Biomedical subjects

H G Smid

Publications and source records attributed to H G Smid.

At least 19 recordsLinked to original sources

PubMed2000

An electrophysiological study of the insertion of overt response choice.

Using electrophysiological measures, the authors studied changes in prestimulus state, stimulus identification, and response-related processing when, in a go/no-go task, forced choice between 2 overt go responses was inserted. The authors observed decreased prestimulus motor preparation (electromyogram), no change in stimulus identification time (selection negativity), a minor increase in response selection time (lateralized readiness potential), a large increase in response preparation time (lateralized readiness potential), a minor effect on response execution time (electromyogram), and a decrease in the activation of a response-inhibition process on no-go trials (frontal event-related potential). The existence of the response-inhibition process was verified by the presence of inverted lateralized readiness potentials on no-go trials. Pure insertion of response choice in a task seems impossible because the choice between activation and inhibition (go/no-go) always seems already present.

Adolescent↗

PubMed2000

Amantadine influences cognitive processing in patients with multiple sclerosis.

We investigated the effect of amantadine on cognitive processing in patients with multiple sclerosis (MS) and fatigue with objective electrophysiological measures. Behavioral methods (Reaction Time, RT) and two different Event Related Potential (ERP) components measuring i) stimulus selection (Selection Negativity, SN) and ii) response selection (Lateralized Readiness Potential, LRP) were employed. Twenty-four patients with clinical definite MS (10 relapsing remitting and 14 secondary progressive) and confirmed fatigue in the past three months (Fatigue Severity Scale (FSS) > 4) were included. Patients were randomized in a double-blind, placebo-controlled cross-over design. We found a difference between the two treatments for ERP measures to stimuli with relevant colour starting at about 200 ms. This negativity had a higher amplitude during amantadine treatment regardless of treatment order. The RT did not differ significantly between the treated and untreated groups. Additional analysis indicated that patients with a disease duration of less than 7 years had a significant test position (practice effect), but no treatment effect, while patients with a longer MS duration showed no practice effect, but rather an improved reaction speed and increased ERP amplitude effects when treated with amantadine. The present findings suggest that amantadine exerts beneficial effects on early cognitive processes in patients with MS, but appears to be limited to subjects with a longer duration of the disease.

Adult↗

PubMed2000

Impairment and recovery of elementary cognitive function induced by hypoglycemia in type-1 diabetic patients and healthy controls.

Although neuroendocrine changes after induction of hypoglycemia, in patients with diabetes and healthy persons, are thoroughly investigated, cognitive adaptation processes are still insufficiently understood. Changes in cognitive functions are mainly investigated by psychometric tests, which represent a summation of different cognitive processes. We aimed at dissecting cognitive adaptation into single components, i.e. stimulus selection, response choice, and reaction speed during a hyperinsulinemic hypoglycemic clamp in patients with type-1 diabetes and matched healthy controls. Using novel neurophysiological analyses, the event-related potentials of early stimulus selection (selection negativity) and response selection (lateralized readiness potential) were studied, in addition to reaction time (RT). A total of 12 diabetic patients and 12 normal volunteers were studied while receiving a hyperinsulinemic hypoglycemic clamp. RTs and the event-related potentials related to stimulus selection and response selection were significantly delayed during hypoglycemia in both groups, whereas early evoked potentials (P100) were unaltered. This suggests that hypoglycemia delays stimulus selection, with the consequence that also central and motor processing are delayed. In addition, patients with diabetes showed an earlier negative shift over the frontal cortex, which, when compared with the controls, reveals better adaptation to hypoglycemia in frontal cortical brain regions. After restoration of euglycemia stimulus selection, response selection and RT returned to baseline level in the type-1 group. In the control group, however, response selection and RTs were still delayed. This suggests that type-1 patients, possibly because of the past occurrence of hypoglycemic events, might be able to better cope with the hypoglycemic state than healthy volunteers who lack such a history. In summary, our data demonstrate, for the first time, that cognitive adaptation processes to an experimental hypoglycemic episode can clearly be dissected into their single components.

Adrenocorticotropic Hormone↗

PubMed1999

Representations in human visual short-term memory: an event-related brain potential study.

Behavioral measures and event-related potentials (ERPs) were recorded from 12 subjects while performing three delayed matching-to-sample tasks. The task instructions indicated whether stimulus locations, shapes or conjunctions of locations and shapes had to be memorized and matched against a probe. Memory load was varied trial-by-trial by cueing one or two of three stimuli for memorization, followed by one probe. ERPs during memorization and probe response times varied as a function of memory load (posterior negative and anterior positive ERP wave), but not between single-feature and conjunction of features, nor was an interaction found between them. The results support and extend hypotheses by Luck and Vogel (Luck, S.J. and Vogel, E.K., The capacity of visual working memory for features and conjunctions. Nature, 390 (1997) 279-281.) that visual short-term memory stores conjunctions of features rather than single features.

