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Iconic Memory

In the classic Atkinson & Shiffrin model, the first box is labeled sensory registers. These are more commonly called the sensory stores today. The sensory stores are like brief delay systems associated with each sense. They preserve the pattern of stimulation before it enters attention. The sensory stores are sensory systems, but they are also memory systems because they preserve information after the external stimulus is gone. Other names for the sensory stores are sensory buffers, or very short term stores.

What does "icon" mean? What are characteristics of iconic memory?

Iconic memory is the sensory store for vision. The term icon means form or image. Ulric Neisser (1967) proposed this label to convey the idea that iconic memory preserves an exact duplicate of the image falling on the retina. Iconic memory was first reported in the modern era by George Sperling (1963). Sperling tested subjects by flashing several rows of letters on a screen for a split second to see how many letters they could read after very short exposures.

What is a tachistoscope? What did Sperling do?

Sperling used a tachistoscope (pronounced tuh-KISS-tuh-scope), an instrument invented by Volkmann in 1859 to replace a then-current methodology of using electric sparks to produce brief visual exposures. The tachistoscope used a camera-like shutter to flash a picture onto a screen for a brief time measured in milliseconds (thousandths of a second). In Sperling's experiment, subjects saw an array of letters like this flashed very briefly on a screen:

W P X T
M R C S
L H Y D

Subjects were asked to read as many letters as possible during the brief flash. Usually they could only read 3 or 4 letters. Next Sperling tried a variation of his experiment called the partial report method. After he flashed the letters he sounded a high, medium, or low tone. Depending on which tone was sounded, the subject read the high, medium, or low row of letters.

When was the tone sounded, in the "partial report" experiment?

Because the tone came only after the flash of letters, you might think it would not do the subjects any good. However, Sperling found that as long as the tone was sounded within 250 milliseconds (a quarter second) of the flash, subjects could report 3 out of 4 letters from any row. Apparently they preserved a memory of the entire image for a quarter second.

What function does iconic memory serve?

Why does this memory system exist? Eye movements take about a quarter second, and during a sudden eye movement, visual information from the eye to the brain is interrupted. During this quarter second, the iconic memory system preserves information from the last place where the eye stopped (the previous "eye fixation"). Therefore iconic memory helps maximize useful information available to the visual system, preserving information from one eye fixation while the eye moves to the next fixation point.

When did Sperling's subjects think the tone was sounded?

Partly because of iconic memory, we are unaware of the gap between eye fixations. Similarly, Sperling's subjects were unaware of the gap between the flash of letters and the tone. They simply waited for a tone and read the appropriate row. They believed the image of letters was still showing on the screen when the tone sounded. Actually they were reading the letters from their iconic image when the tone sounded.

What is another "sensory storage" system for visual information, and how long does it last?

The iconic image is complete. It contains all the sensory information available from the retina of the eye. However, it lasts only a fraction of a second and cannot be conjured up voluntarily at a later time. Probably the location of the iconic image is the circuitry of the retina itself. Complex experimental techniques reveal that individual sensory traces or "pictures" can also be preserved in visual processing areas of the brain for up to five minutes. Unlike iconic images, the image memories which last for several minutes are "accessible to higher level processes" such as attempts to remember a picture (Ishai and Sagi, 1995). This second system might be the one used by people with so-called photographic memory, called eidetikers, discussed later in this chapter.


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