Biomedical subjects

Andreas Keil

Publications and source records attributed to Andreas Keil.

At least 19 recordsLinked to original sources

PubMed2006

The neural correlates of feature-based selective attention when viewing spatially and temporally overlapping images.

We used dense-array EEG to study the neural correlates of selective attention to specific features of objects that spatially overlapped an unattended image. Participants viewed superimposed images (horizontal and vertical bars differing in color) and attended to one image to identify bar width changes in specific locations. Images were frequency tagged so attention directed to unique parts of the stimuli could be tracked. Steady-state visual evoked potentials were used to quantify attention-related neural activity. As expected, selectively attending to specific parts of the attended image enhanced brain activity related to the attended element, and left unchanged activity elicited by spatially overlapping unattended stimuli. Under specific conditions, however, we found increased activity to unattended stimuli. The specificity of the selective attention effects presented herein, however, may be limited under certain complex stimulus conditions.

Adolescent↗

PubMed2006

Alpha-band activity reflects reduction of mental effort in a comparison task: a source space analysis.

Comparison processes contribute to many core phenomena of social cognition research. Whenever humans judge a given target, they rely on comparisons with a pertinent standard. We propose that comparison processes may be so ubiquitous because they reduce mental effort. To investigate this possibility, we used dense-array Electroencephalogram (EEG) recordings together with a minimum norm source projection approach. As the dependent variable, we examined changes in parietal alpha (8-12 Hz) amplitude during a judgment task. Spectral changes in the alpha frequency range have been reliably related to attentional load, cognitive arousal, or mental effort. Two groups of participants (n=22) were procedurally primed to solve a series of target judgments in a more comparative (experimental group) versus more absolute (control group) manner. While the participants performed the critical judgment tasks, we recorded changes in alpha amplitude. Continuous EEG was transformed into a spherical source space using the minimum norm (L2) estimate and spectral changes were subsequently calculated in the source domain. Statistical parametric mapping in combination with permutation statistics was employed to map regions showing significant group differences. Results demonstrate that comparative processing was associated with smaller changes in alpha amplitude than absolute processing. This difference was most pronounced at parietal source locations, where alpha reduction was at a maximum. Temporal analysis suggested that this effect was present particularly during task preparation and execution. We conclude that comparative information processing may reduce mental effort in judgment tasks.

Adult↗

PubMed2006

Early cortical facilitation for emotionally arousing targets during the attentional blink.

BACKGROUND: The present study aimed to investigate the time course of electrocortical facilitation for affectively arousing written words during the so-called 'attentional blink' (AB) period in a rapid serial visual presentation (RSVP) task. The AB refers to a period of reduced awareness for second-target stimuli following a first target by an interval of about 200-500 ms. Pleasant, neutral, and unpleasant written verbs were used as second targets in an 8.6-Hz RSVP paradigm that contained affectively neutral words as distractors. Replicating and extending behavioral studies, we expected that emotional second targets would be associated with better identification accuracy and greater electrocortical activity, compared with neutral targets. RESULTS: The steady-state visual evoked potential was recorded using 129 scalp electrodes. The time-varying energy at the presentation frequency of 8.6 Hz was extracted as a continuous measure of electrocortical activity related to the RSVP stream. Behavioral data showed that at an inter-target interval of 232 ms, the report for emotionally arousing (pleasant and unpleasant) words was more accurate than for neutral control words. This result was mirrored by the electrocortical response at posterior sensors, which showed rapid amplitude enhancement (120-270 ms after T2 onset) for pleasant and unpleasant targets specifically. CONCLUSION: The present data suggest that identification facilitation for emotionally arousing target words in the AB is related to rapid enhancement of sensory processing. Affectively arousing information is preferentially selected at the level of early perceptual analysis, leading to facilitation at later stages of processing, including consolidation in working memory and visual awareness.

Adolescent↗

PubMed2006

Acquisition of affective dispositions in dementia patients.

