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 ...

Evoked potentials to brief 1,000-Hz tones presented to either the left or the right ear were recorded from 30 electrodes arrayed over the head. These recordings were submitted to two different forms of source analysis: brain electric source analysis (BESA) and variable-resolution electromagnetic tomography (VARETA). Both analyses showed that the dominant intracerebral sources for the late auditory-evoked potentials (50–300 ms) were in the supratemporal plane and lateral temporal lobe contralateral to the ear of stimulation. The analyses also suggested the possibility of additional sources in the frontal lobes.

Although subtle anatomical anomalies long precede the onset of clinical symptoms in Alzheimer's disease, their impact on the reorganization of brain networks underlying cognitive functions has not been fully explored. A unique window into this reorganization is provided by presymptomatic cases of familial Alzheimer's disease (FAD). Here we studied neural circuitry related to semantic processing in presymptomatic FAD cases by estimating the intracranial sources of the N400 event-related potential (ERP). ERPs were obtained during a semantic-matching task from 24 presymptomatic carriers and 25 symptomatic carriers of the E280A presenilin-1 (PS-1) mutation, as well as 27 noncarriers (from the same families). As expected, the symptomatic-carrier group performed worse in the matching task and had lower N400 amplitudes than both asymptomatic groups, which did not differ from each other on these variables. However, N400 topography differed in mutation carrier groups with respect to the noncarriers. Intracranial source analysis evinced that the presymptomatic-carriers presented a decrease of N400 generator strength in right inferior-temporal and medial cingulate areas and increased generator strength in the left hippocampus and parahippocampus compared to the controls. This represents alterations in neural function without translation into behavioral impairments. Compared to controls, the symptomatic-carriers presented a similar anatomical shift in the distribution of N400 generators to that found in presymptomatic-carriers, albeit with a larger reduction in generator strength. The redistribution of N400 generators in presymptomatic-carriers indicates that early focal degeneration associated with the mutation induces neural reorganization, possibly contributing to a functional compensation that enables normal performance in the semantic task.

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