Biomedical subjects

Martin Eimer

Publications and source records attributed to Martin Eimer.

At least 37 records · Page 2Linked to original sources

PubMed2004

Effects of hand posture on preparatory control processes and sensory modulations in tactile-spatial attention.

OBJECTIVE: Event-related brain potentials (ERPs) were measured to investigate spatial coordinate systems involved in the control of preparatory tactile-spatial orienting, and in subsequent attentional modulations of somatosensory processing. METHODS: On each trial, a visual precue directed attention to the left or right hand, where infrequent tactile targets had to be detected. Hands were positioned either close together or wide apart. ERPs were recorded in the cue-target interval and in response to attended and unattended tactile non-targets. RESULTS: A frontal anterior directing attention negativity (ADAN) and a posterior late directing attention positivity (LDAP) were elicited in the cue-target interval contralateral to the direction of an attentional shift. The ADAN was unaffected by hand posture, but the LDAP was attenuated when hands were close together. N140 amplitudes were enhanced in response to tactile stimuli presented to the attended hand, and this effect was more pronounced when hands were wide apart. CONCLUSIONS: ADAN and LDAP are linked to separable anterior and posterior attentional control systems, which use coordinate systems based on somatotopic and external space, respectively. Effects of spatial attention on somatosensory stimulus processing are affected by variations in body posture. SIGNIFICANCE: Our results demonstrate that representations of body locations in external space play a central role in the control of tactile attention.

Adult↗

PubMed2004

ERP evidence for cross-modal audiovisual effects of endogenous spatial attention within hemifields.

Previous ERP studies have uncovered cross-modal interactions in endogenous spatial attention. Directing attention to one side to judge stimuli from one particular modality can modulate early modality-specific ERP components not only for that modality, but also for other currently irrelevant modalities. However, past studies could not determine whether the spatial focus of attention in the task-irrelevant secondary modality was similar to the primary modality, or was instead diffuse across one hemifield. Here, auditory or visual stimuli could appear at any one of four locations (two on each side). In different blocks, subjects judged stimuli at only one of these four locations, for an auditory (Experiment 1) or visual (Experiment 2) task. Early attentional modulations of visual and auditory ERPs were found for stimuli at the currently relevant location, compared with those at the irrelevant location within the same hemifield, thus demonstrating within-hemifield tuning of spatial attention. Crucially, this was found not only for the currently relevant modality, but also for the currently irrelevant modality. Moreover, these within-hemifield attention effects were statistically equivalent regardless of the task relevance of the modality, for both the auditory and visual ERP data. These results demonstrate that within-hemifield spatial attention for one task-relevant modality can transfer cross-modally to a task-irrelevant modality, consistent with spatial selection at a multimodal level of representation.

Acoustic Stimulation↗

PubMed2004

Masked prime stimuli can bias "free" choices between response alternatives.

Stimuli presented below the threshold of awareness can systematically influence choice responses determined by the instructed stimulus-response (S-R) mapping (task set). In this study, we investigated whether such stimuli will also bias a free choice between two response alternatives under conditions in which this choice subjectively appears to be internally generated and free. Participants had to respond to targets preceded by masked arrow primes. Left-pointing and right-pointing arrow targets required left or right responses, whereas randomly interspersed "free-choice" targets indicated that the participants were free to choose either response. Although masked primes could not be consciously discriminated, they systematically affected not only performance to arrow targets, but also the free choice between response alternatives. This demonstrates that apparently "free" choices are not immune to nonconsciously triggered biases. However, in blocks in which no specific S-R mapping was imposed, masked primes did not affect free-choice performance, indicating that these effects are not automatic but are determined by currently active task sets.

Adolescent↗

PubMed2003

Modulations of early somatosensory ERP components by transient and sustained spatial attention.

