Biomedical subjects

M N Verbaten

Publications and source records attributed to M N Verbaten.

At least 19 recordsLinked to original sources

PubMed2000

The ability to activate and inhibit speeded responses: separate developmental trends.

When children grow older they respond faster and are less susceptible to interference caused by task-irrelevant information. These observations suggested the hypothesis that a global mechanism may account for developmental change in the speed of responding and that inhibitory function may underlie the ability to activate speeded responses. The current study examined these issues by comparing the performance of 4 age groups (5-, 8-, and 11-year-olds and young adults) on a battery of 6 speeded performance tasks, 4 of which required the inhibition of response activation. An analysis of reaction and inhibition times supported a hypothesis of generalized developmental changes in response activation, but revealed a less pronounced development of inhibition. A nonselective mechanism of response inhibition seems to be fully developed during early childhood.

Adult↗

PubMed2000

Habituation: an event-related potential and dipole source analysis study.

The goal of this study was to investigate habituation processes in the brain, and in particular, to identify the brain structures involved in these processes. Therefore, event-related potentials (ERPs) were recorded in response to a series of repeated, task-irrelevant, salient stimuli presented against the background of a cognitive task which required the subjects to attend to a fixation sign on a TV screen. Stimuli were presented randomly in one of four possible quadrants of the TV screen. Dipole analyses were performed in order to localize the source of stimulus repetition effects (habituation). As expected, there was a decrease in amplitude of the N1 and P3 components as a function of stimulus repetition; habituation of N1 occurred faster than that of the P3 component. It is suggested from the results of dipole analyses that the N1 effect concerns diffuse cortical activation. The P3 habituation effect seemed to involve the temporal cortex especially.

Adult↗

PubMed2000

The effects of a sub-anaesthetic dose of ketamine on human selective attention.

A growing number of studies demonstrate that antagonists of the N-methyl-D-aspartate (NMDA) receptors can induce a broad range of psychophysiological anomalies in healthy subjects similar to those observed in schizophrenia. In this study, the effect of a sub-anaesthetic dose of the non-competitive NMDA antagonist, ketamine, on human selective attention was explored. It was hypothesized that ketamine would induce in healthy subjects psychophysiological anomalies that are commonly observed in schizophrenic patients, such as reduced P300 amplitude and a reduction of both mismatch negativity (MMN) and processing negativity (PN). In a double-blind randomized placebo-controlled design, healthy male volunteers (n = 18) were challenged with a sub-anaesthetic dose of ketamine (0.3 mg/kg i.v.) after which they were tested in a selective attention task. In this task, two types of stimuli were evenly presented to the left or right ear: standard tones (80%) and deviant tones (20%) of either 1000 or 1100 Hz. The duration of a stimulus (95 dB) was 50 ms, the interstimulus intervals were randomized between 1750 and 2150 ms. The volunteer was instructed to push a button as quickly as possible after hearing the deviant tone in a specified ear. Ketamine did not alter performance of the subjects: in both the placebo and drug condition their reaction times for and percentages of hits and false alarms did not differ. Ketamine did, however, reduce PN and the P300 amplitude (both in general and to deviant stimuli in particular). However, no drug effect on MMN was found. In addition, ketamine enhanced the N100 amplitude to deviant stimuli. In conclusion, ketamine induces some of the attentional deficits in healthy controls that are observed in schizophrenic patients. Consequently, reduced glutamatergic activity in the brain may be involved in some of the symptoms of schizophrenia.

Adult↗

PubMed2000

On the processing of spatial frequencies as revealed by evoked-potential source modeling.

