Marc Green

Seminar Available on this topic.

Causes of Memory Unreliability

1. Memory is "blurred"

There are several reasons for this. One is that images in our mind's eye are never as clear as an actual perception. If you try to recall your bedroom, you can get a general image of the location of large and significant objects, their shapes, colors, etc. But the image is not nearly as detailed as what you would see if you were actually viewing the room.

People are much better at discriminating between two objects when they are physically present than when one is present and the other is in memory. Two colors which are easily distinguishable when presented side by side may be confused when one or both must be recalled in memory. In fact, while humans can distinguish thousands (some say millions) of physically present colors, one study suggests that they can identify only 17 in memory.

Color is a particular good example of memory's low resolution. While there are thousands of discriminable colors, color memory is very limited. Research suggests that people group colors into about 11 categories: white, black, red, green, blue, yellow, brown, orange, purple, pink and gray. Memory will easily distinguish between colors of different categories (red vs. blue) but will have a very difficult time distinguishing shades within a category (blue-green vs. blue-violet.)

Color memory also has some biases. People typically remember colors as being closer to a purer color category, so an orange-red will be remembered as a more perfect red. For example, people often think of tomatoes as being red, but check them the next time you go to the supermarket: they are usually very orangey. People also remember colors as being brighter than they actually were. I have a gray Toyota Camry, similar to the one shown on the left. After shopping when I return to the parking lot, I often mistakenly head toward a "silver" Camry like the one shown on the right in the belief that it must be my car. Viewed next to each other, gray and silver cars are quite distinct. In memory, they are not.

Another problem is that memory often stores perceptual information in verbal form rather than as an image. That is, a person might see a blue car. This information is stored in memory as the words "blue" and "car." Later, the person will not be able to identify the shade of blue because memory has only stored the fact that the car was blue. Worse, the car perception may have been stored as "dark" and the person will not be able to distinguish among dark colors - dark blue, black, etc. This process of converting images to word-like forms is automatic and people are unaware when they do it.

Several authors have concluded that memory simply encodes the general gist of a scene. For example, memory may code a memory of a person as short, tall, young, fat, thin, old, white, black, rugged, etc. or some combination. If coded as "tall" and "fat", the person will have difficulty discriminating among different tall, fat people. Moreover, the witness might well characterize the person as being jolly, since fat people are often stereotyped as being jolly. Memory reconstruction (see below) often uses general knowledge and expectations to fill in blanks of specific memories. In short, the eyewitness confuses information stored in semantic and autobiographical memories.

2. Memory Fills In The Gaps

Memory is a reconstruction, not a record. As noted, memory traces are, at best, highly impoverished versions of the original percept. The eyewitness will often have insufficient information in the memory itself, so the reconstruction must invoke pieces of information from other sources. There are two main sources of additional information: 1) pre-existing schemas and 2) other memories. People understand the world through "schemas" and "scripts," stereotyped mental models of objects and events. When they recognize a situation, either in perception or in memory, they invoke the most applicable schema or script and may unconsciously fill in missing information in order to complete the reconstruction.

Further, people confuse information sources. The example at the top of this page provides a classic case of the fact that people often confuse the source of their memories. In this case, the eyewitness confused two actual events. In other cases, people confuse actual events with imagined memories. For many years, for example, I believed my earliest childhood memory to be of myself on the sand in Miami Beach. My parents had moved there when I was very young, but returned to Pittsburgh after only a year.

3. Memory systematically distorts perception

Memory tends to distort perception in systematic ways. For example, people tend to remember colors as being brighter and more saturated than they actually were. Other studies show that people who are asked to recall vehicle speeds tend to overestimate slow speeds and to underestimate fast ones. Additional studies show systematic biases in remembering distance and size. Lastly, as noted above, memory also biases toward expected events.

4. Memory is personal

Human memory does not exist so that an observer may accurately report previously seen events. The actual, physical events are merely grist for the mill of interpretation. Each witness extracts an interpretation that is meaningful in terms of his own beliefs, experiences and needs. Once the interpretation occurs, the events themselves become relatively unimportant. Moreover, since each person interprets the events in terms of his own world view, different eyewitnesses observing the same event may have different interpretations and different memories. To put it succinctly:

"We do not see what we sense. We see what we think we sense. Our consciousness is presented with an interpretation, not the raw data. Long after presentation, an unconscious information processing has discarded information, so that we see a simulation, a hypothesis, an interpretation; and we are not free to choose" (Norretranders, 1999).

Eyewitnesses incorporate information learned after the event into memory. For example, they may talk to another witness and use information from the conversation to fill in their reconstruction of the events. They may do this by combining two memories into one or by using bias or expectations of what probably was seen.

As people recall an event over and over, they drop details from earlier versions and add new details to later versions. All things being equal, accuracy declines with each new version, at least until an asymptote is reached. In some cases, however, an eyewitness accuracy is lower when questioned immediately after a traumatic event.

References

Baddeley, A. (2004). Your Memory: A User's Guide. Richmond Hill, Canada: Firefly Books.