In this study, we investigated the acquisition of affective dispositions towards neutral faces in patients with Alzheimer's disease and mixed dementia. Participants viewed four affectively neutral male faces from the International Affective Picture System. Biographical information about the depicted persons was presented, which described them in terms of positive or negative traits varying in intensity. Participants were asked to rate the pictures with respect to emotional valence, arousal, and preference prior to and after the presentation of the biographical information. Following a retention interval of 180 min, pictures were rated again and the biographical information was presented once more. Final ratings were obtained after another interval of 1300 min. As expected, healthy control participants identified the faces and recalled parts of the information in the delayed recall test. They showed pronounced changes in affective ratings. Patients did not recognize the faces in a recognition test after the retention intervals, but valence, arousal, and preference ratings were systematically altered by the affective content of biographies. The results suggest that acquisition and maintenance of implicit affective dispositions are preserved in dementia even when explicit memory is impaired.

Affect↗

PubMed2006

Emotional perception: correlation of functional MRI and event-related potentials.

Dense-array electrocortical and functional hemodynamic measures of human brain activity were collected to assess the relationship between 2 established neural measures of emotional reactivity. Recorded in parallel sessions, the slow-wave late positive potential (LPP) and visual cortical blood oxygen level-dependent (BOLD) signals were both modulated by the rated intensity of picture arousal. The amplitude of the LPP correlated significantly with BOLD intensity in lateral occipital, inferotemporal, and parietal visual areas across picture contents. Estimated strength of modeled regional sources did not correlate significantly with regional BOLD intensity. These data suggest that the enhanced positive slow wave seen over posterior sites during emotional picture processing represents activity in a circuit of visual cortical structures, reflecting a perceptual sensitivity to the motivational relevance of visual scenes.

Adult↗

PubMed2006

Effects of classical conditioning on identification and cortical processing of speech syllables.

The present study was designed to examine effects of learned motivational significance on processing of speech syllables in adults using a classical conditioning paradigm. Aversive white noise (unconditioned stimulus) was paired with two exemplars of /ba/ (conditioned stimulus, CS+) occurring near the category boundary of a 10-item /ba/-da/ continuum. Two corresponding /da/ syllables served as the CS- and indicated absence of white noise. High-density electroencephalogram (EEG) was recorded while participants passively listened to the stimuli. Prior to the EEG session and after intermittent conditioning, participants were asked to identify the syllables in a categorical perception task. Analysis of time-locked electrocortical data revealed amplitude modulations of the N2 (248-312 ms) component as a function of acquired stimulus properties (CS+ versus CS-). Over right hemisphere regions, negativity was specifically enhanced for the CS+ during intermittent conditioning. During extinction, this conditioning effect was paralleled by findings in the time-frequency domain, showing greater oscillatory activity in the gamma-band (25-40 Hz) range, 80-120 ms following onset of the CS+ compared with CS-. A similar pattern emerged in a later time segment of 400-600 ms (30-45 Hz). Aversive conditioning was not reflected in superior categorical perception performance. Our data indicate that electrocortical correlates of speech syllable perception are susceptible to changes induced by contingencies. Physiological differences were not manifest in behavioral advantage for a specific stimulus, i.e., the CS+, however. We conclude that training speech categorization by merely enhancing motivational relevance is not effective for conveying behavioral improvement.

Adolescent↗

PubMed2006

Macroscopic brain dynamics during verbal and pictorial processing of affective stimuli.

Emotions can be viewed as action dispositions, preparing an individual to act efficiently and successfully in situations of behavioral relevance. To initiate optimized behavior, it is essential to accurately process the perceptual elements indicative of emotional relevance. The present chapter discusses effects of affective content on neural and behavioral parameters of perception, across different information channels. Electrocortical data are presented from studies examining affective perception with pictures and words in different task contexts. As a main result, these data suggest that sensory facilitation has an important role in affective processing. Affective pictures appear to facilitate perception as a function of emotional arousal at multiple levels of visual analysis. If the discrimination between affectively arousing vs. nonarousing content relies on fine-grained differences, amplification of the cortical representation may occur as early as 60-90 ms after stimulus onset. Affectively arousing information as conveyed via visual verbal channels was not subject to such very early enhancement. However, electrocortical indices of lexical access and/or activation of semantic networks showed that affectively arousing content may enhance the formation of semantic representations during word encoding. It can be concluded that affective arousal is associated with activation of widespread networks, which act to optimize sensory processing. On the basis of prioritized sensory analysis for affectively relevant stimuli, subsequent steps such as working memory, motor preparation, and action may be adjusted to meet the adaptive requirements of the situation perceived.

