Concurrent recording of steady-state and transient event-related potentials as indices of visual-spatial selective attention.

OBJECTIVES: The present study investigated the effects of spatial attention on concurrently recorded visual event-related potentials (ERPs) and steady-state visual evoked potentials (SSVEPs) to isoluminant color changes embedded in rapidly flickering stimuli. METHODS: EEG was recorded while subjects attended to flickering LEDs in either the right or left visual hemifield and responded by a button press to isoluminant color changes (targets). RESULTS: Target isoluminant color changes elicited the typical ERP components P1, N1, N2 and P3, which were enhanced for attended targets compared to unattended targets. Consistent with previous findings, SSVEP amplitude was enlarged for attended flicker stimuli at posterior electrode sites contralateral to the attended visual hemifield. In addition, significant correlations were found between the N1, N2 and the SSVEP attention effects, whereas no such correlations were found between the P1, P3 and SSVEP attention effects. CONCLUSIONS: Results suggest that the SSVEP and ERP reflect partially overlapping attentional mechanisms that facilitate the discriminative processing of stimuli at attended locations.     

A shift of visual spatial attention is selectively associated with human EEG alpha activity

Event-related potentials and ongoing oscillatory electroencephalogram (EEG) activity were measured while subjects performed a cued visual spatial attention task. They were instructed to shift their attention to either the left or right visual hemifield according to a cue, which could be valid or invalid. Thereafter, a peripheral target had to be evaluated. At posterior parietal brain areas early components of the event-related potential (P1 and N1) were higher when the cue had been valid compared with invalid. An anticipatory attention effect was found in EEG alpha magnitude at parieto-occipital electrode sites. Starting 200 ms before target onset alpha amplitudes were significantly stronger suppressed at sites contralateral to the attended visual hemifield than ipsilateral to it. In addition, phase coupling between prefrontal and posterior parietal electrode sites was calculated. It was found that prefrontal cortex shows stronger phase coupling with posterior sites that are contralateral to the attended hemifield than ipsilateral sites. The results suggest that a shift of attention selectively modulates excitability of the contralateral posterior parietal cortex and that this posterior modulation of alpha activity is controlled by prefrontal regions.     

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.     

Visual event-related potentials index focused attention within bilateral stimulus arrays. I. Evidence for early selection.

Event-related potentials (ERPs) were recorded from the scalp while subjects attended to sequences of bilaterally symmetrical arrays of 4 letters (2 in each visual half-field) that were flashed briefly at intervals of 280-520 msec. These sequences also included randomized presentations of unilateral 'probe' stimuli consisting of irrelevant bars (experiment 1) or potentially relevant letter pairs (experiment 2). The task was to pay attention to the letter pairs in either the left or the right half-field on a given run and to press a button when the two letters matched one another (targets). The ERPs to the bilateral arrays included an early positive wave (P1, peaking at 135 msec) that was enhanced over posterior scalp sites contralateral to the attended visual field. Both types of probe stimulus also elicited a larger early positivity in the P1 latency range (100-200 msec) when delivered to the attended half-field, followed in some cases by a more prolonged positive deflection. Notable for its absence was any sign of an enlarged posterior N1 component (160-200 msec), which was prominent in the ERP to attended-field stimuli in previous studies using randomized sequences of unilateral stimuli. Attended-field targets elicited large N2 and P3 (P300) components, which were greatly reduced or absent when targets occurred in the unattended field. The observed ERP effects were interpreted in terms of early sensory selection during visual spatial attention.     

The location of preceding stimuli affects selective processing in a sustained attention situation

Event-related potentials (ERPs) were recorded to visual and auditory stimuli in a situation where subjects were required to attend selectively to the left or right side for an entire experimental block and to detect occasional target stimuli at attended locations. Stimuli were presented randomly at attended and unattended locations. In exp. 1, visual and auditory stimuli were presented in separate blocks, while in exp. 2, they were presented together and subjects had to detect visual targets at attended locations. Stimuli at attended positions elicited enlarged sensory-evoked potentials and an enhanced negativity at midline electrodes as compared with unattended stimuli. The latter effect was, however, modulated by the location of the preceding stimulus. At frontocentral electrodes, it was larger for stimuli that were preceded by stimuli at the contralateral side as compared with stimuli preceded by stimuli at the same location. It is argued that this effect may be due to a different amount of processing required for the preceding stimulus. When the predecessor is at a to-be-attended location, it has to be processed more intensively which may interfere with the processing of the next stimulus.     

