Effects of spatial selective attention on the steady-state visual evoked potential in the 20-28 Hz range

Enhancement and suppression in the visual field under perceptual load

The perceptual load theory of attention proposes that the degree to which visual distractors are processed is a function of the attentional demands of a task-greater demands increase filtering of irrelevant distractors. The spatial configuration of such filtering is unknown. Here, we used steady-state visual evoked potentials (SSVEPs) in conjunction with time-domain event-related potentials (ERPs) to investigate the distribution of load-induced distractor suppression and task-relevant enhancement in the visual field. Electroencephalogram (EEG) was recorded while subjects performed a foveal go/no-go task that varied in perceptual load. Load-dependent distractor suppression was assessed by presenting a contrast reversing ring at one of three eccentricities (2, 6, or 11°) during performance of the go/no-go task. Rings contrast reversed at 8.3 Hz, allowing load-dependent changes in distractor processing to be tracked in the frequency-domain. ERPs were calculated to the onset of stimuli in the load task to examine load-dependent modulation of task-relevant processing. Results showed that the amplitude of the distractor SSVEP (8.3 Hz) was attenuated under high perceptual load (relative to low load) at the most proximal (2°) eccentricity but not at more eccentric locations (6 or 11°). Task-relevant ERPs revealed a significant increase in N1 amplitude under high load. These results are consistent with a center-surround configuration of load-induced enhancement and suppression in the visual field.

SWIFT: a novel method to track the neural correlates of recognition

Isolating the neural correlates of object recognition and studying their fine temporal dynamics have been a great challenge in neuroscience. A major obstacle has been the difficulty to dissociate low-level feature extraction from the actual object recognition activity. Here we present a new technique called semantic wavelet-induced frequency-tagging (SWIFT), where cyclic wavelet-scrambling allowed us to isolate neural correlates of object recognition from low-level feature extraction in humans using EEG. We show that SWIFT is insensitive to unrecognized visual objects in natural images, which were presented up to 30s, but is highly selective to the recognition of the same objects after their identity has been revealed. The enhancement of object representations by top-down attention was particularly strong with SWIFT due to its selectivity for high-level representations. Finally, we determined the temporal dynamics of object representations tracked by SWIFT and found that SWIFT can follow a maximum of between 4 and 7 different object representations per second. This result is consistent with a reduction in temporal capacity processing from low to high-level brain areas.

Slow biasing of processing resources in early visual cortex is preceded by emotional cue extraction in emotion-attention competition

In our previous studies on competition for attentional processing resources in early visual cortex between a foreground task and distracting emotional background images we found that emotional background images withdraw attentional resources from the foreground task after about 400 ms. Costs in behavioral data and a significant reduction of the steady state visual evoked potential (SSVEP) amplitude that was elicited by the foreground task lasted for several hundred milliseconds. We speculated that the differential effect in SSVEP amplitudes is preceded by the extraction of the emotional cue. Event related potential (ERP) studies to emotional and neutral complex images identified an early posterior negativity (EPN) as a robust neural signature of emotional cue extraction. The late positive potential (LPP) was related to in-depth processing of the emotional image. We extracted ERPs that were evoked by the onset of background images concurrently with the SSVEP that was elicited by the foreground task. Emotional compared to neutral background pictures evoked a more negative EPN at about 190 ms and a more positive LPP at about 700 ms after image onset. SSVEP amplitudes became significantly smaller with emotional background images after about 400 ms lasting for several hundred ms. Interestingly, we found no significant correlations between the three components, indicating that they act independently. Source localizations resulted in nonoverlapping cortical generators. Results suggest a cascade of perceptual processes: Extraction of the emotional cue preceded biasing of attentional resources away from the foreground task towards the emotional image for an evaluation of the picture content.

Affective engagement and subsequent visual processing: effects of contrast and spatial frequency

The present study examined if viewing affective stimuli alters subsequent visual processing, as indexed by steady-state visual potentials (ssVEPs) and behavioral performance in an orientation discrimination task. Participants viewed task-irrelevant but emotionally arousing pictures from the International Affective Picture System (1 s) followed by a target stimulus stream consisting of low (2 cpd) or high-spatial frequency (6 cpd) Gabor patches, flickering at a temporal rate of 14 Hz. Luminance contrast of the patches gradually increased for the first half and decreased for the second half of the total duration, resulting in a waxing-waning pattern of stimulus contrast. The authors found that the waveform envelope of 14 Hz-ssVEPs corresponded to time-varying stimulus contrast. Analyses compared medium- and high-contrast time segments, as a function of emotional content and spatial frequency. Results showed greater ssVEP amplitudes for patches with high compared to medium contrast. Viewing emotionally arousing pictures selectively enhanced the ssVEP amplitudes for low-spatial frequency target patches and attenuated the ssVEP evoked by high-spatial frequency patches, across contrast levels. Response times were slower for patches following unpleasant pictures rather than pleasant and neutral, and error rates mirrored the interaction of emotional content and spatial frequency observed in the ssVEP data. Together, the present results suggest that additive gain mechanisms and early visual pathways may mediate costs and benefits of emotional engagement for subsequent sensory processing.

