Attention to motion enhances processing of both visual and auditory stimuli: an event-related potential study

The present event-related potential (ERP) study investigated whether attending to a particular direction of motion similarly enhances the processing of auditory and visual stimuli. ERPs were recorded while participants perceived horizontally moving visual and auditory stimuli. Attention was manipulated by asking participants to detect an infrequent target stimulus that was of a specified modality (either visual or auditory) and that moved in a specified direction (either leftward or rightward). Stimuli moving in the attended direction elicited ERPs that were more negative than ERPs to stimuli moving in the unattended direction. This difference started around 140 ms post stimulus onset for visual and around 120 ms for auditory stimuli. The auditory effect had a frontal scalp topography, whereas the visual effect was distributed parieto-occipitally. Later parts of the difference waves were maximal at centro-parietal electrodes for both modalities. Crossmodal effects of attention to motion from one modality to the other could not be detected. The results are discussed with regard to hierarchical models of attention.     

Visual event-related potentials index focused attention within bilateral stimulus arrays. II. Functional dissociation of P1 and N1 components.

Event-related potentials (ERPs) were recorded from 12 subjects as they attended to the left or right hemifield of a visual display while fixating a central point. Stimuli were presented to the left or right visual fields on separate trials (unilateral stimuli) or to both fields simultaneously (bilateral stimuli). In different conditions, the stimulus sequences contained only bilateral stimuli, only unilateral stimuli, or a mixture of unilateral and bilateral stimuli. Bilateral stimuli elicited an enhanced positivity lasting from about 75 to 250 msec that was largest at posterior electrode sites contralateral to the attended hemifield. The early phase of this attention-related positivity appeared to be an enhancement of the exogenous P1 component. In contrast, both the posterior P1 and N1 components were enhanced in response to attended unilateral stimuli. Moreover, the N1 attention effect was reduced when the preceding stimulus contained elements in the attended field. It was concluded that modulations of the N1 and P1 components in these experiments represent different aspects of visual spatial attention: N1 may represent the orienting of attention to a task-relevant stimulus, whereas P1 may represent a facilitation of early sensory processing for items presented to a location where attention is already focused.     

Selective attention and multisensory integration: multiple phases of effects on the evoked brain activity

We used event-related potentials (ERPs) to evaluate the role of attention in the integration of visual and auditory features of multisensory objects. This was done by contrasting the ERPs to multisensory stimuli (AV) to the sum of the ERPs to the corresponding auditory-only (A) and visual-only (V) stimuli [i.e., AV vs. (A + V)]. V, A, and VA stimuli were presented in random order to the left and right hemispaces. Subjects attended to a designated side to detect infrequent target stimuli in either modality there. The focus of this report is on the ERPs to the standard (i.e., nontarget) stimuli. We used rapid variable stimulus onset asynchronies (350-650 msec) to mitigate anticipatory activity and included "no-stim" trials to estimate and remove ERP overlap from residual anticipatory processes and from adjacent stimuli in the sequence. Spatial attention effects on the processing of the unisensory stimuli consisted of a modulation of visual P1 and N1 components (at 90-130 msec and 160-200 msec, respectively) and of the auditory N1 and processing negativity (100-200 msec). Attended versus unattended multisensory ERPs elicited a combination of these effects. Multisensory integration effects consisted of an initial frontal positivity around 100 msec that was larger for attended stimuli. This was followed by three phases of centro-medially distributed effects of integration and/or attention beginning at around 160 msec, and peaking at 190 (scalp positivity), 250 (negativity), and 300-500 msec (positivity) after stimulus onset. These integration effects were larger in amplitude for attended than for unattended stimuli, providing neural evidence that attention can modulate multisensory-integration processes at multiple stages.     

Abnormal early stages of task stimulus processing in children with attention-deficit hyperactivity disorder--evidence from event-related gamma oscillations.