Adult↗

PubMed1999

An event-related brain potential study of visual selective attention to conjunctions of color and shape.

What cognitive processes underlie event-related brain potential (ERP) effects related to visual multidimensional selective attention and how are these processes organized? We recorded ERPs when participants attended to one conjunction of color, global shape and local shape and ignored other conjunctions of these attributes in three discriminability conditions. Attending to color and shape produced three ERP effects: frontal selection positivity (FSP), central negativity (N2b), and posterior selection negativity (SN). The results suggested that the processes underlying SN and N2b perform independent within-dimension selections, whereas the process underlying the FSP performs hierarchical between-dimension selections. At posterior electrodes, manipulation of discriminability changed the ERPs to the relevant but not to the irrelevant stimuli, suggesting that the SN does not concern the selection process itself but rather a cognitive process initiated after selection is finished. Other findings suggested that selection of multiple visual attributes occurs in parallel.

Adult↗

PubMed1998

Successful verbal encoding into episodic memory engages the posterior hippocampus: a parametrically analyzed functional magnetic resonance imaging study.

The medial temporal lobe (MTL) is essential for episodic memory encoding, as evidenced by memory deficits in patients with MTL damage. However, previous functional neuroimaging studies have either failed to show MTL activation during encoding or they did not differentiate between two MTL related processes: novelty assessment and episodic memory encoding. Furthermore, there is evidence that the MTL can be subdivided into subcomponents serving different memory processes, but the extent of this functional subdivision remains unknown. The aim of the present functional magnetic resonance imaging (fMRI) study was to investigate the role of the MTL in episodic encoding and to determine whether this function might be restricted to anatomical subdivisions of the MTL. Thirteen healthy volunteers performed a word list learning paradigm with free recall after distraction. Functional images acquired during encoding were analyzed separately for each participant by a voxel-wise correlation (Kendall's tau) between the time series of the T2*-signal intensity and the number of subsequently recalled words encoded during each particular scan. Of the 13 participants, 11 showed voxel clusters with statistically significant, positive correlations in the posterior part of the hippocampus. Across participants, an ANOVA on the number of voxels with significant, positive correlations within individually defined volumes of interest confirmed a statistically significant difference in activation for anterior versus posterior regions of the hippocampus. However, no differences between left and right hippocampal activation were revealed. Thus, these findings demonstrate that successful encoding into episodic memory engages neural circuits in the posterior part of the hippocampus.

Adolescent↗

PubMed1998

Event-related potentials of verbal encoding into episodic memory: dissociation between the effects of subsequent memory performance and distinctiveness.

Episodic memory encoding and distinctiveness detection were examined using event-related potentials (ERP) in a single-trial word list learning paradigm with free recall following distraction. To manipulate distinctiveness, encoding of high- and very low-frequency words was contrasted. Amplitudes of the N400 and late positive component (LPC) were larger for low- than for high-frequency words, and ERPs were more positive for subsequently recalled than not recalled words. This subsequent memory effect was dissociated from the correlates of distinctiveness by polarity for the N400 and by time course for the LPC and dissociable into two effects. The data suggest that the first subsequent memory effect, which occurred for both word categories, is more directly related to episodic memory formation, whereas the second effect, which occurred for high-frequency words only, is related to processes influencing episodic encoding success indirectly.

Adult↗

PubMed1997

Selective attention to conjunctions of color and shape of alphanumeric versus non-alphanumeric stimuli: a comparative electrophysiological study.

We compared multi-dimensional selection on the basis of the color, the global shape and the local shape of alphanumeric (letters) and non-alphanumeric (non-letters) stimuli. We investigated whether letters are selected on the basis of name codes or on the basis of highly familiar local shape codes. Participants responded to a single conjunction of color, global shape and local shape occurring in a randomized stream of other conjunctions of these attributes. Dependent variables were reaction time and measures derived from event-related brain potentials (onset latencies and peak amplitudes of the occipital selection negativity, SN). The SN results showed that, for both letters and non-letters, color and global shape were selected first and local shape was selected later. Reaction times were faster, and SN to the local shape occurred earlier for letters than for non-letters. The SN to the local shape of letters was larger than the SN to the local shape of non-letters. In contrast, the SN to the global shape of letters was smaller than the SN to the global shape of non-letters. Selection of the global shape of letters, but not of non-letters, depended on whether they occurred in the relevant color. Selection of the color of both letters and non-letters was independent of shape relevance, and selection of the local shape of both letters and non-letters was independent of color relevance. These results suggest that, (1) both letter and non-letter shapes are initially analyzed in a feature-specific manner; and (2) letters are selected for task-directed processing on the basis of highly familiar local shape codes and not on the basis of name codes.