Influences of encoding and retrieval on the steady-state visual evoked potential

The steady-state visual evoked potential (SSVEP) is a powerful tool to study basic perceptual functions in the human electroencephalogram (EEG) (e.g. selective attention). The present study aimed at finding support for the SSVEP's sensitivity to higher cognitive functions (e.g. memory). We tagged pictures of meaningless objects with 15 Hz within a combined subsequent-memory and recognition-memory design. The EEG was recorded from 128 electrodes. The subsequent-memory design revealed that SSVEPs are influenced by encoding-related processes. In particular, successful encoding was associated with higher SSVEP amplitudes at central-parietal scalp electrodes and reduced activity at left occipital sites. Recognition memory led to higher SSVEP amplitudes at right temporal scalp regions and to an amplitude reduction at midline electrodes. For the first time it was demonstrated that the SSVEP technique is a useful tool to study memory encoding and retrieval. SSVEPs might reflect the ongoing dynamics within an externally driven and widespread task-relevant network. This offers remarkable possibilities for future research on electrophysiological correlates of mnemonic functioning.

Selective attention to task-irrelevant emotional distractors is unaffected by the perceptual load associated with a foreground task

A number of studies have shown that emotionally arousing stimuli are preferentially processed in the human brain. Whether or not this preference persists under increased perceptual load associated with a task at hand remains an open question. Here we manipulated two possible determinants of the attentional selection process, perceptual load associated with a foreground task and the emotional valence of concurrently presented task-irrelevant distractors. As a direct measure of sustained attentional resource allocation in early visual cortex we used steady-state visual evoked potentials (SSVEPs) elicited by distinct flicker frequencies of task and distractor stimuli. Subjects either performed a detection (low load) or discrimination (high load) task at a centrally presented symbol stream that flickered at 8.6 Hz while task-irrelevant neutral or unpleasant pictures from the International Affective Picture System (IAPS) flickered at a frequency of 12 Hz in the background of the stream. As reflected in target detection rates and SSVEP amplitudes to both task and distractor stimuli, unpleasant relative to neutral background pictures more strongly withdrew processing resources from the foreground task. Importantly, this finding was unaffected by the factor 'load' which turned out to be a weak modulator of attentional processing in human visual cortex.

Neural strategies for selective attention distinguish fast-action video game players

We investigated the psychophysical and neurophysiological differences between fast-action video game players (specifically first person shooter players, FPS) and non-action players (role-playing game players, RPG) in a visual search task. We measured both successful detections (hit rates) and steady-state visually evoked EEG potentials (SSVEPs). Search difficulty was varied along two dimensions: number of adjacent attended and ignored regions (1, 2 and 4), and presentation rate of novel search arrays (3, 8.6 and 20 Hz). Hit rates decreased with increasing presentation rates and number of regions, with the FPS players performing on average better than the RPG players. The largest differences in hit rate, between groups, occurred when four regions were simultaneously attended. We computed signal-to-noise ratio (SNR) of SSVEPs and used partial least squares regression to model hit rates, SNRs and their relationship at 3 Hz and 8.6 Hz. The following are the most significant results: RPG players’ parietal responses to the attended 8.6 Hz flicker were predictive of hit rate and were positively correlated with it, indicating attentional signal enhancement. FPS players’ parietal responses to the ignored 3 Hz flicker were predictive of hit rate and were positively correlated with it, indicating distractor suppression. Consistent with these parietal responses, RPG players’ frontal responses to the attended 8.6 Hz flicker, increased as task difficulty increased with number of regions; FPS players’ frontal responses to the ignored 3 Hz flicker increased with number of regions. Thus the FPS players appear to employ an active suppression mechanism to deploy selective attention simultaneously to multiple interleaved regions, while RPG primarily use signal enhancement. These results suggest that fast-action gaming can affect neural strategies and the corresponding networks underlying attention, presumably by training mechanisms of distractor suppression.

Time courses of attentional modulation in neural amplification and synchronization measured with steady-state visual-evoked potentials

Endogenous attention modulates the amplitude and phase coherence of steady-state visual-evoked potentials (SSVEPs). In efforts to decipher the neural mechanisms of attentional modulation, we compared the time course of attentional modulation of SSVEP amplitude (thought to reflect the magnitude of neural population activity) and phase coherence (thought to reflect neural response synchronization). We presented two stimuli flickering at different frequencies in the left and right visual hemifields and asked observers to shift their attention to either stimulus. Our results demonstrated that attention increased SSVEP phase coherence earlier than it increased SSVEP amplitude, with a positive correlation between the attentional modulations of SSVEP phase coherence and amplitude. Furthermore, the behavioral dynamics of attention shifts were more closely associated with changes in phase coherence than with changes in amplitude. These results are consistent with the possibility that attention increases neural response synchronization, which in turn leads to increased neural population activity.

Precise mapping of the somatotopic hand area using neuromagnetic steady-state responses

The body surface is represented in somatotopically organized maps in the primary somatosensory cortex. Estimating the size of the hand area with neuromagnetic source analysis has been used as a metric for monitoring neuroplastic changes related to training, learning, and brain injury. Commonly, results were significant as group statistics only because source localization accuracy was limited by factors such as residual noise and head motion. In this study we aimed to develop a robust method for obtaining the somatotopic map of the hand area in individuals using the bootstrap framework. Furthermore, a comprehensive analysis of the different factors affecting the accuracy of the obtained maps was provided. We applied vibrotactile touch stimuli to the tip of the index finger or the ring finger of the right hand and recorded the 22-Hz steady-state response using MEG. Single equivalent dipole sources were localized in contralateral left somatosensory cortex. Bootstrap resampling revealed the confidence intervals for the source coordinates using a single block of 5 min MEG recording. Residual noise in the averaged evoked response predominantly affected source localization, and the related confidence interval was reciprocally related to the signal-to-noise ratio. Apparently, head movements within a block of MEG recording contributed less to the variability of source localization in cooperative volunteers. The results of the current study indicate that significant separations of index finger and ring finger representations along the somatotopic map can be revealed in an individual using bootstrap framework.

