The time course of feature-selective attention inside and outside the focus of spatial attention.

Research on attentional selection of stimulus features has yielded seemingly contradictory results. On the one hand, many experiments in humans and animals have observed a "global" facilitation of attended features across the entire visual field, even when spatial attention is focused on a single location. On the other hand, several event-related potential studies in humans reported that attended features are enhanced at the attended location only. The present experiment demonstrates that these conflicting results can be explained by differences in the timing of attentional allocation inside and outside the spatial focus of attention. Participants attended to fields of either red or blue randomly moving dots on either the left or right side of fixation with the task of detecting brief coherent motion targets. Recordings of steady-state visual evoked potentials elicited by the flickering stimuli allowed concurrent measurement of the time course of feature-selective attention in visual cortex on both the attended and the unattended sides. The onset of feature-selective attentional modulation on the attended side occurred around 150 ms earlier than on the unattended side. This finding that feature-selective attention is not spatially global from the outset but extends to unattended locations after a temporal delay resolves previous contradictions between studies finding global versus hierarchical selection of features and provides insight into the fundamental relationship between feature-based and location-based (spatial) attention mechanisms.

Division and spreading of attention across color.

Biological systems must allocate limited perceptual resources to relevant elements in their environment. This often requires simultaneous selection of multiple elements from the same feature dimension (e.g. color). To establish the determinants of divided attentional selection of color, we conducted an experiment that used multicolored displays with four overlapping random dot kinematograms that differed only in hue. We manipulated (i) requirement to focus attention to a single color or divide it between two colors; (ii) distances of distractor hues from target hues in a perceptual color space. We conducted a behavioral and an electroencephalographic experiment, in which each color was tagged by a specific flicker frequency and driving its own steady-state visual evoked potential. Behavioral and neural indices of attention showed several major consistencies. Concurrent selection halved the neural signature of target enhancement observed for single targets, consistent with an approximately equal division of limited resources between two hue-selective foci. Distractors interfered with behavioral performance in a context-dependent fashion but their effects were asymmetric, indicating that perceptual distance did not adequately capture attentional distance. These asymmetries point towards an important role of higher-level mechanisms such as categorization and grouping-by-color in determining the efficiency of attentional allocation in complex, multicolored scenes.

Attentional Modulation in Early Visual Cortex: A Focused Reanalysis of Steady-state Visual Evoked Potential Studies.

Steady-state visual evoked potentials (SSVEPs) are a powerful tool for investigating selective attention. Here, we conducted a combined reanalysis of multiple studies employing this technique in a variety of attentional experiments to, first, establish benchmark effect sizes of attention on amplitude and phase of SSVEPs and, second, harness the power of a large data set to test more specific hypotheses. Data of eight published SSVEP studies were combined, in which human participants (n = 135 in total) attended to flickering random dot stimuli based on their defining features (e.g., location, color, luminance, or orientation) or feature conjunctions. The reanalysis established that, in all the studies, attention reliably enhanced amplitudes, with color-based attention providing the strongest effect. In addition, the latency of SSVEPs elicited by attended stimuli was reduced by ∼4 msec. Next, we investigated the modulation of SSVEP amplitudes in a subset of studies where two different features were attended concurrently. Although most models assume that attentional effects of multiple features are combined additively, our results suggest that neuronal enhancement provided by concurrent attention is better described by multiplicative integration. Finally, we used the combined data set to demonstrate that the increase in trial-averaged SSVEP amplitudes with attention cannot be explained by increased synchronization of single-trial phases. Contrary to the prediction of the phase-locking account, the variance across trials of complex Fourier coefficients increases with attention, which is more consistent with boosting of a largely phase-locked signal embedded in non-phase-locked noise.

Attentional Enhancement of Tracked Stimuli in Early Visual Cortex Has Limited Capacity.

