Visual Short-Term Memory Complexity

Several recent studies have explored the nature and limits of visual short-term memory (VSTM) (e.g. Luck & Vogel, 1997). A general VSTM capacity limit of about 3 to 4 letters has been found, thus confirming results from earlier studies (e.g. Cattell, 1885; Sperling, 1960). However, Alvarez and Cavanagh (2004) have raised the question that the capacity of VSTM is dependent on visual complexity rather than the number of objects. We hypothesise that VSTM capacity is dependent on both the objective and subjective complexity of visual stimuli. Contrary to Alvarez and Cavanagh, who argue for the role of objective complexity, it seems that subjective complexity - which is dependent on the familiarity of the stimulus - plays a more important role than the objective visual complexity of the objects stored.
In two studies, we explored how familiarity influences the capacity of VSTM. 1) In children learning to read, we found an increase in VSTM capacity for letters while the capacity for line drawings (Snodgrass & Vanderwart, 1980) remained unchanged. 2) In further investigations, we found that Japanese readers had a larger VSTM capacity for Japanese Kanji symbols than non-Japanese readers while the capacity for letters and pictures remained similar. Our results indicate that VSTM capacity for familiar items is larger irrespective of visual complexity.