Information Theory, Inference, and Learning ... - Inference Group

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Copyright Cambridge University Press 2003. On-screen viewing permitted. Printing not permitted. http://www.cambridge.org/0521642981You can buy this book for 30 pounds or $50. See http://www.inference.phy.cam.ac.uk/mackay/itila/ for links.490 40 — Capacity of a Single NeuronWhat is interesting is (for large K) the number of points N such thatalmost any labelling can be realized. The ratio T (N, K)/2 N is, for N < 2K,still greater than 1/2, and for large K the ratio is very close to 1.For our purposes the sum in equation (40.9) is well approximated by theerror function,K∑( ) ( )N K − (N/2)≃ 2 N Φ √ , (40.10)k N/20where Φ(z) ≡ ∫ z−∞ exp(−z2 /2)/ √ 2π. Figure 40.9 shows the realizable fractionT (N, K)/2 N as a function of N and K. The take-home message is shown infigure 40.9c: although the fraction T (N, K)/2 N is less than 1 for N > K, it isonly negligibly less than 1 up to N = 2K; there, there is a catastrophic dropto zero, so that for N > 2K, only a tiny fraction of the binary labellings canbe realized by the threshold function.ConclusionThe capacity of a linear threshold neuron, for large K, is 2 bits per weight.A single neuron can almost certainly memorize up to N = 2K randombinary labels perfectly, but will almost certainly fail to memorize more.40.4 Further exercises⊲ Exercise 40.4. [2 ] Can a finite set of 2N distinct points in a two-dimensionalspace be split in half by a straight line• if the points are in general position?• if the points are not in general position?Can 2N points in a K dimensional space be split in half by a K − 1dimensional hyperplane?Exercise 40.5. [2, p.491] Four points are selected at random on the surface of asphere. What is the probability that all of them lie on a single hemisphere?How does this question relate to T (N, K)?Exercise 40.6. [2 ] Consider the binary threshold neuron in K = 3 dimensions,and the set of points {x} = {(1, 0, 0), (0, 1, 0), (0, 0, 1), (1, 1, 1)}. Finda parameter vector w such that the neuron memorizes the labels: (a){t} = {1, 1, 1, 1}; (b) {t} = {1, 1, 0, 0}.Find an unrealizable labelling {t}.⊲ Exercise 40.7. [3 ] In this chapter we constrained all our hyperplanes to gothrough the origin. In this exercise, we remove this constraint.How many regions in a plane are created by N lines in general position?Exercise 40.8. [2 ] Estimate in bits the total sensory experience that you havehad in your life – visual information, auditory information, etc. Estimatehow much information you have memorized. Estimate the informationcontent of the works of Shakespeare. Compare these with the capacity ofyour brain assuming you have 10 11 neurons each making 1000 synapticconnections, and that the capacity result for one neuron (two bits perconnection) applies. Is your brain full yet?Figure 40.10. Three lines in aplane create seven regions.
Copyright Cambridge University Press 2003. On-screen viewing permitted. Printing not permitted. http://www.cambridge.org/0521642981You can buy this book for 30 pounds or $50. See http://www.inference.phy.cam.ac.uk/mackay/itila/ for links.40.5: Solutions 491⊲ Exercise 40.9. [3 ] What is the capacity of the axon of a spiking neuron, viewedas a communication channel, in bits per second? [See MacKay andMcCulloch (1952) for an early publication on this topic.] Multiply bythe number of axons in the optic nerve (about 10 6 ) or cochlear nerve(about 50 000 per ear) to estimate again the rate of acquisition sensoryexperience.40.5 SolutionsSolution to exercise 40.5 (p.490). The probability that all four points lie on asingle hemisphere isT (4, 3)/2 4 = 14/16 = 7/8. (40.11)
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