Keeping track of the location of multiple moving objects is one of the well documented functions of visual attention. However, the mechanism of attentional selection that supports such continuous tracking is unclear. In particular, it has been proposed that target selection in early visual cortex occurs in parallel, with tracking errors arising because of attentional limitations at later processing stages. Here, we examine whether, instead, total attentional capacity for enhancement of early visual processing of tracked targets is shared between all attended stimuli. If the magnitude of attentional facilitation of multiple tracked targets was a key limiting factor of tracking ability, then one should expect it to drop systematically with increasing set-size of tracked targets. Human observers (male and female) were instructed to track two, four, or six moving objects among a pool of identical distractors. Steady-state visual evoked potentials (SSVEPs) recorded during the tracking period revealed that the processing of tracked targets was consistently amplified compared with the processing of the distractors. The magnitude of this amplification decreased with increasing set size, and at lateral occipital electrodes it closely followed inverse proportionality to the number of tracked items, suggesting that limited attentional resources must be shared among the tracked stimuli. Accordingly, the magnitude of attentional facilitation predicted the behavioral outcome at the end of the trial. Together, these findings demonstrate that the limitations of multiple object tracking (MOT) across set-sizes stem from the limitations of top-down selective attention already at the early stages of visual processing.SIGNIFICANCE STATEMENT The ability to selectively attend to relevant features or objects is the key to flexibility of perception and action in the continuously changing environment. This ability is demonstrated in the multiple object tracking (MOT) task where observers monitor multiple independently moving objects at different locations in the visual field. The role of early attentional enhancement in tracking was previously acknowledged in the literature, however, the limitations on tracking were thought to arise during later stages of processing. Here, we demonstrate that the strength of attentional facilitation depends on the number of tracked objects and predicts successful tracking performance. Thus, it is the limitations of attentional enhancement at the early stages of visual processing that determine behavioral performance limits.

Attentional modulation as a mechanism for enhanced facial emotion discrimination: The case of action video game players.

Action video game players (AVGPs) outperform nonvideo game players (NVGPs) on a wide variety of attentional tasks, mediating benefits to perceptual and cognitive decision processes. A key issue in the literature is the extent to which such benefits transfer beyond cognition. Using steady-state visual evoked potentials (SSVEP) as a neural measure of attentional resource allocation, we investigated whether the attentional benefit of AVGPs generalizes to the processing of rapidly presented facial emotions. AVGPs (n = 36) and NVGPs (n = 32) performed a novel, attention-demanding emotion discrimination task, requiring the identification of a target emotion in one of two laterally presented streams of emotional faces. The emotional faces flickered at either 2.0 Hz or 2.5 Hz. AVGPs outperformed NVGPs at detecting the target emotions regardless of the type of emotion. Correspondingly, attentional modulation of the SSVEP at parieto-occipital recording sites was larger in AVGPs compared with NVGPs. This difference appeared to be driven by a larger response to attended information, as opposed to a reduced response to irrelevant distractor information. Exploratory analyses confirmed that this novel paradigm elicited the expected pattern of event-related potentials associated with target detection and error processing. These components did not, however, differ between groups. Overall, the results indicate enhanced discrimination of facial emotions in AVGPs arising from enhanced attentional processing of emotional information. This presents evidence for the attentional advantage of AVGPs to extend beyond perceptual and cognitive processes.

Dynamic Interplay between Reward and Voluntary Attention Determines Stimulus Processing in Visual Cortex.

Reward enhances stimulus processing in the visual cortex, but the mechanisms through which this effect occurs remain unclear. Reward prospect can both increase the deployment of voluntary attention and increase the salience of previously neutral stimuli. In this study, we orthogonally manipulated reward and voluntary attention while human participants performed a global motion detection task. We recorded steady-state visual evoked potentials to simultaneously measure the processing of attended and unattended stimuli linked to different reward probabilities, as they compete for attentional resources. The processing of the high rewarded feature was enhanced independently of voluntary attention, but this gain diminished once rewards were no longer available. Neither the voluntary attention nor the salience account alone can fully explain these results. Instead, we propose how these two accounts can be integrated to allow for the flexible balance between reward-driven increase in salience and voluntary attention.

Conscious awareness modulates processing speed in the redundant signal effect.