Emotionally arousing stimuli compete for attention with left hemispace

Rapid interaction of the emotional and attentional networks is critical for adaptive behavior. Here, we examined the effects of emotional stimulation on hemifield attention allocation using event-related potential and behavioral measures. Participants performed a visual-discrimination task on nonemotional targets presented randomly in the left or right hemifield. A brief task-irrelevant emotional (pleasant or unpleasant; 150-ms duration) or neutral picture was presented centrally 350 ms before the next target (150-ms duration). Unpleasant stimuli interfered with the left visual field attention capacity, slowing behavioral responses to attended left field stimuli. In keeping with the behavioral data, event-related potential responses to nonemotional attended left field stimuli were reduced over the right parietal regions when preceded by an unpleasant event. The results provide electrophysiological and behavioral evidence that unpleasant, emotionally arousing stimuli interfere with the right hemisphere-dependent attention capacity.     

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.     

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.     

Oscillatory activity in parietal and dorsolateral prefrontal cortex during retention in visual short‐term memory: Additive effects of spatial attention and memory load

We used whole‐head magnetoencephalography to study the representation of objects in visual short‐term memory (VSTM) in the human brain. Subjects remembered the location and color of either two or four colored disks that were encoded from the left or right visual field (equal number of distractors in the other visual hemifield). The data were analyzed using time‐frequency methods, which enabled us to discover a strong oscillatory activity in the 8–15 Hz band during the retention interval. The study of the alpha power variation revealed two types of responses, in different brain regions. The first was a decrease in alpha power in parietal cortex, contralateral to the stimuli, with no load effect. The second was an increase of alpha power in parietal and lateral prefrontal cortex, as memory load increased, but without interaction with the hemifield of the encoded stimuli. The absence of interaction between side of encoded stimuli and memory load suggests that these effects reflect distinct underlying mechanisms. A novel method to localize the neural generators of load‐related oscillatory activity was devised, using cortically‐constrained distributed source‐localization methods. Some activations were found in the inferior intraparietal sulcus (IPS) and intraoccipital sulcus (IOS). Importantly, strong oscillatory activity was also found in dorsolateral prefrontal cortex (DLPFC). Alpha oscillatory activity in DLPFC was synchronized with the activity in parietal regions, suggesting that VSTM functions in the human brain may be implemented via a network that includes bilateral DLPFC and bilateral IOS/IPS as key nodes. Hum Brain Mapp, 2009. © 2009 Wiley‐Liss, Inc.     

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.     

Luminance and spatial attention effects on early visual processing

Event-related potentials (ERPs) were recorded from healthy subjects in response to unilaterally flashed high and low luminance bar stimuli presented randomly to left and right field locations. Their task was to covertly and selectively attend to either the left or right stimulus locations (separate blocks) in order to detect infrequent shorter target bars of either luminance. Independent of attention, higher stimulus luminance resulted in higher ERP amplitudes for the posterior N95 (80–110 ms), occipital P1 (110–140 ms), and parietal N1 (130–180 ms). Brighter stimuli also resulted in shorter peak latency for the occipital N1 component (135–220 ms); this effect was not observed for the N1 components over parietal, central or frontal regions. Significant attention-related amplitude modulations were obtained for the occipital P1, occipital, parietal and central N1, the occipital and parietal P2, and the parietal N2 components; these components were larger to stimuli at the attended location. In contrast to the relatively short latencies of both spatial attention and luminance effects, the first interaction between luminance and spatial attention effects was observed for the P3 component to the target stimuli (350–750 ms). This suggests that interactions of spatial attention and stimulus luminance previously reported for reaction time measures may not reflect the earliest stages of sensory/perceptual processing. Differences in the way in which luminance and attention affected the occipital P1, occipital N1 and parietal N1 components suggest dissociations among these ERPs in the mechanisms of visual and attentional processing they reflect. Nonetheless, scalp current density mappings of the attention effects throughout the latency ranges of the P1 and N1 components show the most prominent attention-related activity to be in lateral occipital scalp areas. Such a pattern is consistent with the spatially selective filtering of information into the ventral stream of visual processing which is reponsible for complex feature analysis and object identification.     

