Bettina Forster

Redundant target effect and intersensory facilitation from visual-tactile interactions in simple reaction time

In a simple reaction time (RT) task, normal observers responded faster to simultaneous visual and tactile stimuli than to single visual or tactile stimuli. RT to simultaneous visual and tactile stimuli was also faster than RT to simultaneous dual visual or tactile stimuli. The advantage for RT to combined visual-tactile stimuli over RT to the other types of stimulation could be accounted for by intersensory neural facilitation rather than by probability summation. The direction of gaze (and presumably of visual attention) to space regions near to or far from the site of tactile stimulation had no effect on tactile RT. However, RT to single or dual tactile stimuli was fastest when observers could see the sites of tactile stimulation on their hands both directly and through a mirror at the same time. All these effects can be ascribed to the convergence of tactile and visual inputs onto neural centers which contain flexible multimodal representations of body parts.

Covert manual response preparation triggers attentional shifts: ERP evidence for the premotor theory of attention

The premotor theory of attention claims that the preparation of goal-directed action and shifts of attention are closely linked, because they are controlled by shared sensorymotor mechanisms. Until now, support for this theory has come primarily from studies demonstrating links between saccade programming and attention shifts. The present event-related brain potential (ERP) study demonstrated that attentional orienting processes are also elicited during the covert preparation of unimanual responses. ERPs were recorded in the interval between a visual response-hand selection cue and a subsequent visual Go/Nogo signal when participants prepared to lift their left or right index finger. Lateralised ERP components elicited during response preparation were very similar to components previously observed during instructed endogenous attention shifts, indicating that analogous attentional orienting processes are activated in both cases. Somatosensory ERP components (P90, N140) were enhanced when task-irrelevant tactile probes were delivered during response preparation to the hand involved in an anticipated response, even when probes were presented well in advance of response execution. These results suggest that attentional shifts are triggered during unimanual response preparation, as predicted by the premotor theory. This link between manual response programming and attention is consistent with the hypothesis that common mechanisms are involved in the control of attention and action.

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.

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.

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.

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.

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.

An erp investigation on visuotactile interactions in peripersonal and extrapersonal space: Evidence for the spatial rule

The spatial rule of multisensory integration holds that cross-modal stimuli presented from the same spatial location result in enhanced multisensory integration. The present study investigated whether processing within the somatosensory cortex reflects the strength of cross-modal visuotactile interactions depending on the spatial relationship between visual and tactile stimuli. Visual stimuli were task-irrelevant and were presented simultaneously with touch in peripersonal and extrapersonal space, in the same or opposite hemispace with respect to the tactile stimuli. Participants directed their attention to one of their hands to detect infrequent tactile target stimuli at that hand while ignoring tactile targets at the unattended hand, all tactile nontarget stimuli, and any visual stimuli. Enhancement of ERPs recorded over and close to the somatosensory cortex was present as early as 100 msec after onset of stimuli (i.e., overlapping with the P100 component) when visual stimuli were presented next to the site of tactile stimulation (i.e., perihand space) compared to when these were presented at different locations in peripersonal or extrapersonal space. Therefore, this study provides electrophysiological support for the spatial rule of visual–tactile interaction in human participants. Importantly, these early cross-modal spatial effects occurred regardless of the locus of attention. In addition, and in line with previous research, we found attentional modulations of somatosensory processing only to be present in the time range of the N140 component and for longer latencies with an enhanced negativity for tactile stimuli at attended compared to unattended locations. Taken together, the pattern of the results from this study suggests that visuotactile spatial effects on somatosensory processing occur prior and independent of tactile–spatial attention.

Temporal discrimination of cross-modal and unimodal stimuli in generalized dystonia.

Background: Motor and nonmotor timing functions and cross-modal processing of visual–tactile signals may be linked to basal ganglia. These neural structures are thought to be dysfunctional in dystonia. Objective: To test whether cross-modal stimulation influences deficits of temporal discrimination in dystonia. Methods: Eight patients with generalized dystonia and 10 control subjects were asked to discriminate whether pairs of unimodal (tactile or visual) and cross-modal (visual and tactile) stimuli were simultaneous or sequential and, in the latter case, which stimulus preceded the other. Visual stimuli consisted of red lights and tactile stimuli of non-noxious electrical shocks. Intervals between stimuli in each pair were increased from 0 to 400 msec (in steps of 10 msec). Results: Patients with dystonia recognized the asynchrony between the experimental stimuli and judged correctly which stimulus in a pair came first, at significantly longer intervals than did controls. Moreover, differences in performance between patients and controls were maximal for cross-modal stimuli. The defective performance of patients with dystonia in the cross-modal combinations showed a high positive correlation with the severity of symptoms. Conclusion: Patients with generalized dystonia present with difficulties both in timing functions and in cross-modal processing of visual–tactile stimuli.

Vision and gaze direction modulate tactile processing in somatosensory cortex: evidence from event-related brain potentials

Several behavioural studies have shown that directing one’s gaze at a body part reduces detection speed and enhances discrimination of tactile stimuli at that location. We investigated how vision of a body part stimulated and manipulations of gaze direction affect tactile processing. Participants’ gaze was directed to one of their hands, with vision of this hand either available or prevented in different experiments. They had to detect infrequent tactile targets among non-targets. Somatosensory event-related brain potentials were recorded in response to stimulation of the hand towards which gaze was directed (G+ trials) and in response to stimulation of the other hand (G– trials). When vision (V+) of the hand gaze was directed at was available (G+V+), an early positivity overlapping with the P45 and N80 component was observed for G+V+ trials relative to G–V– trials. In contrast, when the hands were occluded from view (V–), an enhanced N140 component followed by a late negativity was observed for G+V– as compared to G–V– trials. It is suggested that vision of the body part stimulated can modulate processing in primary somatosensory cortex (S1), while effects of gaze direction in the absence of vision of the body part touched are located in higher order somatosensory areas. Such effects of vision and gaze on tactile processing may be mediated by pathways from multimodal brain regions to somatosensory cortex.