For a witness to remember something, they first need to see it and notice it. It goes without saying that you cannot remember something if you did not see it. But is it possible to see something, but not notice it?

In producing their initial statements, both Lila and Seth had to make use of their memory for the crime. In Week 2, you considered the reliability of witness memory, particularly for certain types of information.

The branch of psychology that studies the mental processes used by the brain is known as cognitive psychology, and it divides the way the brain deals with information into sets of different processes:

Perception - refers to how the mind interprets the information that is received by our senses (such as sight and hearing). For example, the light that enters our eyes is transformed by the brain into electrical impulses that are analysed to allow us to make sense of the world around us.Attention - is the processes involved in noticing different things around us. As you sit here reading this page, it is likely that the only thing you are attending to is this text, but of course you are also perceiving a great deal of additional information about the world around you. Attentional processes therefore allow us to focus on just part of our environment.Memory - allows us to encode, store and then later retrieve the information that has been perceived and attended to. Of course, not all the information we see and notice is remembered, and even the information that is stored in memory may be quite different to what we actually saw.

So far you have been concentrating on the memory of an eyewitness, but now you'll consider the roles played by perception and attention. One key question to ask here, is what do witnesses tend to notice, and is it possible for a witness to completely miss a significant event that happens right in front of them?

Psychologists refer to the inability to attend to something fairly significant that happens right in front of you as 'inattentional blindness'.

The amazing thing about inattentional blindness is that your mind can perceive what is happening perfectly well, but you do not attend to it. In other words, your eyes can see the mouse and your brain can process and interpret what your eyes are seeing, but for some reason you do not attend to this information and so are not conscious that it is there.

Probably the best known demonstration of inattentional blindness comes from a study by Simons and Chabris (1999) known as the 'gorilla in the midst'. Our giant mouse video is in fact a recreation of the stimuli used in that study. Simons and Chabris (1999), asked participants to watch a film that began with two teams of players (one wearing white shirts and the other black) each passing a ball between the members of their team. The participants were asked to count how many times the members of one of the teams passed the ball. About 45 seconds into the film, a woman wearing a gorilla costume enters the scene from the left, walks through the teams of players and exits to the right. Overall, just 44% of participants noticed the woman in the gorilla suit. This means that over half of the participants must have seen the gorilla (in that they were looking at the screen when she appeared) but failed to attend to it, with the result that they did not even notice it.

The power of the inattentional blindness effect has implications for eyewitnesses viewing a crime. For example, if participants are too fixated on a folder to notice a giant mouse, eyewitnesses might be too fixated on one part of the crime to notice something significant happening elsewhere. Before looking at the implications of inattentional blindness, we'll explore an interesting research tool that has been used to study the phenomenon.

The eyetracker is able to work out the exact point of a computer monitor that someone is looking at, and to track this point of focus as the person looks at different points of the screen.

Eyetracking has also proven invaluable to psychologists, as it provides a record of exactly what a participant focuses on. In the 'Eyetracking' video, the participant watches the 'Observation test' video showing her 'inattentional blindess' while her eyes are being tracked. The computer shows where the participant is looking by drawing a blue line. You will see that the participant is fixated on the red folder as it is passed around, and does not focus on the mouse at all.

Change blindness is the term used to describe a situation in which someone fails to notice that a key element of their surroundings (including the identity of the person right in front of them) has changed.

One of the most important demonstrations of change blindness was a study by Simons and Levin (1998), which started with one researcher initiating a conversation with a participant. After about 15 seconds, two other researchers carrying a door pushed between the first researcher and the participant so that their view of one another was blocked. While the view was blocked, the first researcher swapped places with one of the researchers carrying the door. Once the door had passed, the new researcher then continued the conversation with the participant.

Only seven of the fifteen participants noticed that the person they were having a conversation with had changed, the other eight participants continued the conversation with the other researcher as if nothing had happened. In other words, those eight participants were 'blind' to the 'change' that had taken place.

The implications of change blindness for an eyewitness viewing a crime are very worrying indeed - it could mean that the witness confuses the identities of those involved.

This problem was demonstrated by Nelson et al. (2011) who showed participants a film in which a female student finishes studying in a student lounge and leaves to buy a drink. While she's out of the room, another woman (Actor A) enters and takes some money from the student's table. Two different endings to this scene were filmed. In the first ending, involving 'no change', Actor A leaves the room, turns a corner and walks through an exit. In the second ending, involving a 'change', when Actor A turns the corner she is replaced by a woman (Actor B) of similar appearance to the first.

Of the 374 participants who viewed the 'change' film, only 17 (4.5%) noticed that the identity of the actor changed. Even more worryingly, although 92 of these participants correctly identified Actor A in a line-up, 98 participants selected Actor B. This means that not only can an eyewitness confuse the identity of the perpetrator, they can then go on to identify the innocent person in a line-up.