The dynamic allocation of attention to emotion: simultaneous and independent evidence from the late positive potential and steady state visual evoked potentials

Emotional stimuli capture and hold attention without explicit instruction. The late positive potential (LPP) component of the event related potential can be used to track motivated attention toward emotional stimuli, and is larger for emotional compared to neutral pictures. In the frequency domain, the steady state visual evoked potential (ssVEP) has also been used to track attention to stimuli flickering at a particular frequency. Like the LPP, the ssVEP is also larger for emotional compared to neutral pictures. Prior work suggests that both the LPP and ssVEP are sensitive to "top-down" manipulations of attention, however the LPP and ssVEP have not previously been examined using the same attentional manipulation in the same participants. In the present study, LPP and ssVEP amplitudes were simultaneously elicited by unpleasant and neutral pictures. Partway through picture presentation, participants' attention was directed toward an arousing or non-arousing region of unpleasant pictures. In line with prior work, the LPP was reduced when attention was directed toward non-arousing compared to arousing regions of unpleasant pictures; similar results were observed for the ssVEP. Thus, both electrocortical measures index affective salience and are sensitive to directed (here: spatial) attention. Variation in the LPP and ssVEP was unrelated, suggesting that these measures are not redundant with each other and may capture different neurophysiological aspects of affective stimulus processing and attention.

Square or sine: finding a waveform with high success rate of eliciting SSVEP

Steady state visual evoked potential (SSVEP) is the brain's natural electrical potential response for visual stimuli at specific frequencies. Using a visual stimulus flashing at some given frequency will entrain the SSVEP at the same frequency, thereby allowing determination of the subject's visual focus. The faster an SSVEP is identified, the higher information transmission rate the system achieves. Thus, an effective stimulus, defined as one with high success rate of eliciting SSVEP and high signal-noise ratio, is desired. Also, researchers observed that harmonic frequencies often appear in the SSVEP at a reduced magnitude. Are the harmonics in the SSVEP elicited by the fundamental stimulating frequency or by the artifacts of the stimuli? In this paper, we compare the SSVEP responses of three periodic stimuli: square wave (with different duty cycles), triangle wave, and sine wave to find an effective stimulus. We also demonstrate the connection between the strength of the harmonics in SSVEP and the type of stimulus.

On multiple alternating steady states induced by periodic spin phase perturbation waveforms

Direct measurement of neural currents by means of MRI can potentially open a high temporal resolution (10-100 ms) window applicable for monitoring dynamics of neuronal activity without loss of the high spatial resolution afforded by MRI. Previously, we have shown that the alternating balanced steady state imaging affords high sensitivity to weak periodic currents owing to its amplification of periodic spin phase perturbations. This technique, however, requires precise synchronization of such perturbations to the radiofrequency pulses. Herein, we extend alternating balanced steady state imaging to multiple balanced alternating steady states for estimation of neural current waveforms. Simulations and phantom experiments show that the off-resonance profile of the multiple alternating steady state signal carries information about the frequency content of driving waveforms. In addition, the method is less sensitive than alternating balanced steady state to precise waveform timing relative to radiofrequency pulses. Thus, multiple alternating steady state technique is potentially applicable to MR imaging of the waveforms of periodic neuronal activity.

Temporal trade-off effects in sustained attention: dynamics in visual cortex predict the target detection performance during distraction

Models of visual selective attention have suggested that the representation of specific features characterizing a target object is enhanced in the visual cortex, at the cost of competing task-irrelevant information. In psychophysical studies, however, such attentional enhancement has been shown to result in reduced perceptual sensitivity when maintained over periods of several seconds. Two experiments examined the relationship between target detection behavior and electrocortical facilitation in human visual cortex during sustained attention under competition, in near real time. Steady-state visual evoked potentials (ssVEPs) were used in a change detection paradigm, in which a stream of flickering grating stimuli containing target events was fully overlapping with distractor faces (experiment 1) or competing complex scenes (experiment 2), covering the same part of the visual field. Beamformer source localization was used to test plausibility of lower-tier visual cortex involvement in modulation of the ssVEP signal. Results of both experiments suggest that early overallocation of visual cortical resources to the attended stimulus stream is associated with rapid reduction of electrocortical facilitation and poor change detection across the entire trial. By contrast, temporally balanced dynamics in visual cortex predicted accurate change detection. Together, the present results support models of sustained selective attention that emphasize competition for resources in lower-tier visual cortex. These models can be extended by a temporal dimension, on which attentive behavior is characterized by frugal resource sharing across the viewing time.

Visual attention: the past 25 years

This review focuses on covert attention and how it alters early vision. I explain why attention is considered a selective process, the constructs of covert attention, spatial endogenous and exogenous attention, and feature-based attention. I explain how in the last 25 years research on attention has characterized the effects of covert attention on spatial filters and how attention influences the selection of stimuli of interest. This review includes the effects of spatial attention on discriminability and appearance in tasks mediated by contrast sensitivity and spatial resolution; the effects of feature-based attention on basic visual processes, and a comparison of the effects of spatial and feature-based attention. The emphasis of this review is on psychophysical studies, but relevant electrophysiological and neuroimaging studies and models regarding how and where neuronal responses are modulated are also discussed.

