamic topographic components model.

Möcks' topographic component model (TCM) (Möcks, J. Topographic components model for event-related potentials and some biophysical considerations. IEEE Trans. Biomed. Eng., 1988a, 35: 482-484; Möcks, J. Decomposing event-related potentials: a new topographic components model. Biol. Psychol., 1988b, 26: 199-215) decomposes event-related potentials into components uniquely determined by their respective amplitude profiles across replicates, assuming a constant topography and wave shape for each component. To accommodate possible changes in the component expression across conditions, a dynamic version of TCM is investigated which further admits component modulation in time scale. Twenty test problems were synthesized, incorporating two arbitrary topographies each activated with its own arbitrary wave shape modified, across two conditions, in amplitude, onset and duration. Seventeen problems were perfectly solved, with substantial success on the remaining three, confirming that component jitter or stretching can even help component identification.

Brain↗
PubMed1997

Principal component analysis of event-related potentials: misallocation of variance revisited.

Misallocating variance, in event-related potential analysis, refers to attributing an experimental effect to components not actually affected. A vector interpretation of the relationship between mathematically derived and true underlying components shows that misallocation depends exclusively on incorrect identification of the affected component. Simulations, using seven imperfect rotations, confirmed all predictions from the vector interpretation concerning the presence, direction, and size of misallocated variance. Contrary to principal component analysis (PCA). Möcks's topographic component model (TCM) is not subject to rotation problems. These two methods were compared over 100 simulations in which the components had constant waveforms and topographics across participants. The group effect was always detected, but only PCA and not TCM showed significance on other components, except when their random weights happened to differ between groups.

Analysis of Variance↗
PubMed1995

Cerebral source localization paradigms: spatiotemporal source modeling.

Spatiotemporal source modeling (STSM) attempts to interpret multichannel scalp recordings in terms of their contributing brain areas. Localization accuracy is generally limited by spatially correlated residual noise and by modeling simplifications. For signals emanating simultaneously from several sources, however, correct interpretation of the source structure is the prime concern. The discussion of various STSM methods emphasizes this validity issue. Methods are proposed for systematic search of alternate source models also consistent with the data, in order to prevent unwarranted conclusions, and for combining information from different montages and from data variability across trials to impose severe objective criteria for consistency of a source model with the data.

Brain Mapping↗
PubMed1995

Signal detection in averaged evoked potentials: Monte Carlo comparison of the sensitivity of different methods.

Many clinical and research applications rely on detecting evoked potential (EP) signal or EP differences between conditions. Statistical methods for objective signal detection should be sensitive to the presence of signal, but must provide the user strict control on tolerated false alarm rate. The respective sensitivities of 6 signal detection methods were compared through several Monte Carlo simulations involving 2 autocorrelation structures, 5% and 1% significance levels, 8, 10 or 12 replications per study, and increasing signal to noise ratio. The signal detection methods compared were: (1) the Record Orthogonality Test by Permutations (ROT-p), a variant of the Residual Orthogonality Test (Achim et al., 1988), that provides an unbiased estimate of the energy of the signal present in the averaged data, (2) the Tsum2 permutation test of Karniski et al. (1994), (3) a Principal Component Analysis method (PC1) consisting of a t test on the weights of the first principal component, (4) multiple t tests on amplitudes with empirical adjustment for global false alarm rate, and (5-6) the test of Guthrie and Buchwald (1991) on length of consecutive t tests significant at P < 0.05 or 0.01 per-test. The first 3 methods did not exceed their nominal false alarm rate and clearly outperformed the last 3, with the ROT-p method being significantly more sensitive than all others under almost all conditions.

Electrophysiology↗
PubMed1994

Zaidel's model of interhemispheric dynamics: empirical tests, a critical appraisal, and a proposed revision.

Zaidel has proposed a model of intra- and interhemispheric dynamics which attempts to explain field and hand effects in visual reaction time experiments. The model includes prototypes of two extreme cases: a "callosal relay" pattern, in which interhemispheric transfer is inferred, and a "direct access" pattern, in which each hemisphere is inferred to execute the task independently. Five visual reaction time experiments were carried out to test six predictions which Zaidel has made within the framework of his model. Some of these predictions were supported but others were rejected. These findings, in conjunction with findings of other researchers, were critically analyzed in view of determining the extent to which Zaidel's model can be revised in order to fit the ensemble of relevant findings. It was concluded that the model can indeed be revised but that this requires a redefinition of conditions indicative of presence or absence of interhemispheric relay. A new model is proposed assuming exclusively excitatory callosal relay and focussing on magnitude and valence of field effects obtained at each hand, considered in their interrelation.

