[Comp-neuro] two underlying questions about noise
rinkus at comcast.net
rinkus at comcast.net
Wed Jul 30 17:48:46 CEST 2008
Dr. Bower and all,
Thanks for this wonderful discussion with such a grand charter on noise and synchrony. I see two different questions re noise underlying the discussion.
1. Is the pattern of firing observed for any particular principal cell (PC) noisy?
2. Is noise (randomness) used by the system in the process of storing information?
Regarding the first, given that the typical PC has about 10,000 afferent synapses and that the experimenter knows very little about the simultaneous state of those 10,000 inputs, this is a hard question to answer with certainty. I agree that we'd see a lot less, and possibly very little, randomness in the PC's firing pattern if we had complete knowledge of its 10,000 inputs.
Regarding the second question, in my earlier post, I described a simple mechanism whereby noise is used by the system to ensure that a certain property--namely, that similar inputs map to similar codes (SISC)--exists over the set of codes stored in a given patch of cortex. By 'codes', I mean specifically, 'sparse distributed codes'.
My working hypothesis is that the neuromodulatory systems of NE and/or Ach might instantiate this noise mechanism (cf. work by Isakova, Kimura, Tateno, Hasselmo, Vankov, Clayton, Dayan, Bouret, Sara, and many others). When the input to the patch is detected to be novel, one/both of NE/Ach are flooded into the patch, swamping out the effects of specific synaptic inputs, resulting in choosing a small set of PC's in the patch essentially at random.
However, if we could immediately give the same input again to the patch, no novelty (perfect familiarity) would be detected, thus no Ach/NE would be flooded to the patch and the specific synaptic inputs would dominate, which would with very high probability (with little randomness) cause the same set of PC's (code) to become active (recognition).
Thus, in my model, noise (manifest as momentary Ach/NE levels) is most definitely used during construction of memories. However, after construction, if we could test, with complete control over the PC's inputs, the firing pattern would show very little noise.
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