On Visual Working Memory

Suppose you're preparing dinner and you realize that you'd like to add some more tomatoes, onions, and mushrooms to the salad that you're making. You have no trouble storing images of these items in your head and locating them in the fridge. Suppose instead that you're eating dinner and you decide that you absolutely must have another bottle of red wine for your guests, but you really want the cote du rhone, and not the boring merlot. Again, you can quite easily go and grab the bottle based simply on its label, but in this latter situation, the amount of visual information: the detail that must be stored for comparison upon reaching your liquor cabinet is far greater.

In both cases, you're employing some form of working visual memory. This form of memory is thought to be highly plastic, short-term storage. It is possible however, that it might have different characteristics. The forms that working memory might take are (1) a set of fixed "slots," each having a discreet capacity or (2) a set of dynamic slots which can be more tailored to the specific use.

The fixed-slots concept is similar to the pictures taken by current digital cameras. The images are always the same number of (mega)pixels, no more, no less. While the dynamic-slots concept is more akin to being able to vary the number of pixels in an image depending on demand. For instance, if I was taking a picture of a pure red wall, I wouldn't need any more than 1 pixel because every part of the image would be identical. However, if you were taking a picture of a Jackson Pollack painting, you might want to combine several digital camera images to get a really accurate rendering of the details held therein, a more flexible allocation of working-memory resources.

The scenarios I've highlighted above don't really serve to answer the question of which form of working memory our brains employ because in both cases, we can imagine either form working just fine (as long as we accept the notion that the complexity of the wine bottle label is within the capacity limits of the fixed-slots). However, a recent paper published in Nature claims to have employed a savvy enough experimental approach to disentangle the two possibilities.

The research, published by Steven J. Luck (who has done quite a bit of excellent work in the field of Visual Neuroscience in general) and a colleague, Weiwei Zhang, is sadly quite brief, having clearly been edited for length by the editors at Nature. In fact, this curtailed version is quite difficult to follow, given the subtlety of the approach that the authors used, but their essential point comes through.

The approach they take is a common one. They measure human performance on a variety of working-memory tasks and attempt to fit these data to models with different assumptions. In this paper, they compare how well the data can be fit to a fixed-slots model versus a dynamic-slots model. Although they conclude that the dynamic-slots model simply doesn't explain the data as well, and they thus discard the notion of flexible resource allocation, one of the final sentences betrays what they must admit: "This model does not completely eliminate the concept of resources, because the slots themselves are a type of resource." In other words, it is possible to allocate multiple slots to the same item object-to-be-held-in-memory. However, it does appear that the slots themselves are of a fixed size.

This result places limits on the possible anatomical underpinnings of working-memory, and makes predictions about how one might expect human beings to perform in other, working-memory tasks. It will be interesting to see if the conclusions that these authors reached will be borne out in future work.