Results from an Experiment: The Time Course of Visual Short-Term Memory

The first experiment I ran on the Web has finally made it into print. Rather fittingly, it has been published in a Web-based journal: The Public Library of Science One.

Visual Memory is a Scrawny Creature

That experiment, The Time Course of Visual Short-Term Memory, was part of a larger study probing a fundamental question about memory: why is visual working (short-term) memory so lousy? In recent years, visual memory folk like Edward Vogel and George Alvarez have debated whether we can store as many as four items in visual memory, while on the other hand researchers looking more at verbal memory, such as Nelson Cowan, have been arguing over whether verbal memory can store only four items. There are memory tricks that can allow you to keep a hundred words in short-term memory; nobody has reported any similar tricks for visual memory.

There are many other ways in which visual memory is piddly compared to verbal memory, and I go into them in depth in the paper. Interestingly, previous researchers have not made much out of this difference, possibly because people seem to work on either visual memory or verbal memory, but not both.

Does Verbal Memory Explain the Differences between Humans and Apes?

One possibility that occurred to me is that if verbal memory in fact is considerably more robust and more useful than visual memory, that would endow verbal animals (i.e., adult humans) with significant advantages over non-verbal animals (e.g., human infants and all other animals). Just as writing has allowed some human cultures to supplement our limited memory capacity -- try doing a complicated math problem in your head; the real limitation is memory -- language could allow us to supplement limited non-verbal memory systems.

In fact, I found that many of the differences between adult humans on the one side and young children and apes on the other are found in tasks with large working memory demands. More examples are given in the paper, but this includes theory of mind tasks.

Is Verbal Memory Really Better?

Of course, this is fruitless speculation unless visual working memory is really inferior. The problem is that visual and verbal memory capacity is tested in somewhat different ways. The easiest way to test verbal memory capacity is to give people a list of words to remember and then ask them to repeat that list back (this forms an important part of many IQ tests).

This is obviously impossible with visual memory tests.

In a visual memory test, the participant is usually shown several images to remember. Then, after a delay, they are shown another image and asked if that is the same as one of the original images. Notice that you can be right 50% of the time just by guessing. Thus, to get a good measure, you need to do this many times.

Proactive Interference

This brings up the specter of proactive interference. I have written about proactive interference recently and won't belabor it here. The basic intuition is that if you do many trials of a memory test, it becomes hard to remember which stimuli were on which trial. So if you have been asked to remember circles of different colors, and then you are asked if the last trial contained a blue circle, you might remember that you have seen a blue circle recently but not remember if it was on the last trial or not.

So if visual working memory capacity tasks require many trials and verbal working memory tasks do not, one possible reason for the poor performance observed for visual working memory might be greater proactive interference.

Nope -- not proactive interference

The short version of the results of the published paper is that proactive interference does decrease measured capacity for visual working memory, but not by very much (about 15%). So it cannot account for the differences between visual and verbal working memory. The search must go on.

I hope to describe how the Web-based experiment contributed to this result in a future post. But interested readers can also read the paper itself. It is fairly short and reasonably non-technical.

Participate in current Web-based experiments at my lab by clicking here.

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Hartshorne, J.K. (2008). Visual working memory capacity and proactive interference. Public Library of Science One