Visual working memory is a subsystem of working memory that is responsible for encoding, maintaining, and retrieving visual information and is critical for effective cognitive functioning. In the field of visual working memory, a lot of research focuses on studying its limited capacity, with one of the goals being to determine what are the basic units of storage. The conclusions of research in this area are not unanimous. Some studies suggest that the basic unit of storage is an individual visual feature, others that it is a visual object, and still others that it is a combination of individual features and objects. The aim of our research was to examine this in more detail, taking into account the theoretical model of two systems, which assumes that the capacity of visual working memory is limited at two levels – a representational system responsible for establishing representations of visual information and an activation system responsible for maintaining representations of visual information. 64 participants took part in our study, performing a visual working memory span task in which they had to remember the colors, shapes, or both characteristics of visual objects displayed on one or both sides of the visual field. The comparison of the estimated working memory capacity in separate and integrated display of colors and shapes in unilateral and bilateral conditions and the derived indices are consistent with the interpretation that the basic storage unit in the representation system is an integrated object, while the basic storage unit in the activation system is an individual visible feature. Our findings provide new insight into the understanding of the limited capacity of visual working memory at two levels, as the two-system model explains the storage limitation with the number of integrated objects and individual visible properties, and sets new guidelines for further research.
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