Neural basis of superior performance of action videogame players in an attention-demanding task

Steady-state visual evoked potentials (SSVEPs) were recorded from action videogame players (VGPs) and from non-videogame players (NVGPs) during an attention-demanding task. Participants were presented with a multi-stimulus display consisting of rapid sequences of alphanumeric stimuli presented at rates of 8.6/12 Hz in the left/right peripheral visual fields, along with a central square at fixation flashing at 5.5 Hz and a letter sequence flashing at 15 Hz at an upper central location. Subjects were cued to attend to one of the peripheral or central stimulus sequences and detect occasional targets. Consistent with previous behavioral studies, VGPs detected targets with greater speed and accuracy than NVGPs. This behavioral advantage was associated with an increased suppression of SSVEP amplitudes to unattended peripheral sequences in VGPs relative to NVGPs, whereas the magnitude of the attended SSVEPs was equivalent in the two groups. Group differences were also observed in the event-related potentials to targets in the alphanumeric sequences, with the target-elicited P300 component being of larger amplitude in VGPS than NVGPs. These electrophysiological findings suggest that the superior target detection capabilities of the VGPs are attributable, at least in part, to enhanced suppression of distracting irrelevant information and more effective perceptual decision processes.

Social vision: sustained perceptual enhancement of affective facial cues in social anxiety

Heightened perception of facial cues is at the core of many theories of social behavior and its disorders. In the present study, we continuously measured electrocortical dynamics in human visual cortex, as evoked by happy, neutral, fearful, and angry faces. Thirty-seven participants endorsing high versus low generalized social anxiety (upper and lower tertiles of 2104 screened undergraduates) viewed naturalistic faces flickering at 17.5 Hz to evoke steady-state visual evoked potentials (ssVEPs), recorded from 129 scalp electrodes. Electrophysiological data were evaluated in the time-frequency domain after linear source space projection using the minimum norm method. Source estimation indicated an early visual cortical origin of the face-evoked ssVEP, which showed sustained amplitude enhancement for emotional expressions specifically in individuals with pervasive social anxiety. Participants in the low symptom group showed no such sensitivity, and a correlational analysis across the entire sample revealed a strong relationship between self-reported interpersonal anxiety/avoidance and enhanced visual cortical response amplitude for emotional, versus neutral expressions. This pattern was maintained across the 3500 ms viewing epoch, suggesting that temporally sustained, heightened perceptual bias towards affective facial cues is associated with generalized social anxiety.

Competitive effects on steady-state visual evoked potentials with frequencies in- and outside the α band

Multiple concurrently presented stimuli are thought to compete for neuronal processing resources. Such competitive stimulus interactions can be investigated by "frequency tagging" each stimulus with an individual temporal frequency. In this case, all stimuli will drive distinct steady-state visual evoked potentials (SSVEPs), hence allowing for an assessment of the distribution of processing resources. Here, we investigated whether competitive effects on SSVEP amplitudes are dependent upon the choice of tagging frequency of either the driving stimulus or a close-by competing stimulus. In particular, we were interested whether changes in amplitude are specific to a 10-Hz SSVEP, as it has been suggested that tagging frequencies within the alpha band drive uniquely characterized neural networks. If this was the case, an additional competition might be introduced when two stimuli are tagged with frequencies within the alpha band and thus compete for processing resources in similar networks. Additionally, we tested whether effects on SSVEP amplitude differ when the competing stimulus is tagged with a frequency of 12 Hz that produces a perceptible flicker when compared to an imperceptible 60-Hz flicker. We found a significant decrease in amplitude of 10- and 15-Hz SSVEPs upon presentation of the competing stimulus regardless of its tagging frequency. Our results clearly indicate that an SSVEP with a frequency within the alpha band and a 15-Hz SSVEP show similar sensitivity to effects of competition. Furthermore, the observed effects of competition on SSVEP amplitude occur independently of flicker perceptibility.

Differential roles of frequency-following and frequency-doubling visual responses revealed by evoked neural harmonics

Frequency-following and frequency-doubling neurons are ubiquitous in both striate and extrastriate visual areas. However, responses from these two types of neural populations have not been effectively compared in humans because previous EEG studies have not successfully dissociated responses from these populations. We devised a light-dark flicker stimulus that unambiguously distinguished these responses as reflected in the first and second harmonics in the steady-state visual evoked potentials. These harmonics revealed the spatial and functional segregation of frequency-following (the first harmonic) and frequency-doubling (the second harmonic) neural populations. Spatially, the first and second harmonics in steady-state visual evoked potentials exhibited divergent posterior scalp topographies for a broad range of EEG frequencies. The scalp maximum was medial for the first harmonic and contralateral for the second harmonic, a divergence not attributable to absolute response frequency. Functionally, voluntary visual-spatial attention strongly modulated the second harmonic but had negligible effects on the simultaneously elicited first harmonic. These dissociations suggest an intriguing possibility that frequency-following and frequency-doubling neural populations may contribute complementary functions to resolve the conflicting demands of attentional enhancement and signal fidelity--the frequency-doubling population may mediate substantial top-down signal modulation for attentional selection, whereas the frequency-following population may simultaneously preserve relatively undistorted sensory qualities regardless of the observer's cognitive state.

