Covert unimanual response preparation triggers attention shifts to effectors rather than goal locations

The premotor theory of attention postulates that during response preparation, attention shifts are elicited towards the goal of a prepared movement. Support for this claim comes from research demonstrating enhanced performance at the location of upcoming saccades. To investigate whether attention shifts occur towards effectors or goal locations during the covert preparation of unimanual movements, we recorded event related brain potentials (ERPs) to task-irrelevant tactile probes that were presented while participants prepared to move one hand towards the index finger of the other hand, as directed by visual response cues presented at the start of each trial. These cues specified either the effector or the goal location of an upcoming movement. The somatosensory N140 component was enhanced when probes were presented to the effector hand relative to the goal hand, regardless of cue instructions. Analogous modulations of the N80 component were only present with effector cues. These results demonstrate a close link between covert response preparation and attention shifts, and strongly suggest that attention shifts are directed to the effector, and not to the goal location of manual movements.     

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.     

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.     

Maintenance of tactile short-term memory for locations is mediated by spatial attention

According to the attention-based rehearsal hypothesis, maintenance of spatial information is mediated by covert orienting towards memorized locations. In a somatosensory memory task, participants simultaneously received bilateral pairs of mechanical sample pulses. For each hand, sample stimuli were randomly assigned to one of three locations (fingers). A subsequent visual retro-cue determined whether the left or right hand sample was to be memorized. The retro-cue elicited lateralized activity reflecting the location of the relevant sample stimulus. Sensory processing during the retention period was probed by task-irrelevant pulses randomized to locations at the cued and uncued hand. The somatosensory N140 was enhanced for probes delivered to the cued hand, relative to uncued. Probes presented shortly after the retro-cue showed greatest attentional modulations. This suggests that transient contributions from retrospective selection overlapped with the sustained effect of attention-based rehearsal. In conclusion, focal attention shifts within tactile mnemonic content occurred after retro-cues and guided sensory processing during retention.     

An event-related brain potential study of cross-modal links in spatial attention between vision and touch

Event-related potential (ERP) evidence for the existence of cross-modal links in endogenous spatial attention between vision and touch was obtained in an experiment where participants had to detect tactile or visual targets on the attended side and to ignore the irrelevant modality and stimuli on the unattended side. For visual ERPs, attentional modulations of occipital P1 and N1 components were present when attention was directed both within vision and within touch, indicating that links in spatial attention from touch to vision can affect early stages of visual processing. For somatosensory ERPs, attentional negativities starting around 140 ms poststimulus were present at midline and lateral central electrodes when touch was relevant. No attentional somatosensory ERP modulations were present when vision was relevant and tactile stimuli could be entirely ignored. However, in another task condition where responses were also required to infrequent tactile targets regardless of their location, visual-spatial attention modulated somatosensory ERPs. Unlike vision, touch apparently can be decoupled from attentional orienting within another modality unless it is potentially relevant.     

Adverse effects of viewing the hand on tactile-spatial selection between fingers depend on finger posture

Primary somatosensory cortex (S1) is known to rapidly adapt to task demands and to intermodal information (e.g. from vision). Here, we show that also intramodal information (i.e. posture) can affect tactile attentional selection processes and the intermodal effects of vision on those processes at S1 stages of processing. We manipulated the spatial separation between adjacent fingers, that is, thumb and index finger where close, far apart, or touching. Participants directed their attention to either the index finger or thumb to detect infrequent tactile targets at that location while either they saw their fingers or these were covered from view. In line with the previous results, we found that attentional selection affected early somatosensory processing (P45, N80) when fingers were near and this attention effect was abolished when fingers were viewed. When fingers were far or touching, attentional modulations appeared reliably only from the P100, and furthermore, enhanced tactile-spatial selection was found when touching fingers were viewed. Taken together, these results show for the first time a profound effect of finger posture on attentional selection between fingers and its modulations by vision at early cortical stages of processing. They suggest that the adverse effects of vision on tactile attention are not driven by a conflict between the selected information in vision (two fingers) and touch (one finger) and imply that external spatial information (i.e. finger posture) rapidly affects the organisation of primary somatosensory finger representations and that this further affects vision and tactile-spatial selection effects on S1.     

