The Nature of Memory Explained

Memory—the glue that holds our thoughts, impressions, and experiences—is generally defined as the capacity to retain information over time, once the information has been learned.

Memory is of course very important to any information processing system because it underlies the ability to learn. There are close links between learning and memory. Something that is learned is lodged in memory, and we can only remember things learned in the past.

With neurologically intact memory system, we can store our past experiences and draw on them. In this way, we can deal with new situations that bear certain similarity to old ones in the way we have in the past, and not have to solve each new problem from scratch.

Types of Memory Systems and Characteristics

Any discussion of memory must include the notion that there is no single type of memory. Cognitive research in this area of cognitive psychology has led to consistent finding: the existence of functionally distinct memory systems. Before we delve deeper, it is apt to lay out some of the characteristics and features of these individual memory systems.

Memories are characterized with the following variables:

  1. duration, which pertains to how long information remains retrievable in a memory system;
  2. capacity, which involves how much information the memory system can retain; and
  3. coding, which means the particular type of information the system contains.

Now, we summarize the various memory systems.

Sensory Memory

Sensory memory is the repository for incoming stimuli (i.e., raw, unanalyzed data that are derived from the senses are held very briefly). The purpose of sensory memory is to maintain the representation of a stimulus long enough so that it can be recognized. Although you may have glanced at a visual scene for only a very brief time, say, 100 milliseconds, a representation of the scene is preserved in sensory memory for longer than that. This gives the information a chance to be salient and operated upon by selection and pattern recognition mechanisms.

Sensory memory Opens in new window consists in various forms, mainly one for each of the five senses and each has different characteristics. Iconic memory Opens in new window, for instance, is a visual sensory memory. It holds a brief “snapshot” of what you have just looked at. Iconic memory has a very short duration; it lasts only about 250 to 300 milliseconds (Averbach & Sperling, 1961; Sperling, 1960).

Echoic memory Opens in new window, an auditory sensory store, is another form of sensory memory. You can think of it as an “echo” of what you have just heard. It lasts considerably longer than iconic memory, on the order of several seconds longer (Darwin, Turvey & Crowder, 1972).

There other forms of sensory memory, however, these two are the most researched. The coding or representation of information in sensory memory thus varies with the modality. Iconic memory stores visual representations; echoic memory, auditory ones.

Working Memory

Working memory (also sometimes called short-term memory (STM)) is the ability to hold a limited amount of information on-line over the short term while the information is being actively used or processed. Some researchers often use both terms interchangeably, as I also in this entry.

As aptly hinted by its name, information in short-term memory is only stored there briefly. However, the duration of items residing in working memory is much longer than that of items residing in sensory memory.

Working memory Opens in new window is limited to storing just a handful of items. On average, individuals can retain about seven items, give or take two, commonly denoted as 7+(or -2). This limit has come to be called the “magical number seven” (Miller, 1956).

In common parlance, you might think of working memory as the mental equivalent of a cross between Twitter and Snapchat. Like Twitter (restricted to 280 characters), working memory is limited in its capacity, in this case to approximately seven items. Like Snapchat, which disappears after 10 seconds, information is only held in working memory for a relatively short amount of time, on the order of seconds to minutes.

Long-Term Memory

Long-term memory is the system we use to remember information for extended periods. When you remember the name of your fifth grade teacher or the correct route to class, you’re using your long-term memory Opens in new window. Most psychologists believe that the capacity of long-term memory is effectively unlimited.

There are no limits to what we can potentially remember, but not everything we experience gets stored. What types of memories are stored in long-term memory?

At the broadest level, we can distinguish between two general types of long-term memory: declarative (or explicit) memory Opens in new window and non-declarative or implicit memory Opens in new window, which differ largely on the level of conscious awareness devoted to processing.

Declarative memories are those for which we have conscious awareness, while non-declarative memories are acquired below such awareness (Schacter, 1987). Further, declarative memories come in two types: episodic and semantic.

Episodic memories are those memories that concern specific life events or episodes, and contain temporal and spatial information, such as the memory of the details surrounding your first kiss (learn more here Opens in new window).

Alternatively, semantic memories concern facts and general knowledge acquired throughout our lives, but lack distinct temporal or spatial information, such as knowing that the capital of France is Paris, but being unable to remember the exact event in which you learned this information (learn more here Opens in new window). On the other hand, non-declarative memories encompass procedural memories Opens in new window, which include memories for performing skills that often cannot be verbally expressed, such as knowing how to ride a bike.

Memory has also been defined in terms of how it is structured. Traditionally, a multi-store model Opens in new window of memory has been widely accepted among scholars. Long-term (LTM) Opens in new window and short-term memory (STM) Opens in new window are generally considered to be two forms of storage in such a model.

Although STM and LTM Opens in new window are often considered to be two separate storage spaces, researchers have also argued that they can be viewed as elements on a continuum (Melton, 1963) and that information can often be held in both stores simultaneously (Waugh & Norman, 1965).

Stages of Memory in the Learning Process

Memory is the central mechanism in the learning process, such that all information encountered by an individual is encoded, stored, and retrieved from memory. Therefore, although learning may be the process of encoding, storage, and retrieval of information, memory is the processor that makes it possible.

Memory processing can be broken down into three stages.


The first stage is encoding, which occurs when new information or experiences are initially acquired and transformed into mental representations in the brain.


Storage is the second stage by which information is stored into long-term memory and converted into a usable form. A related process is consolidation—another process by which information is transferred from short-term memory to long-term memory.

Consolidation “strengthens” information so that it lasts longer. In other words, consolidation gradually stabilizes the information over hours, days, and even years, making them resistant to interference from other competing information.

Once in long-term memory, information may not be used immediately. We say that the information is in a state of storage. Stored information is represented, but not available for immediate use. As alluded to above, this information may sit in limbo for a very long time.

Some have proposed that this second stage be further broken down into two substages: stabilization and enhancement (Walker and Stickgold, 2006), while others argue that the consolidation process also involves qualitative changes or transformations in memory representations (for review see Payne, 2011; Payne and Kensinger, 2010).


When information that resides within long-term memory is needed, a retrieval process, the third stage, takes place. Retrieval is the act of accessing needed data and making it available for use.

Psychologists who are interested in learning focus on encoding and storage (consolidation), whereas those interested in memory concentrate on retrieval. However, all these processes depend on each other. In other words, they work hand-in-hand.