OBJECTIVES AND METHODS: Visually evoked potentials (VEPs) are known to be sensitive to spatial frequency, especially in the time range between 50 and 100 ms post-stimulus. In two experiments we localized the cortical activity elicited by stimuli of varying spatial frequency in scalp-recorded brain potentials, using multi-electrode recordings and dipole-source analysis. RESULTS: Low spatial frequencies (<1 c/d) activated relatively lateral occipital areas, the orientation of the neural ensembles involved being predominantly perpendicular to the scalp surface. In contrast, high spatial frequencies (>4 c/d) induced activation of more medial occipital areas with the predominant orientation of the sources being much more parallel to the scalp surface. Furthermore, at about 100 ms latency the lateral-occipital response to low spatial frequencies was stronger in the right hemisphere; no such asymmetry was found for the responses to the high spatial frequencies. These findings were consistent across varying recording conditions, individual subjects, subject populations, stimulus characteristics (grating orientation, grating vs. checkerboard), and task conditions (active vs. passive). CONCLUSION: The results indicate that there are differences in sensitivity to specific spatial frequencies between primary and secondary visual areas, as well as between the right and the left hemispheres.

Adult↗

PubMed2000

Attentional capacity, a probe ERP study: differences between children with attention-deficit hyperactivity disorder and normal control children and effects of methylphenidate.

In the present study it was investigated whether the smaller P3s in attention-deficit hyperactivity disorder (ADHD) children are caused by a shortage of capacity underlying P3 processes or whether they are due to a capacity allocation problem. Also, effects of methylphenidate on these processes were investigated. Performance and event-related potentials (ERPs) of 14 ADHD and 14 control children were measured using an irrelevant-probe technique. Three types of task irrelevant visual probes (standards, deviants, and novels) were presented against the background of two visual tasks that varied in task difficulty. The parietal P3 wave was measured in response to task stimuli and probes. ADHD subjects made significantly fewer correct detections than normal controls in both the easy and the hard tasks. Controls showed an enhanced P3 to task-relevant stimuli in the hard task, whereas ADHD children did not. Probe (novel) P3 amplitudes decreased from the easy to the hard task to the same extent in both groups. Methylphenidate enhanced the percentage of correct responses and task P3 amplitudes in both the easy and the hard task but probe P3 amplitudes were not influenced by methylphenidate. It was concluded that ADHD children do not suffer from a shortage in attentional capacity; rather, the evidence is in favor of a problem with capacity allocation. Furthermore, methylphenidate had enhancing effects on performance and ERPs, but did not improve the capacity-allocation deficit.

Adolescent↗

PubMed1999

P50 suppression and prepulse inhibition of the startle reflex in humans: a correlational study.

BACKGROUND: Sensory gating is an important feature of the normally functioning brain. When not operating correctly, it can contribute to different kinds of psychiatric illnesses by flooding the higher brain functions with useless information. Over the years, two paradigms have evolved to quantify the amount of sensory gating: the prepulse inhibition (PPI) of the startle reflex and the suppression of the P50 evoked potential. To enable comparison across studies it is important to find out to what extent these paradigms reflect the same processes. In the present study, this relationship was explored. METHODS: Thirty-one healthy male volunteers with no personal or family history of mental illness were tested on their ability to suppress the P50 wave and to inhibit the startle reflex. RESULTS: A significant positive correlation was found between PPI and P50 suppression mainly early in testing, when habituation of the startle reflex is taking place. Furthermore, a significant negative correlation was found between P50 suppression in the second half of testing and the habituation of the startle reflex. CONCLUSIONS: PPI and P50 suppression are correlated at an early stage of testing, when the process of habituation of the startle reflex is active. The role of the habituation in the correlation between these two measures needs to be further explored.

Acoustic Stimulation↗

PubMed1999

Caffeine and stroop interference.

In two experiments the hypothesis that caffeine reduces Stroop interference was tested. In the first experiment interference was measured as the reduction in performance when subjects had to indicate the numerosity of strings of incongruent digits, relative to neutral-symbol strings. In the second experiment the incongruent condition consisted of naming the color of words referring to incongruent colors, and was compared to color naming of neutral strings. In the number-digit task 250 mg caffeine reduced interference at the level of error rates, relative to placebo. In the color-word task interference was reduced at the level of reaction times. These results were obtained with blocked presentations of incongruent and neutral conditions. The color-word experiment also contained a condition in which neutral and incongruent trials were mixed within one sequence. In this mixed condition caffeine still reduced overall reaction times, but no longer specifically interference. It is argued that this dissociation reflects a caffeine-induced increase in flexibility. The results are discussed in relation to failed previous attempts to demonstrate increased selectivity under caffeine using non-Stroop tasks, the importance of including pretreatment sessions to detect artificial effects, and the possible contribution of withdrawal effects.

