August 8, 2008
Why Shocking Images Stick in Short-Term Memory
You'd probably remember seeing a man with pink hair more
than you could recall the guy walking next to him with brown hair. That's
because our brains best remember attention-grabbing images, according to a new
Previous research has implied an upper limit to the number
of visual images a person can store in short-term memory, but a new study found
that memory capacity is much more flexible.
"Before people have had this idea that visual memory has
a very simple limit: three or
four items and that's it," said researcher Paul Bays,
a neuroscientist at University College London. "What
my research shows is that there isn't that upper limit, but instead there's a
single resource that's shared out between items. What happens is that the items
that stand out get more of this resource, and so are remembered with greater
The study, funded by the British biomedical research fund Wellcome
Trust, will be detailed in the Aug. 7 issue of the journal Science.
Bays and his University College
London colleague Masud Husain showed subjects computer screens displaying
colored squares or lines for one second. After the screen went blank, one
object would reappear, but slightly shifted in position. The scientists then
asked the participants what direction the object had moved in.
The researchers found that the more objects there were in
the image, the harder it was for subjects to remember the original position of
the changed item. However, the scientists did not measure a sharp drop off in
performance at a limit of three of four objects, as previous research would
"To see whether attention was having an effect we had
one item flash very briefly before they all disappeared," Bays told LiveScience. "We found that if the
flashed item was then the one that was moved, subjects were better at
Bays said this experiment revealed a previously unknown
aspect of working
memory. Rather than having a fixed limit on the number of visual items we
can recall, we have a total memory capacity, which gets subdivided based on how
much attention we pay each object, with the more striking and noteworthy taking
up more of our working memory resources.
"It just seemed like something was lacking in the
research that had been done so far," Bays said. "People had only
looked at the question in terms of do you remember it or don't you remember it.
You need to ask how accurately do you remember it?"
He predicts that his discovery about visual memory may apply
to other types of memory, including the number of digits or words we can
Research in those fields has also generally found fixed
ceilings on the number of items short-term memory can hold: generally about
four things, with a capacity of about seven if certain tricks, such as chunking
information in groups, are used.
"I suspect the same thing will apply - that in fact
this limit doesn't really exist but it's just because people haven't been looking
at this thing the right way," Bays said. "I think you'll find our
memory is more flexible than that."