To investigate when and how spatial attention affects somatosensory processing, event-related brain potentials (ERPs) were recorded in response to mechanical tactile stimuli delivered to the left and right hand while attention was directed to one of these hands. The attended hand either remained constant throughout an experimental block (sustained attention), or was changed across successive trials (transient attention). Attentional modulations of the N140 component and a sustained 'processing negativity' for attended stimuli were observed in both attention conditions. However, attentional effects on earlier somatosensory components differed systematically. Sustained attention resulted in a contralateral negativity overlapping with the N80 component, while transient attention was reflected by a bilateral positivity overlapping with the P100 component. This dissociation indicates that sustained and transient attention affect different somatosensory areas. It is suggested that sustained attention can modulate tactile processing within primary somatosensory cortex (S1), while effects of transient attention are located beyond S1. Overall, results demonstrate that spatial selectivity in touch is mediated by activity modulations in modality-specific somatosensory cortex.

Adult↗

PubMed2003

Response facilitation and inhibition in subliminal priming.

The research reviewed in this article has investigated with behavioural, electrophysiological, and functional imaging methods how subliminally presented masked prime stimuli affect response-related processes. An initial response activation triggered by these primes was found to be followed by an inhibition of this response tendency, provided that the initial activation was strong enough to exceed an 'inhibition threshold'. This biphasic pattern is assumed to reflect the presence of self-inhibitory circuits in motor control. In contrast to endogenous response inhibition, observed when response-relevant signals are consciously perceived, this exogenous mode of response inhibition appears to be mediated by corticostriate rather than by prefrontal mechanisms. Overall, results demonstrate that inhibitory mechanisms are involved in the control of response processes, even when motor activations are triggered by stimuli that are not accessible to conscious awareness.

Dominance, Cerebral↗

PubMed2003

Shifts of attention in light and in darkness: an ERP study of supramodal attentional control and crossmodal links in spatial attention.

Crossmodal links in spatial attention, uncovered by recent behavioural and electrophysiological studies, have been interpreted as evidence for supramodal processes controlling shifts of attention. However, previous experiments have usually been conducted in illuminated environments. Continuously available visuo-spatial information might result in shifts of attention being primarily guided by visible information, even when another modality is task-relevant. The present ERP study evaluated this. A symbolic auditory cue directed attention to the left or right hand. Participants had to detect infrequent tactile targets delivered to the cued hand, while ignoring any visual stimuli. Stimuli were presented either in a lit environment or in darkness. Although continuous ambient visuo-spatial information was eliminated in the latter condition, processing of task-irrelevant visual events was still modulated by spatial attention for the tactile task. Moreover, ERP correlates of attentional shifts in the cue-target interval were similar for both illumination conditions. This was further confirmed in a follow-up experiment where the darkness condition was repeated without any peripheral visual stimulation ever occurring. These findings demonstrate that the ERP correlates of crossmodal attention (both preparatory effects in the cue-target interval, and also modulations of stimulus-evoked components) do not depend on selection being guided by ambient visible information in a lit environment. They suggest instead that spatial shifts of attention are controlled supramodally.

Acoustic Stimulation↗

PubMed2003

The processing of emotional facial expression is gated by spatial attention: evidence from event-related brain potentials.

To investigate whether the processing of faces and emotional facial expression can be modulated by spatial attention, ERPs were recorded in response to stimulus arrays containing two faces and two non-face stimuli (houses). In separate trials, attention was focused on the face pair or on the house pair, and facial expression was either fearful or neutral. When faces were attended, a greater frontal positivity in response to arrays containing fearful faces was obtained, starting about 100 ms after stimulus onset. In contrast, with faces located outside the attentional focus, this emotional expression effect was completely eliminated. This differential result demonstrates for the first time a strong attentional gating of brain processes involved in the analysis of emotional facial expression. It is argued that while an initial detection of emotionally relevant events mediated by the amygdala may occur pre-attentively, subsequent stages of emotional processing require focal spatial attention. The face-sensitive N170 component was unaffected by emotional facial expression, but N170 amplitudes were enhanced when faces were attended, suggesting that spatial attention can modulate the structural encoding of faces.