Evidence for the influence of unaware signals on behaviour has been reported in both patient groups and healthy observers using the Redundant Signal Effect (RSE). The RSE refers to faster manual reaction times to the onset of multiple simultaneously presented target than those to a single stimulus. These findings are robust and apply to unimodal and multi-modal sensory inputs. A number of studies on neurologically impaired cases have demonstrated that RSE can be found even in the absence of conscious experience of the redundant signals. Here, we investigated behavioural changes associated with awareness in healthy observers by using Continuous Flash Suppression to render observers unaware of redundant targets. Across three experiments, we found an association between reaction times to the onset of a consciously perceived target and the reported level of visual awareness of the redundant target, with higher awareness being associated with faster reaction times. However, in the absence of any awareness of the redundant target, we found no evidence for speeded reaction times and even weak evidence for an inhibitory effect (slowing down of reaction times) on response to the seen target. These findings reveal marked differences between healthy observers and blindsight patients in how aware and unaware information from different locations is integrated in the RSE.

Too little, too late, and in the wrong place: Alpha band activity does not reflect an active mechanism of selective attention.

Selective attention focuses visual processing on relevant stimuli in order to allow for adaptive behaviour despite an abundance of distracting information. It has been proposed that increases in alpha band (8-12 â€‹Hz) amplitude reflect an active mechanism for distractor suppression. If this were the case, increases in alpha band amplitude should be succeeded by a decrease in distractor processing. Surprisingly, this connection has not been tested directly; specifically, studies that have investigated changes in alpha band after attention-directing cues have not directly assessed the neuronal processing of distractors. We concurrently recorded alpha activity and steady-state visual evoked potentials (SSVEPs) to assess the processing of target and distractor stimuli. In two experiments, participants covertly shifted attention to one of two letter streams (left or right) to detect infrequent target letters 'X' while ignoring the other stream. In line with previous findings, alpha band amplitudes contralateral to the unattended location increased compared to a pre-cue baseline. However, there was no suppression of SSVEP amplitudes elicited by unattended stimuli, while there was a pronounced enhancement of SSVEPs elicited by attended stimuli. Furthermore, and crucially, changes in alpha band amplitude during attention shifts did not precede those in SSVEPs and hit rates in both experiments, indicating that changes in alpha band amplitudes are likely to be a consequence of attention shifts rather than the other way around. We conclude that these findings contradict the notion that alpha band activity reflects mechanisms that have a causal role in the allocation of selective attention.

Top-Down Attention Is Limited Within but Not Between Feature Dimensions.

In natural vision, processing of spatial and nonspatial features occurs simultaneously; however, the two types of attention in charge of facilitating this processing have distinct mechanisms. Here, we tested the independence of spatial and feature-based attention at different stages of visual processing by examining color-based attentional selection while spatial attention was focused or divided. Human observers attended to one or two of four fields of randomly moving dots presented in both left and right visual hemifields. In the focused attention condition, the target stimulus was defined both by color and location, whereas in the divided attention condition stimuli of the target color had to be attended in both hemifields. Sustained attentional selection was measured by means of steady-state visual evoked potentials elicited by each of the frequency-tagged flickering dot fields. Additionally, target and distractor selection was assessed with ERPs to these stimuli. We found that spatial and color-based attention independently modulated the amplitude of steady-state visual evoked potentials, confirming independent top-down influences on early visual areas. In contrast, P3 amplitudes elicited only by targets and distractors of the attended color were subject to space-based enhancement, suggesting increasing integration of spatial and feature-based selection over the course of perceptual processing.

Rapid adaptive adjustments of selective attention following errors revealed by the time course of steady-state visual evoked potentials.

Directing attention to task-relevant stimuli is crucial for successful task performance, but too much attentional selectivity implies that new and unexpected information in the environment remains undetected. A possible mechanism for optimizing this fundamental trade-off could be an error monitoring system that immediately triggers attentional adjustments following the detection of behavioral errors. However, the existence of rapid adaptive post-error adjustments has been controversially debated. While preconscious error processing reflected by an error-related negativity (Ne/ERN) in the event-related potential has been shown to occur within milliseconds after errors, more recent studies concluded that error detection even impairs attentional selectivity and that adaptive adjustments are implemented, if at all, only after errors are consciously detected. Here, we employ steady-state visual evoked potentials elicited by continuously presented stimuli to precisely track the emergence of error-induced attentional adjustments. Our results indicate that errors lead to an immediate reallocation of attention towards task-relevant stimuli, which occurs simultaneously with the Ne/ERN. Single-trial variation of this adjustment was correlated with the Ne/ERN amplitude and predicted adaptive behavioral adjustments on the post-error trial. This suggests that early error monitoring in the medial frontal cortex is directly involved in eliciting adaptive attentional adjustments.

