memory (and SOMs)

yan king yin (dont spam) y.k.y at lycos.com
Sat Feb 23 12:37:26 EST 2002


>> > I agree with this too. For example, one area in the brain thought to
>> > be critical for consolidation of short-term memory is the hippocampal
>> > formation and associated entorhinal cortex. The iterative structure
>> > looks like this:
>>
>> You should be careful that consolidation is different from recall, and
>> the hippocampus is required for consolidation, but not recall. There
>> is also the additional complication that hippocampal lesions usually
>> result in a few years of retrograde amnesia as well.
>
>Perhaps consolidation is different, and perhaps not. How does an
>experimenter know whether a memory has been consolidated unless the
>test subject is also able to recall it? Either way, the hoippocampus
>appears to be important in recall of memories that have just been
>acquired (short-term). The famous example of being able to remember 7
>plus or minus 2 digits of a phone number is sensitive to hippocampal
>damage.

I think there may be two forms of short-term memory. A normal person
can store much more than 7 digits in the so-called phonological loop,
when they keep reciting them in their mind. I think this is the working
memory that is located in the prefrontal cortex. On the other hand, when
the digit span task is disrupted by other tasks then the number of digits
that can be retained becomes "7 plus or minus 2". This kind of memory
is episodic in nature and involves the hipocampus. H.M. had *normal*
digit span abilities.

I think if you ask a subject to remember a number of items, and after
a few minutes ask them to recall them, when their mind has become
occupied with other things, then they can only recall 7 +/- 2. That is
not working memory but is episodic and probably involves the
hippocampus.

>> I suppose you're talking about memory recall here, but hippocampus
>> is not involved in recall of long term memories. Though you may argue
>> that this is the recall mechanism of recent memories that is stored in the
>> hippocampus.
>
>Right. I think everyone agrees that long-term storage is somehow
>distributed around cortex, not in hippocampus. Also, there are several
>kinds of memories that appear to be little if at all affected by
>hippocampal damage.
>
>> Another simplified schematic pathway is as follows: cortex -> EC ->
>> dentate gyrus -> CA1 -> CA3 -> Subiculum -> EC -> cortex, where
>> CA=hippocampus. So it looks like a loop and memory might pass through
>> this loop to be stored in the cortex. It is also possible that memory is
>> processed in other parts of the cortex and the hippocampus only
>> mediates the consolidation process.
>
>I think you mean dentate->CA3->CA1...

Thanks for the correction =)

>> I think the central question is in where and how is memory stored
>> in the cortex.
>
>I do not necessarily think that is the central question. The long-term
>storage might be the very last step in the process. Lots of other
>steps are probably very important. For example, decisions about which
>experiences are worth storing and which are not are probably pretty
>important steps. We don't store everything, only some things. Some
>theories suggest that the selection of events for storage takes place
>in hippocampus, and involves a comparison of new information with
>previously buffered information. If the new information is
>sufficiently different, then it gets processed and eventually stored.
>If it's very similar to previous information, it may get ignored
>because there's already a copy of it. There is a dual role of memory
>processing at work, where similar information gets compressed into a
>single representation, whereas novel information gets differentiated
>from previous information. The hippocampus appears to be involved in
>both of these processes, without which the cortex would just fill up
>with a lot of extraneous information that isn't much use to the
>animal, and was stored in a very inefficient manner.
>lished, but I saw his Neuroscience poster.

The problem is that we have very little understanding of cortical circuits,
so we cannot formulate a theory about long-term memory in the cortex,
including any form of pre-processing. What is needed is a detailed
map of cortical circuits. This is not easy, but if we want to understand
the brain then its a prerequisite.

According to one study, there are about 32 visual areas in the macaque
visual cortex, connected by 305 connections. Now it seems that these
visual maps cannot be the result of self-organization, so they must be
genetically specified. If this can happen to the visual cortex, the rest of
the cortex might be organized this way too. By simple division we can
estimate that there could be 100 to several hundreds of  "maps" in the
cortex. That is the number of circuits we have to figure out!






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