Biomedical subjects

Andrea Antal

Publications and source records attributed to Andrea Antal.

At least 19 recordsLinked to original sources

PubMed2006

Task-related temporal and topographical changes of cortical activity during ultra-rapid visual categorization.

The aim of our study was to provide electrophysiological evidence about the modulation of the categorization process by task requirements in the human brain. Event-related potentials (ERP) were recorded during three different categorization tasks using matched stimulus sets. In all cases, the subjects were required to differentiate between "animal" and "non-animal" stimuli. In the first task (two-choice task), they were asked to press corresponding buttons to each stimulus types. The second task was a go/no-go paradigm, only animal stimuli required motor response. The third task was a counting task; participants had to count the animal stimuli without any motor response. The reaction times in the go/no-go paradigm were significantly shorter. ERP differences between animal and non-animal pictures in the go/no-go task also appeared earlier and were localized at more posterior scalp positions compared to the two-choice task. Comparing animal responses in the two-choice task and in the go/no-go paradigm, we found a significant difference in the 130- to 170-ms time window over the fronto-central, centro-parietal regions. Similar differences were found between the responses to animal pictures in the two-choice task and in the counting paradigm. We used brain electric source analysis (BESA) algorithm on difference waves to localize the best fitting dipoles and determine the localization of brain areas contributing to scalp potential differences. The results show that different task requirements evoke different activity in the medial part of the temporal pole. The data we provided here draw attention to the careful handling of results obtained from categorization experiments, because different task requirements can affect the early categorization process itself.

Adult↗

PubMed2006

Temporal pattern of source activities evoked by different types of motion onset stimuli.

The aim of this study was to compare the time course of motion-related source activities evoked by the onset of different kinds of visual motion stimuli in human subjects. Event-related potentials (ERP) were recorded from 64 scalp electrodes in ten healthy subjects while they were viewing four different types of motion stimuli (translation, rotation, expansion and contraction). Following a new approach combining a current density reconstruction with clustering algorithms, source maxima in the time range from 50 to 400 ms after the onset of the visual stimulus were localized and the time courses of activation were elaborated. Six regions contributed significantly to source activity, half originating in the occipital lobe and half in the right parietal and right temporal cortex. The comparison of their time courses led to the following conclusions: (i) the different kinds of motion stimuli activated about the same areas of the brain but with different temporal patterns. (ii) Mainly parietal and extrastriate areas, but not V1/V2, were significantly involved in the differentiation of different kinds of motion. (iii) Contrasting the different kinds of motion onsets, responses from parietal areas were found mainly before those from lateral occipital areas. (iv) The classically defined N2 and P2 components were significantly different among the four motion conditions, but not P1. The N2 motion-related component was elicited not only by lateral occipital areas and middle temporal areas but also by right parietal areas. (v) The rotation condition evoked a novel component P180, concomitant with an increased activity in the left middle temporal gyrus.

Adult↗

PubMed2006

Dopaminergic modulation of long-lasting direct current-induced cortical excitability changes in the human motor cortex.

Dopaminergic mechanisms participate in N-methyl-D-aspartate (NMDA) receptor-dependent neuroplasticity, as animal experiments have shown. This may be similar in humans, where dopamine influences learning and memory. We tested the role of dopamine in human cortical neuroplasticity. Changes of excitability were induced by transcranial direct current stimulation (tDCS). D2 receptor blocking by sulpiride abolished the induction of after-effects nearly completely. D1 activation alone in the presence of D2 receptor blocking induced by co-administration of sulpiride and pergolide did not re-establish the excitability changes induced by tDCS. This suggests that D2 receptors play a major supporting role in inducing neuroplasticity in the human motor cortex. Enhancement of D2 and, to a lesser degree, D1 receptors by pergolide consolidated tDCS-generated excitability diminution until the morning after stimulation. The readiest explanation for this pattern of results is that D2 receptor activation has a consolidation-enhancing effect on tDCS-induced changes of excitability in the human cortex. The results of this study underscore the importance of the dopaminergic system for human neuroplasticity, suggest a first pharmacological add-on mechanism to prolong the excitability-diminishing effects of cathodal tDCS for up to 24 h after stimulation, and thus render the application of tDCS practicable in diseases displaying enhanced cortical excitability, e.g. migraine and epilepsy.

