Abstract
The ability to maintain representations in the absence of external sensory stimulation, such as in working memory, is critical for guiding human behavior. Human functional brain imaging studies suggest that visual working memory can recruit a network of brain regions from visual to parietal to prefrontal cortex. In this review, we focus on the maintenance of representations during visual working memory and discuss factors determining the topography of those representations. In particular, we review recent studies employing multi-voxel pattern analysis (MVPA) that demonstrate decoding of the maintained content in visual cortex, providing support for a “sensory recruitment” model of visual working memory. However, there is some evidence that maintained content can also be decoded in areas outside of visual cortex, including parietal and frontal cortex. We suggest that the ability to maintain representations during working memory is a general property of cortex, not restricted to specific areas, and argue that it is important to consider the nature of the information that must be maintained. Such information-content is critically determined by the task and the recruitment of specific regions during visual working memory will be both task- and stimulus-dependent. Thus, the common finding of maintained information in visual, but not parietal or prefrontal, cortex may be more of a reflection of the need to maintain specific types of visual information and not of a privileged role of visual cortex in maintenance.
Highlights
Working memory commonly refers to our ability to maintain and manipulate stimulus representations, typically for a short period of time, in the absence of the ongoing presence of that stimulus (Baddeley and Hitch, 1974)
The representation of information during visual working memory may be highly related to visual imagery
We have reviewed functional magnetic resonance imaging (fMRI) studies employing multivoxel decoding during working memory
Summary
Working memory commonly refers to our ability to maintain and manipulate stimulus representations, typically for a short period of time, in the absence of the ongoing presence of that stimulus (Baddeley and Hitch, 1974). Emrich et al (2013) found that the ability to decode multiple items in memory decreased significantly with increasing load in early visual cortex and MT+, but could not decode remembered items in parietal cortex, even in those areas that showed loadsensitive delay period activity These results argue strongly for the sensory recruitment model and suggest that neither elevated nor load-sensitive delay activity is a sufficient marker for maintained representations in working memory. Given the evidence just discussed, it may be that this ability is not limited to visual cortex, but that any particular cortical region can be recruited for maintenance, depending on the nature of the information maintained To test this idea, we presented participants sequentially with two visual objects before presenting a retro-cue (indicating which sample to hold in memory) and asked them to perform one of two different tasks after a delay period (Lee et al, 2013). The ability to maintain information is dissociable from the presence or absence of delay activity and elevated activity may reflect separate functions related to attention, motor preparation or executive control
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