Top-down task sets for combined features: behavioral and electrophysiological evidence for two stages in attentional object selection

We studied whether visual search for targets defined by a combination of features from different dimensions is guided by separately represented target features or by an integrated representation of the target objects. In Experiment 1, participants searched for target singleton bars that were defined by a specific combination of color (red or blue) and size (small or large). The target arrays were preceded by cue arrays that contained a spatially uninformative color/size singleton. Behavioral spatial-cueing effects indicative of attentional capture were triggered only by cues that matched both target-defining features, but not by partially target-matching cues, suggesting that attention was guided by integrated object representations. However, the presence of reliable N2pc components for partially matching cues demonstrated that these cues did capture attention, in line with independent feature-based guidance of attention. This dissociation between the electrophysiological and behavioral markers of attentional capture was confirmed in Experiment 2, in which targets were defined by a color/size disjunction. Our results suggest that the attentional selection of targets that are defined by a combination of features is a two-stage process: Attention is initially captured by all target-matching features, but is then rapidly withdrawn from nontarget objects that share some but not all features with the current target.

The impact of task relevance and degree of distraction on stimulus processing

Background

The impact of task relevance on event-related potential amplitudes of early visual processing was previously demonstrated. Study designs, however, differ greatly, not allowing simultaneous investigation of how both degree of distraction and task relevance influence processing variations. In our study, we combined different features of previous tasks. We used a modified 1-back task in which task relevant and task irrelevant stimuli were alternately presented. The task irrelevant stimuli could be from the same or from a different category as the task relevant stimuli, thereby producing high and low distracting task irrelevant stimuli. In addition, the paradigm comprised a passive viewing condition. Thus, our paradigm enabled us to compare the processing of task relevant stimuli, task irrelevant stimuli with differing degrees of distraction, and passively viewed stimuli. EEG data from twenty participants was collected and mean P100 and N170 amplitudes were analyzed. Furthermore, a potential connection of stimulus processing and symptoms of attention deficit hyperactivity disorder (ADHD) was investigated.

Results

Our results show a modulation of peak N170 amplitudes by task relevance. N170 amplitudes to task relevant stimuli were significantly higher than to high distracting task irrelevant or passively viewed stimuli. In addition, amplitudes to low distracting task irrelevant stimuli were significantly higher than to high distracting stimuli. N170 amplitudes to passively viewed stimuli were not significantly different from either kind of task irrelevant stimuli. Participants with more symptoms of hyperactivity and impulsivity showed decreased N170 amplitudes across all task conditions. On a behavioral level, lower N170 enhancement efficiency was significantly correlated with false alarm responses.

Conclusions

Our results point to a processing enhancement of task relevant stimuli. Unlike P100 amplitudes, N170 amplitudes were strongly influenced by enhancement and enhancement efficiency seemed to have direct behavioral consequences. These findings have potential implications for models of clinical disorders affecting selective attention, especially ADHD.

Active suppression after involuntary capture of attention

After attention has been involuntarily captured by a distractor, how is it reoriented toward a target? One possibility is that attention to the distractor passively fades over time, allowing the target to become attended. Another possibility is that the captured location is actively suppressed so that attention can be directed toward the target location. The present study investigated this issue with event-related potentials (ERPs), focusing on the N2pc component (a neural measure of attentional deployment) and the Pd component (a neural measure of attentional suppression). Observers identified a color-defined target in a search array, which was preceded by a task-irrelevant cue array. When the cue array contained an item that matched the target color, this item captured attention (as measured both behaviorally and with the N2pc component). This capture of attention was followed by active suppression (indexed by the Pd component), and this was then followed by a reorienting of attention toward the target in the search array (indexed by the N2pc component). These findings indicate that the involuntary capture of attention by a distractor is followed by an active suppression process that presumably facilitates the subsequent voluntary orienting of attention to the target.

Top-down control of audiovisual search by bimodal search templates

To test whether the attentional selection of targets defined by a combination of visual and auditory features is guided in a modality-specific fashion or by control processes that are integrated across modalities, we measured attentional capture by visual stimuli during unimodal visual and audiovisual search. Search arrays were preceded by spatially uninformative visual singleton cues that matched the current target-defining visual feature. Participants searched for targets defined by a visual feature, or by a combination of visual and auditory features (e.g., red targets accompanied by high-pitch tones). Spatial cueing effects indicative of attentional capture were reduced during audiovisual search, and cue-triggered N2pc components were attenuated and delayed. This reduction of cue-induced attentional capture effects during audiovisual search provides new evidence for the multimodal control of selective attention.