Adult↗

PubMed2005

Position-specificity of facial adaptation.

We investigated the representation of objects' position at the higher, shape-selective stages of visual processing by testing the position-specificity of the behavioural and neural effects of facial adaptation. Here, we show that facial after-effects evoked by adaptation to both upright and upside-down faces are significantly larger when the adaptor and test faces are presented on the same retinal position than when they are displayed in different hemifields. Our event-related potential recordings revealed that adaptation effects measured on the amplitude of the N170 event-related potential component over the hemisphere that was contralateral to the test face stimulus also show strong position-specificity. These findings suggest that face adaptation effects are only partially translation invariant and facial after-effects measured with peripheral test stimuli primarily reflect the adaptation processes in the contralateral hemisphere.

Adaptation, Physiological↗

PubMed2005

Transcranial direct current stimulation and the visual cortex.

Neuroplastic changes are defined as enduring changes in the organization of the central nervous system, such as the strength of connections, representational patterns, or neuronal properties, either morphological or functional. In recent years, new tools have emerged to induce and manipulate ongoing neuroplastic changes by external stimulation, either by modification of synchronized neuronal activity or modulation of the spontaneous firing rate. The first is performed by transcranial magnetic stimulation (TMS), the latter by direct current stimulation (tDCS). tDCS as a tool aims to induce prolonged neuronal excitability and activity alterations in the human brain via alterations of the neuronal membrane potential and results in prolonged synaptic efficacy changes. Apart from its impressive persistent excitability effects, it is a non-invasive method and can be applied painlessly. Most likely that up- or downregulation of different cortical areas by tDCS will open a new branch in the area of visual psychophysics.

Animals↗

PubMed2005

Electrophysiological correlates of visual adaptation to faces and body parts in humans.

The existence of facial aftereffects suggests that shape-selective mechanisms at the higher stages of visual object coding -- similarly to the early processing of low-level visual features -- are adaptively recalibrated. Our goal was to uncover the ERP correlates of shape-selective adaptation and to test whether it is also involved in the visual processing of human body parts. We found that prolonged adaptation to female hands -- similarly to adaptation to female faces -- biased the judgements about the subsequently presented hand test stimuli: they were perceived more masculine than in the control conditions. We also showed that these hand aftereffects are size and orientation invariant. However, no aftereffects were found when the adaptor and test stimuli belonged to different categories (i.e. face adaptor and hand test, or vice versa), suggesting that the underlying adaptation mechanisms are category-specific. In accordance with the behavioral results, both adaptation to faces and hands resulted in a strong and category-specific modulation -- reduced amplitude and increased latency -- of the N170 component of ERP responses. Our findings suggest that shape-selective adaptation is a general mechanism of visual object processing and its neural effects are primarily reflected in the N170 component of the ERP responses.

Adaptation, Physiological↗

PubMed2005

Modulating parameters of excitability during and after transcranial direct current stimulation of the human motor cortex.

Weak transcranial direct current stimulation (tDCS) of the human motor cortex results in excitability shifts which occur during and after stimulation. These excitability shifts are polarity-specific with anodal tDCS enhancing excitability, and cathodal reducing it. To explore the origin of this excitability modulation in more detail, we measured the input-output curve and motor thresholds as global parameters of cortico-spinal excitability, and determined intracortical inhibition and facilitation, as well as facilitatory indirect wave (I-wave) interactions. Measurements were performed during short-term tDCS, which elicits no after-effects, and during other tDCS protocols which do elicit short- and long-lasting after-effects. Resting and active motor thresholds remained stable during and after tDCS. The slope of the input-output curve was increased by anodal tDCS and decreased by cathodal tDCS. Anodal tDCS of the primary motor cortex reduced intracortical inhibition and enhanced facilitation after tDCS but not during tDCS. Cathodal tDCS reduced facilitation during, and additionally increased inhibition after its administration. During tDCS, I-wave facilitation was not influenced but, for the after-effects, anodal tDCS increased I-wave facilitation, while cathodal tDCS had only minor effects. These results suggest that the effect of tDCS on cortico-spinal excitability during a short period of stimulation (which does not induce after-effects) primarily depends on subthreshold resting membrane potential changes, which are able to modulate the input-output curve, but not motor thresholds. In contrast, the after-effects of tDCS are due to shifts in intracortical inhibition and facilitation, and at least partly also to facilitatory I-wave interaction, which is controlled by synaptic activity.