Behavioral performance follows the time course of neural facilitation and suppression during cued shifts of feature-selective attention

A central question in the field of attention is whether visual processing is a strictly limited resource, which must be allocated by selective attention. If this were the case, attentional enhancement of one stimulus should invariably lead to suppression of unattended distracter stimuli. Here we examine voluntary cued shifts of feature-selective attention to either one of two superimposed red or blue random dot kinematograms (RDKs) to test whether such a reciprocal relationship between enhancement of an attended and suppression of an unattended stimulus can be observed. The steady-state visual evoked potential (SSVEP), an oscillatory brain response elicited by the flickering RDKs, was measured in human EEG. Supporting limited resources, we observed both an enhancement of the attended and a suppression of the unattended RDK, but this observed reciprocity did not occur concurrently: enhancement of the attended RDK started at 220 ms after cue onset and preceded suppression of the unattended RDK by about 130 ms. Furthermore, we found that behavior was significantly correlated with the SSVEP time course of a measure of selectivity (attended minus unattended) but not with a measure of total activity (attended plus unattended). The significant deviations from a temporally synchronized reciprocity between enhancement and suppression suggest that the enhancement of the attended stimulus may cause the suppression of the unattended stimulus in the present experiment.

Steady-state visually evoked potentials: focus on essential paradigms and future perspectives

After 40 years of investigation, steady-state visually evoked potentials (SSVEPs) have been shown to be useful for many paradigms in cognitive (visual attention, binocular rivalry, working memory, and brain rhythms) and clinical neuroscience (aging, neurodegenerative disorders, schizophrenia, ophthalmic pathologies, migraine, autism, depression, anxiety, stress, and epilepsy). Recently, in engineering, SSVEPs found a novel application for SSVEP-driven brain-computer interface (BCI) systems. Although some SSVEP properties are well documented, many questions are still hotly debated. We provide an overview of recent SSVEP studies in neuroscience (using implanted and scalp EEG, fMRI, or PET), with the perspective of modern theories about the visual pathway. We investigate the steady-state evoked activity, its properties, and the mechanisms behind SSVEP generation. Next, we describe the SSVEP-BCI paradigm and review recently developed SSVEP-based BCI systems. Lastly, we outline future research directions related to basic and applied aspects of SSVEPs.

Neural mechanisms of intermodal sustained selective attention with concurrently presented auditory and visual stimuli

We investigated intermodal attention effects on the auditory steady-state response (ASSR) and the steady-state visual evoked potential (SSVEP). For this purpose, 40-Hz amplitude-modulated tones and a stream of flickering (7.5 Hz) random letters were presented concurrently. By means of an auditory or visual target detection task, participants’ attention was directed to the respective modality for several seconds. Attention to the auditory stream led to a significant enhancement of the ASSR compared to when the visual stream was attended. This attentional modulation was located mainly in the right superior temporal gyrus. Vice versa, attention to the visual stream especially increased the second harmonic response of the SSVEP. This modulation was focused in the inferior occipital and lateral occipitotemporal gyrus of both hemispheres. To the best of our knowledge, this is the first demonstration of amplitude modulation of the ASSR and the SSVEP by intermodal sustained attention. Our results open a new avenue of research to understand the basic neural mechanisms of intermodal attention in the human brain.

Dynamic crossmodal links revealed by steady-state responses in auditory-visual divided attention

Frequency tagging has been often used to study intramodal attention but not intermodal attention. We used EEG and simultaneous frequency tagging of auditory and visual sources to study intermodal focused and divided attention in detection and discrimination performance. Divided-attention costs were smaller, but still significant, in detection than in discrimination. The auditory steady-state response (SSR) showed no effects of attention at frontocentral locations, but did so at occipital locations where it was evident only when attention was divided between audition and vision. Similarly, the visual SSR at occipital locations was substantially enhanced when attention was divided across modalities. Both effects were equally present in detection and discrimination. We suggest that both effects reflect a common cause: An attention-dependent influence of auditory information processing on early cortical stages of visual information processing, mediated by enhanced effective connectivity between the two modalities under conditions of divided attention.

Using frequency tagging to quantify attentional deployment in a visual divided attention task

Frequency tagging is an EEG method based on the quantification of the steady state visual evoked potential (SSVEP) elicited from stimuli which flicker with a distinctive frequency. Because the amplitude of the SSVEP is modulated by attention such that attended stimuli elicit higher SSVEP amplitudes than do ignored stimuli, the method has been used to investigate the neural mechanisms of spatial attention. However, up to now it has not been shown whether the amplitude of the SSVEP is sensitive to gradations of attention and there has been debate about whether attention effects on the SSVEP are dependent on the tagging frequency used. We thus compared attention effects on SSVEP across three attention conditions-focused, divided, and ignored-with six different tagging frequencies. Participants performed a visual detection task (respond to the digit 5 embedded in a stream of characters). Two stimulus streams, one to the left and one to the right of fixation, were displayed simultaneously, each with a background grey square whose hue was sine-modulated with one of the six tagging frequencies. At the beginning of each trial a cue indicated whether targets on the left, right, or both sides should be responded to. Accuracy was higher in the focused- than in the divided-attention condition. SSVEP amplitudes were greatest in the focused-attention condition, intermediate in the divided-attention condition, and smallest in the ignored-attention condition. The effect of attention on SSVEP amplitude did not depend on the tagging frequency used. Frequency tagging appears to be a flexible technique for studying attention.

Prolonged reduction of electrocortical activity predicts correct performance during rapid serial visual processing

When two targets are shown in a rapid temporal stream of distractors, performance for the second target (T2) is typically reduced when presented between 200 and 500 ms after the first (T1). The present study used the steady-state visual evoked potential (ssVEP), a continuous index of electrocortical facilitation, to compare brain responses in trials with correct versus incorrect T2 responses. We found a reduction of the electrocortical response following T1 in trials with correct T2 identification. By contrast, incorrect T2 trials were characterized by enhanced electrocortical amplitude. Amplitude attenuation predictive of successful T2 report was sustained over time, suggesting a reduction of resources allocated to the distractor stream in correct trials. Across intertarget intervals, T2 performance was a linear function of the ssVEP amplitude reduction in correct trials, weighted by the stimulus onset asynchrony.

