Monday, December 30, 2013

Secrets of the Holy Grail, Part IV

Part I, II, III, IV, V

Abstract

In Part III, I described the hierarchical structure of sequence memory and I explained why patterns are the key to sequence learning. In this post, I explain invariant object recognition, the difference between short and long-term memory and how to catch a liar. But first, a word about remembering and forgetting.

Remembering and Forgetting

Unlike patterns which, once learned, remain permanently in memory, learned sequences are slowly forgotten, i.e., disconnected, unless they are repeated often. Repetition strengthens the connections between nodes in a sequence. If the strength of a connection reaches a predetermined threshold, it becomes permanent. There are two ways the connections in a sequence can be repeated, via sensory stimulation or internal playback. So even sequences that receive little sensory stimulation can become permanent if they are played back internally. The latter happens each time the brain focuses on a particular branch in the memory hierarchy.

In Part III, I wrote that the sequence learner starts with short intervals before moving on to progressively longer intervals. That is the first of the three fitness criteria used in sequence learning. Forgetting is the second. A node in a sequence under construction is often presented with multiple successor candidates. Initially, the learning system has no way of knowing which of the candidates are legit, if any. One way to eliminate bad candidates is to slowly forget them. A node cannot survive unless it is frequently reinforced via sequence repetition. But this raises a serious question, what happens to infrequent sequences or to sequences that never repeat? The answer is that they must be repeated (replayed) internally in order to be retained. However, the only way to really be sure that they are good or bad is to use the third fitness criterion: find out if they lead to a contradiction (see Catching Liars below).

Invariant Object Recognition

Hold your hand in front of your eyes and slowly rotate your wrist. As you do so, your visual cortex is presented with a sequence of images. Even though each successive image is different from the others, your brain does not think of each image as representing a different object. Somewhere, in your cortex, you still know that you are looking at your hand regardless of its orientation or distance from your eyes. This is called invariant object recognition, probably the most important perceptual capability of the brain. It holds the key to understanding several other aspects of perception such as attention and short-term memory.

Catching Liars

When you rotate your hand, your brain sees the successive patterns as one object because they are linked together within a single package called a branch. The branch is a bundle of linked pattern sequences. But how does the brain know that one sequence should be tied to another in order to form a bundle? The answer turns out to be rather simple: if two sequences have two or more nodes in common, the branch mechanism automatically links them together. The problem is that any of the shared nodes may have been acquired in error. How can we tell? The answer lies in the timing between shared nodes. If two sequences belong to the same branch, their predictions must match. If there is a mismatch, one of the nodes is a liar and must be discarded. Consider the two sequences in the diagram below.
The arrows represent the direction of pattern activations. The red circles are normal sequence nodes and the green circles symbolize shared nodes. The total recorded interval between the two shared nodes must be the same is both sequences. If not, there is a contradiction and the culprit is eliminated. If there is agreement, it means the two sequences represent different facets of the same object and thus belong to the same invariant branch.

In human and animal brains, testing for sequence contradictions occurs during sleep. The reason is that the sequences must be replayed internally during the test and this would disrupt the brain's normal activity. I believe that catching thieves (see Part II) and liars is what is happening during so-called REM sleep.

Short-Term Memory

Psychologists and neurologists have maintained in the past that memory is divided into two separate areas, one for long-term memories and one for short-term memories. My position is that there is only one memory structure for both short and long-term memories. Short-term memory is merely the currently activated branch, i.e., the one under attention. The brain can focus on only one thing at a time, that is to say, only one branch in the sequence hierarchy can be activated at a time, a phenomenon that magicians and pickpockets have exploited over the years. Furthermore, a branch can only be active for up to about 12 seconds at a time, after which attention must be switched to another branch.

Coming Up

As I promised in Part I, in my next post, I will reveal where I got my knowledge of the brain's organization.

See Also

The Holy Grail of Robotics
Raiders of the Holy Grail
Jeff Hawkins Is Close to Something Big

2 comments:

Jaroslav KuboŇ° said...

Great article! Looking forward to the next one already ;-)

Louis Savain said...

Jaroslav,

My next post in this series will be the most surprising and revealing. Many won't like it. LOL.