Adult↗

PubMed2005

Anodal transcranial direct current stimulation of prefrontal cortex enhances working memory.

Previous studies have claimed that weak transcranial direct current stimulation (tDCS) induces persisting excitability changes in the human motor cortex that can be more pronounced than cortical modulation induced by transcranial magnetic stimulation, but there are no studies that have evaluated the effects of tDCS on working memory. Our aim was to determine whether anodal transcranial direct current stimulation, which enhances brain cortical excitability and activity, would modify performance in a sequential-letter working memory task when administered to the dorsolateral prefrontal cortex (DLPFC). Fifteen subjects underwent a three-back working memory task based on letters. This task was performed during sham and anodal stimulation applied over the left DLPFC. Moreover seven of these subjects performed the same task, but with inverse polarity (cathodal stimulation of the left DLPFC) and anodal stimulation of the primary motor cortex (M1). Our results indicate that only anodal stimulation of the left prefrontal cortex, but not cathodal stimulation of left DLPFC or anodal stimulation of M1, increases the accuracy of the task performance when compared to sham stimulation of the same area. This accuracy enhancement during active stimulation cannot be accounted for by slowed responses, as response times were not changed by stimulation. Our results indicate that left prefrontal anodal stimulation leads to an enhancement of working memory performance. Furthermore, this effect depends on the stimulation polarity and is specific to the site of stimulation. This result may be helpful to develop future interventions aiming at clinical benefits.

Adult↗

PubMed2005

Lactate infusion fails to improve semantic categorization in Alzheimer's disease.

Impaired neuronal energy metabolism, oxidative changes and microvascular abnormalities lead to altered lactate levels in Alzheimer's dementia. The aim of the present study was to assess whether intravenous sodium-lactate, a metabolic alternative and vasodilator that is thought to improve cognition, advances the cognitive performance of Alzheimer patients. Semantic categorization paradigm was used to present the electrophysiological correlates of natural scene categorization of Alzheimer patients before and after intravenous saline or sodium-lactate infusion. Mean amplitudes of event-related potentials (ERPs) were measured in two time windows before and after the treatments; two negative components (N1 between 150 and 250 ms and N2 between 400 and 600 ms) and one positive component (P2 between 250 and 400 ms) were identified. The negative components were more negative for the non-animal trials than for the animal trials while the positive component was similar for both categories. After the lactate treatment the amplitudes of the negative components became more negative mainly for the non-animal trials while the amplitude of the positive component turned more positive for the animal trials, however these changes were not significant. No changes have been observed after normal saline infusion. These results suggest that, contrary to its anticipated beneficial effects, sodium-lactate fails to significantly improve semantic categorization processes in Alzheimer's disease and this enhancement can be detected by recording ERPs. The effect of sodium-lactate to slightly improve semantic memory might be based on its positive effect on cardio- and cerebro-vascular function and neuronal metabolism.

Aged↗

PubMed2004

Direct current stimulation over MT+/V5 modulates motion aftereffect in humans.

While there is strong evidence for the central role of the human MT+/V5 in motion processing, its involvement in motion adaptation is still the subject of debate. We used transcranial direct current stimulation (tDCS) to test whether MT+/V5 is part of the neural network involved in the long-term adaptation-induced motion after-effect in humans. It was found that both cathodal and anodal stimulation over MT+/V5 resulted in a significant reduction of the perceived motion after-effect duration, but had no effect on performance in a luminance-change-detection task used to determine attentional load during adaptation. Our control experiment excluded the possibility that the observed MT+/V5 stimulation effects were due to a diffused modulation of the early cortical areas, i.e. by the stimulation applied over MT+/V5. These results provide evidence that external modulation of neural excitability in human MT+/V5 affects the strength of perceived motion after-effect and support the involvement of MT+/V5 in motion adaptation processes.

Adaptation, Physiological↗

PubMed2004

Oscillatory brain activity and transcranial direct current stimulation in humans.