Vision enhances selective attention to body-related information

Viewing ones’ own hands while directing attention to one of the hands leads to earlier attentional modulations of somatosensory processing than when hands are not visible. This effect of vision on tactile-spatial selection could be explained by vision providing additional information about the location of the hands in external space. The present study investigated whether vision of the hands also affected tactile-attentional mechanisms when the relative locations of the hands were irrelevant. Participants silently counted infrequent tactile or auditory deviants in an alternating stream of tactile and auditory stimuli while ignoring stimuli in the other modality, when their hands were either visible or covered from view. Modality-selective attentional modulations of ERPs to tactile stimuli (when touches vs. tones were attended) were already present for the time range of the N80 component when hands were visible, but there were only later modulations (starting at N140) when hands were covered. This suggests that, rather than being restricted to tasks requiring spatial selection between body parts, vision of the hands can facilitate attention toward the body in far more general terms. In contrast to tactile stimuli, attentional modulations of ERPs to auditory stimuli (when tones vs. touches were attended) were not reliably affected by viewing the hands. This suggests that the primary purpose of visual facilitation may be to enhance the processing of body-related information only.     

Covert attention in touch: behavioral and ERP evidence for costs and benefits

To investigate the mechanism underlying tactile spatial attention, reaction times (RTs) and event-related potentials (ERPs) were recorded in response to mechanical stimuli delivered to the hands. At the start of each trial cues indicated either the correct (valid) or incorrect (invalid) tactile stimulus location or were uninformative (neutral). RT costs (suppression of invalid compared to neutral trials) were found to be larger than benefits (enhancement of valid compared to neutral trials). ERPs showed that costs and benefits contribute equally to attentional modulations of the somatosensory N140 component, whereas these were largely due to costs at longer latencies. These results differ from the pattern of attentional ERP modulations previously found for vision and audition, where costs precede benefits, and therefore suggest that the mechanisms of attentional selectivity in touch might be different from attentional processes in other modalities.     

ERP investigation of transient attentional selection of single and multiple locations within touch

Mechanisms underlying pure tactile attentional selection were investigated. Tactile imperative stimuli were preceded by symbolic tactile cues directing attention to the left or right (directional cues), or to both hands (non-directional cues). Comparison of ERP waveforms on directional and non-directional cue trials showed that attentional modulations at N140 and P200 components reflect mainly enhancement of stimuli at the attended, while longer latency modulations reflect mainly suppression of processing of stimuli at the unattended location. This pattern of results differs from analogous studies involving other modalities suggesting that different mechanisms underlie pure tactile attention. Furthermore, ERP waveforms on non-directional cue trials were enhanced in comparison to directional cue trials at the P100 component and at longer latencies, indicating that tactile attentional mechanisms may differ when attending to one compared to multiple locations.     

Do ERP components triggered during attentional orienting represent supramodal attentional control?

Lateralized ERP components triggered during cued shifts of spatial attention (anterior directing attention negativity [ADAN], late directing attention positivity [LDAP]) have been observed during visual, auditory, and tactile attention tasks, suggesting that these components reflect supramodal attentional control processes. This interpretation has recently been called into question by the finding that the ADAN is absent in response to auditory attention cues. Here we demonstrate that ADAN and LDAP components are reliably elicited in a purely unimodal auditory attention task where auditory cues are followed by auditory imperative stimuli. The fact that the ADAN is not restricted to task contexts where visual or tactile stimuli are relevant is consistent with the hypothesis that this component is linked to supramodal attentional control.     

The attentional selection of spatial and non-spatial attributes in touch: ERP evidence for parallel and independent processes

To investigate the functional relationship between spatial and non-spatial attentional selectivity in somatosensory processing, event-related potentials (ERPs) were recorded to mechanical tactile stimuli, which were delivered to the right or left hand, and were low or high in frequency (Experiment 1), or soft or strong in intensity (Experiment 2). Participants' task was to attend to a specific combination of one stimulus location and one non-spatial attribute. Spatial attention was reflected in enhanced N140 components followed by a sustained attentional negativity. ERP effects of non-spatial attention (enhanced negativities to the attended frequency or intensity) were observed in the same latency range, suggesting that the attentional selection of relevant spatial and non-spatial attributes occurs in parallel. Most importantly, ERP correlates of attention directed to stimulus frequency and intensity were unaffected by the current focus of spatial attention. In contrast to vision, where the selective processing of non-spatial attributes is hierarchically dependent on selection by location, but similar to auditory attention, spatial and non-spatial attentional selectivity appear to operate independently in touch.     

