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Memory and consciousness are among the greatest mysteries of the mind.
How is it that we can consciously recollect events that have happened to
us in the past? Many people have been tempted to compare human memory with
the memory storage devices of computers, and this analogy has indeed proved
beneficial in the understanding of certain aspects of memory. But while
computers, like people, can retrieve information, they do not have any of
the curious feelings or experiences that memory gives us.

Consider some of these experiences: everybody has had the feeling of
deja vu, where we have a strong sensation that something we are actually
experiencing for the first time is familiar. Or consider the ‘tip-of-the-tongue’
phenomenon when we know that a word or name is stored in our memory, but
we cannot retrieve it. Often, a person can even make accurate judgments
about what a word sounds like or how long it is, yet still be unable to
retrieve it. Here is a situation in which we are conscious that our memory
is providing a great deal of information about a word, but not consciousness
of the word itself.

It is almost taken for granted in traditional accounts of memory that
remembering is accompanied by a sense or experience of the past, and also
that the act of remembering is accompanied by a conscious feeling or awareness
of the past. In fact, this simple view allowed psychologists to ignore consciousness,
which for many years during the behaviourist era of the 1940s and 1950s
was virtually taboo as a subject of study. But renewed interest in conscious
phenomena, together with new experimental techniques, have made consciousness
a flourishing research topic. A new perspective has emerged on the role
consciousness plays in our behaviour which is leading cognitive psychologists
to overturn many laws of memory previously thought to be unassailable.

Novelists have been keener than psychologists to ponder the relationship
between memory and consciousness. One of the most graphic accounts of the
experience of memory is in Marcel Proust’s novel Remembrance of Things Past.
As a result of eating a piece of madeleine cake with his tea, Proust’s hero
had a strong feeling of familiarity and a sensation that the cake was reminding
him of some past event that he was unable to reconstruct. He describes how
no sooner had he tasted the cake ‘than a shudder ran through me and I stopped,
intent upon the extraordinary thing that was happening to me. An exquisite
pleasure had invaded my senses, something isolated, detached, with no suggestion
of its origin.’

He knew that the curious sensations he was having were not simply due
to the cake: it wasn’t that the cake was so delicious that it could be causing
those strange experiences. ‘I sensed that it was connected with the taste
of the tea and the cake, but that it infinitely transcended those savours,
could not, indeed, be of the same nature.’ Instead, he immediately attributed
the sensations to memory: he knew that a memory was trying to force its
way into his consciousness. Eventually he was able to draw the memory out
into consciousness and was transported back to an occasion in his youth
when his grandmother had given him a piece of the same cake. This simple
event, happening late in the hero’s life, summoned up long-lost memories
of the utmost vividness for him.

The central question that has motivated recent research is whether the
relationship between conscious experience and memory is as simple as is
usually thought. Are we always aware that we are remembering? Psychologists
and neurologists have been intrigued to discover some circumstances in which
awareness of remembering can happen without the retrieval of any actual
memory, as, for example, in Proust’s case, and some in which memories appear
to be able to express themselves in the absence of awareness. Thus the apparently
intimate relationship between the retrieval of memories, on the one hand,
and the conscious experience of memory, on the other, can be broken.

Consider, for example, people who can retrieve information without being
aware of it. Neurologists have known for many years that patients with damage
to the temporal lobes of the brain can lose their memory. Such patients
are typically unable to consciously recall even recent events. One patient,
‘ HM’, was subjected to scores of psychological tests on consecutive days.
By the second day he had usually forgotten everything about the tests, and
could not remember seeing them before.

Yet at the same time, amnesiac patients such as HM are clearly influenced
by unconscious memories of past events. For example, when amnesiacs read
a list of words, including, say, the word ‘reed’, and are later asked to
remember those words, they have great difficulty doing so. But if they are
asked instead to spell the spoken word, they will be more likely to spell
it as ‘reed’ rather than the more common ‘read’. The word is in memory to
the extent that it influences their spelling, but is unavailable for conscious
recall.

Memories that reveal themselves in the absence of awareness have been
termed ‘implicit memories’, and the study of implicit memory has become
particularly interesting to psychologists. In many situations, ‘normal’
people who cannot consciously recollect an event can be shown nonetheless
to retain some unconscious trace of that event. We appear to ‘know’ far
more than we can consciously report.