OBJECTIVES: Attention-related differences in early stages of stimulus processing were assessed in healthy controls and children with attention-deficit hyperactivity disorder (ADHD) by analyzing phase-locked gamma band (31-63 Hz) responses to auditory stimuli in a selective-attention task. METHODS: A total of 28 children aged 9-12 years (ADHD and matched healthy controls) pressed a button in response to each target stimulus presented at the attended side (right or left). Auditory gamma band responses (GBRs) within 0-120 ms were analyzed at 8 electrodes with wavelet transform. Effects of attended channel, stimulus type, and group were evaluated for GBR power and phase-locking. RESULTS: For both groups, GBRs had a frontal-central distribution, were significantly larger and more strongly phase-locked to target than to non-target stimuli, and did not differentiate the attended from the unattended channel. ADHD children produced larger and more strongly phase-locked GBRs than controls only to right-side stimuli, irrespective of whether these were the attended or the ignored stimuli. CONCLUSIONS: The association between auditory GBR and motor task stimulus in children suggests that phase-locked gamma oscillations may reflect processes of sensory-motor integration. ADHD-related deviations of GBRs indicate that early mechanisms of auditory stimulus processing are altered in ADHD, presumably as a result of impaired motor inhibition.     

Orienting and maintenance of spatial attention in audition and vision: an event-related brain potential study

We examined the effects of orienting and maintenance of attention on performance and event-related brain potentials (ERPs) in audition and vision. Our subjects selectively attended to sounds or pictures in one location (Maintenance of attention) or alternated the focus of their auditory or visual attention between left and right locations (Orienting of attention) in order to detect and press a response button to infrequent targets among the attended stimuli. Reaction times were longer in the Auditory Orienting condition and hit rates were lower and false alarm rates higher in the Visual Orienting condition than in the corresponding Maintenance conditions. Comparison of ERPs to the attended and unattended stimuli in the Auditory and Visual Orienting and Maintenance conditions revealed attention-related modulations of ERPs. In each modality, ERPs to attended stimuli were negatively displaced in relation to unattended stimuli at 100-250 ms from stimulus onset. These negative differences (Nds) showed modality-specific distributions and they were larger over the hemisphere contralateral to the attended sounds and pictures than over the ipsilateral hemisphere. Moreover, the Nd was larger in the Auditory Orienting condition than in the Auditory Maintenance condition, while no such difference was observed in the visual modality. In addition to the Nd, attended visual stimuli elicited a late positive response (LPR) in both Orienting and Maintenance conditions. In contrast to our recent functional magnetic resonance imaging (fMRI) study employing the same experimental paradigm and indicating orienting-related activity in the frontal and parietal cortices, no ERP responses specifically related to orienting were found in either modality.     

Spatial gradients of visual attention: behavioral and electrophysiological evidence

The spatial distribution of visual attention was investigated by measuring target detectability (d') and event-related brain potentials (ERPs) to stimuli at varying distances from an attended locus. Vertical bars were flashed rapidly in random order to 1 of 3 locations: one in each of the lateral visual fields and one on the vertical meridian above the fixation point. Subjects maintained eye fixation while directing their attention to 1 of the 3 locations for the duration of each 1.75 min run. Their primary task was to detect infrequent, shorter target bars at the attended location. A secondary task was to respond to shorter target bars at either of the 2 unattended locations if they 'happened to notice them' (without trying to detect them). ERPs and d' scores were obtained to the lateral field stimuli both when they were specifically attended (primary task), as well as when attention was focused upon midline or opposite-field flashes (secondary task). Both d' scores and the amplitudes of the P135 and N190 waves decreased progressively as attention was directed to locations increasingly distant from a given lateral stimulus. These results support 'gradient' models of the spatial distribution of visual attention.     

A modified oddball paradigm "cross-modal delayed response" and the research on mismatch negativity

A modified oddball paradigm was developed to facilitate the focus of attention and to minimize target effects on deviant-related components of auditory and visual event-related potentials (ERPs) elicited with long interstimulus intervals. Subjects were required to focus on either the visual or auditory stimulus in each stimulus block. Deviant-related components were obtained by subtracting ERPs of the standard stimulus from that of the deviant stimulus for each modality with each stimulus condition. Results showed that auditory mismatch negativity (MMN) and a visual early deviant related negativity (DRN1) were elicited both when stimuli were attended and unattended. In contrast, N2b and P3 were produced only under the attended condition. In comparison of attended MMN and unattended MMN at three time windows (100-150 ms, 150-200 ms, and 200-250 ms) of MMN zone, different scalp distributions were shown, depending on the time windows. This result suggests that the attended auditory MMN is a mixed wave, consisting of genuine MMN, N2b, and possible P165. The effect of attention on MMN may stem from the contamination of these overlapping components. With the present paradigm, at least three sensory memory traces have to be maintained simultaneously in multiple sensory modalities to support automatic processing.     