ERP correlates of tactile spatial attention differ under intra- and intermodal conditions

To investigate whether the mechanisms underlying endogenous tactile spatial attention differ under pure tactile compared to mixed modality conditions event-related brain potentials (ERPs) were recorded to bilateral tactile and visual cues and tactile imperative stimuli. In the cue-stimulus interval the anterior directing attention negativity (ADAN) was present contralateral to the side of the attentional shift. Importantly, under pure tactile conditions this component persisted until imperative stimulus onset, while it diminished under intermodal conditions. Furthermore, post-tactile stimulus onset attentional modulations were present for the P100 component and later latencies under intermodal conditions. In contrast, under pure tactile conditions attentional modulations only emerged for the N140 component and later latencies. It is suggested that mechanisms underlying attentional orienting and selection are not entirely supramodal but depend in part on the modalities involved.     

ERP correlates of tactile spatial attention differ under intra- and intermodal conditions

To investigate whether the mechanisms underlying endogenous tactile spatial attention differ under pure tactile compared to mixed modality conditions event-related brain potentials (ERPs) were recorded to bilateral tactile and visual cues and tactile imperative stimuli. In the cue-stimulus interval the anterior directing attention negativity (ADAN) was present contralateral to the side of the attentional shift. Importantly, under pure tactile conditions this component persisted until imperative stimulus onset, while it diminished under intermodal conditions. Furthermore, post-tactile stimulus onset attentional modulations were present for the P100 component and later latencies under intermodal conditions. In contrast, under pure tactile conditions attentional modulations only emerged for the N140 component and later latencies. It is suggested that mechanisms underlying attentional orienting and selection are not entirely supramodal but depend in part on the modalities involved.     

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.     

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.     

Explicit eye movements failed to facilitate the precision of subsequent attentional localization

This study investigated the usefulness of explicit spatial coordinates from eye movements for the precision of covert shifts of attention within dense arrays of items. Observers shifted their attention covertly from one item to the next in response to a series of beeps and reported the color of the disc on which the series ended, providing an estimate of the accuracy of the “attentional walk”. We compared performance in this task when only covert shifts of attention were done to performance when observers first executed an explicit eye movement to the starting point of the attentional walk before beginning the covert attentional walk. The hypothesis was that the eye movement would activate explicit coordinates of the starting point of the attentional walk within brain systems that are involved in controlling both shifts of attention and eye movements. This in turn would provide an anchor for the attentional walk, thereby improving performance. The evidence did not support this hypothesis. Performance was no better with an explicit eye movement prior to the attentional walk than without one. This suggests that covert orienting—shifting attention—and overt orienting—shifting the eyes—access the same coordinate system and therefore activating new coordinates interferes with the old ones, no matter what the system of orienting is.     

Preparatory states in crossmodal spatial attention: spatial specificity and possible control mechanisms

We used event-related functional magnetic resonance imaging to study the neural correlates of endogenous spatial attention for vision and touch. We examined activity associated with attention-directing cues (central auditory pure tones), symbolically instructing subjects to attend to one hemifield or the other prior to upcoming stimuli, for a visual or tactile task. In different sessions, subjects discriminated either visual or tactile stimuli at the covertly attended side, during bilateral visuotactile stimulation. To distinguish cue-related preparatory activity from any modulation of stimulus processing, unpredictably on some trials only the auditory cue was presented. The use of attend-vision and attend-touch blocks revealed whether preparatory attentional effects were modality-specific or multimodal. Unimodal effects of spatial attention were found in somatosensory cortex for attention to touch, and in occipital areas for attention to vision, both contralateral to the attended side. Multimodal spatial effects (i.e. effects of attended side irrespective of task-relevant modality) were detected in contralateral intraparietal sulcus, traditionally considered a multimodal brain region; and also in the middle occipital gyrus, an area traditionally considered purely visual. Critically, all these activations were observed even on cue-only trials, when no visual or tactile stimuli were subsequently presented. Endogenous shifts of spatial attention result in changes of brain activity prior to the presentation of target stimulation (baseline shifts). Here, we show for the first time the separable multimodal and unimodal components of such preparatory activations. Additionally, irrespective of the attended side and modality, attention-directing auditory cues activated a network of superior frontal and parietal association areas that may play a role in voluntary control of spatial attention for both vision and touch. Electronic Publication     