Adult↗
PubMed1993

Perceptual context and the selective attention effect on auditory event-related brain potentials.

Two experiments examined the effect of the perceptual context established through tonal grouping on neuroelectric responses during selective listening. Subjects monitored one of the extreme pitches in four-pitch tone sequences and detected rare longer tones of the designated pitch. In the first study, tonal grouping was manipulated by changing the tonal separation between the extreme pitches and their nearest neighbor, keeping the extreme pitches constant. Grouping increased the negativity of the attended-unattended difference potential. A second study examined the effect of grouping on the attention-related negativity when it opposed the effect of physical similarity. The proximity of extreme pitches varied, keeping the middle pitches constant. The effect of grouping on event-related potentials (ERPs) for the middle pitches was varied according to the tone being attended. ERPs for the middle tone adjacent to the attended tone became more negative with grouping, whereas ERPs for the distant middle tone became less negative with grouping. These effects suggest that the attention-related negativity is sensitive to contextual information.

Adult↗
PubMed1991

Methodological considerations for the evaluation of spatio-temporal source models.

The effects of physiological noise and modelling procedure on spatio-temporal source modelling (STSM) solutions were examined by adding EEG noise (3-4% of total energy) to synthetic signals from 3 dipoles producing topographies that partially cancelled at the surface of an homogeneous sphere. Three patterns of source activation profiles were each associated with two EEG records. The STSM solutions were subjected to statistical tests that detect signal left in the residuals. All models substantially accounted for noise. Essentially correct models left no consistent signal in the residuals, but other statistically acceptable models were also occasionally found. Unambiguous correct solutions were found for moderately correlated source activations. Statistically adequate 2-source and 3-source models were found for the 4 data sets incorporating nearly parallel activations of 2 sources. When all 3 sources had comparable amplitudes, the 2-source models represented compromises containing gross mislocalizations but correct 3-source models were found. When one of the parallel sources was attenuated by 75%, only 2-source models satisfactorily approximated the 2 main sources. The localization errors in the "correct" models ranged from 2.5% to 13% of the sphere radius. Thus, even with a perfect signal propagation model, STSM could at best claim approximate localizations from data containing structured noise.

Brain↗
PubMed1989

Human intracerebral potentials associated with target, novel, and omitted auditory stimuli.

We recorded late auditory potentials from lateral and medial regions in the frontal, temporal and parietal lobes of patients with temporal lobe epilepsy implanted with horizontal depth electrodes. Tone sequences were presented in three tasks: 1) auditory target detection in a tone sequence, 2) target detection with interspersed novel stimuli, and 3) detection of stimulus omissions. At frontal sites, potentials to targets showed a triphasic response with peak latencies around 200, 270 and 350 ms. At temporal sites, potentials consisted of a generally positive 285 ms peak which was sometimes accompanied by a negative peak at 200 ms or at 400 ms. At parietal sites, potentials were generally triphasic with latencies of about 230, 300, and 370 ms. At most sites, potentials evoked by novel stimuli had shorter latencies than those evoked by targets. The frontal and parietal potentials were either absent or strongly attenuated during stimulus omissions. The results lend further support to the multiple generator hypothesis of late potentials and suggest that some of the cerebral sources of the late potentials are stimulus dependent while others are not.

Acoustic Stimulation↗
PubMed1988

Methods for separating temporally overlapping sources of neuroelectric data.

The localization of intracranial sources of EEG or MEG signals can be misled by the combined effect of several sources, as illustrated by simulated MEG data in which two of the three dipolar sources have slightly out of phase activity and partly complementary scalp topographies. These data were analysed by three different source localization methods. Fitting a single source to each sequential topography worked perfectly when only one source was active; this could also account for as much as 95% of the spatial variance of topographies resulting from two overlapping sources, although the solution was then far from any source. A principal component analysis approach followed by an oblique rotation (fitting one source to the spatial aspect of each component) correctly localized two of the sources but severely mislocated the source that was never active alone. Spatio-temporal source modeling (simultaneously fitting a set of sources to all consecutive topographies) correctly localized all three sources, provided that the parameter optimization method could escape sub-optimal local minima of the error function. Temporally overlapping sources can thus be separated and correctly identified if the mathematical model is adequate and the optimization procedure is well adapted.