Acoustic Stimulation↗

PubMed2006

Fear but not awareness predicts enhanced sensory processing in fear conditioning.

It is not clear whether enhanced cortical processing of reinforced stimuli as reported by neuroimaging studies is due to expectancy of an aversive event alone or to activation of the fear system. The present study investigated cortical and autonomic responses of aware participants using an instructed fear conditioning design. Steady-state visual evoked fields (ssVEF) and heart rate change were recorded to assess sensory processing and activation of the fear system by reinforced (CS+) and nonreinforced stimuli (CS-). Participants who showed heart rate acceleration demonstrated increased ssVEFs in visual and parietal cortex during CS+ in acquisition trials. Heart rate decelerators did not show enhanced cortical activation with respect to the CS+. Participants in both groups reported awareness of CS-US contingencies. Awareness of stimulus contingency in fear conditioning seems not to be sufficient to elicit enhanced visual cortical processing.

Adult↗

PubMed2005

Modulation of the C1 visual event-related component by conditioned stimuli: evidence for sensory plasticity in early affective perception.

Previous research has demonstrated optimized processing of motivationally significant stimuli early in perception. In the present study, the time course and underlying mechanisms for such fast differentiation are of interest. We investigated the involvement of the primary visual cortex in affective evaluation of conditioned stimuli (CSs). In order to elicit learning within the visual system we chose affective pictures as unconditioned stimuli and used laterally presented gratings as CSs. Using high-density electroencephalography, we demonstrated modulation of the C1 visual event-related component for threat-related stimuli versus neutral stimuli, which increased with continuing acquisition of affective meaning. The differentiation between aversive and neutral visual stimuli occurred as early as 65-90 ms after stimulus onset and suggested involvement of the primary visual areas in affective evaluation. As an underlying mechanism, we discuss short-term reorganization in visual cortex, enabling sensory amplification of specific visual features that are related to motivationally relevant information.

Adult↗

PubMed2005

Cortical activation during Pavlovian fear conditioning depends on heart rate response patterns: an MEG study.

In the present study, we examined stimulus-driven neuromagnetic activity in a delayed Pavlovian aversive conditioning paradigm using steady state visual evoked fields (SSVEF). Subjects showing an accelerative heart rate (HR) component to the CS+ during learning trials exhibited an increased activation in sensory and parietal cortex due to CS+ depiction in the extinction block. This was accompanied by a selective orientation response (OR) to the CS+ during extinction as indexed by HR deceleration. However, they did not show any differential cortical activation patterns during acquisition. In contrast, subjects not showing an accelerative HR component but rather unspecific HR changes during learning were characterized by greater activity in left orbito-frontal brain regions in the acquisition block but did not show differential SSVEF patterns during extinction. The results suggest that participants expressing different HR responses also differ in their stimulus-driven neuromagnetic response pattern to an aversively conditioned stimulus.

Adult↗

PubMed2004

Additive effects of emotional content and spatial selective attention on electrocortical facilitation.

Affectively arousing visual stimuli have been suggested to automatically attract attentional resources in order to optimize sensory processing. The present study crosses the factors of spatial selective attention and affective content, and examines the relationship between instructed (spatial) and automatic attention to affective stimuli. In addition to response times and error rate, electroencephalographic data from 129 electrodes were recorded during a covert spatial attention task. This task required silent counting of random-dot targets embedded in a 10 Hz flicker of colored pictures presented to both hemifields. Steady-state visual evoked potentials (ssVEPs) were obtained to determine amplitude and phase of electrocortical responses to pictures. An increase of ssVEP amplitude was observed as an additive function of spatial attention and emotional content. Statistical parametric mapping of this effect indicated occipito-temporal and parietal cortex activation contralateral to the attended visual hemifield in ssVEP amplitude modulation. This difference was most pronounced during selection of the left visual hemifield, at right temporal electrodes. In line with this finding, phase information revealed accelerated processing of aversive arousing, compared to affectively neutral pictures. The data suggest that affective stimulus properties modulate the spatiotemporal process along the ventral stream, encompassing amplitude amplification and timing changes of posterior and temporal cortex.