Cross-modal links in endogenous spatial attention are mediated by common external locations: evidence from event-related brain potentials

Recent behavioural and event-related potential (ERP) studies reported cross-modal links in spatial attention between vision, audition and touch. Such links could reflect differences in hemispheric-activation levels associated with spatial attention to one side, or more abstract spatial reference-frames mediating selectivity across modalities. To distinguish these hypotheses, ERPs were recorded to lateral tactile stimuli, plus visual (experiment 1) or auditory stimuli (experiment 2), while participants attended to the left or right hand to detect infrequent tactile targets, and ignored other modalities. In separate blocks, hands were either in a crossed or uncrossed posture. With uncrossed hands, visual stimuli on the tactually attended side elicited enhanced N1 and P2 components at occipital sites, and an enhanced negativity at midline electrodes, reflecting cross-modal links in spatial attention from touch to vision. Auditory stimuli at tactually attended locations elicited an enhanced negativity overlapping with the N1 component, reflecting cross-modal links from touch to audition. An analogous pattern of results arose for crossed hands, with tactile attention enhancing auditory or visual responses on the side where the attended hand now lay (i.e. in the opposite visual or auditory hemifield to that enhanced by attending the same hand when uncrossed). This suggests that cross-modal attentional links are not determined by hemispheric projections, but by common external locations. Unexpectedly, somatosensory ERPs were strongly affected by hand posture in both experiments, with attentional effects delayed and smaller for crossed hands. This may reflect the combined influence of anatomical and external spatial codes within the tactile modality, while cross-modal links depend only on the latter codes. Electronic Publication     

Spatial tuning of tactile attention modulates visual processing within hemifields: an ERP investigation of crossmodal attention

Recent event-related brain potential (ERP) studies have revealed crossmodal links in spatial attention, but have not yet investigated differences in the spatial tuning of attention between task-relevant and irrelevant modalities. We studied the spatial distribution of attention in vision under conditions where participants were instructed to attend to the left or right-hand in order to detect infrequent targets, and to entirely ignore visual stimuli presented via LEDs at two eccentricities in the left or right hemifield. Hands were located close to two of these four LEDs in different blocks. Visual N1 amplitudes were enhanced when visual stimuli in the cued hemifield were close to the attended hand, relative to visual stimuli presented at the other location on the same side. These within-hemifield attentional modulations of visual processing demonstrate that crossmodal attention is not distributed diffusely across an entire hemifield. The spatial tuning of tactile attention transfers crossmodally to affect vision, consistent with spatial selection at a multimodal level of representation.     

Perceptual and decisional attenuation of tactile perception during the preparation of self- versus externally-generated movements

We investigated tactile perception during the execution of self- versus externally-generated movements. In a first experiment, we established the temporal characteristics of the movements of interest. In a second experiment, participants had to try to detect a short gap in an otherwise continuous vibratory stimulus delivered to their right wrist under conditions of rest, throwing (i.e., self-initiated movement), or catching a basketball (i.e., externally-generated movement). Our hypothesis was that different patterns of tactile sensitivity (d′) and response bias (criteria c and c′) would be observed as a function of the timing of gap delivery (i.e., during movement preparation or movement execution) and the type of movement (self- or externally-generated). A third experiment investigated tactile perception at rest while participants adopted different hand postures. This experiment also tested the simple preparation of the self-/externally-generated movements versus the observation of these targeted movements as performed by the experimenter. Due to sensory suppression, participants were significantly less sensitive in detecting the gap in tactile stimulation while executing the movement. Preparing to catch the ball only triggered a shift in response bias (i.e., participants were more liberal/conservative when reporting the gap in stimulation), but no change in perceptual sensitivity was observed, as compared to rest. Preparing to make a ball-throwing movement resulted in a significant decrement in tactile sensitivity, as well as a shift in participants’ criterion toward their being more conservative, when responding to the presence of the target. Similar decrements were observed for the observation of self-initiated movement preparation, but not for the observation of their externally-generated counterparts. Taken together, these results demonstrate that different forms of attenuation influence tactile perception, depending on the type of movement that is executed: perceptual and decisional attenuation for self-initiated movements, but only decisional attenuation for externally-generated movements. These results suggest that the movement preparation sensorimotor contingencies are already modulated in prefrontal decision-related cortical brain areas.