Adult↗

PubMed1999

Perceptual and response interference in children with attention-deficit hyperactivity disorder, and the effects of methylphenidate.

Fourteen children with attention-deficit hyperactivity disorder (ADHD) and 14 normal control children were compared with respect to stimulus- and response-related processes. Subjects with ADHD took part in two additional sessions under methylphenidate or placebo. In both experiments, performance and electrophysiological measures such as the P2, N2, and P3 components of event-related potential and electromyogram (EMG) activity were measured during an Eriksen flanker task. In both groups of children, reaction times (RTs) to arrow stimuli incongruent with the target were longer than those to neutral stimuli (response interference), which were again slower than RTs to target-alone stimuli (perceptual interference). Children with ADHD made more errors to incongruent stimuli and showed more response interference. For correct responses, no differences between the groups in response interference effects on reaction time, P2, N2, and P3 latency, or EMG onset were found. Methylphenidate had a general enhancing effect on accuracy but did not specifically reduce interference from the flanking stimuli. Methylphenidate had no effects on RT, N2 and P2 latency, P3 amplitude or latency, or EMG activity. The conclusion that methylphenidate did not influence response processes contrasts sharply with findings reported by authors using the Sternberg memory search task.

Adolescent↗

PubMed1998

Differences in plasma concentrations of the D- and L-threo methylphenidate enantiomers in responding and non-responding children with attention-deficit hyperactivity disorder.

Children with attention-deficit hyperactivity disorder who are categorized as responders or non-responders to methylphenidate (MPH) on the basis of their electrophysiological P3 response in a selective attention task differ in metabolic patterns of the D- and L-threo MPH enantiomers. Non-responders showed significantly higher plasma concentrations of both D- and L-threo MPH enantiomers.

Analysis of Variance↗

PubMed1998

Caffeine and visuo-spatial attention.

The effects of two doses of caffeine (1.5 and 3 mg/kg) on various aspects of visual selective attention were investigated in 24 healthy human subjects. Specific task conditions were compared to provide measures of selectivity for a location in the visual field, of distractibility, of selectivity among response alternatives, and of strategic influences. In two out of three tasks, caffeine speeded responses significantly. However, these effects did not differ across conditions within-task, so there was no indication that they were to due to (a) specific effect(s) on one or more of the attentional sub-functions. The results suggest that the beneficial effects of caffeine in low-load conditions cannot be attributed to reduced distractibility or increased suppression of task-irrelevant response tendencies.

Adult↗

PubMed1998

The effects of low dose ketamine on sensory gating, neuroendocrine secretion and behavior in healthy human subjects.

Recently, much interest has been given to the role of glutamatergic N-methyl-D-aspartate receptors (NMDA) in sensory gating, such as prepulse inhibition (PPI) and reduction of the P50 evoked response potential (ERP). Currently, mainly animal data are available describing the role of NMDA receptors in these stimulus evaluation processes. Human data are virtually lacking and are potentially important, for instance for the understanding of sensory gating deficits observed in schizophrenia. Therefore, the effects of the NMDA antagonist ketamine, in a dose of 0.3 mg/kg i.v., on concurrent assessment of PPI and P50 reduction was studied in 18 healthy male volunteers. Ketamine was administered in a pseudo-steady state model with a subacute loading dose. In addition, the effects of ketamine on behavior, vital signs, homovanillic acid (HVA) plasma levels and secretion of cortisol and luteinizing hormone (LH) were also determined. Ketamine did not significantly alter PPI or the reduction of the P50 ERP. A small but significant increase in Brief Psychiatric Rating Scale (BPRS) total scores and BPRS composite scores "thinking disorder" and "withdrawal/retardation" was observed. Several subjects experienced visual perceptional alterations, but complex hallucinations did not occur. Ketamine induced mild analgesia and coordination problems. In addition, ketamine induced a marked rise in cortisol secretion, while LH secretion was not affected. Finally, systolic and diastolic, blood pressure and heart rate increased during ketamine infusion. Although in humans NMDA receptors may not be involved in the regulation of PPI and P50 reduction, the most likely explanation for the lack of effect of ketamine on these sensory gating paradigms is the dose used in this experiment. However, using a higher dose is hampered by the aspecificity of racemic ketamine. Future studies should use the enantiomer S-ketamine, which is more specific to NMDA receptors, to evaluate the involvement of NMDA receptors in these neurophysiological processes further.