Adolescent↗

PubMed2003

The spatial distribution of attentional selectivity in touch: evidence from somatosensory ERP components.

OBJECTIVE: Somatosensory event-related brain potentials (ERPs) were measured to investigate the spatial distribution of selective attention in touch, and whether the focus of tactile attention can be split between non-contiguous areas of the body surface. METHODS: On each trial, vibratory tactile stimuli were delivered to one of 4 possible locations of the right hand. Participants had to attend to either one or two locations in order to detect infrequently presented target stimuli there. ERPs were recorded to tactile non-targets at attended and unattended locations. RESULTS: Attention directed to one finger versus another was reflected by amplitude modulations of the sensory-specific P100 component and a subsequent attentional negativity (Nd). These effects were smaller for within-finger as compared to between-finger selection. When attention was directed simultaneously to non-adjacent fingers, ERPs in response to stimuli delivered to spatially and anatomically intervening fingers showed no attentional modulations whatsoever. CONCLUSIONS: Allocating tactile-spatial attention to one finger versus another affects early modality-specific somatosensory processing stages, and these effects of within-hand attentional selectivity decrease gradually with increasing distance from the current attentional focus. Unlike vision, the focus of tactile attention can be split, and directed simultaneously to non-adjacent areas, thus excluding spatially and anatomically intermediate regions from attentional processing.

Adult↗

PubMed2003

Early posterior ERP components do not reflect the control of attentional shifts toward expected peripheral events.

Previous experiments investigating ERP correlates of anticipatory attention shifts triggered by central symbolic cues have identified a contralateral "early directing attention negativity," which was assumed to be generated by processes involved in the control of spatial orienting. Here we demonstrate that this component is not directly linked to the control of attentional shifts, but instead reflects the selection of task-relevant aspects of cue stimuli. In contrast, later ERP components triggered during covert attentional shifts are insensitive to physical cue attributes, and thus appear to be genuine electrophysiological correlates of covert attentional control mechanisms.

Adult↗

PubMed2003

Anterior and posterior attentional control systems use different spatial reference frames: ERP evidence from covert tactile-spatial orienting.

To investigate whether processes controlling preparatory covert shifts of spatial attention operate within external and anatomically defined spatial coordinates, lateralized event-related potentials components sensitive to the direction of attentional shifts were measured in response to visual precues directing attention to the relevant location of tactile events. Participants had to detect infrequent tactile targets delivered to the hand located on the cued side. In different blocks, hands were uncrossed or crossed, so that external and anatomical codes specifying task-relevant locations were either congruent or incongruent. With uncrossed hands, an anterior directing attention negativity and a posterior directing attention positivity were elicited in the cue-target interval contralateral to the side of a cued attentional shift. Although the posterior effect was unaffected by hand posture, the anterior effect was delayed and reversed polarity with crossed relative to uncrossed hands. This pattern of results provides new evidence that different spatial coordinate systems may be used by separable attentional control processes. It is suggested that a posterior process operates on the basis of external spatial coordinates, whereas an anterior process is based primarily on anatomically defined spatial codes.

Adult↗

PubMed2003

The role of spatial attention in the processing of facial expression: an ERP study of rapid brain responses to six basic emotions.

To investigate the time course of emotional expression processing, we recorded ERP responses to stimulus arrays containing neutral versus angry, disgusted, fearful, happy, sad, or surprised faces. In one half of the experiment, the task was to discriminate emotional and neutral facial expressions. Here, an enhanced early frontocentral positivity was elicited in response to emotional as opposed to neutral faces, followed by a broadly distributed positivity and an enhanced negativity at lateral posterior sites. These emotional expression effects were very similar for all six basic emotional expressions. In the other half of the experiment, attention was directed away from the faces toward a demanding perceptual discrimination task. Under these conditions, emotional expression effects were completely eliminated, demonstrating that brain processes involved in the detection and analysis of facial expression require focal attention. The face-specific N170 component was unaffected by any emotional expression, supporting the hypothesis that structural encoding and expression analysis are independent processes.