Re-entrant projections modulate visual cortex in affective perception: evidence from Granger causality analysis

Re-entrant modulation of visual cortex has been suggested as a critical process for enhancing perception of emotionally arousing visual stimuli. This study explores how the time information inherent in large-scale electrocortical measures can be used to examine the functional relationships among the structures involved in emotional perception. Granger causality analysis was conducted on steady-state visual evoked potentials elicited by emotionally arousing pictures flickering at a rate of 10 Hz. This procedure allows one to examine the direction of neural connections. Participants viewed pictures that varied in emotional content, depicting people in neutral contexts, erotica, or interpersonal attack scenes. Results demonstrated increased coupling between visual and cortical areas when viewing emotionally arousing content. Specifically, intraparietal to inferotemporal and precuneus to calcarine connections were stronger for emotionally arousing picture content. Thus, we provide evidence for re-entrant signal flow during emotional perception, which originates from higher tiers and enters lower tiers of visual cortex.

Neuronal encoding of the distance traversed by covert shifts of spatial attention

Neurons in monkey medial superior temporal cortex selectively respond to the patterned visual motion in optic flow that simulates observer self-movement. We trained monkeys in a task that required behavioral responses indicating the location of a precue or the simulated heading direction in a subsequent optic flow stimulus. Medial superior temporal neuronal responses contained transient peaks at latencies proportionate to the distance from the precue to the heading direction represented by the subsequent optic flow. We conclude that these response transients reveal neural mechanisms underlying covert shifts of spatial attention and that the varying latency of these transients reflect the time required for reorientation between attentional targets.

Effects of intermodal attention on the auditory steady-state response and the event-related potential

The aim of the present study was to simultaneously measure and compare intermodal attention effects in event-related brain potentials (ERPs) and auditory steady-state responses (ASSRs). For this purpose, 40-Hz amplitude modulated tones and a visual fixation cross were presented concurrently. By means of target detection tasks either on the sounds or on the fixation cross, participants' attention was directed to the respective modality. Attended sounds elicited a negative difference (Nd) in the ERP relative to unattended sounds. Nd was divided into an early and a late part as often observed for intramodal attention. Moreover, attention to the sounds led to a significant enhancement of the ASSR. This modulation of the ASSR by intermodal attention is demonstrated for the first time in the EEG. The present data suggest that ASSRs could provide a useful tool for the investigation of the neural dynamics of intermodal attentional processes.

Attention facilitates multiple stimulus features in parallel in human visual cortex

Successfully locating a dangerous or desirable object within a cluttered visual scene is a commonplace yet highly adaptive skill. In the laboratory, this ability is modeled by visual search experiments in which subjects try to find a target item surrounded by an array of distracting stimuli. Under certain conditions, targets that are distinguishable from distractors by virtue of having a particular combination of shared sensory features (e.g., a particular color and orientation) can be found rapidly regardless of the number of distractors. To explain this highly efficient localization of feature-conjunction targets, "guided search" theories propose that attention is directed in parallel to the individual features that define the target, which then stands out from the distractors because of additive facilitation of its feature signals. Here we recorded frequency-tagged potentials evoked in human visual cortex and found that color and orientation features of target stimuli are indeed facilitated by attention in a parallel and additive manner. This additive feature-enhancement mechanism, reported here for the first time, not only enables rapid guided search but also plays a broader role in directing and sustaining attention to multi-feature objects and keeping them perceptually distinct from background clutter.

Look who's talking: the deployment of visuo-spatial attention during multisensory speech processing under noisy environmental conditions

In a crowded scene we can effectively focus our attention on a specific speaker while largely ignoring sensory inputs from other speakers. How attended speech inputs are extracted from similar competing information has been primarily studied in the auditory domain. Here we examined the deployment of visuo-spatial attention in multiple speaker scenarios. Steady-state visual evoked potentials (SSVEP) were monitored as a real-time index of visual attention towards three competing speakers. Participants were instructed to detect a target syllable by the center speaker and ignore syllables from two flanking speakers. The study incorporated interference trials (syllables from three speakers), no-interference trials (syllable from center speaker only), and periods without speech stimulation in which static faces were presented. An enhancement of flanking speaker induced SSVEP was found 70-220 ms after sound onset over left temporal scalp during interference trials. This enhancement was negatively correlated with the behavioral performance of participants -- those who showed largest enhancements had the worst speech recognition performance. Additionally, poorly performing participants exhibited enhanced flanking speaker induced SSVEP over visual scalp during periods without speech stimulation. The present study provides neurophysiologic evidence that the deployment of visuo-spatial attention to flanking speakers interferes with the recognition of multisensory speech signals under noisy environmental conditions.

Towards an independent brain-computer interface using steady state visual evoked potentials

OBJECTIVE

Brain-computer interface (BCI) systems using steady state visual evoked potentials (SSVEPs) have allowed healthy subjects to communicate. However, these systems may not work in severely disabled users because they may depend on gaze shifting. This study evaluates the hypothesis that overlapping stimuli can evoke changes in SSVEP activity sufficient to control a BCI. This would provide evidence that SSVEP BCIs could be used without shifting gaze.