Neural correlates of pre-attentive and attentive processing of visual changes

To identify electrophysiological correlates of pre-attentive and attentive processing of visual changes, we compared event-related brain potentials in response to color changes at attended and unattended spatial locations using a visual S1-S2 matching task. The results showed that compared to no change, change stimuli elicited occipito-temporal positivity at around 100-160 ms (change-related positivity) and subsequent central negativity at around 220-300 ms (N270). Change-related positivity was observed in response to changes at both attended and unattended locations, while N270 was observed only when attention was directed to the location of the changes. These results suggest that change-related positivity reflects the pre-attentive processing of visual changes and N270 reflects the attentive processing of visual changes in the human brain.     

Is susceptibility to perceptual migration and fusion modality-specific or multimodal?

A previous paper reported high susceptibility to spatial migration (allochiria) of tactile stimuli in about 25% of healthy individuals (High Error subjects). When synchronous stimuli touched the two hands, if the unattended stimulus was temporally modulated when the attended one was not (and was thus more salient than the latter), it "migrated" to and fused with or replaced the stimulus on the attended hand. When subjects rated similarity of the attended stimulus accompanied by a distractor to each stimulus alone, scaling distributions tested against a sampling model showed most High Error subjects experienced fused stimuli, others experienced replacement and Low Error subjects experienced neither. We argued that these migrations are equivalent to allochiria and that this underlies neglect and extinction. This study assessed whether the individual difference is modality-specific or not. In auditory and visual equivalents of the tactile rating experiment, the difference between High and Low Error subjects was replicated in audition, but no migration occurred in vision. However, when two words were briefly presented visually before a mask with cued report of one, letter migrations to equivalent locations did occur and the individual difference was reproduced. This constitutes the first report of individual differences in auditory fusion and visual letter migration. Migration occurred in egocentric coordinates but apparently preserved structural homology. Different migration rates between the modalities paralleled relative salience of the unattended to the attended stimulus. The multimodality of the individual difference suggests that its source is supramodal, in deficient binding of perceptual content to location.     

Intra-modal and cross-modal spatial attention to auditory and visual stimuli. An event-related brain potential study.

This study investigated cross-modal interactions in spatial attention by means of recording event-related brain potentials (ERPs). Noise bursts and light flashes were presented in random order to both left and right field locations separated by 60 degrees in free-field. One group of subjects was instructed to attend selectively to the noise bursts (attend-auditory group), and a second group attended only to the flashes (attend-visual group). On different runs attention was directed to either the right or left field stimuli of the designated modality. In the attend-auditory group, noise bursts at the attended location elicited a broad, biphasic negativity (Nd) beginning at 70 ms. The cross-modal spatial attention effect on the auditory ERPs in the attend-visual group was very similar in morphology, but the Nd was reduced in amplitude relative to the intra-modal effect. In the attend-visual group, flashes at the attended location elicited enhanced early (100-200 ms) and late (200-350 ms) ERP components relative to unattended-location flashes. The cross-modal effect in the attend-auditory group included small but significant enhancements of early components of the visual ERPs. It was concluded that spatial attention has a multi-modal organization such that the processing of stimuli at attended locations is facilitated at an early, sensory level, even for stimuli of an unattended modality.     

Can attention be directed to opposite locations in different modalities? An ERP study.

