Eric M. Van (ericmvan) wrote,
Eric M. Van
ericmvan

A Model for Attention-Switching, Part V: The Selection Mechanism

So, how are attendums selected for attention?

If you were designing a system from scratch, your first thought might be to make this a top-down process.  You would build a module in the executive control center in the prefrontal cortex that continually monitored the salience level of every attendum in active memory, and selected the one that was most salient.

The fundamental problem with such a design is that it’s a lot of circuitry.  Is there something simpler that could do at least as good a job?

Well, here’s an idea.  Why not make it a bottom-up process?  Let the attendums compete for entry into consciousness.  Without worrying about the details of the actual neural mechanism, we can think of all the attendums in a sort of rugby scrum at the gateway to consciousness.  Each attendum has strength and vigor proportional to its salience, so the most salient attendum is likeliest to win the scrum and gain a foothold in consciousness.  It will, however, be continually subject to being overmastered and supplanted by another attendum of comparable or newly superior strength.  (Those familiar with Gerald M. Edelman’s concept of “neural Darwinism” will recognize the inspiration for designing a brain based on Darwinian principles of selection.)

There would appear to be one major bug in such a design: the most salient attendum is not certain to be the one selected.  This is inherent in any advantageously efficient bottom-up design.  Any bottom-up mechanism that always resulted in the most salient attendum being selected would be functionally equivalent to our top-down system, and would require as much circuitry, or even more (instead of the cortex monitoring the salience of every attendum, each attendum would have to monitor the salience of every other).  The fundamental tradeoff here is to simplify the selection mechanism by allowing a probabilistic selection among attendums based on their relative strength.

Why do I believe that the brain has made such a tradeoff?  Because it’s not really a tradeoff at all.  The “bug” is in fact a feature.  It is a good idea to sometimes let an apparently less salient attendum win the battle for consciousness.  And that is because the salience programs are all essentially out of date.  As we saw in Part IV, they can only be created or revised when the attendum is in consciousness, because the creation of the salience program requires the good and bad feelings that are only present in consciousness.  The salience program of every attendum is thus based on the state of affairs the last time we attended to it.  And things may well have changed since then.

This of course reflects our subjective experience of attentional switching.  We sometimes (I originally said “often,” but that’s true only at one end of a personality spectrum we’re about to discuss) find ourselves thinking about something we hadn’t thought of in a while, and when we do, we sometimes discover that events or insights that have happened in the interim have changed its relevance or importance.  A truly efficient brain would to a lot of “checking in” on apparently less-salient attendums to see if their salience programs needed updating.  And a terrifically simple way to accomplish that is to simply let them sometimes win the battle for consciousness.

Now, there is one more thing to consider, and then we’ll have a complete model.  And that is that there are good times and bad times to let a less salient attendum win the battle for consciousness.  When we are sitting and doing nothing is a very good time for “letting our mind wander,” to unexpectedly find ourselves thinking about something that we’ve been regarding as relatively less important, perhaps even trivial.  When we are fleeing from a bear, on the other hand, is an extraordinarily bad time to attend to anything other than fleeing from the bear.

What we need then, is a way of controlling the contrast among the salience tags.  When we’re sitting daydreaming, we can imagine the contrast turned all the way down, so that the strongest salience tags are not much stronger than the weakest, with the difference in fact being not significantly greater than the built-in “margin of error” that is inherent in the competitive mechanism.  The playing field is thus essentially leveled.  When we’re attending to a project for work (or a blog post), we can imagine the contrast turned up about half way, so that only reasonably strong attendums have any chance of seizing control from it (one that always has a good chance is “I need to pee.”)  When we’re running from a bear, the contrast would be all the way up, so that the difference between running from the bear and the next most salient attendum would be greater than any margin of error in the selection process.  Hence the thought of stopping to urinate would not cross our minds regardless of the urgency of our need.

We can model this mathematically.  Imagine that the salience tags range from 1 to 10 in strength, and that the “margin of error” in the competition for consciousness is 8, which is to say that the salience-1 tags compete anywhere from 1 to 9 and the salience-10 tags compete anywhere from 2 to 10.  There is thus ordinarily only a very slight advantage for the most salient attendums over the least.  This is the baseline daydreaming state, with the contrast turned all the way down.

Now imagine that we had a way of effectively multiplying all the salience tags.  Let’s multiply them all by 2.  Now they range from 2 to 20, and the salience-1 tags have an effective strength of 2 to 10, and the salience-10 tags have an effective strength of 12 to 20.  So there is no chance of a salience-1 attendum ever winning the consciousness battle.  This is a reasonable minimal level of concentration.

With a multiplying signal of 10, the salience-9 tags would be something like 83-91 and the salience-10 tags would be 92-100.  This is running from the bear.

I like to thing in terms of a salience gradient, the steepness of the line you would draw if you plotted the effective salience of every attendum from weakest to strongest and connected them.  With no multiplying signal, the slope of this line, the salience gradient, is very shallow.  What the multiplying signal does is increase the steepness of this line, the salience gradient, and hence the likelihood of the most salient attendums winning the battle for consciousness.

So, what might this multiplicative signal be?  We’ll answer that next.

Tags: neuroscience
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