Adult↗

PubMed1998

Localization of spatial attention processes with the aid of a probe technique.

A sudden visual onset is thought to 'attract attention to its location' within less than 100 ms. We attempted to measure the effect of this attentional process on the event-related potential (ERP) to a probe presented about 140 ms after the onset, and to delineate the spatiotemporal characteristics of such an effect, if any. ERPs were recorded from 30 channels from 6 subjects while they performed a target detection task. Both targets and probes could be located in each of the 4 quadrants (eccentricities 6.1 degrees and 7 degrees, respectively). For a given single target, the subsequent probe was either presented near the location of the target ('valid target') or at the diagonal opposite ('invalid target'). Appropriate 'neutral' conditions (probes preceded by no target, or by simultaneous targets in all quadrants) were applied, and ERPs to probes were corrected for the contribution of the ERPs to targets. The earliest effect of (in)validity was found at about 120 ms after probe onset for lower field probes. This effect consisted of enhanced posterior positivity for valid relative to neutral relative to invalid conditions. This positivity was superposed on a contralateral, extrastriate negative ongoing wave peaking at about 150 ms ('N150'). Source localization suggested that the (in)validity effects originate from deep medial parietal areas. The source corresponding to the N150 activity was not influenced by (in)validity. An earlier deflection to the probe at 80 ms ('NP80') depended on location, but not on (in)validity, and seemed to be of striate origin. Results are discussed in terms of a model postulating an attention-independent 'input module' from which activation is fed to a 'location module' embodying the actual attention mechanism.

Adult↗

PubMed1998

Abnormal saccadic eye movements in autistic children.

The saccadic eye movements, generated during a visual oddball task, of autistic children, normal children, children with attention deficit disorder and hyperactivity (ADDH), and dyslexic children were examined to determine whether autistic children differed from these other groups in saccadic frequency. Autistic children made more saccades during the presentation of frequent stimuli (than normals and ADDH children), and between stimulus presentations. Also, unlike the normal and dyslexic groups, their saccadic frequency did not depend on stimulus type. This abnormal pattern of saccades may negatively influence the ability to attend to stimuli, and thereby learning processes. Suggestions are made with respect to possible abnormalities in subcortical mechanisms involved in saccade generation.

Attention↗

PubMed1998

Associations between event-related potentials and measures of attention and inhibition in the Continuous Performance Task in children with ADHD and normal controls.

OBJECTIVES: First, to differentiate between inattention and impulsivity based on type of errors made in the AX version of the Continuous Performance Task (CPT), and second, to investigate whether differences in performance between children with attention-deficit hyperactivity disorder (ADHD) and normal controls also occur in specific forms of brain activity, namely event-related potentials (ERPs), presumably related to inattention and impulsivity or inhibition. METHOD: Sixteen ADHD and 16 normal control children performed the CPT-AX. ERPs were recorded at occipital (Oz), parietal (Pz), central (Cz), and frontal (Fz) leads. RESULTS: The ADHD children had a higher CPT-Inattention score and showed smaller parietal positive waves at a latency of approximately 300 msec in reaction to target stimuli, target P3s, likewise indicating less attention. In contrast, they showed neither higher CPT-Impulsivity nor a smaller frontocentral negative wave at about 200 msec (N2); the N2 is generally seen as reflecting inhibition. A subgroup of children with ADHD and oppositional defiant disorder (n = 6) had smaller N2 waves than controls, however. CONCLUSIONS: The ADHD group studied showed deficits in attention but not in impulsivity (or inhibition).

