Short Term Memory

is (by general consensus) the number of concepts an individual can hold in "short term memory". (If you are run over by a truck with an 8-digit license plate, you probably won't be able to 'get' that last digit as it whizzes by.)
- I think six is a better approximation.

You might try practicing in order to improve this limit. For instance, by taking a quick look a an auto hubcap design then look away and try to remember what the number of its symmetry was (most are from 3 to 48). When it gets higher than ~7 you'll just be guessing.
This doesn't help at all to improve your skill ......

The reason involves the brains hardware configuration. It has a "temporal bus" rather than a "physical bus" of 8,16,32,or 64 wires. It runs at about twenty Hertz (that's hertz not megahertz) and will "bus" about seven times per clock cycle.

What ? the heck does that mean?

We detect changes in things ... not sameness. If you stare fixedly at a picture on a wall you will see it disappear from view. Your eyes must wander over the picture constantly to stimulate the rods and cones on the retina. Unless there is a change, the chemical reaction within the rod or cone comes to a stop and no further electrical transmission is made to the brain. Hence, the object disappears.
Your nerves take about one twentieth of a second to 'reset'.

So, to make a non-jerky motion picture you need about 22 frames per second.

And, to hear you need about the same frequency. A tone at too low a frequency (7, 10, 15, etc.) is perceived as a series of individual sounds whereas something at 60 cycles per second is perceived as a steady hum. You can no longer differentiate between individual cycles. They blur into one another.
This is one reason middle C on a piano is not in the center of the piano. Putting lower notes on the keyboard gives almost musically useless junk. (Although a famous piano maker has done just that - to laughable consequences.)

O.K. ... Frequency ~ 20 cycles per second ...

Alright ... there is also a transmission velocity along the nerve path for information running around in your head and body. Let's deal with the head.

To hold seven things in consciousness at one time as an undifferentiated whole which you can spit back out on demand, that whole must fit into a one twentieth of a second frame. And that "size" is determined by the width of your brain in concert with general nerve impulse velocity.

In this manner:

Each of the seven things represents an understood concept, i.e. one which has been put into a hierarchical structure as shown previously. That hierarchy takes up physical space (namely a distance from one neuron (axon,soma,dendrite) to another to another to another - the length of which is unknown but which might go anywhere in the brain).

Taking a typical length to be on the order of one head width, i.e. about 6 inches, seven concepts understood times 6 inches = 42 inches of signal travel. In one twentieth of a second we should get 42 inches of signal travel for a general transmission velocity of about 70 feet per second (ballpark).

There are probably about a zillion other factors of lesser importance to be considered but generally ...

The number of things in short term memory is too basic to be determined by anything other than fundamentals. ... you can "back calculate" the brain's signal velocity from such principles.

Now, the seven things are held as one thing, one unit, which is all you or any animal can think, see or do at once. So that,

dog, cat, rat, kangaroo, chimp, cricket, Elvis = SEVEN
... becomes ...
dogcatratkangaroochimpcricketelvis = ONE

Bring back the ONE from SHORT TERM MEMORY and snip it apart into its SEVEN components while holding it in conscious display.

Seven is [1] responsible for [2] the size of [3] sentence[4] structures [5] as well [6].

If we put too many concepts in a sentence we begin to lose track of the "train of thought". We can dodge this problem by using conjunctions which is sort of like starting a new sentence but one which is related to the previous part. When I was ~15 I read a sentence by Emmanuel Kant in his antinomies which contained, I think, 66 words. I searched in vain for 45 minutes just to locate the noun verb and object. And ended up putting the book down with the declaration "If you can't express yourself clearly, you don't know what you are talking about". (This goes for me as well ; )

Even when we write the sentence down we try to put about seven things on a line. Look at the confusion which results when you type an average of , say , twenty words per line. No one can read it efficiently. And of course 6 or 7 sentences makes a respectable paragraph. But 6 or 7 paragraphs don't make a good chapter (there must be another division in there somewhere - help me out on this - ahh ... yes ... a PAGE !).

Babies don't hold seven things in short term memory so they show none in conscious display. They don't have any formed concepts to put there at all. When their knowledge field begins to fill up, first it's one word (mama, dada, numnum, etc.), then it's two words (go store, me candy!, toy broken), then three (Me- go- to store, I- want- candy), finally multiple structures (I want to eat some candy... now!).

What I mean by Conscious Display

Conscious display is the main 'focus' of the brains attention. It has many such foci at any given instant. (It's a multi-tasking 'mutha. See naman.htm)

The other focuses constitute the "subconscious". They're ready to take over the main role at a moments notice. For instance, you're driving and thinking about something else and suddenly notice that you're a few miles down the road without any recollection of having traversed that distance. Now if, during that 'unmonitored' drive, some emergency situation occurred, the subconscious focus doing the driving would have overridden your main focus and taken over. (Look out, stupid. Stop thinking about HER and watch where you're going.)

The main focus of attention commands the greatest resources and is most efficient in dealing with reality. It's as if you had a set of prioity levels assigned to each separate focus which changes in response to external stimuli. At any rate, each focus is continually 'running into and through every other focus'. (They are, after all, using the same wiring for data transmission.) During multi-tasking, all manner of "weird" connections are made which make no sense and which have to be 'graded' on their importance and meaningfulness. The arbiter (standard) of importance-truth is external reality without which our minds would float off into ideas too fanciful to be of any use. This "arbiter" is not another program running along with the other foci but is simply the relationship of a focus with the present external incoming data. If the focus concerns that data it rises in importance to become the main focus, i.e. consciousness.

In fact, this is in part what dreaming is - multiple focuses interacting with one another without an external moderator to decide what is important - kinda' like an unmoderated internet discussion group. The rest of dreaming is the integration of the days new data into the minds' present context. (If you think about a dream long enough you will find that every element in your dream comes from something which happened during the previous waking period (true psychosis excluded). Put together in a jumbled half-sensical manner, it is characteristic of what would be called 'dementia' in the waking state.)

So there is a "hierarchy" of conscious, operational foci as well as hierarchies of memory.
hierarchies (your minds' task bar)
DISK hierarchies (mental DOS tree).

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