Jeff, Thanks much for this exposition. It is valuable.
Is this material "beyond my interest level"? Absolutely not. I am very much interested. Is it beyond my current ability to comprehend? Ok -- admitted. I want to learn more. Ralph Dratman On Thu, Aug 29, 2013 at 6:53 PM, Jeff Hawkins <[email protected]> wrote: > “Could you expand a little on what biological problem you're referring to > here? > > -Mike” > > > > Ok, but I suspect it is beyond most people’s interest level, I don’t want > to confuse anyone. But for those that are interested…. > > > > The neurons in the CLA can be in a “predictive state”. Biologically this is > a cell that is depolarized. > > The neurons in the CLA can be in an “active state”. Biologically this is > equivalent to firing or generating one or more spikes. > > These two states are sufficient for learning sequences, but not for temporal > pooling. > > The addition of temporal pooling requires a third state which I don’t like > because it is a little tricky to make it work with real neurons. > > > > When we first implemented the CLA we started with sequence memory and > everything worked fine. After a bunch of testing we added temporal pooling. > With temporal pooling the cells learn to predict their feed forward > activation earlier and earlier. It works like this. First a cell becomes > active due to a feed forward input. It then forms synapses that allow it to > predict its activity one step in advance. Later it becomes active one step > in advance and then forms synapses that allow it to predict its activity two > steps in advance, and so on. (The system doesn’t require discreet steps but > it is easier to think about it that way.) Over repeated training, a cell > learns to be active over longer and longer sequences of patterns. This is > cool for a number of reasons. A cell will learn to be active for as much > time as it can correctly predict its future activity. If the world consists > of a few long repeatable sequences then cells will be active over long > periods of time. The data determines how much pooling a cell can do. The > more pooling that can be done at one level of the hierarchy the easier the > job of the next level. It also suggests why we can learn new tasks very > quickly (i.e. learn a new sequence) but to master something, to make > something second nature, requires many repetitions. I mentioned this in On > Intelligence when I said with practice knowledge gets represented lower and > lower in the hierarchy. As a region gets better at temporal pooling it > frees the memory in the next region for more advanced inference. > > > > The problem is cells that are pooling over time must be active/spiking, not > just depolarized as in sequence learning. When cells become active by > pooling in advance of feed forward activation, it messes up the sequence > memory. The CLA can’t tell the difference between activation because of a > real world feed forward input and activation because of pooling. What > happens is the CLA doesn’t wait for real input and sequences runaway forward > in time. > > > > For pooling to work the CLA needs to distinguish between cell activation due > to feed forward input and cell activation due to pooling. We need two > different states for an active cell. > > > > There is an elegant biological solution to this but the evidence is > equivocal. The solution is: when a cell is activated due to feedforward > input it generates a short burst of action potentials, three to five. It > does this once and then stops. When a cell is activated by pooling it > generates a series of spaced out spikes. Believe it or not there are quite > a few papers that suggest this could be happening. There is evidence of > short bursts prior to a steady firing pattern. The mini-bursts are in the > literature, easy to find. I spoke to several scientists and they report > seeing them. Some claim they see them at the beginning of every trace. > However, others say they never see the mini-busts. The best evidence for > mini-bursts is in layer 5 cells (yes the motor ones that also project up the > hierarchy). These cells are called “intrinsically bursting” cells to > reflect this behavior. For temporal pooling to work I think we also need to > see this mini-bursting behavior in layer 3. Mini-bursts are seen in layer 3 > but not by everybody. The evidence is much spottier. It is possible that > all layer 3 cells exhibit this behavior and scientists are not reporting > them. Perhaps there are different classes of layer 3 cells and only some > mini-burst. I wish the evidence was more conclusive. > > > > For the mini-bursting hypothesis to be correct a cell has to behave > differently when receiving a mini-burst than when receiving regular spaced > spikes. Here too the evidence is good. > > > > The synapses that form on distal dendrite branches (sequence and pooling > memory synapses) are far more effective when they get a burst of quick > spikes in a row. A thin dendrite amplifies the effect of multiple spikes > because thin dendrites don’t leak current quickly and they have low > capacitance. Thus a burst of spikes on multiple synapses may be necessary > for our dendrite segment coincidence detector to work. A single spike won’t > do it. If a cell produces single spikes(not mini-bursts) when activated by > a distal dendrite branch then sequences won’t run away. This is what we > need, it solves our problem! > > > > Conversely, axons that project up the hierarchy form synapses on proximal > dendrites (the SP synapses). Here, because the synapses are close to the > big cell body and the dendrites have large diameters there is large current > leakage and low capacitance. It has been shown that the first arriving > spike on a proximal synapse has a large effect (depolarization) but > subsequent spikes in a mini-burst have a much diminished effect. This is > good because we don’t want the spatial pooler in the higher region to be > overly influenced by the mini-bursts. We want the SP to look at all active > axons equally, those that are mini-bursting and those that are single > spiking via pooling. This is another nice validation of the theory. > > > > If you have followed all of this you