Algorithms↗
PubMed1986

A clinical trial comparing intramuscular haloperidol decanoate and oral haloperidol in chronic schizophrenic patients: efficacy, safety, and dosage equivalence.

Thirty patients with chronic schizophrenia received oral haloperidol and haloperidol decanoate in a two-phase open study. In the first phase, patients were stabilized on haloperidol tablets for 2 weeks, then maintained on a constant daily dose for 2 more weeks. They were then switched to haloperidol decanoate for the second phase. Patients were first stabilized on a monthly dose of haloperidol decanoate and then received this dose for 5 consecutive months. Haloperidol decanoate, administered in monthly injections at 9.4 to 15 times the daily oral dose, was at least as efficacious as oral haloperidol in controlling the symptoms of schizophrenia. There were no serious adverse reactions to either drug. Blood samples were taken from 22 patients during both the oral and the decanoate phases and analyzed for steady state haloperidol concentrations. These determinations demonstrate that the release of haloperidol with the decanoate form is sustained throughout the 4-week dosing interval. Lower plasma drug concentrations were observed during decanoate treatment than during oral treatment. Despite these lower plasma concentrations, patients remained stable on both drug regimens. This finding suggests that haloperidol decanoate injected every 4 weeks can provide control of psychotic symptoms at least as effectively as daily oral haloperidol.

Administration, Oral↗
PubMed1986

Topography and possible physiological significance of EEG amplitude variability in psychosis.

The topography and possible physiological significance of EEG average amplitude and amplitude variability in time were examined in 10 chronic psychotic patients and several groups of healthy volunteers. In confirmation of previous reports, the patients showed more slow (5.2-7.6 Hz) activity, particularly in frontal areas, and less alpha rhythm variability in posterior areas, compared to matched controls, chiefly during the resting state, while eye opening, and particularly structural sensory input, made the groups more similar. The same two EEG variables showed significant correlations with ratings of psychopathology, in the expected direction. The EEG amplitude variability is increased by unstructured perception, probably as an expression of readiness or search for structures in the input, and shows persistent topographic patterns over time in individuals. It appears to reflect a functional dimension which is different from the one represented by the average amplitude and is one of several spontaneous oscillatory functions which are diminished in chronic psychosis, perhaps as consequence of prefrontal malfunction. The results suggest a diminished guiding function by the anterior cortex, and a diminished readiness for perception of exogenous structured information, in chronic psychosis.

Adult↗
PubMed1985

Effect of normal aging on the prolactin response to graded doses of sulpiride and to arginine.

The prolactin (PRL) response to placebo, sulpiride (2.5, 5, 10 and 20 mg im) and arginine HC1 infusion (0.33G/kg) was examined in young (18-25 yrs) and old (65-75 yrs) normal men. Analysis of variance for the sulpiride data showed no significant dose x age group interaction or dose X age group X period interaction. There was, however, a significant age group X period interaction (p less than 0.05). PRL concentrations were significantly lower in the old subjects (N = 9) compared with the young (N = 9) 15 min after 2.5, 5, or 10 mg but not significantly after 20 mg sulpiride or at any other time interval. The areas under the concentration-time curves and the mean individual peak PRL concentrations were not significantly different between the two groups. The pattern of findings suggests a delayed absorption of sulpiride in the elderly rather than a change in pituitary dopamine (DA) receptor sensitivity to account for the lower PRL concentrations at 15 min. Differences in magnitude of the PRL response between the four doses of sulpiride were small and results suggest that the 2.5 mg dose is close to that required to saturate DA receptors on the lactotrophe and that the 10 and 20 mg doses are sufficient to completely block pituitary DA receptors. There was no significant age effect on arginine-induced PRL secretion (N = 11 in each group).(ABSTRACT TRUNCATED AT 250 WORDS)

Adolescent↗