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Biomedical subjects

M Usher

Publications and source records attributed to M Usher.

18 recordsLinked to original sources
PubMed1999

The role of locus coeruleus in the regulation of cognitive performance.

Noradrenergic locus coeruleus (LC) neurons were recorded in monkeys performing a visual discrimination task, and a computational model was developed addressing the role of the LC brain system in cognitive performance. Changes in spontaneous and stimulus-induced patterns of LC activity correlated closely with fluctuations in behavioral performance. The model explains these fluctuations in terms of changes in electrotonic coupling among LC neurons and predicts improved performance during epochs of high coupling and synchronized LC firing. Cross correlations of simultaneously recorded LC neurons confirmed this prediction, indicating that electrotonic coupling in LC may play an important role in attentional modulation and the regulation of goal-directed versus exploratory behaviors.

Action Potentials↗
PubMed1999

The "benefits" of distractibility: mechanisms underlying increased Stroop effects in schizophrenia.

Recent studies of selective attention in schizophrenia patients suggest a particular pattern of single-trial Stroop performance: increased facilitation but not interference in reaction times (RTs), combined with increased error interference. Our Stroop task analysis suggests that this pattern can be explained by a selective attention deficit if one accounts for (1) performance in the congruent condition; (2) the nature of the neutral stimulus; (3) the relationship between accuracy and RT; and (4) response set effects. To test these hypotheses, we examined Stroop performance in 40 DSM-IV schizophrenia patients and 20 healthy control subjects, using a range of neutral stimuli (color patches, noncolor words, color words not in the response set). The findings confirmed several of our predictions and the results were consistent with the hypothesis that abnormal Stroop performance in schizophrenia reflects a failure to adequately attend to the task-appropriate stimulus dimension (color). This inattention affects both the congruent and incongruent conditions and multiple points in the information processing pathway.

Adult↗
PubMed1999

Mechanisms for spatial integration in visual detection: a model based on lateral interactions.

Recent studies of visual detection show a configuration dependent weak improvement of thresholds with the number of targets, which corresponds to a fourth-root power law. We find this result to be inconsistent with probability summation models, and account for it by a model of 'physiological' integration that is based on excitatory lateral interactions in the visual cortex. The model explains several phenomena which are confirmed by the experimental data, such as the absence of spatial and temporal uncertainty effects, temporal summation curves, and facilitation by a pedestal in 2AFC tasks. The summation exponents are dependent on the strength of the lateral interactions, and on the distance and orientation relationship between the elements.

Computer Simulation↗
PubMed1998

Visual synchrony affects binding and segmentation in perception.

The visual system analyses information by decomposing complex objects into simple components (visual features) that are widely distributed across the cortex. When several objects are present simultaneously in the visual field, a mechanism is required to group (bind) together visual features that belong to each object and to separate (segment) them from features of other objects. An attractive scheme for binding visual features into a coherent percept consists of synchronizing the activity of their neural representations. If synchrony is important in binding, one would expect that binding and segmentation are facilitated by visual displays that are temporally manipulated to induce stimulus-dependent synchrony. Here we show that visual grouping is indeed facilitated when elements of one percept are presented at the same time as each other and are temporally separated (on a scale below the integration time of the visual system) from elements of another percept or from background elements. Our results indicate that binding is due to a global mechanism of grouping caused by synchronous neural activation, and not to a local mechanism of motion computation.

Cortical Synchronization↗
PubMed1995

Lateral interactions in primary visual cortex: a model bridging physiology and psychophysics.

Recent physiological studies show that the spatial context of visual stimuli enhances the response of cells in primary visual cortex to weak stimuli and suppresses the response to strong stimuli. A model of orientation-tuned neurons was constructed to explore the role of lateral cortical connections in this dual effect. The differential effect of excitatory and inhibitory current and noise conveyed by the lateral connections explains the physiological results as well as the psychophysics of pop-out and contour completion. Exploiting the model's property of stochastic resonance, the visual context changes the model's intrinsic input variability to enhance the detection of weak signals.

Animals↗
PubMed1994

Phenothiazine treatment and respiratory distress syndrome in a child.

An 11-year-old boy who was treated with a relatively high dose of methotrimeprazine meleate (Levemepromazine) a phenothiazine antipsychotic drug, was admitted to the pediatric intensive care unit suffering from respiratory distress syndrome. He required intensive treatment and support for 13 days. The persistent effects of methotrimeprazine meleate on various organs are typical of the prolonged biological action of the phenothiazine metabolites. The association of phenothiazine overdose and respiratory distress syndrome merits consideration.

Child↗
PubMed1993

A neural model of the dynamic activation of memory.

We study an Attractor Neural Network that stores natural concepts, organized in semantic classes. The concepts are represented by distributed patterns over a space of attributes, and are related by both semantic and episodic associations. While semantic relations are expressed through an hierarchical coding over the attribute space, episodic links are realized via specific synaptic projections. Due to dynamic thresholds expressing neuronal fatigue, the network's behavior is characterized by convergence toward the concept patterns on a short time scale, and by transitions between the various patterns on a longer time scale. In its baseline, undamaged state, the network manifests semantic, episodic, and random transitions, and demonstrates the phenomenon of priming. Modeling possible pathological changes, we have found that increasing the 'noise' level or the rate of neuronal fatigue decreases the frequency of semantic transitions. When neurons characterized by large synaptic connectivity are deleted, semantic transitions decay before the episodic ones, in accordance with the findings in patients with Alzheimer's disease.

Alzheimer Disease↗
PubMed1991

Schematic and situational determinants of depressed and nondepressed students' interpretation of feedback.

We examined whether depressed persons' social skill deficits contribute to their negative cognitions and whether this contribution is independent of their negative schemata. Depressed (n = 60) and nondepressed (n = 60) subjects engaged in group discussions. We assessed subjects' social competence schemata with a questionnaire and subjects' actual level of social competence in the discussion through objective ratings made by codiscussants and outside observers. We found that independently of their negative schemata, depressed subjects' social skill deficits explained a significant portion of the variance in their more negative interpretation of feedback (relative to nondepressed subjects'). This suggests that real deficits in depressed persons' performance compound the effects of their negative schemata and further contribute to their negative cognitions. We also further explored findings by Dykman et al. (1989) and Lewinsohn et al. (1980).

Cognition↗
PubMed1990

Effect of gentamicin on the auditory brainstem evoked response in term infants: a preliminary report.

Seven essentially healthy term infants who received gentamicin starting on the 1st day of life for prolonged rupture of membranes and maternal fever were compared with nine healthy term infants to determine whether this drug induces alterations in the auditory pathway. The auditory pathway was studied on the 3rd day of life by analyzing brainstem auditory evoked potentials elicited by a click stimulus presented at the infant's ears. Latencies of components III and V, interval I-III, and interval I-V were significantly prolonged in the gentamicin group, indicating impairment of the central component of the auditory pathway. Peak and trough serum gentamicin levels all fell within the recommended therapeutic range. The study indicates that short course gentamicin therapy in healthy newborn infants can lead to abnormality of auditory function.

Bacterial Infections↗
PubMed1977

Comparison of Tc-99m methylene diphosphonate with Tc-99m pyrophosphate in the detection of acute myocardial infarction: concise communication.

Myocardial scans were obtained in ten patients with proven myocardial infarction. Scintigraphy on consecutive days was performed with technetium-99m methylene diphosphonate and technetium-99m pyrophosphate. Images obtained with the two tracers were generally equal in quality. Those with PPi exhibited denser uptake, while background activity was lower with MDP.

Diphosphates↗