Imagine that in a psychology experiment you are shown a list of words
(this is called the ‘study’ list) that includes, for example, the word ‘mystery’.
The study episode is followed some weeks later by a test. It is unlikely
that you would then be able to remember many of the words. But if you were,
instead, shown the word fragment ‘ ys e y’, and simply asked to complete
this fragment, you are more likely to produce ‘mystery’ as a completion
than you would if you had not read that word previously. Although you cannot
consciously remember the word, it has left behind some record in your mind.

Psychologists have now invented a variety of such implicit memory tests.
For instance, if a word is flashed in front of you for about 0.035 seconds,
it will be difficult to name the word, because the exposure duration is
so short. But if you had read the word, say, 24 hours previously, you would
be much more likely to name it. Again, this is an implicit memory test:
the instructions do not require you to consciously remember any previous
study episode. Your ability to say the right word indicates that a prior
episode can influence future behaviour without awareness.

More traditional memory tests are explicit because they do require the
person to consciously remember a prior episode. The most common of all memory
tests, free recall, is typical of this: some time after reading a list of
words, the person is asked to remember the words they saw previously. Another
commonly used explicit memory test is recognition. Here, several words are
presented and the person is asked to consciously remember which of them
they had seen earlier.

So results from implicit tests show that memory and consciousness can
readily be separated. Several researchers have claimed that such findings
provide evidence for distinct memory systems in the brain. One of the most
forceful proponents of this view is Daniel Schacter of Harvard University.
He believes that quite different memory systems of the brain underlie performance
on explicit and implicit tests, and that the conscious experience of memory
is associated with only one of those systems, the explicit one. The strongest
reason for supposing there are two independent systems comes from the discovery
that a variety of factors have very different effects on performance in
explicit and implicit memory tests.

Some of the major examples of these factors are worth considering, because
they show that many of the cherished generalisations about memory that psychologists
have held to be correct are false. For a start, it has been known for many
years that memory for pictures is better than memory for words when explicit
memory tests are used. Subjects can recall a large number of objects better
if shown pictures of the objects rather than a list of the names of the
objects. For instance, it is easier to remember an elephant from a picture
than from the word ‘elephant’.

But this fact about memory had always been demonstrated using explicit
memory tests. It turns out to be possible to demonstrate the opposite effect,
of words being easier to remember than pictures, if memory is assessed by
an implicit test, such as completing word fragments. Such demonstrations
were first made and largely ignored in the early 1970s, but have now been
confirmed by more recent studies.

The results of implicit memory tests disrupt another long-standing view
of memory. Psychologists have shown countless times that people quickly
forget a list of words studied once when they are assessed using an explicit
test such as free recall. Within a comparatively short time, little information
can be consciously retrieved. Yet an implicit test, even six months after
the learning episode, shows that people retain an impressive amount of detail
about the material.

Psychologists have also long known that interference is a major source
of forgetting in explicit tests: for many years, this was taken to be a
ubiquitous feature of memory, and the explanation of why forgetting is often
so rapid. If you spend some time learning one set of information, and then
some time learning a second set, the second set will interfere with the
first and make it harder to remember. But recent research has failed to
find such interference when implicit tests are used to measure retention.
This implies that some other – as yet unknown – mechanism also contributes
to forgetting.

A final factor concerns what is called the levels of processing effect.
If people are shown a list of words and asked to remember them at some later
time, their performance is strongly affected by what they had to do with
the words in the list. If they are asked simply to read the words and then
perform some trivial task such as counting how many letters were in each
word, subsequent memory performance will be poor. On the other hand, if
they had to process the meaning of the words, for example by finding a synonym
for each word, subsequent performance on memory tests will be greatly improved.

Advertisers constantly take advantage of this effect: they know that
if they can persuade us to concentrate on the meaning of their slogans,
we remember them much better. But the interesting feature of this ‘levels-of-processing’
effect is that it shows up only on explicit tests of memory. On implicit
tests, it makes no difference what a person did with the words in the study
list. So this constitutes another piece of evidence that implicit and explicit
memory are distinct systems.

Mapping memories in the brain

Some researchers have been so convinced by these differences that they
have tried to localise the two systems in the brain. A new and very exciting
procedure for monitoring the brain is called positron-emission tomography
(PET). In this technique, the subject is injected with some radioactively-labelled
water which is taken up by cells in the brain and which emits particles
that can be detected in a positron detector. Areas where many particles
are being emitted correspond to parts of the brain that are particularly
active and have a high rate of blood flow and oxygen uptake.