Adult↗

PubMed2004

Mapping the brain's orchestration during speech comprehension: task-specific facilitation of regional synchrony in neural networks.

BACKGROUND: How does the brain convert sounds and phonemes into comprehensible speech? In the present magnetoencephalographic study we examined the hypothesis that the coherence of electromagnetic oscillatory activity within and across brain areas indicates neurophysiological processes linked to speech comprehension. RESULTS: Amplitude-modulated (sinusoidal 41.5 Hz) auditory verbal and nonverbal stimuli served to drive steady-state oscillations in neural networks involved in speech comprehension. Stimuli were presented to 12 subjects in the following conditions (a) an incomprehensible string of words, (b) the same string of words after being introduced as a comprehensible sentence by proper articulation, and (c) nonverbal stimulations that included a 600-Hz tone, a scale, and a melody. Coherence, defined as correlated activation of magnetic steady state fields across brain areas and measured as simultaneous activation of current dipoles in source space (Minimum-Norm-Estimates), increased within left- temporal-posterior areas when the sound string was perceived as a comprehensible sentence. Intra-hemispheric coherence was larger within the left than the right hemisphere for the sentence (condition (b) relative to all other conditions), and tended to be larger within the right than the left hemisphere for nonverbal stimuli (condition (c), tone and melody relative to the other conditions), leading to a more pronounced hemispheric asymmetry for nonverbal than verbal material. CONCLUSIONS: We conclude that coherent neuronal network activity may index encoding of verbal information on the sentence level and can be used as a tool to investigate auditory speech comprehension.

Acoustic Stimulation↗

PubMed2004

Large-scale neural correlates of developmental dyslexia.

Recent work in the field of developmental dyslexia has emphasized the large-scale neural aspects of the disorder as measured by means of contemporary imaging techniques, electrophysiology, and post-mortem analyses. This article presents findings from these research domains and comprehensively reviews their relevance with respect to the behavioral and cognitive profiles of dyslexia. Large-scale alterations were observed in the perisylvian region across paradigms. Convergent evidence has also been reported in terms of hemispheric balance. Specifically, deviances in cerebral asymmetry associated with atypical organization of the left hemisphere were found in both children and adults with dyslexia. Emerging research encompassing high-temporal resolution methods such as magnetoencephalography (MEG) suggests right-hemisphere involvement and points to the complexity of the developmental disorder. A combined approach of structural imaging and MEG, and most importantly theory driven behavioral tasks may shed light on dynamics and trajectories of the neurobiology of dyslexia.

Adult↗

PubMed2004

Aberrant brain dynamics in schizophrenia: delayed buildup and prolonged decay of the visual steady-state response.

In schizophrenia, aberrant brain activity has been reported both during stimulus processing and at rest. Evoked response amplitude is a function of both the number and synchronization of neurons firing in relation to a stimulus. It is at present unclear whether schizophrenia patients have normal synchronization of neural activity in relation to stimulus processing, and whether the amount and time course of synchronization is related to their evoked response amplitudes. EEG brain dynamics in response to visual steady-state stimulation were assessed in 12 schizophrenia and 12 healthy subjects at three stimulation durations (2, 4, and 6 s). Group differences in the visual evoked potential, the visual steady-state response, and the local coherence of the visual steady-state response were evaluated over time. Schizophrenia patients had smaller and delayed event-related potentials. Moreover, they had a slower buildup of steady-state amplitude following stimulation onset and a prolonged decrease after stimulation offset. Groups did not differ during mid-segments of steady-state stimulation. Increase in coherence to stimulation onset did not differ between-groups, but coherence decay of the visual steady-state response following stimulus offset was delayed in schizophrenia patients. The initial response to visual stimulation among schizophrenia subjects, therefore, may be reduced in amplitude due to weak signal strength, not poor coordination between distant cortical regions. The prolonged recovery function of schizophrenia patients' visual system may indicate abnormal nonlinearity in neural response. These findings have implications understanding the nature of evoked response differences between schizophrenia and normal groups especially in repetitive stimulus paradigms.

Adolescent↗

PubMed2004

Motivated attention in emotional picture processing is reflected by activity modulation in cortical attention networks.