Cortical summation and attentional modulation of combined chromatic and luminance signals.

Cortical networks that process colour and luminance signals are often studied separately, although colour appearance depends on both colour and luminance. In fact, objects in everyday life are very rarely defined by only colour or only luminance, necessitating an investigation into combined processing of these signals. We used steady-state visual evoked potentials (SSVEPs) to investigate (1) cortical summation of luminance and chromatic contrast and (2) attentional modulation of neural activity driven by competing stimuli that differ in chromoluminant content. Our stimuli combined fixed amounts of chromatic contrast from either of the two cone-opponent mechanisms (bluish and yellowish; reddish and greenish) with two different levels of positive luminance contrast. Our experiments found evidence of non-linear processing of combined colour and luminance signals, which most likely originates in V1-V3 neurons tuned to both colour and luminance. Differences between luminance contrast of stimuli were found to be a key determinant for the size of feature-based voluntary attentional effects in SSVEPs, with colours of lower contrast than the colour they were presented with receiving the highest level of attentional modulation. Our results indicate that colour and luminance contrast are processed interdependently, both in terms of perception and in terms of attentional selection, with a potential candidate mediating their link being stimulus appearance, which depends on both chromaticity and luminance.

Neural mechanisms of divided feature-selective attention to colour.

Feature-based attentional selection of colour is challenging to investigate due to the multidimensional nature of colour-space. When attending concurrently to features from different feature dimensions (e.g. red and horizontal), the attentional selections of the separate dimensions are largely independent. Therefore, if colour constitutes multiple independent feature dimensions for attentional purposes, concurrently attending to two colours should be effective and independent of the specific configuration of target and distractor colours. Here, observers attended concurrently to two out of four fully overlapping random dot kinematograms of different colours, and the allocation of attention to each colour was assessed separately by recordings of steady-state visual evoked potentials. The magnitude of attention effects depended on colour proximity and was well described by a simple model which suggested that colour space is rescaled in an adaptive manner to achieve attentional selection. In conclusion, different spatially overlaid colours can be attended concurrently with an efficiency that is determined by their configuration in colour space, supporting the idea that (at least in terms of hue) colour acts as a single dimension for attentional purposes.

Global Enhancement but Local Suppression in Feature-based Attention.

A key property of feature-based attention is global facilitation of the attended feature throughout the visual field. Previously, we presented superimposed red and blue randomly moving dot kinematograms (RDKs) flickering at a different frequency each to elicit frequency-specific steady-state visual evoked potentials (SSVEPs) that allowed us to analyze neural dynamics in early visual cortex when participants shifted attention to one of the two colors. Results showed amplification of the attended and suppression of the unattended color as measured by SSVEP amplitudes. Here, we tested whether the suppression of the unattended color also operates globally. To this end, we presented superimposed flickering red and blue RDKs in the center of a screen and a red and blue RDK in the left and right periphery, respectively, also flickering at different frequencies. Participants shifted attention to one color of the superimposed RDKs in the center to discriminate coherent motion events in the attended from the unattended color RDK, whereas the peripheral RDKs were task irrelevant. SSVEP amplitudes elicited by the centrally presented RDKs confirmed the previous findings of amplification and suppression. For peripherally located RDKs, we found the expected SSVEP amplitude increase, relative to precue baseline when color matched the one of the centrally attended RDK. We found no reduction in SSVEP amplitude relative to precue baseline, when the peripheral color matched the unattended one of the central RDK, indicating that, while facilitation in feature-based attention operates globally, suppression seems to be linked to the location of focused attention.

Attentional Selection of Feature Conjunctions Is Accomplished by Parallel and Independent Selection of Single Features.

