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Three points for today

Sensory memory (SM) contains highly transient information about the dynamic sensory array.

Stabilizing the contents of SM allows identification of external stimuli, which in turn permits access to knowledge.

Stabilization occurs when the contents of SM are extracted and stored in Short Term Memory (STM)

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The world and our representation of it

A. External world Dynamic (in motion with respect to us). That is, something is always happening. Representations must be created fast.

B. Internal representation of the world Begins with receptor surface. Receptor surface codes information about what is out there in the world.

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Question

Can the receptor surface drive all responses?

That is, can all responses be prompted and guided only by information available at the receptor surface?

E.g., shadow passing over squirrel causes squirrel to run for shelter – without thinking.

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Which behaviours could receptors control?

Behaviours based on distinctions available at receptors - e.g., for vision:

light intensity wavelength (hue) contrast (light vs. dark)

Nothing involving learning.

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Which behaviours can receptors not control?

Behaviours based on abstract properties – for vision, these properties include:

size shape category name

These properties are computed based on information extracted from SM.

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Behaviours receptor surface cannot control

Most human behaviours of interest require information about those abstract properties. But because the world is dynamic, SM is continually being refreshed.

Representations in SM don’t last long enough for analysis of abstract properties.

We need to extract and stabilize these representations, in order to find out what is out there. How do we do that?

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How do we extract information from SM?

Two issues:

1. How does the extraction process work?

2. What is the nature of the stimulus representation that results?

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How does the extraction process work?

We’ll begin by looking at some of the functional characteristics of SM that will influence our model of the extraction process. We’ll ask:

How long do representations in SM last? How many representations can SM contain?

Then, we’ll look at Rumelhart’s model .

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Functional characteristics of sensory memory

Sperling (1960)

tachistoscopic procedure: very brief exposure of a matrix of letters (because letters are a convenient stimulus)

subject’s task is to report what’s in the matrix.

until Sperling, whole report technique used.

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80 years of Whole Report results

Consider a typical display of 3 rows of 4 letters per row.

Subjects in the whole-report task typically reported 4 or 5 out of 12 letters.

Did that mean that SM could contain only 4 or 5 letters at one time?

Or did the rest of the display decay during readout of the first 4 or 5?

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J V E C

M B S A

Y L K R

J V E C M

“J – V – E – C – M”

Theory 1: that’s all we can get into SM

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J V E C

M B S A

Y L K R

“J – V – E – C – M – – That’s all I remember.”

J V E C

M B S A

Y L K R

Theory 2 – rest decays during read-out:

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Sperling’s (1960) Partial Report Technique

After display disappears, a tone signals which row to report.

High tone, top row. Medium tone, middle row, etc.

Logic: if you can report a randomly selected row, you could have reported the other rows, too if a different tone had sounded.

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Sperling’s results:

76% correct in partial report condition.

Suggests that most of a complex display is available in SM immediately after offset.

76% of 4 letters = 3 letters. Why not 4 or 5 as in whole report condition?

After 5 seconds, most of array has decayed.

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Characteristics of Sensory Memory

We asked about the duration and capacity of Sensory Memory. Sperling’s data tell us:

Duration – under ideal circumstances (display followed by darkness), a second or two.

Capacity – up to 75% of a complex visual display.

Now, let’s look at Rumelhart’s model…

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Rumelhart’s model of the extraction process

Extraction process takes information from SM to STM.

Process operates in parallel on all contents of SM (unless task instructions say focus).

Process influenced by stimulus quality – better quality, faster process.

Explains Sperling’s results.

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J V E C

M B S A

Y L K R

“M – B – S – …”

J V E C

M B S A

Y L K R

Rumelhart’s model

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The nature of the stimulus representation

In SM, the representation is unprocessed – it contains information about environmental energy levels (e.g., where the image is light or dark). But the representation in STM is abstract. That means it has some form. What is that form?

Think of the difference between muffin mix in the mixing bowl and muffins. The mixing bowl is SM. The muffins are the contents of STM. How do we get from mix to muffins?

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The nature of the stimulus representation

Three suggestions, in historical order:

Template theory

Feature theory

Structural description theory

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Template theory

information extracted from SM processed to give stimulus representation in form of a picture of the object being viewed.

memory also contains pictures of objects.

recognition involves searching memory for the picture that matches current stimulus representation.

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A problem with template theory

How do you recognize objects at an unfamiliar orientation?

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Feature theory

all visual objects are composed of features

a stimulus representation is created by extracting features from the visual object

memory contains representations in the form of lists of features of recognizable objects

recognition consists of matching feature sets

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Problems with feature theory

1. What are the features (that is, what are the visual features of objects that we supposedly extract from visual SM)?

• nobody knows

2. What about spatial relations among features?

• not specified in the theory

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Chair?

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Template and Feature theories compared

Template theory deals with whole objects. Templates do not have component parts. A template is like a photograph of an object. If you have a photo of a Lego building, you cannot separate the individual bricks out. The photo is all one piece, though the actual object represented is made of many pieces. Feature theory deals with representations that have internal structure – the whole is made up of many parts. Pairs of objects may share some features and not others.

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Structural descriptions

A feature theory with spatial relations among features specified in description.

Biederman’s Recognition by Components theory:

Geons – geometric ions (primitives)

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In column B, the geons can be recovered, so the objects can be identified. Not so in column C.

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Problems with SDT

Difficult to specify geons that compose some objects.

• what are the geons for a loaf of bread?

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Selection is the responsibility of attention – see next chapter. But note:

William James:

Selection is the function of consciousness.

Consciousness refers to experiencing the world.

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Which comes first?

Do we have consciousness because we have stable representations?

Or do we have stable representations because we have consciousness (so are able to choose objects and responses)?