Research Article: Visual Search Elicits the Electrophysiological Marker of Visual Working Memory

Date Published: November 26, 2009

Publisher: Public Library of Science

Author(s): Stephen M. Emrich, Naseem Al-Aidroos, Jay Pratt, Susanne Ferber, Alex O. Holcombe.

Abstract: Although limited in capacity, visual working memory (VWM) plays an important role in many aspects of visually-guided behavior. Recent experiments have demonstrated an electrophysiological marker of VWM encoding and maintenance, the contralateral delay activity (CDA), which has been shown in multiple tasks that have both explicit and implicit memory demands. Here, we investigate whether the CDA is evident during visual search, a thoroughly-researched task that is a hallmark of visual attention but has no explicit memory requirements.

Partial Text: We use visual working memory (VWM) processes to integrate information across events such as eye blinks and eye movements, where sensory input to the visual system is interrupted [1]–[4]. Studies of VWM have repeatedly demonstrated that the number of detailed representations that can be maintained over short periods of time is limited to three to four visual objects [5], [6]. Memory capacity can be measured using a change-detection task: observers see a display containing a variable number of colored items and are asked to remember as many items as possible. After a brief delay, a probe appears and observers say whether a change occurred. Recent electrophysiological results have revealed that during the delay period of these change-detection tasks, maintaining memory items in a lateralized display is associated with greater negativity over the channels contralateral to the attended side. This difference in amplitude between contralateral and ipsilateral channels, called the contralateral delay activity (CDA), or the sustained posterior contralateral negativity (SPCN) [7], [8], reflects the encoding and maintenance of items in VWM and predicts individual differences in VWM capacity [8]–[11]. As such, the CDA provides a marker of VWM engagement that reflects the involvement of this cognitive resource in a given task. The goal of the present study was to test whether or not the CDA can be recorded during visual search, a task with continuous visual input that is typically considered a hallmark of attentional, rather than VWM, processing [9].

The aim of the current study was to determine whether the electrophysiological marker of VWM encoding and maintenance is present during visual search, a task that has continuous visual input and virtually no explicit memory demands (other than to remember the target being sought). Our results reveal search-related activity (i.e., the CSA) that is largely indistinguishable from the CDA observed in a four-item, memory-dependent change-detection task. That is, both tasks demonstrate a large, negative, sustained voltage difference between ipsilateral and contralateral channels. Furthermore, the distribution of activity was similar for the two tasks, indicating that the CSA during search and the CDA during a VWM change-detection task likely represent the engagement of the same neural and cognitive resources.



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