Adolescent↗

PubMed1998

Are abnormal event-related potentials specific to children with ADHD? A comparison with two clinical groups.

Children with attention deficit disorder and hyperactivity (ADHD) were compared with two other clinical groups, namely, children with autism and children with dyslexia, with respect to several peaks of the ERP. By using these other clinical groups, it was studied whether amplitude differences between children and ADHD and normal control children, which were found in an earlier study, were specific to children with ADHD. ERPs were measured in response to stimuli in an auditory and a visual oddball task. Only with respect to the P3 and Pz, measured in response to deviant auditory stimuli, did the children with ADHD show smaller amplitudes than both autistic children and those with dyslexia. It was concluded that a smaller amplitude of this peak is specific to children with ADHD.

Acoustic Stimulation↗

PubMed1997

Effects of methylphenidate on event-related potentials and performance of attention-deficit hyperactivity disorder children in auditory and visual selective attention tasks.

Attention-deficit hyperactivity disorder (ADHD) children participated in a double-blind placebo-controlled study in which the effects of a dosage of 15 mg methylphenidate (MPH) on auditory and visual selective attention tasks was determined by presenting frequent (90%) and infrequent (10%) stimuli in both relevant and irrelevant input channels. The subject's task was to respond to the infrequent tones in the relevant input channel. Processing activity (negativity and positivity) was assessed for both tasks. N1, P2, N2, and P3b peaks were scored in the auditory task and N1, P1, N2, P2, P3(1), and P3b peaks were scored in the visual task. Effects of MPH were more prevalent in the visual than in the auditory condition. In the visual condition MPH enhanced the percentage of hits, caused higher central, parietal, and occipital P3b amplitudes to attended stimuli (both standards and deviants), and also enhanced the frontal processing negativity (PN). In the auditory task MPH did not influence performance, but it enhanced the frontal PN as well as the parietal and occipital P3b amplitudes to all stimulus types. In ADHD children, MPH ameliorates some, but not all, deficits and also improves processing where no differences with normal children are present.

Adolescent↗

PubMed1997

Event-related potentials and performance of attention-deficit hyperactivity disorder: children and normal controls in auditory and visual selective attention tasks.

Attention-deficit hyperactivity disorder (ADHD) children and normal controls (7-13 yrs old) performed an auditory and visual selective attention task. Subjects were instructed to respond to the infrequent (10%) stimuli in the relevant channel. Processing negativity (PN) and several other ERP peaks were scored at the midline electrodes. In the auditory task, controls had more correct detections (hits), less false alarms, larger P3b amplitudes to nontarget stimuli (but not to hits), a larger central PN and larger early frontal positivity (100-250 ms) to target stimuli than ADHD subjects. In the visual modality, controls had more correct detections, less false alarms, larger P3b amplitudes to nontarget stimuli (but not to hits), and larger frontal P3(1) amplitudes to infrequent than to frequent stimuli. It was hypothesized that in ADHD children in both the auditory and the visual task, there is a deficit in the activation of the P3b process. Incorrect triggering of the P3b process might be caused by disturbances in other aspects of the attention process, preceding the P3b.

Adolescent↗

PubMed1997

Processing capacity and the frontal P3.

In the present study, unexpected, task-irrelevant, visual stimuli (probes) were presented against the background of one of two visual tasks. Subjects either performed an easy (low task load) or hard (high task load) visual task. During task performance probes were presented consisting of standard (80%), slightly deviating (deviants, 10%) and highly deviating stimuli (rares, 10%). The rare stimuli induced both a frontal (peak latency 448 ms) and a parietal P3 (peak latency 502 ms). The latter P3 was smaller during the high load condition, but in contrast, the frontal P3 was not influenced by task-load. The results of the present study suggest that the frontal and the parietal P3 are generated by two different neural sources, of which only the latter is capacity-limited.

Adult↗