Adolescent↗

PubMed2002

An ERP study on the time course of emotional face processing.

Using event-related brain potentials (ERPs), we investigated the time course of facial expression processing in human subjects watching photographs of fearful and neutral faces. Upright fearful faces elicited a frontocentral positivity within 120 ms after stimulus presentation, which was followed by a broadly distributed sustained positivity beyond 250 ms post-stimulus. Emotional expression effects were delayed and attenuated when faces were inverted. In contrast, the face-specific N170 component was completely unaffected by facial expression. We conclude that emotional expression analysis and the structural encoding of faces are parallel processes. Early emotional ERP modulations may reflect the rapid activation of prefrontal areas involved in the analysis of facial expression.

Adolescent↗

PubMed2002

Cross-modal interactions between audition, touch, and vision in endogenous spatial attention: ERP evidence on preparatory states and sensory modulations.

Recent behavioral and event-related brain potential (ERP) studies have revealed cross-modal interactions in endogenous spatial attention between vision and audition, plus vision and touch. The present ERP study investigated whether these interactions reflect supramodal attentional control mechanisms, and whether similar cross-modal interactions also exist between audition and touch. Participants directed attention to the side indicated by a cue to detect infrequent auditory or tactile targets at the cued side. The relevant modality (audition or touch) was blocked. Attentional control processes were reflected in systematic ERP modulations elicited during cued shifts of attention. An anterior negativity contralateral to the cued side was followed by a contralateral positivity at posterior sites. These effects were similar whether the cue signaled which side was relevant for audition or for touch. They also resembled previously observed ERP modulations for shifts of visual attention, thus implicating supramodal mechanisms in the control of spatial attention. Following each cue, single auditory, tactile, or visual stimuli were presented at the cued or uncued side. Although stimuli in task-irrelevant modalities could be completely ignored, visual and auditory ERPs were nevertheless affected by spatial attention when touch was relevant, revealing cross-modal interactions. When audition was relevant, visual ERPs, but not tactile ERPs, were affected by spatial attention, indicating that touch can be decoupled from cross-modal attention when task-irrelevant.

Adult↗

PubMed2002

The electrophysiology of tactile extinction: ERP correlates of unconscious somatosensory processing.

We examined the electrophysiological correlates of left-sided tactile extinction in a patient with right-hemisphere damage. Computer-controlled punctate touch was presented to the left, right or both index fingers in an unpredictable sequence. The patient reported his conscious tactile percept ("left", "right" or "both"). He showed extinction on 75% of bilateral trials, reporting only right stimulation for these. Somatosensory evoked potentials for unilateral stimulation showed early components over contralateral somatosensory areas (P60 and N110) for either hand. In contrast to the results observed for age-matched controls, the patient's P60 was smaller in amplitude for left-hand touch over the right hemisphere than for right-hand touch over the intact hemisphere. Bilateral trials with extinction revealed residual P60 and N110 components over the right hemisphere in response to the extinguished left touch. These results demonstrate residual unconscious somatosensory processing of extinguished touch. They also suggest that tactile extinction can be caused by attenuation rather than elimination of somatosensory responses in the damaged hemisphere, with an underlying deficit even on unilateral trials.

Aged↗

PubMed2002

Crossmodal links in spatial attention are mediated by supramodal control processes: evidence from event-related potentials.