OBJECTIVES: An event-related brain potential (ERP) study investigated whether spatially selective processing in vision and audition is controlled by a single supramodal system or by independent modality-specific systems. METHODS: Event-related brain potentials were recorded in response to visual and auditory stimuli at attended and unattended locations. In the 'Attend Same' condition, attention was directed to a single location in both modalities, while in the 'Attend Opposite' condition, visual and auditory attention had to be directed into opposite directions. RESULTS: Sensory-specific effects of attention on visual and auditory ERPs reflecting attentional modulations of perceptual processing were obtained in the 'Attend Same' condition, but not the 'Attend Opposite' condition. Beyond 200 ms post-stimulus, attentional ERP effects were also found in the 'Attend Opposite' condition. CONCLUSION: Results are inconsistent with the view that spatially selective processing is controlled by independent modality-specific systems. Effects of spatial attention on visual and auditory perceptual processing are closely linked, suggesting the existence of a supramodal attentional control system. At post-perceptual levels, attentional control may be more flexible.     

Stepping out of the spotlight: MMN attenuation as a function of distance from the attended location

In this report we present neurophysiological evidence that spatial separation between attended and unattended sound sources influences a listener's ability to register changes in sounds presented outside the focus of attention. Standard and deviant stimuli were presented at three azimuth locations. Participants were asked to press a key whenever they heard a deviant at a designated location. Mismatch negativity waves were generated for deviants at the attended location and were attenuated for deviants occurring 30 degrees away from the attended location. Mismatch negativities were not observed at distances of 60 degrees or more. The results are consistent with a spotlight model of auditory attention in which the processing of stimuli outside the attentional focus is attenuated as a function of increasing distance from the focus.     

Foveal and parafoveal spatial attention and its impact on the processing of facial expression: an ERP study

ObjectiveThe aim was to investigate the effects of spatial attention, stimulus location, and emotional expression on the processing of face stimuli.MethodsHealthy volunteers performed two sustained attention tasks, focusing their attention on either fixation (foveal location) or on a location several degrees above fixation (parafoveal location). Photographs of faces with either a neutral or a fearful expression were presented randomly at one of these locations. Event-related potentials were recorded in response to the face stimuli.ResultsSpatial attention modulated the early P1 and N1 components for both foveally and parafoveally presented stimuli. Emotional expression effects already occurred at the P1 stage in response to attended foveal stimuli, but only arose from 220 ms post-stimulus onwards in response to attended parafoveal stimuli. When faces were unattended, emotional expression effects were still present for parafoveal stimuli, whereas they were completely absent for foveal stimuli.ConclusionsThe modulations of the P1/N1 components by fearful faces are due to an early sensory modulation of extrastriate generators, and reflect an enhanced allocation of attention to threat-related stimuli.SignificanceFuture studies should take into account the possibility that spatial attention effects and effects of emotional expression processing are different for different positions in the visual field.     

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.     

Cross-modal selective attention to visual and auditory stimuli modulates endogenous ERP components

In two experiments event-related potentials (ERPs) to visual and auditory stimuli were measured in 12 healthy subjects. A cross-modal and delayed response paradigm was used that allows ERPs to be obtained separately to attended and unattended stimuli under conditions in which unattended stimuli are less likely to be covertly or randomly attended. The results showed: (1) N1 enhancement with attention for standard stimuli in auditory and visual modalities and for deviant stimuli in the visual modality; (2) The onset time and scalp distribution of both the N1 for attend condition and Nd1 were similar regardless of standard or deviant stimuli in the auditory and visual modality; the onset time of Nd1 elicited by auditory and visual deviant stimuli was earlier than that of the unattended N1, and their scalp distributions were different; and (3) The Nd1 components elicited by auditory and visual deviant stimuli were distributed over the respective primary sensory areas, but Nd1 components evoked by auditory and visual standard stimuli were distributed over the frontal scalp. These results suggest that the attended N1 enhancement is primarily caused by a component with endogenous origins and that the early attention effect occurs before the exogenous components. The results support the view that the cross-modal attention to deviant stimuli modulates modality-specific processing in the brain, whereas attention to standard stimuli affects modality-nonspecific or supramodal brain systems.     

