We used an electrophysiological measure of selective stimulus processing (the steady-state visual evoked potential, SSVEP) to investigate feature-specific attention to color cues. Subjects viewed a display consisting of spatially intermingled red and blue dots that continually shifted their positions at random. The red and blue dots flickered at different frequencies and thereby elicited distinguishable SSVEP signals in the visual cortex. Paying attention selectively to either the red or blue dot population produced an enhanced amplitude of its frequency-tagged SSVEP, which was localized by source modeling to early levels of the visual cortex. A control experiment showed that this selection was based on color rather than flicker frequency cues. This signal amplification of attended color items provides an empirical basis for the rapid identification of feature conjunctions during visual search, as proposed by ''guided search'' models.electrophysiology ͉ feature-based attention ͉ steady-state evoked potential ͉ visual search W hen we search for a target item having a unique color (e.g., red) within an array of distracter items of a different color (e.g., green), the target appears to ''pop-out'' from the background of distracters. This pop-out effect is seen when targets are distinguished from uniform distracters by virtue of an easily discriminated visual feature such as color, orientation, size, curvature, etc. (1, 2). Under conditions where the pop-out effect occurs, the time to detect target presence is rapid and does not vary with the number of background distracters. In contrast, when a target is defined by a particular combination of two or more features (e.g., color and size) and is intermingled with distracter items having different combinations of those features, target detection is typically slower and increases as a function of the number of distracters. These findings have led to proposals that feature conjunctions must be identified by serially examining each item in the display (3), although parallel models of search can also account for the sloping set-size functions of conjunction search (4).Under certain conditions, it has been found that visual search for conjunction targets may be highly efficient and yield flat rather than sloping set-size functions (4). Such efficient search is possible when the two features defining the conjunction are highly discriminable from the distracter features. To account for such findings, Wolfe and colleagues (5) have proposed a ''guided search'' model, according to which the sensory representations of items having the relevant features are facilitated by attention, and target items having both attended features stand out by virtue of having a double-dose of facilitation. An alternative proposal is that items having the nontarget features are suppressed, again leading to pop-out of the targets (6). The evidence is mixed, however, as to whether attention can produce a global facilitation (or suppression) of a particular feature such as color across an entire array ...

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 featureselective 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.human EEG | neural mechanisms of shifting | steady-state visual evoked potentials | feature-based attention | random dot kinematogram S hifting and focusing attention on a certain location, object, or feature is a key element in the extraction of sensory information to allow for adaptive behavior. The distribution of attentional resources and the underlying temporal neural mechanisms of attentional shifting are still not well understood. Some previous studies have measured event-related potentials (ERPs) during the cue-target interval (1-5). This approach, however, allows one to investigate only the neural mechanisms of cue processing and target expectation and, thus, the activity of a cortical control network (1); it cannot provide information on the temporal dynamics of neural facilitation and/or suppression in early visual processing areas that are involved in the processing of the new to-be-attended visual stimulus. Knowledge of these dynamics in early visual processing areas seems pivotal for understanding the effect of top-down control mechanisms in attentional shifts because behavioral performance is closely linked to the modulation of the cortical evoked activity representing the newly attended stimulus (6). If visual processing were a strictly limited resource, which is distributed by selective attention, enhancement of an attended stimulus should be accompanied by an equal suppression of the unattended stimulus. However, in previous studies, we showed that shifting attention to either one of two lateral flickering stimuli was purely facilitatory, with ...

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.

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.

We investigated mechanisms of concurrent attentional selection of location and color using electrophysiological measures in human subjects. Two completely overlapping random dot kinematograms (RDKs) of two different colors were presented on either side of a central fixation cross. On each trial, participants attended one of these four RDKs, defined by its specific combination of color and location, in order to detect coherent motion targets. Sustained attentional selection while monitoring for targets was measured by means of steady-state visual evoked potentials (SSVEPs) elicited by the frequency-tagged RDKs. Attentional selection of transient targets and distractors was assessed by behavioral responses and by recording event-related potentials to these stimuli. Spatial attention and attention to color had independent and largely additive effects on the amplitudes of SSVEPs elicited in early visual areas. In contrast, behavioral false alarms and feature-selective modulation of P3 amplitudes to targets and distractors were limited to the attended location. These results suggest that feature-selective attention produces an early, global facilitation of stimuli having the attended feature throughout the visual field, whereas the discrimination of target events takes place at a later stage of processing that is only applied to stimuli at the attended position.

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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.

It is well-established that attention can select stimuli for preferential processing on the basis of non-spatial features such as color, orientation, or direction of motion. Evidence is mixed, however, as to whether feature-selective attention acts by increasing the signal strength of to-be-attended features irrespective of their spatial locations or whether it acts by guiding the spotlight of spatial attention to locations containing the relevant feature. To address this question, we designed a task in which feature-selective attention could not be mediated by spatial selection. Participants observed a display of intermingled dots of two colors, which rapidly and unpredictably changed positions, with the task of detecting brief intervals of reduced luminance of 20% of the dots of one or the other color. Both behavioral indices and electrophysiological measures of steady-state visual evoked potentials showed selectively enhanced processing of the attended-color items. The results demonstrate that feature-selective attention produces a sensory gain enhancement at early levels of the visual cortex that occurs without mediation by spatial attention.