[edit: I am referring to the brain here, not Artificial Neural Nets.]
Hmmm, I disagree with the randomness. A lot of projections in the brain are ordered before pruning. Indeed, given the degree of order, and the percentage of neurons pruned, it would be impossible to establish that much order with pruning alone.
it would be impossible to establish that much order with pruning alone.
That seems straightforwardly falsified by ANNs like lotterytickets or weight agnostic NNs, where the weights are randomized and all the learning is done by adding/pruning away connections (you can think of it as searching in fully-connected net space). Most on point here would be Ramanujan et al 2019 which proves that as random NNs get bigger, you expect there to be a sub-network which computes whatever you need (which makes sense, really, why wouldn’t there be? NNs express such rich classes of functions, that if you can pick and choose from within a very large NN, it’d make sense you could sculpt whatever you need).
To clarify, I meant, within a human brain. Not within an artificial neural net. The human brain is highly ordered before birth by genetically determined chemotaxis signals guiding axon growth. Synapses are formed within relatively small regions of where the guided axons have ended up. This results in a large capacity for ‘fine’ adjustments, while having a preset large scale structure which is very similar between individuals. The amount of this large-scale order seen in the human brain, as evidenced by the broad similarity of Brodmann regions between individuals, is the thing I’m claiming couldn’t be established by pruning neurons.
I don’t think you do. Let me rephrase: the weights are picked at random, under a distribution biased by molecular cues, then pruned through activity dependent mechanisms.
In other words, our disagreement seems to count as an instance of Bertrand’s paradox.
“Wiring is actually largely random before the critical periods that prunes most synapses, after which what remains is selected to fit the visual properties of the training environment”
Yes, I think I technically agree with you. I just think that describing the cortex’s connections as “largely random” gives a misleading sense of the pattern. Randomness actually plays a relatively small and constrained role in brain wiring even before learning occurs.
The analogy I’ve come up with is:
Original growth and connection
For an analogy of the scale involved, I describe this as a neuron being like a house in America. That house grows a neuron guided by chemotaxis to a particular block of buildings on the other coast.
Once there, the neuron forms a synapse according to a chemical compatibility rule. In this analogy, let’s say that the neuron in our example must connect to an address ending in 3.
Refinement
Picking a different member of the closest ten allowed options (respecting the rule of ending in 3 and respecting the neighborhood boundary) according to the Hebbian rule. The Hebbian rule is “Neurons that fire together, wire together.” The highest temporally synchronous candidate from among the set will be chosen for a new connection.
Existing connections with poor temporal synchronicity will get gradually weaker.
Synchronicity changes over time, and thus the set of connections fluctuates.
Pruning
Of the candidates matched with following refinement, those which are consistently poorly synchronized will be considered ‘bad’. The poor quality connections will weaken until below threshold, then be pruned (removed).
A neuron with no connections above threshold for a lengthy period of time will be killed (also called pruning). Connections can break and later be reformed, but neurons which are killed are not replaced.
Not bad! But I stand by « random before (..) » as a better picture in the following sense: neurons don’t connect once to an address ending in 3. It connects several thousands of times to an address ending in 3. Some connexion are on the door, some on windows, some on the roof, one has been seen trying to connect to the dog, etc. Then it’s pruned, and the result looks not that far from a crystal. Or a convnet.
(there’s also long lasting silent synapses and a bit of neurogenesis, but that’s details for another time)
Yes, that’s fair. I think we’ve now described the situation well enough that I don’t think future readers of this thread will end up with a wrong impression.
To expand on Ilio’s point: the connection point on the “building” (recipient neuron’s dendrites) matters a lot because the location of of the synapse on the dendrites sets a floor and ceiling on the strength of the connection which cannot be exceeded by weight modifications due to temporal synchronicity.
Also, yes, there is neurogenesis ongoing throughout the lifespan. Never long range (e.g. cross country in our metaphor), only short range (within same metropolitan area). The long range connections are thus special in that they are irreplaceable.