Salience-based selection: attentional capture by distractors less salient than the target

Current accounts of attentional capture predict the most salient stimulus to be invariably selected first. However, existing salience and visual search models assume noise in the map computation or selection process. Consequently, they predict the first selection to be stochastically dependent on salience, implying that attention could even be captured first by the second most salient (instead of the most salient) stimulus in the field. Yet, capture by less salient distractors has not been reported and salience-based selection accounts claim that the distractor has to be more salient in order to capture attention. We tested this prediction using an empirical and modeling approach of the visual search distractor paradigm. For the empirical part, we manipulated salience of target and distractor parametrically and measured reaction time interference when a distractor was present compared to absent. Reaction time interference was strongly correlated with distractor salience relative to the target. Moreover, even distractors less salient than the target captured attention, as measured by reaction time interference and oculomotor capture. In the modeling part, we simulated first selection in the distractor paradigm using behavioral measures of salience and considering the time course of selection including noise. We were able to replicate the result pattern we obtained in the empirical part. We conclude that each salience value follows a specific selection time distribution and attentional capture occurs when the selection time distributions of target and distractor overlap. Hence, selection is stochastic in nature and attentional capture occurs with a certain probability depending on relative salience.

Specific fear modulates attentional selectivity during visual search: electrophysiological insights from the N2pc

In the present study, we used high-density EEG during a visual search task to investigate the dynamics of spatial attention to fear-relevant targets and background stimuli in small animal phobia during a visual search task. Twenty-five spider fearful (22 females) and 25 healthy nonfearful participants (19 females) were measured, while searching for discrepant objects in visual arrays. Compared to nonfearful participants, spider fearful individuals showed a more enhanced posterior N2pc to spider (vs. butterfly) targets in an array of flowers. Furthermore, spider fearful participants showed enhanced hypervigilance for all presented stimuli compared to controls as reflected by enhanced N1 amplitudes (160-200 ms). Our findings provide neural evidence for early, enhanced selective spatial attention for fear-relevant stimuli.

A common neural mechanism for preventing and terminating the allocation of attention

Much is known about the mechanisms by which attention is focused to facilitate perception, but little is known about what happens to attention after perception of the attended object is complete. One possibility is that the focus of attention passively fades. A second possibility is that attention is actively terminated after the completion of perception so that the brain can be prepared for the next target. The present study investigated this issue with event-related potentials in humans, focusing on the N2pc component (a neural measure of attentional deployment) and the Pd component (a neural measure of attentional suppression). We found that active suppression occurred both to prevent the allocation of attention to known distractors and to terminate attention after the perception of an attended object was complete. In addition, the neural measure of active suppression was correlated with a behavioral measure of trial-to-trial variations in the allocation of attention. Active suppression therefore appears to be a general-purpose mechanism that both prevents and terminates the allocation of attention.

On the time course of bottom-up and top-down processes in selective visual attention: an EEG study

According to recent models on visual attention, both the salience of signals (bottom-up) and the intention to search for particular stimuli (top-down) are determinants for attentional selection. We investigated these mechanisms by varying the top-down set of participants that had to detect either luminance or orientation changes of two symmetrically located bars. Irrelevant changes impaired target detection when they were presented spatially separated to the relevant change. Initial attentional selection was represented in posterior N1 asymmetries and was determined by both the relative salience of orientation changes and a subsequent intentional bias towards relevant stimuli. Only when salient orientation changes interfered with luminance target selection in the N1 time window did an N2pc occur. Thus, the selection of relevant information proceeds in a network whose activation is induced by a dynamic interplay of bottom-up and top-down processes.

Neuronal correlates of multiple top-down signals during covert tracking of moving objects in macaque prefrontal cortex

Resistance to distraction is a key component of executive functions and is strongly linked to the prefrontal cortex. Recent evidence suggests that neural mechanisms exist for selective suppression of task-irrelevant information. However, neuronal signals related to selective suppression have not yet been identified, whereas nonselective surround suppression, which results from attentional enhancement for relevant stimuli, has been well documented. This study examined single neuron activities in the lateral PFC when monkeys covertly tracked one of randomly moving objects. Although many neurons responded to the target, we also found a group of neurons that exhibited a selective response to the distractor that was visually identical to the target. Because most neurons were insensitive to an additional distractor that explicitly differed in color from the target, the brain seemed to monitor the distractor only when necessary to maintain internal object segregation. Our results suggest that the lateral PFC might provide at least two top-down signals during covert object tracking: one for enhancement of visual processing for the target and the other for selective suppression of visual processing for the distractor. These signals might work together to discriminate objects, thereby regulating both the sensitivity and specificity of target choice during covert object tracking.