One way of looking at memory is to divide it up into three stages:
encoding, storage, and retrieval.
Encoding is about whether the eyewitness actually acquired any memory of
the incident in the first place. Storage is about whether they can keep that memory intact
over time. And retrieval is about whether they can report that memory at a future date.
If someone can't remember something, it could be that they never had that information
in the first place - that is, the information never even became a memory, that's a
failure of encoding. It could be that the information did make it into memory, but they've
since forgotten that memory, it's no longer there to be retrieved, so that's a failure
of storage. Or a memory could still be present in their brains but they can't access it,
they can't retrieve it, so that's a failure of retrieval.
As we've discussed, memory is a creative, constructive process. It's not like filming
what happened and replaying the film footage at a later date. We've already seen narrative
conventions and time-slice errors and how they can affect memory accuracy. However,
there are many other types of errors that can occur at every stage of the memory process.
Let's start at the encoding stage. Well the first thing to understand is that what
we experience, what we think we perceive about the real world around us, is largely fake
to begin with. Presuming that there is an objective reality out there, what we perceive
is actually only a subjective reconstruction of that reality created by our brains. Let's
have a look at a demonstration. I'm going to show you two pictures from the crime scene
that will be shown in an alternating sequence.
Can you spot the differences between the two pictures?
If you're one of the people who didn't spot the difference, here are the two images
again, but this time with the grey screen that was inserted between the images, removed.
Most people wouldn't immediately have spotted that very large and obvious change where we'd
removed an entire car from the picture. So, your impression of viewing those images was
that you were looking at the whole of the picture in all its detail. It doesn't feel
like you're only seeing a part of the picture. In reality, however, that's exactly what's
happening. Your brain was focussing in on particular narrow elements of the picture,
and essentially guessing what was in the bits you weren't attending to and filling them
in for you to give the impression of a complete picture. Hence many people fail to notice
immediately when we remove, for example, an entire car from the image.
What this sort of effect tells us is that our experience of the world shouldn't always
be trusted. What we see around us is no more than an elaborate reconstruction that our
brain makes, interpreting information from our senses. What we're seeing is not the
objective reality of what is out there. There's a correspondence between what we experience
and the objective reality, but it's far from perfect.
Our brain constructs an impression of the outside world that is generally convincing,
but it does sometimes fall down. And this is what is happening when we view visual illusions.
In fact, two people can experience the same event quite differently. Have a look at this
classic image by Fischer in 1968. Some of you will see an old woman, but some of you
will see a young woman. And others will see the old man.
So - not everyone interpreted that image in the same way. Different people saw different things.
Your experience of the world around you is coloured by expectation and open to
influence. So, if you witness a crime it's entirely possible that you might misperceive
some elements of the events in the first place, even before you have to remember them. The
circumstances of the crime, like whether the offender has a weapon, can also influence
how the witness remembers the crime.
The weapon focus effect suggests that when a weapon is involved, the witness's memory
of the criminal's physical appearance is impaired because the witness's attention
is drawn to the weapon and away from the criminal's face. This is something that has been demonstrated
in laboratory studies, where researchers have been trying to find out why this happens.
One explanation for this effect is that witnesses are so scared by the weapon that they fixate
on the source of the threat to the exclusion of more peripheral cues. Another explanation
is that the weapon is an unusual or surprising item and so it draws the witness's attention
away from other things in the scene. Jonathan Fawcett, of Dalhousie University, and his
colleagues, they did a study combining all previous research in what's called a meta-analysis,
which suggested that there's some empirical support for both of these explanations.
The weapon focus effect can be harder to reproduce in real world settings, leading to some people
to suggest that it's only something that happens under simulated conditions. However,
recent research indicates that the problem with the real world work is that it is compromised
by other issues. For example, a suspect with a weapon is more likely to be bolder in terms
of getting closer to a witness or staying at the scene longer, where the negative effect
of the weapon on the eyewitness's memory of the suspect's face is cancelled out by
the fact they had a better opportunity to see the suspect. When Jonathan Fawcett examined
this in their meta-analysis, they did find a weapon focus effect that was unaffected
by whether the event was simulated or real.
So what does the weapon focus effect mean for eyewitness testimony? Well, we need to be
aware of the potential for a witness's memory accuracy to be worse in situations where a
weapon is present, though the size of this effect depends on a number of interacting
factors that need to be taken into account, such as length of time that passes before
the memory is retrieved, the duration of the original event, and the level of threat. Another
implication is that it may be possible to train people likely to be exposed to situations
involving weapons, such as police officers, to avoid this bias.
So, that's one example of a bias that affects the encoding stage of eyewitness memory.
However, every step of the memory process is vulnerable to distortion. In the next presentation
we'll talk about some factors that can affect the storage and retrieval stages.