[edit: I am referring to the brain here, not Artificial Neural Nets.] Hmmm, I disagree with the randomness. A lot of projections in the brain are ordered before pruning. Indeed, given the degree of order, and the percentage of neurons pruned, it would be impossible to establish that much order with pruning alone.
https://www.cell.com/fulltext/S0092-8674(00)80565-6
That seems straightforwardly falsified by ANNs like lottery tickets or weight agnostic NNs, where the weights are randomized and all the learning is done by adding/pruning away connections (you can think of it as searching in fully-connected net space). Most on point here would be Ramanujan et al 2019 which proves that as random NNs get bigger, you expect there to be a sub-network which computes whatever you need (which makes sense, really, why wouldn’t there be? NNs express such rich classes of functions, that if you can pick and choose from within a very large NN, it’d make sense you could sculpt whatever you need).
To clarify, I meant, within a human brain. Not within an artificial neural net. The human brain is highly ordered before birth by genetically determined chemotaxis signals guiding axon growth. Synapses are formed within relatively small regions of where the guided axons have ended up. This results in a large capacity for ‘fine’ adjustments, while having a preset large scale structure which is very similar between individuals. The amount of this large-scale order seen in the human brain, as evidenced by the broad similarity of Brodmann regions between individuals, is the thing I’m claiming couldn’t be established by pruning neurons.
For more info: https://www.lesswrong.com/posts/Wr7N9ji36EvvvrqJK/response-to-quintin-pope-s-evolution-provides-no-evidence?commentId=r72N4LxupbmTtgN3i
I don’t think you do. Let me rephrase: the weights are picked at random, under a distribution biased by molecular cues, then pruned through activity dependent mechanisms.
In other words, our disagreement seems to count as an instance of Bertrand’s paradox.
Yes, I think I technically agree with you. I just think that describing the cortex’s connections as “largely random” gives a misleading sense of the pattern. Randomness actually plays a relatively small and constrained role in brain wiring even before learning occurs.
The analogy I’ve come up with is: Original growth and connection For an analogy of the scale involved, I describe this as a neuron being like a house in America. That house grows a neuron guided by chemotaxis to a particular block of buildings on the other coast. Once there, the neuron forms a synapse according to a chemical compatibility rule. In this analogy, let’s say that the neuron in our example must connect to an address ending in 3.
Refinement Picking a different member of the closest ten allowed options (respecting the rule of ending in 3 and respecting the neighborhood boundary) according to the Hebbian rule. The Hebbian rule is “Neurons that fire together, wire together.” The highest temporally synchronous candidate from among the set will be chosen for a new connection. Existing connections with poor temporal synchronicity will get gradually weaker. Synchronicity changes over time, and thus the set of connections fluctuates.
Pruning Of the candidates matched with following refinement, those which are consistently poorly synchronized will be considered ‘bad’. The poor quality connections will weaken until below threshold, then be pruned (removed). A neuron with no connections above threshold for a lengthy period of time will be killed (also called pruning). Connections can break and later be reformed, but neurons which are killed are not replaced.
Not bad! But I stand by « random before (..) » as a better picture in the following sense: neurons don’t connect once to an address ending in 3. It connects several thousands of times to an address ending in 3. Some connexion are on the door, some on windows, some on the roof, one has been seen trying to connect to the dog, etc. Then it’s pruned, and the result looks not that far from a crystal. Or a convnet.
(there’s also long lasting silent synapses and a bit of neurogenesis, but that’s details for another time)
For those interested in more details, I recommend this video:
Yes, that’s fair. I think we’ve now described the situation well enough that I don’t think future readers of this thread will end up with a wrong impression. To expand on Ilio’s point: the connection point on the “building” (recipient neuron’s dendrites) matters a lot because the location of of the synapse on the dendrites sets a floor and ceiling on the strength of the connection which cannot be exceeded by weight modifications due to temporal synchronicity. Also, yes, there is neurogenesis ongoing throughout the lifespan. Never long range (e.g. cross country in our metaphor), only short range (within same metropolitan area). The long range connections are thus special in that they are irreplaceable.