METHODS

Subjects viewed a display containing two images that each oscillated at a different frequency. Different conditions used overlapping or non-overlapping images to explore dependence on gaze function. Subjects were asked to direct attention to one or the other of these images during each of 12 one-minute runs.

RESULTS

Half of the subjects produced differences in SSVEP activity elicited by overlapping stimuli that could support BCI control. In all remaining users, differences did exist at corresponding frequencies but were not strong enough to allow effective control.

CONCLUSIONS

The data demonstrate that SSVEP differences sufficient for BCI control may be elicited by selective attention to one of two overlapping stimuli. Thus, some SSVEP-based BCI approaches may not depend on gaze control. The nature and extent of any BCI's dependence on muscle activity is a function of many factors, including the display, task, environment, and user.

SIGNIFICANCE

SSVEP BCIs might function in severely disabled users unable to reliably control gaze. Further research with these users is necessary to explore the optimal parameters of such a system and validate online performance in a home environment.

Face Processing is Gated by Visual Spatial Attention

Human perception of faces is widely believed to rely on automatic processing by a domain-specific, modular component of the visual system. Scalp-recorded event-related potential (ERP) recordings indicate that faces receive special stimulus processing at around 170 ms poststimulus onset, in that faces evoke an enhanced occipital negative wave, known as the N170, relative to the activity elicited by other visual objects. As predicted by modular accounts of face processing, this early face-specific N170 enhancement has been reported to be largely immune to the influence of endogenous processes such as task strategy or attention. However, most studies examining the influence of attention on face processing have focused on non-spatial attention, such as object-based attention, which tend to have longer-latency effects. In contrast, numerous studies have demonstrated that visual spatial attention can modulate the processing of visual stimuli as early as 80 ms poststimulus – substantially earlier than the N170. These temporal characteristics raise the question of whether this initial face-specific processing is immune to the influence of spatial attention. This question was addressed in a dual-visual-stream ERP study in which the influence of spatial attention on the face-specific N170 could be directly examined. As expected, early visual sensory responses to all stimuli presented in an attended location were larger than responses evoked by those same stimuli when presented in an unattended location. More importantly, a significant face-specific N170 effect was elicited by faces that appeared in an attended location, but not in an unattended one. In summary, early face-specific processing is not automatic, but rather, like other objects, strongly depends on endogenous factors such as the allocation of spatial attention. Moreover, these findings underscore the extensive influence that top-down attention exercises over the processing of visual stimuli, including those of high natural salience.

Attentional bias of competitive interactions in neuronal networks of early visual processing in the human brain

Multiple objects in a visual scene compete for neuronal representation. We investigated competitive neuronal dynamics in cortical networks of early visual processing in the human brain. Coloured picture streams flickered at 7.42 Hz, evoking the steady-state visual evoked potential (SSVEP), an electrophysiological response of neuronal populations in early visual areas synchronised by the external pacemaker. While these picture streams were at a fixed location in the upper left and right quadrant, respectively, additional competing picture streams flickering at a different frequency were continuously changing the distance to the stationary streams by slow motion. Analysis of the 7.42 Hz SSVEP amplitude revealed significant amplitude decreases when the competing stimulus was closer than about 4.5 degrees of visual angle. Sources of the SSVEP suppression effect were found in early visual areas of the ventral and dorsal processing streams. Attending the stationary stimulus resulted in no difference in 7.42 Hz SSVEP amplitude regardless of spatial separation to the competing stimulus. Contrary to the predictions of the model, we found co-amplification of the competing stimulus at close spatial proximity accompanied by an increase of an intermodulation frequency, suggesting integrated neuronal processing of target and competing stimuli when both streams are close together.

The influence of cognitive tasks on different frequencies steady-state visual evoked potentials

Previous studies suggested that there exists different neural networks for different frequency bands of steady-state visual evoked potential (SSVEP). What is the effect of the same cognitive task on different frequency SSVEPs? In this work, when a subject was conducting a graded memory task, a 8.3 or 20 Hz flicker was used as a background stimulation. The recorded EEGs were analyzed by the method of steady-state probe topography (SSPT), the results showed that SSVEPs under these two flicker conditions were similar to each other in the various stages of memory process, and were similar to the result of a high alpha band SSVEP as reported before. However, the SSVEP amplitude and latency in the lower frequency band is more clear and stable than that in the higher frequency band. These results suggest that the same cognitive task affects the different frequency SSVEP in a similar way, and the low frequency flicker is a better choice than the high frequency one in such as working memory study.

Time course of competition for visual processing resources between emotional pictures and foreground task

High-arousing emotional stimuli facilitate early visual cortex, thereby acting as strong competitors for processing resources in visual cortex. The present study used an electrophysiological approach for continuously measuring the time course of competition for processing resources in the visual pathway arising from emotionally salient but task-irrelevant input while performing a foreground target detection task. Steady-state visual evoked potentials (SSVEPs) were recorded to rapidly flickering squares superimposed upon neutral and emotionally high-arousing pictures, and variations in SSVEP amplitude over time were calculated. As reflected in SSVEP amplitude and target detection rates, arousing emotional background pictures withdrew processing resources from the detection task compared with neutral ones for several hundred milliseconds after stimulus onset. SSVEP amplitude was found to bear a close temporal relationship with accurate target detection as a function of time after stimulus onset.

