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When people are asked to memorize a long list of words or numbers, they often find that they have an easier time remembering particular items in the sequence relative to others. For example, let’s say that you are asked to memorize the following word list:

hive   ice   lake   oar   year   care   yarn   air   axe   rod   bear   week   door

Someone reads the list out loud to you and, as soon as she's done, you quickly write down all the words you can remember. It is likely that the position of an item in the list will affect your memory for that item — a phenomenon known as the serial-position effect.

REFERENCES: Glanzer, M. & Cunitz, A. R. (1966). Two storage mechanisms in free recall. Journal of Verbal Learning and verbal Behaviour, 5, 351-360.

Murdock, B. B., Jr. (1962). The serial position effect in free recall. Journal of Experimental Psychology, 64, 482-488.

From Postman & Phillips (1965):

Found at this web site: http://intranet.yorkcollege.ac.uk/yc/new/HUMSOC/psycho/unit1/postphil.htm

In general, people tend to remember best the first few items in a list as well as the last item or two; but they forget most of the items in the middle of the list. In other words, there are two serial-position effects:

• the primacy effect, which is the tendency to remember well the first items in a list;
• the recency effect, which is the tendency to remember well the last items in a list.

Can you think of a good explanation for each of the serial-position effects? Cognitive psychologists have explained them by suggesting that they represent the workings of two memory subsystems. The most popular way of thinking about memory was developed by Atkinson and Shiffrin (1968). They developed what has been called the standard (or modal) model of memory (SMM). The SMM conceives of memory as consisting of a system subdivided into three interacting subsystems: sensory memory, short-term memory, and long-term memory. Sensory memory (comprising a set of five "sensory registers," with one register for each of the five major senses) is a subsystem that maintains a copy of a perception in mind for a very brief period of time: less than one second for a visual sensation and up to about two seconds for an auditory (sound) sensation. Short-term memory is a subsystem that maintains a very limited amount of encoded information in mind for up to about fifteen seconds (unless the person consciously attempts to keep the information in mind for a longer period, perhaps by repeating it over and over). Long-term memory is a subsystem that maintains an unlimited amount of information in mind for minutes, hours, days, weeks, months, or years.

According to the SMM, when a word list is read out loud, the sounds of the words are held very briefly in sensory memory. They are attended to, which moves them into short-term memory. In short-term memory, the person repeats the words or uses some other strategy that will allow the words to be transferred to long-term memory. Once in long-term memory, the memorized words will be held anywhere from a minute or so to years, depending on the strength of the memory. The first few items in a word list are well remembered because these are the items most likely to become long-term memories: individuals have time to repeat them (or use some other memory strategy to memorize them), which causes their transfer to the long-term subsystem. Thus, the transfer of encoded information to long-term memory explains the primacy effect. The last couple of items in the word list are very likely to still be in short-term memory. Thus, the storage of encoded information in short-term memory explains the recency effect. The words in the middle of the list no longer are in short-term memory and are unlikely to have been transferred to long-term memory. Thus, these words are least likely to be remembered.

The flow of information through the three memory subsystems of the SMM is illustrated in Figure 1. Characteristics of each subsystem and the major theories of forgetting from each subsystem also are included.

Figure 1. A Sketch of the Standard Model of Memory (Atkinson & Shiffrin, 1968)

What is Sensory Memory?

If you've ever twirled a lighted object (such as a flashlight or a "sparkler") in a circle at night, you've probably noticed that, if you twirled fast enough, you saw a circle of light. This is because of visual sensory memory, which is called iconic memory (the word "icon" means "image" in this context). In this case, you very briefly held in mind a mental image of the light at each point in the path being traced. If twirled fast enough, the individual iconic memories didn't have time to disappear and, therefore, they melded into a "memory circle," which you perceived as an actual circle of light. According to Baddeley (1993):

This effect was used to measure the duration of the visual memory trace as long ago as 1740 by a Swedish investigator, [Johann Andreas] Segner, who attached a glowing ember to a rotating wheel. When the wheel was rotated rapidly, a complete circle could be seen, since the trace left at the beginning of the circle was still glowing brightly by the time the ember returned to its starting point. If the wheel was moved slowly, only a partial circle would be seen, since the trace of the first part had faded by the time the ember returned to its starting point. (p. 18)

By varying the speed of the wheel, Segner was able to estimate the length of iconic memory to be about 1/10th of a second.

The first modern study on iconic memory was performed by George Sperling (1960). In one condition, Sperling asked participants to look at a blank screen onto which he flashed very briefly (1/20th of a second) three rows of four letters each:

G Z O F
D H V J
X R T P

When asked to recall as many letters as possible, most participants named the first 4 to 5 letters. Sperling believed that each participant had stored an image of the entire set of letters but that, by the time they began to "read" the second row of their mental image, it had disappeared.

