By voluntarily directing attention to a specific region of a visual scene, we can improve our perception of stimuli at that location. This ability to focus attention upon specific zones of the visual field has been described metaphorically as a moveable spotlight or zoom lens that facilitates the processing of stimuli within its 'beam'. A long-standing controversy has centred on the question of whether the spotlight of spatial attention has a unitary beam or whether it can be divided flexibly to disparate locations. Evidence supporting the unitary spotlight view has come from numerous behavioural and electrophysiological studies. Recent experiments, however, indicate that the spotlight of spatial attention may be divided between non-contiguous zones of the visual field for very brief stimulus exposures (&<100 ms). Here we use an electrophysiological measure of attentional allocation (the steady-state visual evoked potential) to show that the spotlight may be divided between spatially separated locations (excluding interposed locations) over more extended time periods. This spotlight division appears to be accomplished at an early stage of visual-cortical processing.

Memory impairments constitute an increasing objective and subjective problem with advancing age. The aim of the present study was to investigate the impact of working memory training on memory performance. The authors trained a sample of 80-year-old adults twice weekly over a time period of 3 months. Participants were tested on 4 different memory measures before, immediately after, and 1 year after training completion. The authors found overall increased memory performance in the experimental group compared to an active control group immediately after training completion. This increase was especially pronounced in visual working memory performance and, to a smaller degree, also in visual episodic memory. No group differences were found 1 year after training completion. The results indicate that even in old?old adults, brain plasticity is strong enough to result in transfer effects, that is, performance increases in tasks that were not trained during the intervention.

Adaptive behavior requires the rapid switching of attention among potentially relevant stimuli that appear in the environment. The present study used an electrophysiological approach to continuously measure the time course of visual pathway facilitation in human subjects as attention was shifted from one location to another. Steady-state visual evoked potentials (SSVEPs) were recorded to rapidly flickering lights at attended and unattended locations, and variations in SSVEP amplitude over time were calculated after a cue to shift attention. The build-up of cortical facilitation reflected in SSVEP amplitude was found to bear a close temporal relationship with the emergence of accurate target discriminations at the newly attended location.

Late gamma activity may represent a correlate of widespread cortical networks processing different aspects of emotionally arousing visual objects. In contrast, differences between affective categories in early gamma activity might reflect fast detection of aversive stimulus features.

No abstract

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 ...

The retrieval and formation of cortical object representations seem to require the activation of neuronal cell assemblies, correlated by synchronized neuronal activity in the gamma band range (>20 Hz). In the present electroencephalogram (EEG) study we have analysed induced gamma band activity during the repetition of familiar (meaningful) and unfamiliar (meaningless) line drawings. Results showed a broad posterior distribution of induced gamma band responses (GBRs) after the initial presentation of a familiar stimulus. Repeated presentations of the same picture resulted in a decrease of GBRs, accompanied by a decrease in the number of electrode pairs exhibiting significant phase-locking values. These effects might be linked to a 'sharpening' mechanism within a cell assembly representing a familiar object. In contrast, the re-presentation of primed unfamiliar stimuli was associated with an augmentation of gamma power and an increase in significantly phase-locked pairs of electrodes. These findings might be a signature of the formation of a new cortical network representing an object. Event related potentials (ERPs) showed a decrease in amplitude independent of the stimuli's associative content, and, thus, seem to play a complementary role in repetition priming as compared to high-frequency brain dynamics.