The aim of this study was to induce changes of the oscillatory activity in the visual cortex of healthy human subjects by modulation of neuronal excitability using weak transcranial direct current stimulation (tDCS). tDCS is a non-invasive stimulation method which induces prolonged, polarity-dependent increases or reductions in cortical excitability. An increase in high frequency oscillatory activity in the beta and gamma frequency ranges is closely related in time to the N70 peak of the primary visual evoked potential (VEP), which is an early sensory component of visual activation. Therefore this potential can be used to observe tDCS-induced changes related to oscillatory activity. VEPs were recorded using sinusoidal luminance gratings in an on/off mode before, immediately after and 10, 20, 30 min after the end of 10 min anodal or cathodal stimulation. Cathodal stimulation significantly decreased while anodal stimulation slightly increased the normalized beta and gamma frequency powers. We have shown here that tDCS transiently and reversibly changed the organized cortical activity elicited by visual stimulation. Since gamma activity is also related to a higher level of information processing, tDCS might be a suitable method to affect higher order cognitive processes.

Adult↗

PubMed2004

Light therapy increases visual contrast sensitivity in seasonal affective disorder.

The purpose of this study was to investigate the effects of light therapy on visual contrast sensitivity in patients with seasonal affective disorder (n=10) and healthy control subjects (n=10). Static and dynamic visual contrast sensitivity was measured using a Venus system before and after 4 weeks of light therapy (10,000 lux, 30 min, 5 times a week). Light therapy increased static visual contrast sensitivity in the patients. We found no significant difference between the patients and controls either before or after light therapy. These results raise the possibility that light therapy induces retinal sensitization in seasonal affective disorder.

Adult↗

PubMed2004

No correlation between moving phosphene and motor thresholds: a transcranial magnetic stimulation study.

The aim of this study was to investigate the temporal stability of moving phosphenes and to assess whether moving phosphene thresholds (PTs) correlate with motor thresholds (MTs). Small moving sensations, so-called moving phosphenes, are perceived when V5, an area important for visual motion analysis, is stimulated by transcranial magnetic stimulation (TMS). However, it is still a matter of debate if V5 phosphenes are stable sensations across measurements and if they are a reasonable index of the cortical excitability of V5. Currently, MT is more commonly used as an index of global cortical excitability. However, previous studies have indicated that stationary PTs are suitable alternatives when the primary visual cortex is stimulated by TMS. Using paired-pulse TMS, stationary and moving PTs and applying single pulse TMS, MTs were measured in 11 subjects. PTs were retested in nine subjects 5-7 days later. Stationary and moving PTs were stable within subjects across the two sessions and showed a high inter-correlation. Conversely, PTs and MTs did not correlate. Our results are in agreement with previous studies showing that excitatory measurements of one specific cortex cannot be generalized to the excitability of the whole cortex. Thus, we propose specific measures for cortices of interest: PT for visual experiments and MT for motor experiments.

Adult↗

PubMed2004

Facilitation of probabilistic classification learning by transcranial direct current stimulation of the prefrontal cortex in the human.

The aim of our study was to test if the electrical stimulation of the prefrontal cortex (PFC) could modify probabilistic classification learning (PCL). Transcranial direct current stimulation (tDCS) was administered to the left prefrontal and to the primary visual cortex of 22 healthy subjects while they performed a PCL task. In this task subjects learned which of two outcomes would occur on each trial after presentation of a particular combination of cues. Ten minutes of anodal, but not cathodal, stimulation improved implicit learning only when the left PFC was stimulated. Our results show that implicit PLC can be modified by weak anodal tDCS, which probably increases neural excitability, as has been shown in the motor and visual cortices previously. Our results suggest that further studies on the facilitation of learning and memory processes by tDCS are warranted.

Adult↗

PubMed2004

Facilitation of visuo-motor learning by transcranial direct current stimulation of the motor and extrastriate visual areas in humans.

Performance of visuo-motor tasks requires the transfer of visual data to motor performance and depends highly on visual perception and cognitive processing, mainly during the learning phase. The primary aim of this study was to determine if the human middle temporal (MT)+/V5, an extrastriate visual area that is known to mediate motion processing, and the primary motor cortex are involved in learning of visuo-motor coordination tasks. To pursue this, we increased or decreased MT+/V5, primary contralateral motor (M1) and primary visual cortex excitability by 10 min of anodal or cathodal transcranial direct current stimulation in healthy human subjects during the learning phase of a visually guided tracking task. The percentage of correct tracking movements increased significantly in the early learning phase during anodal stimulation, but only when the left V5 or M1 was stimulated. Cathodal stimulation had no significant effect. Also, stimulation of the primary visual cortex was not effective for this kind of task. Our data suggest that the areas V5 and M1 are involved in the early phase of learning of visuo-motor coordination.