Age-related changes in the attentional control of visual cortex: a selective problem in the left visual hemifield

To what extent does our visual-spatial attention change with age? In this regard, it has been previously reported that relative to young controls, seniors show delays in attention-related sensory facilitation. Given this finding, our study was designed to examine two key questions regarding age-related changes in the effect of spatial attention on sensory-evoked responses in visual cortex--are there visual field differences in the age-related impairments in sensory processing, and do these impairments co-occur with changes in the executive control signals associated with visual spatial orienting? Therefore, our study examined both attentional control and attentional facilitation in seniors (aged 66-74 years) and young adults (aged 18-25 years) using a canonical spatial orienting task. Participants responded to attended and unattended peripheral targets while we recorded event-related potentials (ERPs) to both targets and attention-directing spatial cues. We found that not only were sensory-evoked responses delayed in seniors specifically for unattended events in the left visual field as measured via latency shifts in the lateral occipital P1 elicited by visual targets, but seniors also showed amplitude reductions in the anterior directing attentional negativity (ADAN) component elicited by cues directing attention to the left visual field. At the same time, seniors also had significantly higher error rates for targets presented in the left vs. right visual field. Taken together, our data thus converge on the conclusion that age-related changes in visual spatial attention involve both sensory-level and executive attentional control processes, and that these effects appear to be strongly associated with the left visual field.     

Interactions between spatial attention and global/local feature selection: an ERP study

The present study examined the interaction between spatial attention and global/local feature processing of visual hierarchical stimuli. Event-related brain potentials (ERPs) were recorded from subjects who detected global or local targets at attended locations while ignoring those at unattended locations. Spatial attention produced enhanced occipital P1 and N1 waves in both global and local conditions. Selection of local features enhanced posterior P1, N1 and N2 waves relative to selection of global features. However, the modulations of the P1 and N2 by global/local feature selection were stronger when spatial attention was directed to the left than the right visual fields. The results suggest neurophysiological bases for interactions between spatial attention and hierarchical analysis at multiple stages of visual processing.     

Attention to central and peripheral visual space in a movement detection task: an event-related potential and behavioral study. I. Normal hearing adults

The effects of focussed attention to peripherally and centrally located visual stimuli were compared via an analysis of event-related brain potentials (ERPs) while subjects detected the direction of motion of a white square in a specified location. While attention to both peripheral and foveal stimuli produced enhancements of the early ERP components, the distribution over the scalp of the attention-related changes varied according to stimulus location. The attention-related increase in the amplitude of the N1 wave (157 ms) to the peripheral stimuli was greater over the parietal region of the hemisphere contralateral to the attended visual field. By contrast, the largest effects of foveally directed attention occurred over the occipital regions where the increase was bilaterally symmetrical. Additionally, the effects of attention on the ERPs were significantly larger for moving than for stationary stimuli, and this effect was greater for peripheral than for central attention. A long-latency positive displacement component (300-600 ms) was larger over the right than the left hemisphere during attention to the lateral visual fields, but was symmetrical in amplitude when central stimuli were attended. These results suggest that different pathways are modulated when attention is deployed to different regions of the visual fields. Further, they suggest that the special role of the right hemisphere in spatial attention may be limited to analysis of information in the visual periphery.     

Visual field plasticity in a female with right occipital cortical dysplasia

Brain plasticity refers to its ability to recover after damage. Visual field plasticity is not well recognized. We report a 12-year-old female who first presented with recurrent seizures and was subsequently found to have a large, right occipital cortical dysplasia on magnetic resonance imaging. Her visual field by Goldmann perimetry was totally normal. Visual-evoked potential studies revealed the left hemifield P100 response was detected maximally at the right temporal and parietal regions. A weak but reproducible right hemifield P100 response was located at the right medial skull base. Functional magnetic resonance imaging with flashlight stimulation revealed cerebral activity mainly at the right posterior temporal and parietal lobes and left occipital lobe. These studies suggested that the left hemifield function was located at the right posterior temporal and parietal lobes. The left occipital lobe may also have been reorganized, with a P100 vector pointing out from its inferiomedial base. We reviewed other related reported cases. We believe that visual-evoked potential studies and visual functional magnetic resonance imaging should be performed more liberally for recognition of visual field plasticity.     

