A review of visual memory capacity: Beyond individual items and toward structured representations.

Tulving (2000) provided a concise, general definition of memory as the “neurocognitive capacity to encode, store, and retrieve information,” and suggested the possibility that there are many separate memory systems that fit this definition. Indeed, one of the primary aims of modern memory research has been to identify these different memory systems (Schacter & Tulving, 1994). This approach has lead to an extensive taxonomy of memory systems which are characterized by differences in timing, storage capacity, conscious access, active maintenance, and mechanisms of operation. Early on, William James (1890) proposed the distinction between primary memory – the information held in the ‘conscious present’ – and secondary memory, which consists of information that is acquired, stored outside of conscious awareness, and then later remembered. This distinction maps directly onto the modern distinction between short-term memory (henceforth working memory) and long-term memory (Atkinson & Shiffrin, 1968; Scoville & Milner, 1957; Waugh & Norman, 1965). The most salient difference between these systems is their capacity: the active, working memory system has an extremely limited capacity of only a few items (Cowan, 2001; Cowan, 2005; Miller, 1956), whereas the passive, long-term memory system can store thousands of items (Brady et al. 2008; Standing, 1973; Voss, 2009) with remarkable fidelity (Brady et al., 2008; Konkle et al., 2010a). The emphasis on memory systems and memory processes has been quite valuable in shaping cognitive and neural models of memory. In general, this approach aims to characterize memory systems in a way that generalizes over representational content (Schacter & Tulving, 1994). For example, working memory is characterized by a severely limited capacity regardless of whether items are remembered visually or verbally (e.g., Baddeley, 1986), and long-term memory has a very high capacity whether the items remembered are pictures (e.g., Standing, 1973), words (e.g., Shepard, 1967), or associations (e.g., Voss, 2009). However, generalization across content leaves many basic questions unanswered regarding the nature of stored representations: What is the structure and format of those representations? Research on visual perception takes the opposite approach, attempting to determine what is being represented, and to generalize across processes. For example, early stages of visual representation consist of orientation and spatial frequency features. Vision research has measured the properties of these features, such as their tuning curves and sensitivity (e.g., Blakemore & Campbell, 1969), and shown that these tuning properties are constant across several domains of processing (e.g., from simple detection to visual search). Thus, the intersection between memory and vision is a particularly interesting domain of research because it concerns both the processes of memory and the nature of the stored representations (Luck & Hollingworth, 2008). Recent research within the vision science community at this intersection between memory and vision has been quite fruitful. For example, working memory research has shown an important item/resolution tradeoff: as the number of items remembered increases, the precision with which each one is remembered decreases, possibly with an upper bound on the number of items that may be stored (Alvarez & Cavanagh, 2004; Zhang & Luck, 2008), or possibly without an upper bound (Bays, Catalao & Husain, 2009; Wilken & Ma, 2004). In long-term memory, it is possible to store thousands of detailed object representations (Brady et al. 2008; Konkle et al., 2010b) but only for meaningful items that connect with stored knowledge (Konkle et al., 2010b; Wiseman and Neisser, 1974). Here we review recent research in the domains of visual working memory and visual long-term memory, focusing on how models of these memory systems are altered and refined by taking the contents of memory into account.