day and then close your eyes, an afterimage stays on your retina for a few
moments. This has been callediconic memoryby Ulric Neisser (1967). We talk
about the auditory equivalent of this asechoic memory:forafewmomentsafter
hearing a sound (such as a friend’s voice) we are usually able to ‘‘hear’’ a trace
of that sound in our mind’s ear. Richard Atkinson and Richard Shiffrin (1968)
referred to these immediate sensory memories as being held in asensory buffer.
When you are holding a thought inside your head—such as what you are
about to say next in a conversation, or as you’re doing some mental arithmetic—
it stands to reason that this requires some type of short-term, or immediate,
memory. This kind of memory, the contents of your present consciousness and
awareness, has been called ‘‘working memory’’ by Alan Badelley (1990), and is
similar to what Atkinson and Shiffrin called short-term memory.
Long-term memory is the kind of memory that most of us think of as mem-
ory—the ability to remember things that happened some time ago, or that we
learned some time ago (usually more than a few minutes ago, and up to a life-
time ago). For example, you might have stored in long-term memory images
from your high school graduation, the sound of a locomotive, the capital of
Colorado, or the definition of the word ‘‘protractor.’’ (Actually, in the latter
case, you might not be able to retrieve a definition of a protractor, but rather a
visual image of what one looks like; this is also a form of long-term memory.)
One of the important features of long-term memory is its durability. That is, we
tend to think of long-term memories as staying with us for perhaps an indefi-
nite period of time. We may not always be able to access them when we want
(e.g., when you have somebody’s name on the tip of your tongue but can’t
quite retrieve it), but we have the sense that the memories are ‘‘in there.’’ This is
in contrast to short-term memories, which decay rapidly without rehearsal, and
are not durable unless they somehow are transferred to long-term memory. The
sensory memory/short-term memory/long-term memory distinction appears
to have validity at the neural level.
Psychologists also talk about different types of long-term memory, but it is
not clear that these reflect different neural systems. Rather, they are different
kinds of knowledge stored in long-term memory. It can be useful to make these
distinctions for conceptual purposes. The psychologist Endel Tulving (1985)
makes a distinction between episodic and semantic memory. There is some-
thing different between remembering your eighth birthday and remembering
the capital of Colorado. Your eighth birthday is an episode that you can re-
member, one that occupied a specific time and place. There was also a time and
place when you first learned the capitol of Colorado, but if you’re like most
people, you can’t remember when you learned it, only the fact itself. Similarly,
we remember what words mean, but usually not when and where the learning
occurred.Thisiscalledsemantic memory. Remembering how to ride a bicycle
or tie your shoe is an example of another type of memory calledprocedural
memory.
It is also important to make a distinction betweenmemory storage(or encod-
ing) andmemory retrieval. One of the tricky parts about designing memory
experiments is distinguishing between these operations. That is, if a subject
cannot recall something, we need to distinguish between an encoding failure
296 Daniel J. Levitin