Adult↗

PubMed1997

ERPs to encoding and recognition in two different inter-item association tasks.

EVENT-RELATED potentials were obtained during study and recognition of word pairs in an incidental learning paradigm. Word pairs were studied either by performing a semantic judgment separately for each word (non-associative encoding) or by creating a semantic association between the two words (associative encoding). Only word pairs encoded associatively elicited a reliable dm-effect with a right frontal maximum. Recognition of previously studied word pairs revealed two topographically and temporally distinct old/new effects: an earlier parietal effect which was only reliable for associatively encoded items and a right frontal effect which was of equal magnitude for word pairs from both encoding tasks. The findings suggest that ERP effects of distinct memory processes are differentially influenced by the encoding instructions.

Adult↗

PubMed1997

An electrophysiological study of the selection of the color and shape of alphanumeric characters in response choice.

We examined the functional and temporal organization of task-directed selective processing of the color and shape of alphanumeric characters. Dependent variables were measures derived from event-related brain potentials (onset latency of selection negativity (SN) and the lateralized readiness potential). We found SNs related to the relevance for correct task-performance of the color, the global shape and the local shape of the characters. The onset latencies of these SNs suggested that color and global shape were identified at the same time and earlier than local shape. The absence of an SN to global shape if the color was irrelevant suggested that selection on the basis of the global shape depended hierarchically on that of color. Selection of the local shape was independent of color relevance. The lateralized readiness potentials suggested that response selection occurred when the conjunction of color, global shape and local shape had been identified. No evidence was found for temporal overlap between stimulus identification and response selection. These findings show that selective task-directed processing can be studied at multiple levels in a single paradigm. They are inconsistent with the central assumption of several continuous information transmission models that relevant partial information is used to activate associated responses as soon as it becomes available.

Adult↗

PubMed1997

Positron emission tomography and magnetic resonance imaging for cerebral involvement in patients with systemic lupus erythematosus.

Central nervous system (CNS) involvement in systemic lupus erythematosus (SLE) remains difficult to diagnose, particularly since structural abnormalities may not be revealed by magnetic resonance imaging (MRI). Glucose utilisation was measured by positron emission tomography (PET) in 35 SLE patients to detect signs of CNS involvement. The patients were examined by a standardised neurological examination, a battery of tests to evaluate neuropsychological performance and MRI. Antineuronal antibodies were determined to investigate their putative role in CNS involvement in SLE. Twenty patients had distinct neurological (17) and/or psychiatric (3) symptoms. Ten patients had pronounced cognitive impairment. Neurological and cognitive deficits were thus found to be unrelated disorders in SLE. Global glucose utilisation of SLE patients did not differ significantly from that of normal controls, nor were differences found between SLE patients with or without neurological or cognitive abnormalities. On MRI of the brain, the number and size of white matter lesions correlated with the presence of neurological deficits but were unrelated to the severity of cognitive impairment. Within the normal range, lower global glucose utilisation tended towards lower values with increasing number and size of white matter lesions. Patients with lesions larger than 8 mm also showed distinctly increased IgG anticardiolipin antibody titres, whereas measuring antineuronal antibodies did not reveal any relation to the variables investigated. We conclude that the demonstration of CNS lesions by MRI can contribute confirmatory evidence for CNS involvement in SLE, but PET or the presence of antineuronal antibodies adds little if any information beyond that obtained by clinical examination, neuropsychological testing, and MRI.

Adult↗

PubMed1997

Differentiation of hypoglycaemia induced cognitive impairments. An electrophysiological approach.

Effects of hypoglycaemia on distinct cognitive processes were assessed with event-related brain-potential (ERP) measures and reaction times in a hybrid selective-attention/response-choice task. One group of subjects received a euglycaemia-hypoglycaemia-euglycaemia treatment order and a second group a euglycaemia-placebo(euglycaemia)-euglycaemia sequence of treatments. During hypoglycaemia, ERP measures of selective attention (selection negativity), response choice (lateralized readiness potential) and reaction time were delayed compared with baseline performance. After restoration of euglycaemia, the onset of the selection negativity returned to baseline, whereas the lateralized readiness potential was still delayed, and error frequencies remained elevated. These results suggest that hypoglycaemia delays both the stimulus selection and the motor-response selection. They further suggest that the stimulus-selection process recovers quickly after restoration of euglycaemia, but that the response-selection process does not. Slow shifts in cortical potentials with a broad frontal distribution that occurred during hypoglycaemia, are discussed in relation to frontal lobe mechanisms involved in the control of subordinate, modality-specific selection mechanisms.