Several teams, including that of Michael Posner and his colleagues at
the Washington University School of Medicine in Missouri have been using
PET on subjects who have been required to perform various memory tasks.
Although only in its infancy, the technique has produced evidence that different
parts of the brain are involved in different memory tasks, and could confirm
that the explicit and implicit memory systems are located in different parts
of the brain.

Other researchers, however, have argued that the data from implicit
and explicit learning experiments can be understood without the need for
distinct memory systems. First, the different results produced by implicit
and explicit tests can be understood without needing independent systems,
and secondly, there is evidence that the relationship between memory and
consciousness is more subtle.

Henry Roediger, of Rice University in Houston, has been advocating this
alternative position, and has produced convincing evidence to support it.
The cornerstone of Roediger’s theory is that there are different ways in
which people can mentally process a stimulus such as a word or picture.

Imagine a continuum of descriptions of mental processes or operations.
At one end are processes that deal with the meaning of an item – Roediger
calls these ‘conceptually-driven’ operations. Thinking of a synonym for
a word would be an instance of this. At the other end are operations that
involve ‘data-driven’ processing. One example would be seeing a word for
a fraction of a second and trying to read it. Other tasks involve different
combinations of these two processes.

Roediger has used this notion to account for the results of implicit
and explicit memory experiments. The idea is that memory works best when
the processes that occur when an item is ‘encoded’ in memory are the same
as those that occur when it is retrieved. For example, suppose that the
task the subject performed while studying a list of words involved conceptually-driven
processing, such as thinking of a synonym for each word. If the test involves
free recall,again a conceptually-driven task, good memory performance is
expected on the basis that the same type of mental operations are required.

But if the task had involved counting the number of letters in each
word, a data-driven task, there would be a mismatch between what was done
at study and at retrieval, and memory performance would be poor. This is
how Roediger explains the levels-of-processing effect we saw earlier. He
has shown that many of the differences between implicit and explicit memory
can be understood without the need to postulate different memory systems
in the brain.

But what about people who feel that they are remembering something without
being able to retrieve information from the past? Larry Jacoby, of McMaster
University in Canada, has argued that the feeling of conscious memory can
often be an illusion, and in the process has questioned yet another common
belief about memory – that subjective experiences of memory require the
existence of corresponding memory traces in the brain.

Consider again the experience of Proust’s protagonist. He was having
some conscious experiences which had a powerful effect on him, yet he knew
that the cake itself was not the cause of them. Something about the cake
seemed familiar. He attributed his experiences to memory, and assumed a
memory was trying to force its way into his consciousness.

The odd familiar feeling

This suggests that we use the feeling of familiarity as a clue that
we are retrieving information from the past. If we process a word, for example,
more fluently than it would normally warrant such that it seems more familiar,
then we attribute that familiarity to memory. We ‘remember’ having recently
seen the word, even though the fluency might in reality be due to some other
reason, such as the word having a particular meaning or ‘salience’ for us.
Our conscious experiences of memory are then based on the inferences we
make given such clues. Another clue we seem to use is whether we have a
vivid image or not. If we do, we infer that we must be remembering, even
though the image could have originated from somewhere else.

In one experiment by Jacoby and his colleagues, subjects were shown
a list of words which they were asked to try to remember. Then they were
presented with a test word, and had to say whether they recognised it. The
catch in this experiment was that these test words were presented with different
degrees of clarity, such that some were slightly easier to read than others.
The variation in clarity was sufficiently subtle, however, as to be unnoticeable
to the subjects.

The notion was that the clearer words would have been easier to process
than the less clear ones, and as a result subjects should have been more
likely to say, incorrectly, that they recognised the clearer words, even
if the words had not been previously presented. And this is exactly what
Jacoby found: the subjects were fooled into thinking they were remembering
some test words when in fact they were not. Jacoby’s findings seem to confirm
the idea that the conscious experience of memory can sometimes simply be
the result of an inference we make, based on the quirks of our mental processes.

Experiments on the connection between memory and consciousness are heralding
an exciting era of research in psychology. Already, researchers have had
to revise many of their long-held beliefs about the nature of memory. Whether
the evidence supports the idea that there are separate memory systems in
the brain or not will depend on continuing investigations of people’s conscious
experiences as they retrieve information. We will have to hope that psychologists
are as successful in uncovering the laws of memory as Proust was in recalling
to consciousness ‘this unremembered state, which brought with it no logical
proof, but the indisputable evidence, of its felicity, its reality, and
in whose presence other states of consciousness melted and vanished’.

David Shanks is in the Department of Cognitive Science at the University
of California at San Diego.