Good times for multisensory integration: Effects of the precision of temporal synchrony as revealed by gamma-band oscillations

The synchronous occurrence of the unisensory components of a multisensory stimulus contributes to their successful merging into a coherent perceptual representation. Oscillatory gamma-band responses (GBRs, 30-80 Hz) have been linked to feature integration mechanisms and to multisensory processing, suggesting they may also be sensitive to the temporal alignment of multisensory stimulus components. Here we examined the effects on early oscillatory GBR brain activity of varying the precision of the temporal synchrony of the unisensory components of an audio-visual stimulus. Audio-visual stimuli were presented with stimulus onset asynchronies ranging from -125 to +125 ms. Randomized streams of auditory (A), visual (V), and audio-visual (AV) stimuli were presented centrally while subjects attended to either the auditory or visual modality to detect occasional targets. GBRs to auditory and visual components of multisensory AV stimuli were extracted for five subranges of asynchrony (e.g., A preceded by V by 100+/-25 ms, by 50+/-25 ms, etc.) and compared with GBRs to unisensory control stimuli. Robust multisensory interactions were observed in the early GBRs when the auditory and visual stimuli were presented with the closest synchrony. These effects were found over medial-frontal brain areas after 30-80 ms and over occipital brain areas after 60-120 ms. A second integration effect, possibly reflecting the perceptual separation of the two sensory inputs, was found over occipital areas when auditory inputs preceded visual by 100+/-25 ms. No significant interactions were observed for the other subranges of asynchrony. These results show that the precision of temporal synchrony can have an impact on early cross-modal interactions in human cortex.     

Selective visual-spatial attention alters induced gamma band responses in the human EEG.

OBJECTIVES: The present study was designed to investigate the attentional modulation of gamma band responses in a visual spatial attention task using a 128-channel-EEG-montage. METHODS: Colored rectangles were presented on a screen. After 500 ms an arrow indicated whether subjects had to shift their attention to the left or right half of the screen to detect target stimuli. During the task, either the attended half of the screen rotated horizontally while the unattended part remained motionless, or vice versa. RESULTS: When subjects attended the rotating stimulus, we found significantly higher power in a specific gamma band from 35-51 Hz on parieto-occipital electrode sites contralateral to the stimulation side. In addition, after the onset of the arrow which indicated what side subjects should direct their attention to, the 35-51 Hz response shifted from a broad posterior distribution to an increase of power at parieto-occipital sites contralateral to the to-be-attended side. Furthermore, the rotating stimulus elicited higher gamma band power as compared to the standing stimulus at electrode locations, which may be related to the activity of underlying cortical structures specialized for motion processing. CONCLUSIONS: The present results replicate important parts of previous findings of enhanced gamma power when a moving stimulus was attended.     

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.     

Attentional orienting towards emotion: P2 and N400 ERP effects

Attention can be oriented to different spatial locations yielding faster processing of attended compared to unattended stimuli. Similarly attention can be oriented to a semantic category such as “animals” or “tools”. Words from the attended category will also be recognized faster than words from an unattended category. Here, we asked whether it is possible to orient attention to an emotional category, for example, “negative emotional stimuli”. Furthermore, we investigated which mechanisms facilitate processing of attended stimuli. In an attentional orienting paradigm in which cues are informative with regard to the spatial location, semantic category, or emotional category of subsequent target words, we found attention effects in all three cue conditions. Words at attended locations or of the attended semantic or emotional category were responded to faster than unattended categories. While spatial attention acted upon early visual processing and amplified occipital N1-P2 potentials, semantic cues modulated the N400 amplitude indexing semantic processing. Emotional cues also yielded an N400 modulation; however, in addition, a left anterior P2 effect was observed. The data clearly show that attention can be oriented to emotional categories. Emotional orienting yields facilitated processing of an attended emotional category through the modulation of early and late word processing stages.     

Attentional load affects automatic emotional processing: evidence from event-related potentials

One open question on the relation between attention and emotion concerns the automatic processing of emotional visual stimuli outside the focus of attention. This study examined to what extent the emotional processing at unattended locations is modulated by the processing load at attended locations. Event-related potentials were measured to task-irrelevant unpleasant and neutral pictures briefly presented at peripheral locations while participants performed a visual central task varying in load (low and high load). Unpleasant pictures elicited larger amplitudes of N1-P2 at parietoccipital and occipital sites than that of neutral pictures. This effect was only significant in the low-load condition. Data suggest that brain responses to affective value of task-irrelevant peripheral pictures are modulated by attentional load at fixation.