Adult↗

PubMed1997

The organization of multidimensional selection on the basis of color and shape: an event-related brain potential study.

In this paper, we examine whether color and shape, tied to a single object in space, (1) are identified and selected in series or in parallel, (2) are identified and selected in a dependent, self-terminating manner or in an independent and exhaustive manner, and (3) are conjoined by a feature integration process before or only after an initial stage of separate attribute analyses has finished. We measured response time and the selection negativity (SN) derived from event-related brain potentials when participants responded to a unique conjunction of color and shape in a go/no-go target detection task. The discriminability of the color and the shape of the conjunction was manipulated in three conditions. When color and shape were easy to discriminate, the SNs to color and shape started at the same time. When one attribute was less discriminable the SN to that attribute started later, but not the SN to the complementary attribute. This suggests that color and shape are identified and selected in parallel. In all three discriminability conditions, the SNs to color and shape were initially independent but later interacted. This suggests that color and shape are initially selected independently and exhaustively, after which their conjunction is analyzed. The SN to local shape features started later than that to the conjunction of color and global shape features, which suggests that feature integration can start before the analyses of the separate attributes have finished.

Adult↗

PubMed1997

Flexibility in the structure of human information processing.

The studies reported in this chapter argue for a flexible organization of higher mental processes, including perceptual and attentional systems in the visual modality and attentional systems related to action. There is electrophysiologic evidence that the visual perceptual system has a modularized structure, and that selection of perceived features is adjustable to sensory, conceptual, and utility constraints, both with respect to the hierarchy of selection cues and the transmission of information between perception and action. A distributed neural network subserves these functions and may therefore provide a source for plasticity and reorganization after brain damage by flexibility in the allocation of resources to various stages of stimulus analysis.

Cognition↗

PubMed1996

Mental representations of morphologically complex words: an event-related potential study with adult humans.

Event related brain potentials (ERPs) were recorded from 13 subjects while performing a lexical decision task. Regular and irregular German principles were presented twice (identical repetition) or were primed by their related infinitives (morphological repetition). Regular participles primed by their infinitives displayed ERP waveforms similar to those evoked by identical repetitions, namely a large positivity with an onset latency of about 200 ms. Irregular participles, by contrast, showed no reliable morphological repetition effects. The results are discussed in terms of the controversy between approaches proposing a single representational mechanism for regular and irregular inflection and the "dual mechanism' account that suggests an associative memory for the storage of irregulars and a rule-based symbolic processor for generating regular forms.

Adolescent↗

PubMed1996

A psychophysiological investigation of the selection and the use of partial stimulus information in response choice.

Two alternative explanations were examined for why selective response activation sometimes starts before stimulus identification is complete (e.g., J. O. Miller & S. A. Hackley, 1992) and sometimes starts only after stimulus identification is complete (e.g., R. De Jong, M. Wierda, G. Mulder, & L. J. M. Mulder, 1988). Distinct psychophysiological methods related to stimulus identification and response selection provided evidence suggesting that partial stimulus information is identified but is or is not used before the stimulus is identified more fully, depending on task requirements. This result (a) suggests strategic adaptation of task performance, (b) is inconsistent with particular discrete and continuous models of information processing, and (c) shows the existence of a central selection mechanism that can prevent the automatic activation of responses associated with preliminary available stimulus information.

Adult↗

PubMed1995

The availability versus the use of partial information: multiple levels of selective processing between perception and action.

Temporal overlap of cognitive processes occurs when partial information about a stimulus activates associated responses before the stimulus has been fully identified. Recent evidence obtained with psychophysiological measures suggests that such overlap sometimes does, and sometimes does not occur. In this contribution, we argue that in order to understand why, we have to distinguish between selection of partial information for perception and selection of partial information for control over activation of responses. We discuss several mechanisms and factors that may modulate the temporal overlap of mental processes because they selectively influence the perceptual availability or the use of partial information.

Arousal↗

PubMed1992

A psychophysiological study of the use of partial information in stimulus-response translation.

Ss performed a hybrid go/no-go reaction task in which colored letters were assigned in various ways to 4 finger responses, 2 on each hand. In addition to reaction time, psychophysiological measures were used to assess the duration of stimulus identification and the onset of central and peripheral motor activity. The results suggest that response selection can begin on the basis of 1 stimulus dimension (e.g., color), while the other dimension (e.g., letter form) has not yet been identified. Other results are discussed with regard to "selection for action" (Allport, 1987) and the importance of stimulus-response translation strategies in the use of partial information.

Adult↗