Experiments that study feature-based attention have often examined situations in which selection is based on a single feature (e.g., the color red). However, in more complex situations relevant stimuli may not be set apart from other stimuli by a single defining property but by a specific combination of features. Here, we examined sustained attentional selection of stimuli defined by conjunctions of color and orientation. Human observers attended to one out of four concurrently presented superimposed fields of randomly moving horizontal or vertical bars of red or blue color to detect brief intervals of coherent motion. Selective stimulus processing in early visual cortex was assessed by recordings of steady-state visual evoked potentials (SSVEPs) elicited by each of the flickering fields of stimuli. We directly contrasted attentional selection of single features and feature conjunctions and found that SSVEP amplitudes on conditions in which selection was based on a single feature only (color or orientation) exactly predicted the magnitude of attentional enhancement of SSVEPs when attending to a conjunction of both features. Furthermore, enhanced SSVEP amplitudes elicited by attended stimuli were accompanied by equivalent reductions of SSVEP amplitudes elicited by unattended stimuli in all cases. We conclude that attentional selection of a feature-conjunction stimulus is accomplished by the parallel and independent facilitation of its constituent feature dimensions in early visual cortex. SIGNIFICANCE STATEMENT: The ability to perceive the world is limited by the brain's processing capacity. Attention affords adaptive behavior by selectively prioritizing processing of relevant stimuli based on their features (location, color, orientation, etc.). We found that attentional mechanisms for selection of different features belonging to the same object operate independently and in parallel: concurrent attentional selection of two stimulus features is simply the sum of attending to each of those features separately. This result is key to understanding attentional selection in complex (natural) scenes, where relevant stimuli are likely to be defined by a combination of stimulus features.

Driving steady-state visual evoked potentials at arbitrary frequencies using temporal interpolation of stimulus presentation.

BACKGROUND: Steady-state visual evoked potentials have been utilized widely in basic and applied research in recent years. These oscillatory responses of the visual cortex are elicited by flickering stimuli. They have the same fundamental frequency as the driving stimulus and are highly sensitive to manipulations of attention and stimulus properties. While standard computer monitors offer great flexibility in the choice of visual stimuli for driving SSVEPs, the frequencies that can be elicited are limited to integer divisors of the monitor's refresh rate. RESULTS: To avoid this technical constraint, we devised an interpolation technique for stimulus presentation, with which SSVEPs can be elicited at arbitrary frequencies. We tested this technique with monitor refresh rates of 85 and 120 Hz. At a refresh rate of 85 Hz, interpolated presentation produced artifacts in the recorded spectrum in the form of additional peaks not located at the stimulated frequency or its harmonics. However, at a refresh rate of 120 Hz, these artifacts did not occur and the spectrum elicited by an interpolated flicker became indistinguishable from the spectrum obtained by non-interpolated presentation of the same frequency. CONCLUSIONS: Our interpolation technique eliminates frequency limitations of the common non-interpolated presentation technique and has many possible applications for future research.

Common mechanisms of spatial attention in memory and perception: a tactile dual-task study.

Orienting attention to locations in mnemonic representations engages processes that functionally and anatomically overlap the neural circuitry guiding prospective shifts of spatial attention. The attention-based rehearsal account predicts that the requirement to withdraw attention from a memorized location impairs memory accuracy. In a dual-task study, we simultaneously presented retro-cues and pre-cues to guide spatial attention in short-term memory (STM) and perception, respectively. The spatial direction of each cue was independent of the other. The locations indicated by the combined cues could be compatible (same hand) or incompatible (opposite hands). Incompatible directional cues decreased lateralized activity in brain potentials evoked by visual cues, indicating interference in the generation of prospective attention shifts. The detection of external stimuli at the prospectively cued location was impaired when the memorized location was part of the perceptually ignored hand. The disruption of attention-based rehearsal by means of incompatible pre-cues reduced memory accuracy and affected encoding of tactile test stimuli at the retrospectively cued hand. These findings highlight the functional significance of spatial attention for spatial STM. The bidirectional interactions between both tasks demonstrate that spatial attention is a shared neural resource of a capacity-limited system that regulates information processing in internal and external stimulus representations.

Global facilitation of attended features is obligatory and restricts divided attention.

