Visual working memory (VWM) capacity limitations are estimated to be ~4 items. Yet, it remains unclear why certain items from a given memory array may be successfully retrieved from VWM and others are lost. Existing measures of the neural correlates of VWM cannot address this question because they measure the aggregate processing of the entire stimulus array rather than neural signatures of individual items. Moreover, this cumulative processing is usually measured during the delay period, thereby reflecting the allocation of neural resources during VWM maintenance. Here, we use the steady-state visual evoked potential (SSVEP) to identify the neural correlates of individual stimuli at VWM encoding and test two distinct hypotheses: the focused-resource hypothesis and the diffuse-resource hypothesis, for how the allocation of neural resources during VWM encoding may contribute to VWM capacity limitations. First, we found that SSVEP amplitudes were larger for stimuli that were later remembered than for items that were subsequently forgotten. Second, this pattern generalized so that the SSVEP amplitudes were also larger for the unprobed stimuli in correct compared to incorrect trials. These data are consistent with the diffuse-resource view in which attentional resources are broadly allocated across the whole stimulus array. These results illustrate the important role encoding mechanisms play in limiting the capacity of VWM.
The goal of the current experiment was to elucidate why we are able to retrieve certain items from visual working memory while others are forgotten. Visual working memory (VWM) refers to the encoding, maintenance, manipulation and retrieval of visual representations for immediate use. Despite the importance of VWM in both simple and complex cognitive tasks, capacity limitations associated with VWM are well documented (Cowan, 2001 and Luck and Vogel, 2013). In addition, VWM capacity is further constrained by stimulus factors such as complexity (Alvarez & Cavanagh, 2004), saliency (Melcher & Piazza, 2011), similarity (Awh, Barton, & Vogel, 2007), and set size (Anderson et al., 2011, Bays and Husain, 2008, Bays et al., 2009 and Fukuda et al., 2010). At a basic level, these capacity limitations indicate that when trying to encode, maintain, and retrieve a set of items in and from VWM, only a subset will ultimately be accessible. Although progress has been made in recent years, much remains unknown regarding the origins of this capacity limitation. In the current paper, we propose that constraints on capacity may manifest, in part, during the allocation of VWM resources at the time of encoding. We examine this hypothesis by examining neural signals associated with individual items during VWM encoding, and investigate whether modulations in these signals correlate with the success or failure of the corresponding item being subsequently retrieved from VWM.
