Why do we need sensory memory?

  • Why do we need sensory memory?

Human Memory

It is good to have an end to journey towards; but it is the journey that matters, in the end.

Ursula K. Le Gui

http://www.wnyc.org/shows/radiolab/episodes/2007/06/08

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Stage theory of memory
Assumes humans have 3-stage
Memory

  • Process by which information is:
  • Recording
  • Encoding
  • Stored in the brain
  • Storage
  • Later retrieved
  • Retrieval
  • Eventually (possibly) forgotten

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Our memory is the process by which information is retained for later use. The basic process by which information is processed follows this format: information is acquired and encoded, which leads to storage in the brain, which leads to the possibility of later retrieval (though as you know at test time, is not a guarantee), and the possibility of eventually forgetting the information.

Today, cognitive psychologists like to compare the human mind to a computer and memory to an information-processing system. I think you can appreciate the analogy. Your PC acquires (or receives) input from a keyboard or a mouse; it converts the symbols into a special numeric code; it saves the information on a hard drive, CD, or disk; it then retrieves the data from the disk to be displayed on a screen or sends it to a printer. If the computer crashes, if there’s not enough space on the disk, if the file was deleted, or if you enter the wrong retrieval command, the information becomes inaccessible, or ‘forgotten’.

Three-System Approach to Memory

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Three-System Approach to Memory

  • Three types of memory
  • Sensory memory
  • Only an instant
  • Short-term memory (STM)
  • 15-25 seconds
  • Long-term memory (LTM)
  • Can hold vast quantities of information and relatively permanent

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Using the computer as a model, memory researchers seek to trace the flow of information as it is mental processed. In this Three-System Approach to Memory, a stimulus that registers on our senses can be remembered only if it 1. Draws attention, which brings it into consciousness; 2. Is encoded, or transferred to storage sites in the brain, and 3. Is retrieved for use at a later time.

Within this information-processing memory approach, three types of memory have been distinguished: sensory, short-term and long-term. Sensory memory stores all stimuli that register on the senses, holding literal copies for a brief moment ranging from a fraction of a second to four seconds usually less. Sensations that do not draw attention tend to vanish, but those we ‘notice’ are transferred to short-term memory , another temporary storage system that can hold seven or so items of information for about 20 seconds, less than 1 minute. Although STM fades quickly, information can be held for a longer period of time through repetition and rehearsa or chunkingl. When people talk about attention span, they are referring to short-term memory.

Finally, long-term memory is a somewhat permanent storage system that can hold vast quantities of information for many years. Science writer Isaac Asimov once estimated that LTM takes in a quadrillion separate bits of information in the course of a lifetime. Mathematician John Griffith estimated that, from birth to death, the average person stores five hundred times more information than the Encyclopedia Britannica. When people talk about memory, long-term memory is typically what they have in mind.

We’ll talk about each of these in a little more detail later on.

Information-Processing Model of Memory

Short-term

memory

Stimulus

Sensory

memory

Long-term

memory

Attention

Encoding

Retrieval

Forgetting

Forgetting

Forgetting

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Many events register in sensory memory. Those that are noticed are briefly stored in short-term memory; those that are encoded are transferred to a more permanent facility. As shown forgetting may be caused by failures of attention, encoding, or retrieval.

Note, however, that this is only a model and does NOT mean that the brain has three separate storage bins. This is only one view of how memory works. There is a radically different view. Most computers process instructions in fixed sequence, one linear step at a time. In contrast, the human brain performs multiple operations simultaneously, ‘in parallel’. Thus, some cognitive psychologists have rejected the information-processing model in favor of parallel-processing models in which knowledge is represented in a web-like network of connections among thousands of interacting ‘processing units’ all active at once.

The two main questions we’ll be asking ourselves throughout this chapter are: How are memories stored? And to what extend are our memories of the past faithful to reality?

Sensory Memory

  • Two types
  • Iconic memory
  • Visual
  • Lasts less than a sec
  • Sperling’s tests (1960s)
  • Echoic memory
  • Auditory
  • Fades within 2-3 sec

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Take a flashlight into a dark room, turn it on, shine it on a wall, and wave it quickly in a circular motion. What do you see? If you twirl it fast enough, the light will appear to leave a glowing trail, and you’ll see a continuous circle. The reason: Even though the light illuminates only one point in the circle at a time, your visual system stores a ‘snapshot’ of watch point as you watch the next point. The visual image is called an icon, and the snapshot it stores is called iconic memory.

People typically don’t realize that a fleeting mental trace lingers after a stimulus is removed from view. Nor did cognitive psychologists realize it until George Sperling’s ingenious series of experiments.

Sperling’s Experiment

  • Presented matrix of letters for 1/20 seconds
  • Report as many letters as possible
  • Subjects recalled only half of the letters
  • Was this because subjects didn’t have enough time to view entire matrix?
  • No
  • How did Sperling know this?

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Sperling instructed subjects to stare at the center of a blank screen. Then he flashed an array of the letters for 1/20 of a second and asked subjects to name as many of the letters as possible. Try it for yourself. You’ll probably recall about a a handful of letters. In fact, Sperling found that no matter how large the array was, subjects could name only four or five items. Why? One possibility is that people can register just so much visual input in a single glance – that twelve letters is too much to see in so little time. A second possibility is that all letters registered by the image faded before subjects could report them all. Indeed, many subjects insisted that they were able to ‘see’ the whole array but then forgot some of the letters before they could name them.

Did the information that was lost leave a momentary trace, as subjects had claimed, or did it never register in the first place? To test these alternative hypotheses, Sperling devised the ‘partial-report technique’. Instead of asking subjects to list all the letters, he asked them to name only one row in each array – a row that was not determined until after the array was shown. In this procedure, each presentation was immediately followed by a tone signaling which letters to name: A high-pitched tone indicated the top line; a medium pitch, the middle line; a low pitch, the bottom line.

If the tone was presented very soon the participants would recall most of the letters in the indicated row. But if the delay was more than one quarter of a second, the participants recalled an average of just over one letter per row, indicating how quickly information is lost in the sensory register.

Sperling’s Iconic Memory Experiment

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Sperling’s Iconic Memory Experiment

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Sperling’s Iconic Memory Experiment

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Sperling’s Iconic Memory Experiment

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Sperling’s Experiment

  • Recall was almost perfect
  • Memory for images fades after 1/4 seconds or so, making report of entire display hard to do

http://highered.mcgraw-hill.com/olcweb/cgi/pluginpop.cgi?it=dcr::640::480::/sites/dl/free/0073370207/25025/ICONIC.dcr::Iconic Memory

http://www.mhhe.com/feldmaness8e

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If the saw the entire array, subjects should have been able to report all the letters in a prompted row correctly – regardless of which row was prompted. Sperling was right: subjects correctly recalled 3 letters per row. In other words, all 9 letters, not 4 or 5, were instantly registered in consciousness before fading, held briefly in iconic memory.

To determine how long this type of memory lasts, Sperling next varied the time between the letters and the tone that signaled the row to be recalled. He found that the visual image started to fade as the interval was increased to ¼ of a second. Since this study, researchers have found when it comes to pictures of objects or scenes, words, sentences, and other visual stimuli briefly presented, people form ‘fleeting memories’ that last for just a fraction of a second.

Not an afterimage because Sperling showed he could present the letters to one eye and influence the memory by presenting a bright flash to the other eye. This would not have worked if the visual information was stored on the retina.

Sensory Memory

  • Why do we need sensory memory?

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