In the present study, we presented high (pleasant and unpleasant)- and low (neutral)-arousing emotional pictures in a steady state visual evoked field paradigm while recording the magnetoencephalogram. Applying a minimum norm estimation (MNE) technique, we determined the origin and the strength of the evoked neuromagnetic field. In addition to magnetocortical data, we examined subjective ratings, heart rate change and viewing time to obtain a multivariate data base of emotional experience related to the present paradigm. As evidenced by the MNE, pictures rated as high arousing elicited greater activity in frontoparietal cortical networks than low-arousing pictures, with a right hemispheric predominance. This effect was also observed in occipito-temporal regions but to a lesser extent. Longer viewing times for high-arousing pictures and sustained heart rate deceleration for high-arousing unpleasant pictures indicated that these stimuli were of high motivational relevance compared to neutral pictures. Taken together, we argue that activity in higher-order frontoparietal cortical attention networks is modulated by emotional arousal. In turn, this attention network influences activity in systems performing stimulus processing.

Adult↗

PubMed2004

Identification facilitation for emotionally arousing verbs during the attentional blink.

The present study aimed to examine affective modulation of the "attentional blink" effect during rapid serial visual presentation (RSVP). Pleasant, neutral, and unpleasant written verbs were used as a 2nd target (T2) in an 8.6-Hz RSVP paradigm. Pronounced effects of 1st target (T1)-T2 stimulus onset asynchrony (SOA) were found, showing reduced report accuracy for 232- and 464-ms SOAs. Affectively arousing (pleasant and unpleasant) T2s were associated with enhanced accuracy compared with neutral T2s specifically during short (232 ms) SOAs. In contrast, pleasant and unpleasant T2s rated low in terms of emotional arousal did not show this enhancement. These results suggest that affectively arousing information is selected preferentially from a temporal stream, facilitating processes such as working memory consolidation and action.

Adult↗

PubMed2004

Neuronal synchronization and selective color processing in the human brain.

In the present study, subjects selectively attended to the color of checkerboards in a feature-based attention paradigm. Induced gamma band responses (GBRs), the induced alpha band, and the event-related potential (ERP) were analyzed to uncover neuronal dynamics during selective feature processing. Replicating previous ERP findings, the selection negativity (SN) with a latency of about 160 msec was extracted. Furthermore, and similarly to previous EEG studies, a gamma band peak in a time window between 290 and 380 msec was found. This peak had its major energy in the 55- to 70-Hz range and was significantly larger for the attended color. Contrary to previous human induced gamma band studies, a much earlier 40- to 50-Hz peak in a time window between 160 and 220 msec after stimulus onset and, thus, concurrently to the SN was prominent with significantly more energy for attended as opposed to unattended color. The induced alpha band (9.8-11.7 Hz), on the other hand, exhibited a marked suppression for attended color in a time window between 450 and 600 msec after stimulus onset. A comparison of the time course of the 40- to 50-Hz and 55- to 70-Hz induced GBR, the induced alpha band, and the ERP revealed temporal coincidences for changes in the morphology of these brain responses. Despite these similarities in the time domain, the cortical source configuration was found to discriminate between induced GBRs and the SN. Our results suggest that large-scale synchronous high-frequency brain activity as measured in the human GBR play a specific role in attentive processing of stimulus features.

Adult↗

PubMed2003

Steady-state visual evoked potentials reveal frontally-mediated working memory activity in humans.

Steady-state visual evoked potentials (SSVEPs) reflect power changes at the stimulus driving frequency and have been used to assess brain activity reflecting cognitive processing. Only one study has demonstrated SSVEP modulation associated with working memory (WM), and none have compared the spatial localization of SSVEP modulations during WM performance with other brain imaging methods. Here we examined WM-related activity recorded with dense-array SSVEPs, analyzed using low resolution electromagnetic tomography, and compared the results to our previous findings using functional magnetic resonance imaging (fMRI). WM was associated with increased SSVEP activity over the right dorsolateral prefrontal cortex, paralleling our previous fMRI findings. Frontal WM-related SSVEP power correlated selectively with task performance. These results demonstrate the utility of SSVEPs for studying representational aspects of cognition.

Adolescent↗