see that the mini-burst hypothesis > solves the issues of pooling in a hierarchy and it is supported by a lot > biological evidence. It is a pretty cool explanation for why we see > mini-bursts in layer 5 cells. My only worry is that the evidence for > mini-bursting in layer 3 cells is spotty. If everyone said all layer 3 > cells are intrinsically bursting like forward projecting layer 5 cells I > would be much happier. All in all the theory holds together remarkably > well and I don’t have another one, so I am sticking with it for now. > > > > Of course none of this matters for the SW implementation, but I have found > over and over again that if you stray from the biology you will get lost. > > Jeff > > > > > > From: nupic [mailto:[email protected]] On Behalf Of Michael > Ferrier > Sent: Thursday, August 29, 2013 11:40 AM > To: NuPIC general mailing list. > Subject: Re: [nupic-dev] Inter-layer plumbing > > > >>> There is a biological problem with pooling the way we implemented that I >>> never resolved. So it is a work in progress. > > > > Hi Jeff, > > > > Could you expand a little on what biological problem you're referring to > here? > > > > Thanks! > > > > -Mike > > > _____________ > Michael Ferrier > Department of Cognitive, Linguistic and Psychological Sciences, Brown > University > [email protected] > > > > On Thu, Aug 29, 2013 at 2:29 PM, Jeff Hawkins <[email protected]> wrote: > > Here are some thoughts about how to connect CLA’s in a hierarchy. > > > > Here are some things we know about the brain. > > > > - Layer 3 in the cortex is the primary input layer. (Sometimes input goes > to layer 4 and layer 3, but layer 4 projects mostly to layer 3 and layer 4 > doesn’t always exist. So layer 3 is the primary input layer. It exists > everywhere. We will ignore layer 4 for now.) > > > > - I believe the CLA represents a good model of what is happening in layer 3. > > > > - The output (i.e. axons) of layer 3 cells project up the hierarchy > connecting to the proximal dendrites (SP) of the next region’s layer 3. > > > > - This isn’t the complete picture. The axons of cells in layer 5 (the ones > that project to motor areas) spit in two and one branch also projects up the > hierarchy to layer 3 in the next region. If we aren’t trying to incorporate > motor behavior then we can ignore layer 5 and say input goes from layer 3 to > layer 3 to layer 3, etc. Or CLA to CLA to CLA, etc. > > > > Each cell in layer 3 projects to the next region, so the input to a region > is the output of all the cells in the previous region’s layer 3. If we > consider our default CLA size there would be 64K input bits to the next > level in the hierarchy. Because of the distributed nature of knowledge it > isn’t necessary that all cells in layer 3 project to the next region, as > long as a good portion do we should be ok. But assume they all do. > > > > 64K is a lot of input bits but the SP in the receiving region can take any > number of bits and map them onto any number of columns. That is one of the > nice features of the SP, it can map an input of any dimension and sparsity > to an number of columns. > > > > That’s it for the “plumbing”. Now comes the tricky part. > > > > We, and many others, believe that a large part of how we recognize things in > different forms is the brain assumes that patterns that occur next to each > other in time represent the same thing. This is where the term “temporal > pooler” comes from. We want cells to respond to a sequence of patterns that > occur over time even though the individual patterns don’t have common bits. > The classic case are cells in V1 that respond to a line moving across the > retina. These cells have learned to fire for a sequence of patterns (a line > in different positions as it moves is a sequence). The cell remains active > during the sequence. Thus the outputs of a region are changing more slowing > than the inputs to a region. This basic idea is assumed to be happening > throughout the cortex. Temporal pooling also makes more output bits active > at the same time. So instead of just 40 cells active out of 64K you might > have hundreds. > > > > The CLA was designed to solve the temporal pooling problem. When we were > working on vision problems the temporal pooler was the key thing we were > testing. We have disabled this feature when using the CLA in a single > region because makes the system slower. The temporal pooler without the > “pooling” is still needed for sequence learning. > > > > There is a biological problem with pooling the way we implemented that I > never resolved. So it is a work in progress. > > > > Conclusion: to connect two CLAs together in a hierarchy, all the cells in > the lower region become the input to the next region. But there are some > difficult issues you might need to understand to get good results depending > on the problem. > > Jeff > > > > > > > > From: nupic [mailto:[email protected]] On Behalf Of Tim > Boudreau > Sent: Wednesday, August 28, 2013 4:29 PM > To: NuPIC > Subject: [nupic-dev] Inter-layer plumbing > > > > Is there a general notion of how layers should be wired together, so that > one layer becomes input to the next layer? > > > > It seems like input into one layer is pretty straightforward - in ascii art: > > > > bit bit bit bit bit bit bit bit > > | | | | | > > ------proximal dendrite w/ boost factor---> column > > > > But it's less clear > > - If we have the hierarchy input -> layer 1 -> layer 2, what constitutes an > input bit to layer 2 - the activation of some combination of columns from > layer 1? > > - How information about activation in level 2 should reinforce connections > in layer 1 > > > > Any thoughts? > > > > -Tim > > > > -- > > http://timboudreau.com > > > _______________________________________________ > nupic mailing list > [email protected] > http://lists.numenta.org/mailman/listinfo/nupic_lists.numenta.org > > > > > _______________________________________________ > nupic mailing list > [email protected] > http://lists.numenta.org/mailman/listinfo/nupic_lists.numenta.org > _______________________________________________ nupic mailing list [email protected] http://lists.numenta.org/mailman/listinfo/nupic_lists.numenta.org