In order to test the claim that iconic memory exists and that it stores a detailed visual copy of the original perception for less than a second, Sperling (1960) flashed the 12 letters to other groups of participants but asked them to recall the letters in only one of the three rows. Because it would have taken too long to orally ask them to name, say, the third row of letters (the iconic memory would have disappeared by the time he had finished), Sperling decided to sound tones of different frequencies, with each frequency indicating a particular row to to be recalled: a high-frequency tone to indicate the first row; a medium-frequency tone to indicate the middle row; and a low-frequency tone to indicate the bottom row. He found that, if the tone was sounded immediately after the letters had been flashed on the screen, most participants were able to recall all four letters in the indicated row. Sperling concluded that the participants had a complete iconic memory of the 12 letters — a memory that disappeared in a fraction of a second.

Most studies of sensory memory have looked at iconic memory or echoic (auditory) memory. These studies have demonstrated that sensory memory can be described in terms of five characteristics: the level of awareness at which sensory memories occur, the duration of sensory memories, the capacity of the sensory-memory store, the encoding of sensory memories, and what causes sensory memories to be forgotten.

Level of Awareness of Sensory Memories
Sensory memories typically are processed at the preconscious level of awareness. For example, in Section 1-4, you learned about the so-called cocktail-party phenomenon, in which your attention shifts to a conversation to which you were not attending after something important is said in that conversation (such as a mention of your name). The cocktail-party phenomenon occurs because your sensory memory is processing preconsciously a relatively large amount of sensory information. This processing of sensory information is constant, but it involves only superficial characteristics, such as the intensity of a sensation.

The preconscious processing of information that occurs in sensory memory would fit the definition of subliminal perception, which is defined as perception that occurs below the conscious level of awareness. Thus, subliminal perception allows us to briefly store and superficially process sensory information in order to determine if it is important enough to attend to, as in the cocktail-party phenomenon. However, some have claimed that subliminal perception has complex and long-term effects on our thoughts, motives, and behaviors (see Elliston, 1999). For example, some have claimed that subliminal perception has such a powerful influence that it can cause us to buy products at the store (subliminal persuasion) or to improve our mental functioning (for example, to improve our memory). These claims have little or no scientific evidence to support them (Moore, 1992; Phelps & Exum, 1992; Pratkanis, 1992). Although Sperling's studies as well as those of many others have shown that we have a sensory memory subsystem, which means that we engage in subliminal perception, there is no good evidence that it has a stable and pervasive influence on our behavior beyond the immediate one of shifting our attention to new stimuli.

Some psychoanalysts have claimed that sensory information can be processed unconsciously and that, if it is deemed to be a threat — in the sense that it may be relevant to a repressed conflict — the information is immediately placed into the unconscious level. This process is known as "perceptual defense." It is similar to repression except that, unlike repression, in perceptual defense, information related to repressed material is transferred from the preconscious level to the unconscious level without ever entering consciousness. The evidence for this process is meager: most of the supporting evidence is based on case studies in clinical situations or experimental studies that didn't include adequate controls for extraneous variables (Holmes, 1990).

Duration of Sensory Memories
Sperling's (1960) research demonstrated that iconic memories are stored for a fraction of a second. Echoic memories are thought to last somewhat longer: perhaps as long as 2-3 seconds (Baddeley, 1993). Perhaps you've experienced this when hearing a police siren or a train whistle late at night that suddenly stops: you may still "hear" the echoic memory for a second or two after it has ended.

Capacity of the Sensory Store
When we speak of the capacity of a memory store, we are talking about the amount of information (the number of memories) it can hold at any one instant. The capacity of the sensory stores for each of the senses has not been well studied. There is some evidence suggesting that the iconic store can hold perhaps about 15-20 "icons" at any one time.

Encoding of Sensory Memories
Sensory memories essentially are reproductions of the original perceptions that were processed in the sensory areas of the brain. For example, an iconic memory is a detailed visual image that is similar to the original visual perception; and an echoic memory is an auditory reproduction (an "echo") of the original auditory perception.

Forgetting of Sensory Memories
It is thought that sensory memories consist of physiological changes in the CNS — changes that appear and disappear very rapidly. This explanation of the forgetting of sensory memories is referred to as "decay theory." The physiological or physical change underlying a memory is called an engram[∂] (also known as a "biological memory trace"; Lashley, 1950). Thus, decay theory states that the forgetting of a memory is caused by the disappearance over time of its engram. Once the engram has disappeared, the memory no longer exists anywhere in the memory system: it has "decayed." As stated, the engrams that make up sensory memories are thought to decay very rapidly. Thus, unless a sensory memory is attended to, which then transfers it to short-term memory, it will decay almost immediately. Because we can attend to only a small number of sensory memories, virtually all sensory memories at any one moment disappear within a couple of seconds at most. Thus, we forget almost every perception that we experience (preconsciously or unconsciously) during our lives.

Figure 2 summarizes the discussion of sensory memory.

Figure 2. A Summary of the Major Characteristics of Sensory Memory.

Study Questions