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 also is called iconic memory (the word "icon" refers to "image"). 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 of iconic memory was performed by George Sperling (1960). Sperling asked participants in one group 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 interpreted this finding to mean 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 within 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 forgetting of sensory memories.
Level of Awareness of Sensory Memories
Sensory memories typically are processed at the preconscious level of awareness. For example, in Section 2-1, you learned about the so-called cocktail-party phenomenon, in which your attention shifts to a conversation (one to which you were not attending) after something important is said in that conversation, such as someone mentioning 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 of the information, such as the intensity of a sensation (for instance, whether a flash of light was bright or dim).
The preconscious processing of sensory information 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 may have complex and long-term effects on our thoughts, motives, and behaviors (see Elliston, 1999). For example, some have claimed that subliminal perception in advertisements causes us to buy products later at the store (called subliminal persuasion) or that information provided subliminally can improve our mental functioning (for example, our memory). These claims, however, 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 that works subliminally (preconsciously), there is no good evidence that subliminal perception has a stable and pervasive influence on our behavior beyond the immediate one of shifting our attention to new stimuli.
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. Sensory memories for all five of the traditional senses are thought to be very brief in duration, and never more than a few seconds in length, at most.
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) that it can hold at any one instant. There is some evidence suggesting that the iconic store can hold perhaps about 15-20 "icons" at any one time. The capacities of the other sensory-memory stores have not been well studied because of the difficulty of doing this: these memories last no more than a few seconds and they generally are at the preconscious level.
Encoding of Sensory Memories
Sensory memories essentially are reproductions of the original perceptions that were processed in 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. Thus, there is very little processing (encoding) of sensory memories beyond the simple perceptions that are the end result of relatively superficial sensory processing in various areas of the brain.
Forgetting of Sensory Memories
It is thought that sensory memories consist of physiological changes in the brain — changes that appear and disappear very rapidly. Thus, sensory memories are forgotten very rapidly. This explanation of the forgetting of sensory memories is referred to as decay theory because the neural traces of the memories are thought to 'decay' or disappear. 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 will transfer 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 few seconds at most. Thus, we forget almost every perception that we experience (preconsciously or unconsciously) during our lives.
Figure 1 summarizes the discussion of sensory memory in this section.
Figure 1. A summary of the major characteristics of sensory memory
Study Questions for Section 6-6
- What is iconic memory? In which subsystem of the SMM does it belong?
- What did Sperling (1960) do to show the existence of iconic memory?
- About how long are iconic and echoic memories stored?
- How is sensory memory involved in subliminal perception?
- What is "subliminal persuasion" and what does the evidence show about its effectiveness?
- How are sensory memories encoded (that is, what form do they take)?
- What is an engram?
- What is the major cause of forgetting from sensory memory?