Abstract
Previous work has demonstrated a distinction between maintenance of two types of spatial information in working memory (WM): spatial locations and spatial relations. While a body of work has investigated the neural mechanisms of sensory-based information like spatial locations, little is known about how spatial relations are maintained in WM. In two experiments, we used fMRI to investigate the involvement of early visual cortex in the maintenance of spatial relations in WM. In both experiments, we found less quadrant-specific BOLD activity in visual cortex when a single spatial relation, compared to a single spatial location, was held in WM. Also across both experiments, we found a consistent set of brain regions that were differentially activated during maintenance of locations vs. relations. Maintaining a location, compared to a relation, was associated with greater activity in typical spatial WM regions like posterior parietal cortex and prefrontal regions. Whereas maintaining a relation, compared to a location, was associated with greater activity in the parahippocampal gyrus and precuneus/retrosplenial cortex. Further, in Experiment 2 we manipulated WM load and included trials where participants had to maintain three spatial locations or relations. Under this high load condition, the regions sensitive to locations vs. relations were somewhat different than under low load. We also identified regions that were sensitive to load specifically for location or relation maintenance, as well as overlapping regions sensitive to load more generally. These results suggest that the neural substrates underlying WM maintenance of spatial locations and relations are distinct from one another and that the neural representations of these distinct types of spatial information change with load.
Highlights
Working memory (WM) is the ability to actively maintain and manipulate relevant information that is not currently available as sensory input
A significant difference in accuracy did emerge, t(24) = 3.02, p < 0.01, with accuracy being higher for Relation trials. While these accuracy results do suggest that Location trials were more difficult than Relation trials, the most important feature of our design is that in both trial types, participants were asked to encode and maintain only one piece of information and functional magnetic resonance imaging (fMRI) data was only analyzed for correct trials
We found that bilateral intraparietal sulcus (IPS)/superior parietal lobule (SPL)/inferior parietal lobule (IPL), bilateral frontal eye fields (FEF)/superior frontal junction (SFJ), and left inf-PCS were more active for Location trials compared to Relation trials
Summary
Working memory (WM) is the ability to actively maintain and manipulate relevant information that is not currently available as sensory input. While much research has focused on whether the neural mechanisms underlying WM maintenance of spatial vs object-based vs verbal information are distinct or overlapping (e.g., Shah and Miyake, 1996; Duncan and Owen, 2000; Levy and GoldmanRakic, 2000; Miller and Cohen, 2001; Sala and Courtney, 2007; Chein et al, 2011; Yeo et al, 2014), one commonality exists for all of these modalities: each involves the maintenance of concrete, sensory-based information in WM. Many tasks require that other types of information that are more “abstract” be maintained, such as relationships, rules, or strategies. These forms of abstract information must be extracted from sensory stimuli in some way and maintained in WM. Recent work has begun to suggest that maintaining abstract, non-sensory information in WM relies on distinct neural mechanisms than maintaining concrete, sensory information in WM (Badre, 2008; Montojo and Courtney, 2008; Ackerman and Courtney, 2012; Bahlmann et al, 2014; Ikkai et al, 2014; Libby et al, 2014; Blacker et al, 2016)
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