Contralateral Delay Activity Components Research Articles

Category-based attentional capture can be influenced by color- and shape-dimensions independently in the conjunction search task.

Attention can be involuntarily attracted by a distractor that matches the current attentional control settings (ACSs). However, it remains unclear whether two category-specific ACSs can operate independently. By defining a target as a combination of two prototype-based categories, the present event-related potential (ERP) study investigated how color-category and shape-category ACSs operate within a search task paradigm and the effects of temporal task demands on these ACSs. The matching level between target and distractor was manipulated to separate the effects of each ACS. The relative position between target and distractor was employed to isolate the attentional processing of the distractor from the target. Furthermore, two display durations were used to manipulate the temporal task demands, including a short fixed window (800ms) and a dynamic window extended until the user responded. Our results support a two-stage selection scenario. In early stage, the color- and shape-ACS independently guided attention to task-relevant property (N2pc components) and suppressed attention toward task-irrelevant properties (PD components). In late stage, these two independent ACSs were integrated into a holistic ACS to interfere with the consolidation (contralateral delay activity components) and behavioral performance (accuracy and RTs) of target identification. Moreover, an early N1/P1 component might reflect a preattentive enhancement of relevant information or a preattentive suppression of irrelevant objects. These two category-specific ACSs weights differently in varied temporal task demands. These findings support the idea that independent early processing is followed by integrated late processing, which can be applied to category-based attentional capture with different temporal task demands.

The capacity and resolution of spatial working memory and its role in the storage of non-spatial features

The question whether the storage of spatial locations and other non-spatial features in visual working memory (WM) is based on shared or separate processes remains unresolved. We recorded contralateral delay activity (CDA) components as on-line electrophysiological markers of WM maintenance in two tasks where observers had to retain either the colors or locations of sample stimuli. CDA components were elicited both in the Color and in the Location task, and increasing WM load had identical effects on CDA amplitudes, suggesting shared underlying mechanisms. However, CDA amplitudes were generally larger in the Location Task. Experiment 2 demonstrated that the CDA is sensitive to the resolution demands of spatial WM tasks. CDA amplitudes elicited during the storage of object locations in WM were larger when these locations had to be retained with higher precision. These findings support the hypothesis that spatial and non-spatial features of visual objects are represented in an integrated fashion in WM. The activation of these representations is controlled by space-based attentional control processes, and their spatial resolution can be regulated in line with current task demands.

Intermodal Attention Shifts in Multimodal Working Memory

Attention maintains task-relevant information in working memory (WM) in an active state. We investigated whether the attention-based maintenance of stimulus representations that were encoded through different modalities is flexibly controlled by top-down mechanisms that depend on behavioral goals. Distinct components of the ERP reflect the maintenance of tactile and visual information in WM. We concurrently measured tactile (tCDA) and visual contralateral delay activity (CDA) to track the attentional activation of tactile and visual information during multimodal WM. Participants simultaneously received tactile and visual sample stimuli on the left and right sides and memorized all stimuli on one task-relevant side. After 500 msec, an auditory retrocue indicated whether the sample set's tactile or visual content had to be compared with a subsequent test stimulus set. tCDA and CDA components that emerged simultaneously during the encoding phase were consistently reduced after retrocues that marked the corresponding (tactile or visual) modality as task-irrelevant. The absolute size of cue-dependent modulations was similar for the tCDA/CDA components and did not depend on the number of tactile/visual stimuli that were initially encoded into WM. Our results suggest that modality-specific maintenance processes in sensory brain regions are flexibly modulated by top-down influences that optimize multimodal WM representations for behavioral goals.

Does Contralateral Delay Activity Reflect Working Memory Storage or the Current Focus of Spatial Attention within Visual Working Memory?

During the retention of visual information in working memory, event-related brain potentials show a sustained negativity over posterior visual regions contralateral to the side where memorized stimuli were presented. This contralateral delay activity (CDA) is generally believed to be a neural marker of working memory storage. In two experiments, we contrasted this storage account of the CDA with the alternative hypothesis that the CDA reflects the current focus of spatial attention on a subset of memorized items set up during the most recent encoding episode. We employed a sequential loading procedure where participants memorized four task-relevant items that were presented in two successive memory displays (M1 and M2). In both experiments, CDA components were initially elicited contralateral to task-relevant items in M1. Critically, the CDA switched polarity when M2 displays appeared on the opposite side. In line with the attentional activation account, these reversed CDA components exclusively reflected the number of items that were encoded from M2 displays, irrespective of how many M1 items were already held in working memory. On trials where M1 and M2 displays were presented on the same side and on trials where M2 displays appeared nonlaterally, CDA components elicited in the interval after M2 remained sensitive to a residual trace of M1 items, indicating that some activation of previously stored items was maintained across encoding episodes. These results challenge the hypothesis that CDA amplitudes directly reflect the total number of stored objects and suggest that the CDA is primarily sensitive to the activation of a subset of working memory representations within the current focus of spatial attention.

Direct Electrophysiological Measurement of Attentional Templates in Visual Working Memory

When we look for our children on the playground, internal representations of these important targets must guide our search through the cluttered and chaotic scene. Several theories of attention propose that we hold target representations (i.e., attentional templates) in visual working memory (VWM) to control perceptual attention (Bundesen, Habekost, & Kyllingsbaek, 2005; Desimone & Duncan, 1995). Although this is a foundational theoretical assumption, there is no direct electrophysiological evidence from humans supporting this proposal and recordings from monkeys have yielded mixed results (Chelazzi, Miller, Duncan, & Desimone, 1993, 2001; Kusunoki, Sigala, Gaffan, & Duncan, 2009). This makes it difficult to rule out the classic hypothesis that visual search may operate like a prepared reflex, unguided by VWM representations (Logan, 1978). In the present study, we tested the attentional-template hypothesis by recording event-related potentials (ERPs) from subjects while they searched for targets in complex scenes. On each trial, subjects saw a target-cue array followed by a complex search array (see Figure 1A). We focused our analyses on the ERPs following the cue to determine whether the contralateral-delay activity (CDA) was present. The CDA indexes the maintenance of representations in VWM (e.g., Vogel & Machizawa, 2004), thus providing an ideal tool for testing the hypothesis that we maintain attentional templates in VWM during search. That is, unlike imaging studies demonstrating how brain areas modulate under different task demands (e.g., Soto, Humphreys, & Rotshtein, 2007), we can use the CDA component to definitely show that the same VWM mechanisms relied upon in explicit-memory tasks are engaged in maintaining attentional templates. If templates are maintained in VWM, then we should observe a cue-elicited CDA that continues until search is performed. Furthermore, if the cue-elicited CDA directly measures the attentional template, then CDA amplitude measured prior to the search task should predict subsequent performance. Figure 1 The stimuli, ERP findings, and relationship between the ERPs and behavior. A) Example of the stimulus sequence and the grand-average waveforms from electrodes T5/6, where the effect was maximal, contralateral (red) and ipsilateral (black) to the location ...