Crossmodal links in spatial attention were studied in an experiment where participants had to detect peripheral tactile or visual targets on the attended side, while ignoring all stimuli on the unattended side and in the currently irrelevant modality. Both relevant locations and relevant modalities were specified on a trial-by-trial basis by auditory precues. Spatial orienting in the cue-target interval was reflected in anterior negativities and occipital positivities contralateral to the cued side, either when vision or touch was cued as relevant. These effects resembled previously reported ERP modulations during shifts of visual attention, implicating supramodal mechanisms in the control of spatial attention and demonstrating their independence of cue modality. Early effects of spatial attention on somatosensory and visual ERPs were of equivalent size for currently relevant and irrelevant modalities. Results support the idea that crossmodal links in spatial attention are mediated by supramodal control mechanisms.

Adolescent↗

PubMed2002

Locus of inhibition in the masked priming of response alternatives.

Masked prime stimuli presented immediately before target stimuli in a choice reaction task give rise to behavioral costs when the primes and the target stimuli are mapped to the same response and result in benefits when they are mapped to opposite responses. Researchers assume that this negative compatibility effect reflects inhibitory processes in the control of perceptuomotor links. The authors investigated whether the inhibition operates at the level of abstract central codes or at effector-specific motor stages. In 2 experiments (N = 8 participants in each), left or right hand or foot responses were required to target stimuli that were preceded by masked arrow primes mapped to the same response side as the target stimuli in compatible trials and to the opposite response side in incompatible trials; the primes were irrelevant in neutral trials. In Experiment 1, when the masked primes determined both response side and modality, there was no transfer of negative compatibility effects across response modalities. That finding is inconsistent with a central abstract locus of inhibition and suggests that inhibition operates at effector-specific motor stages. In Experiment 2, primes conveyed only response side information but left response modality uncertain, and negative compatibility effects were elicited for both hand and foot responses, suggesting that partially informative masked primes can trigger a parallel activation and subsequent inhibition of response processes within separate effector systems.

Adult↗

PubMed2002

Temporal dynamics of lateralized ERP components elicited during endogenous attentional shifts to relevant tactile events.

To investigate the temporal dynamics of lateralized event-related brain potential (ERP) components elicited during covert shifts of spatial attention, ERPs were recorded in a task where central visual symbolic cues instructed participants to direct attention to their left or right hand in order to detect infrequent tactile targets presented to that hand, and to ignore tactile stimuli presented to the other hand, as well as all randomly intermingled peripheral visual stimuli. In different blocks, the stimulus onset asynchrony (SOA) between cue and target was 300 ms, 700 ms, or 1,100 ms. Anterior and posterior ERP modulations sensitive to the direction of an attentional shift were time-locked to the attentional cue, rather than to the anticipated arrival of a task-relevant stimulus. These components thus appear to reflect central attentional control rather than the anticipatory preparation of sensory areas. In addition, attentional modulations of ERPs to task-irrelevant visual stimuli were found, providing further evidence for crossmodal links in spatial attention between touch and vision.

Adult↗

PubMed2002

Motor activation with and without inhibition: evidence for a threshold mechanism in motor control.

Masked primes presented prior to a target can trigger response activation processes that may later be subject to inhibition. Evidence for response inhibition has previously been obtained with primes presented at fixation, but not with primes presented in the periphery of the visual field. It is argued that this central-peripheral asymmetry reflects a threshold mechanism in motor control. Foveal masked primes give rise to stronger perceptual representations than do peripheral primes, resulting in stronger response activations. Strong response activations are actively inhibited, whereas weaker activations remain below a hypothetical inhibition threshold. Evidence in favor of this hypothesis is obtained in four experiments that manipulated the perceptual strength of foveal and peripheral primes. Experiments 1 and 2 demonstrate that when the perceptual strength of peripheral primes is gradually increased by delaying mask onset, positive prime-target compatibility effects (reflecting the absence of response inhibition) turn into negative effects (indicating the presence of response inhibition). Experiments 3 and 4 demonstrate that when the perceptual strength of foveal primes is gradually decreased by degradation, negative compatibility effects turn into positive effects. The results are discussed in terms of a simple functional model of early motor control.

Adult↗