Adult↗

PubMed2004

Direct current stimulation over V5 enhances visuomotor coordination by improving motion perception in humans.

The primary aim of this study was to determine the extent to which human MT+/V5, an extrastriate visual area known to mediate motion processing, is involved in visuomotor coordination. To pursue this we increased or decreased the excitability of MT+/V5, primary motor, and primary visual cortex by the application of 7 min of anodal and cathodal transcranial direct current stimulation (tDCS) in healthy human subjects while they were performing a visuomotor tracking task involving hand movements. The percentage of correct tracking movements increased specifically during and immediately after cathodal stimulation, which decreases cortical excitability, only when V5 was stimulated. None of the other stimulation conditions affected visuomotor performance. We propose that the improvement in performance caused by cathodal tDCS of V5 is due to a focusing effect on to the complex motion perception conditions involved in this task. This hypothesis was proven by additional experiments: Testing simple and complex motion perception in dot kinetograms, we found that a diminution in excitability induced by cathodal stimulation improved the subject's perception of the direction of the coherent motion only if this was presented among random dots (complex motion perception), and worsened it if only one motion direction was presented (simple movement perception). Our data suggest that area V5 is critically involved in complex motion perception and identification processes important for visuomotor coordination. The results also raise the possibility of the usefulness of tDCS in rehabilitation strategies for neurological patients with visuomotor disorders.

Adult↗

PubMed2004

Excitability changes induced in the human primary visual cortex by transcranial direct current stimulation: direct electrophysiological evidence.

PURPOSE: Transcranial direct current stimulation (tDCS) has been shown to modify the perception threshold of phosphenes elicited by transcranial magnetic stimulation (TMS). The current study was undertaken to examine whether tDCS, when applied over the occipital cortex, is also able to affect visual-evoked potentials (VEPs), which characterize occipital activation in response to visual stimulation, in a polarity-specific way. METHOD: For this purpose, VEPs evoked by sinusoidal luminance grating in an on/off mode were recorded before, immediately after, and 10, 20, and 30 minutes after the end of 5, 10, or 15 minutes of anodal or cathodal tDCS of the primary visual cortex. RESULTS: Significant effects were observed only when low-contrast visual stimuli were applied. Cathodal stimulation decreased, whereas anodal stimulation increased the amplitude of the N70 component. The effect of cathodal stimulation was significant immediately after and 10 minutes after the end of stimulation, if the stimulation duration was sufficiently long (i.e., 10-15 minutes). An increase of N70 amplitude by anodal stimulation was significant only 10 minutes after the end of the 15 minutes tDCS. Cathodal stimulation tended also to affect the amplitude of the P100 component; however, the effect of stimulation was inverse. The amplitude increased immediately after the end of cathodal stimulation. In contrast, anodal stimulation did not affect the P100. The latencies of the N70 and the P100 were not affected by tDCS. CONCLUSIONS: tDCS appears to be a suitable method of inducing reversible excitability changes in a polarity-specific way, not only in the motor but also in the primary visual cortex. The duration of the induced aftereffects depends not only on stimulation duration but also on stimulation polarity. Cathodal stimulation seems to be more effective, in line with previous reports on the motor cortex.

Adult↗

PubMed2003

Manipulation of phosphene thresholds by transcranial direct current stimulation in man.

Transcranial direct current stimulation (tDCS) can modulate the excitability of the human motor cortex, as revealed by the amplitude of the motor-evoked potentials (MEP). The aim of our study has been to produce localized changes of cerebral excitability of the visual cortex in the intact human by weak anodal and cathodal stimulation. For quantification of current-induced excitability changes, we measured phosphene threshold (PT) using short trains of 5-Hz transcranial magnetic stimulation (TMS) pulses in nine healthy subjects before, immediately after, 10 min, and 20 min after the end of tDCS. PTs are suggested as representative values of visual cortex excitability changes. Reduced PT was detected immediately and 10 min after the end of anodal stimulation, while cathodal stimulation resulted in an opposite effect. Our results show that tDCS elicits a transient, reversible excitability alteration of the visual cortex, thus representing a promising tool for neuroplasticity research.

Adult↗