Hi Jeff, I'm concerned that this scheme will degrade the semantics of the encoding. With all the current scalar encoders, a given bit will be on every time the input value is within a certain range. Each bit also only has this single semantic meaning. Thirdly, most of the bits should stay on if you adjust the input by a small amount.
With the standard fixed encoder, these semantics never change. The downside is that the meaning of the top few bits is (nearly at max plus all the out-of-range values), so you sacrifice some resolution above the max. The current AdaptiveEncoder may drastically change these semantics (what range the bit means) if given a large outlier value. The semantics of every single bit will be altered abruptly, ruining previous learning. This is the big weakness of this encoder. The encoder you're discussing here also changes the semantics, but perhaps in an even worse way due to the randomness. Let's say you start with the min at 0, the max at 101, with a 128 bit input, 21-bits on. Now you give it a value of 175. Are you going to select 25% of the bits and split their semantics? Let's say you pick bit 64. It used to represent the range 55-76 (ish) and now it also represents 155-175. You've doubled the range of values for that bit (ie halving its information content), while the others keep their ranges. The range represented by a bit is also non-contiguous. And all this has happened because one outlier was encountered. It surely makes more sense to degrade the semantics of the bits near the outlier (top or bottom few bits) most, and to do it based on how many outliers occur. if you do this then all the ranges stay continuous, and the bits which suffer dilution are the ones which deserved it most. Adjusting the encoder for outliers is best done gradually and based on the statistics of those outliers. If there's lots of data appearing beyond the current range, spread the encoding out gradually and allow the SP time to adjust its learning. The NonUniformScalarEncoder (NUSE) seems to provide a (half-complete) basis for this idea. In all three cases you're adjusting for the outlier by increasing the ranges which will fire each bit. The AdaptiveEncoder makes a mistake by ignoring the statistics of the outliers, but at least it ends up with the right encoding if the outlier should have been chosen as the initial max (ie all bits are equally representative of the data). You encoder makes this mistake too, and also makes the mistake of creating two populations of bits, one of which is now suddenly responding to two ranges of values. The NUSE learns its encoding based on the data. This is biologically justified. We adjust our sensitivity (in both directions) based on the changing statistics of the physical intensity of the stimulus (light, sound levels, etc), and we also adjust the shape of the response profile in a gradual way. Note that this is different to adjusting the identity of the stimulus (sound frequency, ultraviolet light etc) which is usually brought up whenever this is discussed. We're talking here about encoding a single scalar for a given datum, not changing what the sensor is measuring. Regards, Fergal Byrne Regards, Fergal Byrne On Fri, Nov 1, 2013 at 8:57 PM, Jeff Hawkins <[email protected]> wrote: > Patrick, > Maybe we are thinking about this differently. > > In my understanding there is no "age". In the current encoder a bit in the > encoder represents a small range on the number line. This never changes, > doesn't age. What is new is that same bit now also represents one or more > additional ranges of the number line in addition to the original one. > These > new ranges are either above and/or below the min and max values of the > original encoder. A single bit in the encoder could represent many > different ranges of the number line, as many as we need to cover the full > range of values in the input. No bits are "released". > > In your diagram if all the colored bits were full saturation it would match > what I was thinking. The y axis would represents the encoding of > increasing > values of the input. The diagram could continue expanding vertically > representing increasing values of the input. > Jeff > > -----Original Message----- > From: nupic [mailto:[email protected]] On Behalf Of Patrick > Higgins > Sent: Friday, November 01, 2013 1:14 PM > To: NuPIC general mailing list. > Subject: Re: [nupic-dev] new encoder idea > > Jeff, > > Saturation indicates the bit's age, the oldest one changes to a 0 and a new > random one is selected as the new 5th bit. Green bits are 1 and yellow bits > are 0. Saturation indicates age. It seemed necessary to facilitate > consistent overlap, so I used the age of the bit as the criteria for > deciding which bit is released, to give similar input numbers a similar > SDR. > If its not needed or helpful, I can change the green back to 100% > saturation. > > In the case of t=13 bit 9 is released to 0 and bit 38 is activated to 1. > bit > 38 was chosen randomly, but what if bit 21 was picked instead. Would this > be > acceptable or does the new bit need to be previously unused? > > > > > Patrick > > On Nov 1, 2013, at 11:54 AM, Jeff Hawkins wrote: > > > Patrick, > > I am confused. Why did you go to graded bits? I was expecting to see > > five solid green dots per row. > > Jeff > > > _______________________________________________ > 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 > -- Fergal Byrne <http://www.examsupport.ie>Brenter IT [email protected] +353 83 4214179 Formerly of Adnet [email protected] http://www.adnet.ie
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