In many common situations such as driving an automobile it is advantageous to attend concurrently to events at different locations (e.g., the car in front, the pedestrian to the side). While spatial attention can be divided effectively between separate locations, studies investigating attention to nonspatial features have often reported a "global effect", whereby items having the attended feature may be preferentially processed throughout the entire visual field. These findings suggest that spatial and feature-based attention may at times act in direct opposition: spatially divided foci of attention cannot be truly independent if feature attention is spatially global and thereby affects all foci equally. In two experiments, human observers attended concurrently to one of two overlapping fields of dots of different colors presented in both the left and right visual fields. When the same color or two different colors were attended on the two sides, deviant targets were detected accurately, and visual-cortical potentials elicited by attended dots were enhanced. However, when the attended color on one side matched the ignored color on the opposite side, attentional modulation of cortical potentials was abolished. This loss of feature selectivity could be attributed to enhanced processing of unattended items that shared the color of the attended items in the opposite field. Thus, while it is possible to attend to two different colors at the same time, this ability is fundamentally constrained by spatially global feature enhancement in early visual-cortical areas, which is obligatory and persists even when it explicitly conflicts with task demands.

Sustained multifocal attentional enhancement of stimulus processing in early visual areas predicts tracking performance.

Keeping track of multiple moving objects is an essential ability of visual perception. However, the mechanisms underlying this ability are not well understood. We instructed human observers to track five or seven independent randomly moving target objects amid identical nontargets and recorded steady-state visual evoked potentials (SSVEPs) elicited by these stimuli. Visual processing of moving targets, as assessed by SSVEP amplitudes, was continuously facilitated relative to the processing of identical but irrelevant nontargets. The cortical sources of this enhancement were located to areas including early visual cortex V1-V3 and motion-sensitive area MT, suggesting that the sustained multifocal attentional enhancement during multiple object tracking already operates at hierarchically early stages of visual processing. Consistent with this interpretation, the magnitude of attentional facilitation during tracking in a single trial predicted the speed of target identification at the end of the trial. Together, these findings demonstrate that attention can flexibly and dynamically facilitate the processing of multiple independent object locations in early visual areas and thereby allow for tracking of these objects.

Independent effects of attentional gain control and competitive interactions on visual stimulus processing.

Attention filters behaviorally relevant stimuli from the constant stream of sensory information comprising our environment. Research into underlying neural mechanisms in humans suggests that visual attention biases mutual suppression between stimuli resulting from competition for limited processing resources. As a consequence, processing of an attended stimulus is facilitated. This account makes 2 assumptions: 1) An attended stimulus is released from mutual suppression with competing stimuli and 2) an attended stimulus experiences greater gain in the presence of competing stimuli than when it is presented alone. Here, we tested these assumptions by recording frequency-tagged potentials elicited in early visual cortex that index stimulus-specific processing. We contrasted the processing of a given stimulus when its location was attended or unattended and in the presence or the absence of a nearby competing stimulus. At variance with previous findings, competition similarly suppressed processing of attended and unattended stimuli. Moreover, the magnitude of attentional gain was comparable in the presence or the absence of competing stimuli. We conclude that visuospatial selective attention does not directly modulate mutual suppression between stimuli but instead acts as a signal gain, which biases processing toward attended stimuli independent of competition.

Bottom-up biases in feature-selective attention.

Previous studies of feature-selective attention have focused on situations in which attention is directed to one of two spatially superimposed stimuli of equal salience. While such overlapping stimuli should maximize stimulus interactions, it is still unknown how bottom-up biases favoring one or the other stimulus influence the efficiency of feature-selective attention. We examined the integration of bottom-up contrast and top-down feature-selection biases on stimulus processing. Two fully overlapping random dot kinematograms (RDKs) of light and dark dots were presented on a gray background of intermediate luminance. On each trial, human participants attended one RDK to detect brief coherent motion targets, while ignoring any events in the unattended RDK. Concurrently, through changes in background luminance, stimulus contrast could be set to five different levels: the stimuli could either be equal, or one of the two stimuli could have twice or four times the contrast of the other stimulus. This manipulation introduced a bottom-up bias toward the stimulus with the higher contrast while keeping the difference between the stimuli constant. Stimulus processing was measured by means of steady-state visual evoked potentials (SSVEPs). SSVEP amplitudes generally increased with higher contrast of the driving stimulus. At earlier levels of processing, attention increased the slope of this linear relation, i.e., attention multiplicatively enhanced SSVEP amplitudes. However, at later levels of processing, attention had an additive effect. These effects of attention can be attributed to the differential integration of gain enhancement and inhibitory stimulus competition at different levels of the visual processing hierarchy.