To: jbednar_cs.utexas.edu Subject: Carla Shatz CC: yeh_harlequin.co.uk In-Reply-To: James A. Bednar's message of Thu, 8 Jul 1999 03:37:39 -0500 (CDT) <199907080837.DAA26874@mail.cs.utexas.edu> -----text follows this line----- > First, I agree that her comments to the public do not neccessarily > convey the precise, subtle nuances of everything that is going on > during neural development, and I do not think that she would claim > that they do. That is not the problem. The problem is that she gives a false picture. > Since she is explicitly using vague metaphors like "nerve > cells are kind of like telephones", That is not vague language. We all have a very good idea what telephones are, and how they are connected. > I don't think many people > listening will really think they are hearing every bit of the story. But they will think they get the gross picture right, and theydon't. > Second, I will even grant that some of her comments end up being > misleading when interpreted in a fairly reasonable way. However, I > frankly doubt that anyone else could have summarized that topic for > the nonspecialist any more accurately than she did, and I do not > believe that any of her statements are misleading in context. That is straghtforward stupid. Simply saying that the brain starts from stochastic connectivity would make it much clearer, and more importantly, correct. > I agree that some people might come to such an interpretation based on > what you quoted, particularly non-specialists. I also agree that the > definition of "right few" and "not random" are crucial. > > However, I do not believe that many people will come to interpretation > [1.4] based on her talk, since she quite explicitly states that: > > ...there are just not enough genes in the genome (only about 100,000) > to account for the incredible precision of connectivity present in > the adult brain (> 1000 trillion connections)... > > So she quite clearly and without ambiguity states that there is no > fully-specific genetic plan of connections, nor could there be one. That is plain bullshit. All that this tells you is that there isn't a one connection/ one gene correlation. It does not tell you that the connections are not pre-defined by the genes. > And that's the key question of her research: how can things not be > entirely random if there is no specific blueprint? She doesn't say it is "not entirely random". She says that "none of the connections is random". > As she states, the key issue is that > "neural function" is what does the choosing, not some > genetically-defined, preexisting label. True to some extent. She mislead the reader to think that none of the connections is random as a side issue. > | [1.7] What about the examples that Dr. Shatz quotes, with the neurons > | that select the 'right few'? These example are wrong on two accounts: > | > | They are not accurate. > | Neurons which selects exactly which > | neurons to contact (i.e. they have the same connections in all > | healthy individuals of the same species) haven't been seen > | anywhere in any mamalian brain. > > Here you are interpreting "right few" as some sort of preexisting > genetic label that determines a neurons purpose for all time, which is > a reasonable but incorrect interpretation of her remark in context. > What she means by "right few" is instead functional, i.e. the neuron > that has a certain role in the system. So why she doesn't say it? _You_ interpret it this way, because you already know the facts. A reader that doesn't know the facts doesn't have a way of figuring it out. > However, her > argument requires only broad specificity, fully consistent with that > seen in the cortex. Her argument is that "none of the connections is random". > | [1.10] The second sentence is blatant nonsense: How can any part of > | the brain 'run test patterns' and 'select correct patterns' before it > | itself is connected properly? Dr. Shatz implicitly uses the analogy of > | electrical engineer running tests on an electrical circuit, but in the > | brain we don't have an engineer, and we don't have any mechanism that > | can 'select correct connections', because this mechanism itself would > | have to based on 'the correct connections' > | [Note that I am not saying that there are no activity-dependent > | changes, or that these are not essential for normal development. It is > | just that these cannot be described as 'running test patterns' and > | 'selecting correct connections'.]. > > Ah, here is the big confusion. Actually, running test patterns is one > reasonable description what the brain seems to be doing, though I've > never heard it described that way before. Before eye-opening, the > spontaneous activity in the eye (i.e. the retinal waves) does indeed > seem to act like a test pattern, i.e. like a calibrated set of images > with which to evaluate a system (wong:arn99). Just repeating a nonsense does not make it last of a nonsense. Calling what happens in the brain 'running test pattersn' is trivially nonsense. > So your key criticism must be about the "engineer" metaphor. Here > again "correct" must be interpreted in terms of function, NOT in terms > of some genetic label that the neuron was born with. I didn't say anything about 'genetic label', and certainly not something that the neuron was born with. I am talking about a plan, point. > That is, both > the connections and the neurons are being sorted out at the SAME time; > being the correct neuron depends upon having the correct connections > and vice versa. The rules to sort this out without any need for an > external engineer are quite simple; e.g. see the Hebbian models of > bednar:cns97 and haith:cns97, as well as keesing:nips91. In these > models "correct patterns" of connections are indeed selected, but they > are correct only because of the way that they work, not because of > some predefined genetic label. That is simply stupid. The word 'correct' has a very clear meaning (matching some standard). What you are doing is playing word-games. > > | [1.11] The second sentence also enforces the notion of a plan: It does > | not make sense to talk about 'selecting correct connections' and > | 'eliminating errors', unless there is some plan so 'correct > | connections' are connections according to the plan, and errors are > | not. Thus Dr. Shatz does not leave any doubt in the non-neuroscientist > | reader/listener that she thinks there is a plan for the connections in > | the brain, which specifies all the connections. > > Actually, having a specific plan is just one way to have "correct > connections"; all that she means by "correct" is that it works > correctly, not that it matches some blueprint. This is again just a word-game. Consider an simple > analogy: > > Let's say I'm outside one day, it's hot, and I'm thirsty. I start > walking to find a store to get something to drink. I happen to see > a little kid selling lemonade, so instead of going to a store, I > give some money to the kid, get some lemonade, and go on my way. Your adavantures on ahot day cannot, even in principle, tell us anything about how the brain works or what Dr Shatz means. > | errors"), but maybe in neuroscientists's jargon these have different > | meaning. In this case, that text (and other neuroscience texts) is > | very misleading for other people, which read it using the normal > | definitions of these terms, and hence take it as implicitly saying > | that there is a plan. > > I think that indeed many people would hear "select .. the right few", > etc. and think that there is a strict plan, but when they hear that > there is not enough space in the genome to encode a strict plan, then > they would either: Repeat, that is stupid. The comparison of number of genes to connection tells about a lack of correlation, not whether the connections are pre-defined or not. > in her short talk, despite the impression given by the quotes taken > out of context. Which quote was taken out of context? > > | [1.15] In either case, an underlying reason for the mistake that > | Dr. Shatz (and other neuroscientists) does is the pressure from > | outside the field of neuroscience. This comes both from cognitive > | scientists, which advance theories that are incompatible with > | stochastic connectivity, and from the rest of the public, which > | prefers more 'positive' statements. Thus a statement like "the brain > | contains well over 1000 trillions connections and most of these are > | stochastic", even though much more accurate than the statement > | Dr. Shatz makes, would be received much less favourably both by the > | cognitive scientists and the rest of the public. > > I think her talk was deliberately designed to get across her point to > the nonspecialist, and no assumption of political intrigue or false > optimism is required. So how do you explain a sentence like and "none of them is random!" ? > In fact I frankly doubt that the entire field > of cognitive science has much influence on the field of > neuroscience. Your frank doubt is irrelevant. > I consider it regrettable that they overlap so little and would indeed > rather see an increase in overlap to be desirable. I do not believe > that any of Dr. Shatz's comments contradicted the idea that there is > stochasticity involved. It will contradict if you use the normall meaning of the words, rather than play word-games with them.