You Can Learn to Remember: Change Your Thinking, Change Your Life pdfdrive com

Part of the brain might even register the mushroom shape and evoke images of
atomic explosions. Part will respond to the redness, recalling blood and danger
signs. Most of the memories will be so momentary that we will not notice them,
but many of them will play a part in governing our actions.
Since the nineteenth century scientists have speculated that the rich variety
of our memories could be broken down into separate categories, and that each
one might exist in a different region of the brain. Although their attempts to find
these regions have had limited success, a few of the classifications have
survived. The most important of them distinguishes between sensory memory,
short-term memory (STM) and long-term memory (LTM).
Sensory memory has the shortest duration. The raw information gathered
by our senses – sight, hearing, taste, smell and touch – flows into a sensory store,
which is distributed between different regions of the brain. Each sense has its
own associated region, which is responsible for processing its input. For
example, visual information is dealt with toward the rear of the cortex, while the
primary hearing centre is in the temporal lobe (a part of the cortex at the side of
the brain). There are also so-called association areas in the brain, linking the
sensory regions and allowing all the different inputs to be pulled together into a
coherent whole.

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The amount of information that can be held by the sensory store is
practically unlimited, although the sensory data generally lasts for only a fraction
of a second before it is replaced by new stimuli. An image in the visual cortex –
called an icon – lasts long enough for a modern movie, projected at 24 frames
per second, to seem continuous (the image of each frame is still in the mind
when the next is projected). But a silent movie, projected at its original speed of
18 frames per second, appears to flicker because the icon of each frame has
already begun to fade before the next appears. Auditory information seems to
last longer than data from the other senses, lingering for several seconds before it
fades from our sensory memory.
The sensory store filters the signals from the senses and monitors them at
an unconscious level. The vast majority of sensory information is almost
immediately discarded, but a tiny percentage is selected by the monitoring
procedure because it meets certain criteria – for example, an image may be
intensely coloured, or fast-moving, or an overheard sentence may contain a
familiar name – and is passed on to the short-term memory. This is not a simple
one-step process. To the sensory memory, an apple is nothing more than a red or
green, shiny, round solid. For us to perceive an apple, this information must first
go to the long-term memory – also known as the permanent or reference
memory – to be compared with the elements already there, in an effort to
recognize what it is that we are seeing. Only after some sort of approximate
match is found can the brain create a short-term memory. The whole complicated
sequence is almost instantaneous.
Short-term memory is also known as the active or working memory,
because it depends on the electrochemical activity of excited neurons, and
because it is often used to achieve specific tasks – such as adding up a bill.
Short-term memory generally holds information for only ten to twenty seconds,
but it is vital to any activity that requires conscious thought – even a simple
activity such as understanding a sentence.
However, STM has a limited capacity. It can normally hold approximately
seven pieces of information simultaneously (see
p.51
) – whether they are
numbers, words or images – and any new input displaces whatever is already
there. As a result, a short-term memory is easily lost because of distractions,
either from outside sources or from other thoughts. Nevertheless, if such a mem
ory is sufficiently powerful – because it is the focus of intense concentration, or
is repeated over and over, or is particularly surprising or emotional – it can
become a long-term memory. This happens when the neurological activity
associated with STM changes the physical structure of the brain (see
p.42
), a
change that may last anything from minutes to decades. In fact, it is possible that

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all long-term memories last for life, but that some of them are more dif ficult
than others to access. The memory trace remains some where in the brain, but
we are no longer sure how to find it.
Long-term memory was the subject of research by psychologists and
computer programmers in the 1970s. A distinction was made between different
types of long-term memory stored by the brain: declarative (or explicit)
memories and procedural (or implicit) memories. Declarative memories allow us
to name things, and to recognize what is meant by names. They are the sum of
the facts and information that we have accumulated over the course of our lives.
These include mundane recollections, such as what we had for dinner last night
(which is unlikely to last for more than a few days), and momentous occasions,
such as births and deaths (which will probably last many years). All the
memories that relate to events in our own lives are called episodic. They are
affected by the passage of time and will fade according to how rarely or how
often we recall them, as well as the importance we placed upon the incidents
when they happened. The more powerful an impression an event makes, the
longer our memory of it will last. Factual memory is the name for more
impersonal knowledge, such as mathematical formulae or lines from
Shakespeare. Semantic memory is what gives meaning to all this information, so
that when we remember or hear the phrase “A rose by any other name ... ” we
know that a rose is a flower, that its stem is thorny, that it has an attractive
perfume and is often sent as a romantic token, and so on. Although this system
of classification is popular among some psychologists, others have described it
as artificial, believing that it may not reflect major differences in how the brain
remembers: learning a play by Shakespeare, they argue, is itself an episode in a
student’s life, and the lines may be stored in the same way as the memory of a
birthday party.