The influence of transient spatial attention on the processing of intracutaneous electrical stimuli examined with ERPs

ObjectiveDetermine the influence of transient spatial attention on the processing of intracutaneous electrical stimuli.MethodsElectrical stimuli, a single pulse or five pulses, were presented at the index fingers of the left or right hand. The to-be-attended hand and stimulated finger varied randomly from trial to trial. Participants had to press a foot pedal only when the relevant stimulus, varied between participants, occurred at the attended hand. EEG was measured to extract relevant ERP components.ResultsThe N100 and N150 were enhanced for attended as compared to unattended stimuli. The N100, N150, P260, and the P500 were enlarged for five pulse as compared to single pulse stimuli. The P260, which is thought to reflect a call for attention, was enhanced for unattended as compared to attended stimuli. Source analyses indicate that attentional effects on the N100, N150, and P260 may be related to changes in activity in secondary somatosensory areas and the anterior cingulate cortex.ConclusionsA transient manipulation of spatial attention increases cortical activity induced by attended relative to unattended intracutaneous electrical stimuli, but initially unattended stimuli appear to induce an enhanced orienting effect.SignificanceInitially unattended intracutaneous electrical stimuli seem to induce a call for attention.     

Cortical responses to consciousness of schematic emotional facial expressions: A high‐resolution EEG study

Is conscious perception of emotional face expression related to enhanced cortical responses? Electroencephalographic data (112 channels) were recorded in 15 normal adults during the presentation of cue stimuli with neutral, happy or sad schematic faces (duration: “threshold time” inducing about 50% of correct recognitions), masking stimuli (2 s), and go stimuli with happy or sad schematic faces (0.5 s). The subjects clicked left (right) mouse button in response to go stimuli with happy (sad) faces. After the response, they said “seen” or “not seen” with reference to previous cue stimulus. Electroencephalographic data formed visual event‐related potentials (ERPs). Cortical sources of ERPs were estimated by LORETA software. Reaction time to go stimuli was generally shorter during “seen” than “not seen” trials, possibly due to covert attention and awareness. The cue stimuli evoked four ERP components (posterior N100, N170, P200, and P300), which had similar peak latency in the “not seen” and “seen” ERPs. Only N170 amplitude showed differences in amplitude in the “seen” versus “not seen” ERPs. Compared to the “not seen” ERPs, the “seen” ones showed prefrontal, premotor, and posterior parietal sources of N170 higher in amplitude with the sad cue stimuli and lower in amplitude with the neutral and happy cue stimuli. These results suggest that nonconscious and conscious processing of schematic emotional facial expressions shares a similar temporal evolution of cortical activity, and conscious processing induces an early enhancement of bilateral cortical activity for the schematic sad facial expressions (N170). Hum Brain Mapp, 2010. © 2010 Wiley‐Liss, Inc.     

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.     

On why left events are the right ones: neural mechanisms underlying the left-hemifield advantage in rapid serial visual presentation

When simultaneous series of stimuli are rapidly presented left and right, containing two target stimuli T1 and T2, T2 is much better identified when presented in the left than in the right hemifield. Here, this effect was replicated, even when shifts of gaze were controlled, and was only partially compensated when T1 side provided the cue where to expect T2. Electrophysiological measurement revealed earlier latencies of T1- and T2-evoked N2(pc) peaks at the right than at the left visual cortex, and larger right-hemisphere T2-evoked N2(pc) amplitudes when T2 closely followed T1. These findings suggest that the right hemisphere was better able to single out the targets in time. Further, sustained contralateral slow shifts remained active after T1 for longer time at the right than at the left visual cortex, and developed more consistently at the right visual cortex when expecting T2 on the contralateral side. These findings might reflect better capacity of right-hemisphere visual working memory. These findings about the neurophysiological underpinnings of the large right-hemisphere advantage in this complex visual task might help elucidating the mechanisms responsible for the severe disturbance of hemineglect following damage to the right hemisphere.     

Behavioral and electrophysiological evidence of a right hemisphere bias for the influence of negative emotion on higher cognition

We examined how responses to aversive pictures affected performance and stimulus-locked event-related potentials (ERPs) recorded during a demanding cognitive task. Numeric Stroop stimuli were brief ly presented to either left or right visual hemifield (LVF and RVF, respectively) after a centrally presented aversive or neutral picture from the International Affective Picture System. Subjects indicated whether a quantity value from each Stroop stimulus matched the preceding Stroop stimulus while passively viewing the pictures. After aversive pictures, responses were more accurate for LVF Stroops and less accurate for RVF Stroops. Early-latency extrastriate attention-dependent visual ERPs were enhanced for LVF Stroops. The N2 ERP was enhanced for LVF Stroops over the right frontal and parietal scalp sites. Slow potentials (300-800 msec) recorded over the frontal and parietal regions showed enhanced picture related modulation and amplitude for LVF Stroops. These results suggest that emotional responses to aversive pictures selectively facilitated right hemisphere processing during higher cognitive task performance.