Efficient Coding in Visual Short-Term Memory: Evidence for an Information-Limited Capacity

Previous work on visual short-term memory (VSTM) capacity has typically used patches of color or simple features which are drawn from a uniform distribution, and estimated the capacity of VSTM to be 3-4 items (Luck & Vogel, 1997). Here, we introduce covariance information between colors, and ask if VSTM can take advantage of this redundancy to form a more efficient representation of the displays. We find that observers can successfully remember 5 colors on these displays, significantly higher than the 3 colors remembered when the displays were changed to be uniformly distributed in the final block of the experiment. We suggest that quantifying capacity in terms of number of objects remembered fails to capture factors such as object complexity or statistical redundancy, and that information theoretic measures are better suited to characterizing the capacity of VSTM. We use Huffman coding to model our data, and demonstrate that the data are consistent with a fixed VSTM capacity in bits rather than in terms of number of objects.