The Nature of the Capacity Limitations in Visual Short-Term Memory

Several studies have explored the nature and in particular the limitations of human visual short-term memory (VSTM) (e.g. Luck & Vogel, 1997). A VSTM capacity limit of about 3 to 4 objects has been found, thus confirming results from earlier studies (e.g. Sperling, 1960). However, Alvarez and Cavanagh (2004) have argued that VSTM capacity is more dependent on the visual complexity of the objects rather than the number of objects per se.
We hypothesize that VSTM capacity is not only dependent on the complexity of a given object (like for example curvature and intersections) - what could be coined objective complexity - but more importantly how complex a given stimulus appear to the individual subject - what in contrast could be called subjective complexity. Many studies like Alvarez and Cavanagh seem to describe objective complexity and its influence on capacity limitations, but this may only reflect some of the aspects affecting the limited capacity of VSTM.
In a series of studies, we have explored how the familiarity of different types of stimulus-sets influences the capacity limitations in VSTM. We use a change detection paradigm (Pashler, 1988) to estimate VSTM capacity. In Experiment 1 we test four different groups with different reading skills from novice to expert readers, and in Experiment 2 we test native versus non-native readers of Japanese. In Experiment 3 we test both the capacity and encoding speed of young adults without any prior training as they learn to read either Arabic or Japanese.
Our results indicate that VSTM capacity for familiar items - compared to unfamiliar - is larger, irrespective of their visual complexity, hereby suggesting that visual long-term memory representation and training play an important role in regard to the capacity limitations of VSTM.