X-Message-Number: 3965 Date: Wed, 8 Mar 1995 00:19:47 +0100 (MET) From: Eugen Leitl <> Subject: LifeScan2 There are a lot of technical difficulties associated with the (in vitro) scan of vitrified brain tissue. The task is essentially: 0) to read the neuron type (susceptibility function) 1) to trace the tubular structures of neuron's axons and cell body (graph connectivity, edge weight) 2) distinguish them from the background (glia cells) 3) read the synapse value and sign (weighted edge 2) To 0) Whether this is discernible from shape alone or the membrane proteins type.. No one knows.. (BTW, there are some 50 types of neurons.) To 1),2): One can view the tubular lipid membrane bilayer as an active wire, with the top bandwidth of roughly 1000 s^-1 spikes traveling along myelinated axons (wrapped in Swann cell sheets). After each spike the active membrane (ion pumps driven by ATP) the membrane becomes insusceptable to subsequent signals (refractory time) which enhances signal distinction. Since the signals are electrochemical (tipping a potential gradient to a temporary collapse) the signal propagation velocity is much slower than the speed of light, being only about 100 ms^-1 for saltatory signal propagation in mammals. (Much slower for lower beasties, e.g. like squids). Contrary to common belief, signal coding is analogue: though the spike levels are binary, they get integrated over time slices of some 10 ms at the synapse junction. Frequency encoding's dynamic range of values is limited, but signal encoding robustness is excellent. Alternative codings my be used by higher order cerebral modules: since there is no spatial neuron structuring (logical only) their detection is quite nontrivial. Because of their functional simplicity (binary frequency coded channel with limited bandwidth and a delay which is directly proportional to axon length) their biochemical and structural makeup is simple (provided, there are not unknown (chemical) modulation variables). Ion pumps and ion channel protein density is high, they are uniform and easily accessable (as they are embedded in the membrane lipid bilayer) and can be easily immunostained to enhance imaging contrast. In short, axons alone can be approximated/modeled by weighted edges (graph theory lingo), the weight being the delay. Propagation velocity is a global constant. (Thanks god.) Applying 3d image processing and recognition techniques (edge detection and tracing) upon voxel blocks (critical maximum size determined by the processing systems' memory size) makes for a high hope for easy/successful axon scan/tracing/modeling. The state of the neuron is the probability of firing in the next instant or the overall (time slice) firing frequency. By applying several transmitters (excitatory/inhibitory), which get depleted into the subsynaptic cleft by the synapse upon signal spike transit and diffuse (through passive physical transport) to the postsynaptic membrane of the neuron body and modify the probability of the individual neuron to fire the next instant the whole of NN signal processing is done. The synapse sign is the basic property of a synapse-dumped neurotransmitter: positive (excitatory) or negative (inhibitory), the amount of above to be dumped is the synapse value. Alternatively, the postsynaptical membrane patch might be the location where signal modulation is done. (Might, mark). The susceptibility function of the neuron body to transmitter packet input can be linear or sigmoidal (or (m)any other). Overall function can be modulated by neuromodulators, which' sphere of influence scale is much wider than a synapse. (Read: encompassing many neurons, whole isles of them). What we might well expect that the dynamic range of a synapse is quite limited. While certainly not binary it should not exceed 6 bits, tops 8 bit. The main difficulty in scanning is to attribute such a value to a tiny physical structure (say, 500-100 nm sized) with a tolerable accuracy. Really tough. -- Eugene P.S. These Random Ramblings shall be augmented by a real-world NN-model/encoding, together with a suitable silicon/ME implementation discussion in the next future. Comments? (Puhleeze..) Rate This Message: http://www.cryonet.org/cgi-bin/rate.cgi?msg=3965