Location and features of instructive spatial cues do not influence the time course of covert shifts of visual spatial attention

The time course of shifting visual spatial attention to flickering stimuli in the left and right visual hemifield was investigated. The goal was to test whether an instructive peripheral salient cue located close to the newly to-be-attended location triggers faster shifts per se compared to a central cue. Besides behavioural data an objective electrophysiological measure, the steady-state visual evoked potential (SSVEP) was used to measure the time course of visual pathway facilitation in the human brain for centrally and peripherally cued shifts of spatial attention. Results revealed that both spatial cues resulted in identical time courses of shifts of covert spatial attention. This was true with respect to behavioural data and SSVEP amplitude. Results support the notion that a salient peripheral spatial cue does not automatically produce faster shifts of spatial attention to the to-be-attended location when this cue is informative and embedded in an ongoing stimulation.

The representation of spatial attention in human parietal cortex dynamically modulates with performance

The control and allocation of attention is an essential, ubiquitous neural process that gates our awareness of objects and events in the environment. Neural representations of the locus of spatial attention have been previously demonstrated in parietal cortex. However, the behavioral relevance of these neural representations is not known. While undergoing functional magnetic resonance imaging, subjects performed a covert spatial attention task that yielded a wide range of performance values. Voxels in parietal cortex selective for attended target location also dynamically modulated, becoming more or less responsive as performance levels changed. Surprisingly, this relationship was not linear. Responses peaked at intermediate performance levels and dropped both when performance was very high and when it was very low. Such dynamic modulation may represent a mechanism for organizing neural control signals according to behavioral task demands.

Mountains and valleys: binocular rivalry and the flow of experience

Binocular rivalry provides a useful situation for studying the relation between the temporal flow of conscious experience and the temporal dynamics of neural activity. After proposing a phenomenological framework for understanding temporal aspects of consciousness, we review experimental research on multistable perception and binocular rivalry, singling out various methodological, theoretical, and empirical aspects of this research relevant to studying the flow of experience. We then review an experimental study from our group explicitly concerned with relating the temporal dynamics of rivalrous experience to the temporal dynamics of cortical activity. Drawing attention to the importance of dealing with ongoing activity and its inherent changing nature at both phenomenological and neurodynamical levels, we argue that the notions of recurrence and variability are pertinent to understanding rivalry in particular and the flow of experience in general.

The effect of selective attention on the gamma-band auditory steady-state response

Studies have demonstrated that selective attention can modulate the steady-state evoked potential to repetitive visual and tactile stimulation. However, examinations of the effect of attention on the auditory steady-state response (ASSR) have proven equivocal. The current experiment therefore utilized EEG to examine the effect of attention on the ASSR in healthy humans (n=15). Auditory click trains in the beta (20 Hz) and gamma (40 Hz) ranges were randomly presented binaurally in an oddball discrimination paradigm (each frequency served as the oddball (target) in each of two blocks). A Fast Fourier Transform was used to assess the effect of attention on the ASSR (signal power), and phase consistency across trials was assessed using the phase-locking factor (PLF). As expected, both 20 and 40 Hz targets elicited a robust P300 response, with maximal amplitudes over parietal regions. For the ASSR, it was found that EEG signal power was larger to 40 Hz targets compared to 40 Hz frequent stimuli across all frontocentral electrodes. No differences in signal power were observed during 20 Hz stimulation. Finally, increased PLF values were observed for 40 Hz targets compared to frequent trials. These results provide evidence that selective attention can enhance signal power and phase-locking of the ASSR, particularly to auditory stimulation in the gamma range.

Spatiotemporal analysis of the cortical sources of the steady-state visual evoked potential

This study aimed to characterize the neural generators of the steady-state visual evoked potential (SSVEP) to repetitive, 6 Hz pattern-reversal stimulation. Multichannel scalp recordings of SSVEPs and dipole modeling techniques were combined with functional magnetic resonance imaging (fMRI) and retinotopic mapping in order to estimate the locations of the cortical sources giving rise to the SSVEP elicited by pattern reversal. The time-varying SSVEP scalp topography indicated contributions from two major cortical sources, which were localized in the medial occipital and mid-temporal regions of the contralateral hemisphere. Colocalization of dipole locations with fMRI activation sites indicated that these two major sources of the SSVEP were located in primary visual cortex (V1) and in the motion sensitive (MT/V5) areas, respectively. Minor contributions from mid-occipital (V3A) and ventral occipital (V4/V8) areas were also considered. Comparison of SSVEP phase information with timing information collected in a previous transient VEP study (Di Russo et al. [2005] Neuroimage 24:874-886) suggested that the sequence of cortical activation is similar for steady-state and transient stimulation. These results provide a detailed spatiotemporal profile of the cortical origins of the SSVEP, which should enhance its use as an efficient clinical tool for evaluating visual-cortical dysfunction as well as an investigative probe of the cortical mechanisms of visual-perceptual processing.

Attentional capacity for processing concurrent stimuli is larger across sensory modalities than within a modality

One finding in attention research is that visual and auditory attention mechanisms are linked together. Such a link would predict a central, amodal capacity limit in processing visual and auditory stimuli. Here we show that this is not the case. Letter streams were accompanied by asynchronously presented streams of auditory, visual, and audiovisual objects. Either the letter streams or the visual, auditory, or audiovisual parts of the object streams were attended. Attending to various aspects of the objects resulted in modulations of the letter-stream-elicited steady-state evoked potentials (SSVEPs). SSVEPs were larger when auditory objects were attended than when either visual objects alone or when auditory and visual object stimuli were attended together. SSVEP amplitudes were the same in the latter conditions, indicating that attentional capacity between modalities is larger than attentional capacity within one and the same modality.