Lecture notes for Stat 231: Pattern Recognition and Machine Learning 1. Stat 231. A.L. Yuille. Fall Perceptron Rule and Convergence Proof Capacity of Perceptrons. Multi-layer Perceptrons. Read 5.4, Duda, Hart, Stork.

Lecture notes for Stat 231: Pattern Recognition and Machine Learning 2. Linear Separation N samples where the Can we find a hyperplane in feature space through the origin, that separates the two types of samples

Lecture notes for Stat 231: Pattern Recognition and Machine Learning 3. Linear Separation For the two-class case, simplify by replacing all samples with Then find a plane such that The weight vector is almost never unique. Determine the weight vector that has the biggest margin m(>0), where (Next lecture). Discriminative: no attempt to model probability distributions. Recall that the decision boundary is a hyperplane if the distributions are Gaussian with identical covariance.

Lecture notes for Stat 231: Pattern Recognition and Machine Learning 4. Perceptron Rule Assume there is a hyperplane separating the two classes. How can we find it? Single Sample Perceptron Rule. Order samples Set loop over j, if is misclassified, set repeat until all samples are classified correctly.

Lecture notes for Stat 231: Pattern Recognition and Machine Learning 5. Perceptron Convergence Novikov’s Theorem: the single sample Perceptron rule will converge to a solution weight, if one exists. Proof. Suppose is a separating weight. Then decreases by at least for each misclassified sample. Initialize weight at 0. Then number of weight changes is less than

Lecture notes for Stat 231: Pattern Recognition and Machine Learning 6. Perceptron Convergence Proof of claim. If Using

Lecture notes for Stat 231: Pattern Recognition and Machine Learning 7. Perceptron Capacity The Perceptron was very influencial and unrealistic claims were made about its abilities (1950’s, early 1960’s). The model is an idealized model of neurons. An entire book was published in the mid 1960’s describing the limited capacity of Perceptrons (Minsky and Papert). Some classifications, exclusive or, can’t be performed by linear separation. But, from Learning Theory, limited capacity is good.

Lecture notes for Stat 231: Pattern Recognition and Machine Learning 8. Generalization and Capacity. The Perceptron is useful precisely because it has finite capacity and so cannot represent all classifications. The amount of training data required to ensure Generalization will need to be larger than the capacity. Infinite capacity requires infinite data. Full definition of Perceptron capacity must wait till we introduce Vapnik Chevonenkis (VC) dimension. But the following result (Cover) gives the basic idea..

Lecture notes for Stat 231: Pattern Recognition and Machine Learning 9. Perceptron Capacity Suppose we have n sample points in a d dimensional feature space. Assume that these points are in general position – no subset of (d+1) points lies in a (d-1) dimensional subspace Let f(n,d) be the fraction of the 2^n dichotomies of the n points which can be expressed by linear separation. It can be shown (D.H.S) that f(n,d) =1, for otherwise There is a critical value 2(d+1). f(n,d)=1 for n << 2(d+1), f(n,d) =0 for n >> 2(d+1), transition rapid for large d.

Lecture notes for Stat 231: Pattern Recognition and Machine Learning 10. Capacity and Generalization Perceptron capacity is d+1. The probability of finding a separating hyperplane by chance alignment of the samples decreases rapidly for n > 2(d+1).

Lecture notes for Stat 231: Pattern Recognition and Machine Learning 11. Multi-Layer Perceptrons Multilayer Perceptrons were introduced in the 1980’s to increase capacity. Motivated by biological arguments (dubious). Key Idea: replace the binary decision rule by a Sigmoid function: (Step function as T tends to 0). Input units activity Hidden units Output units Weights connecting the Input units to the hidden units, and the hidden units to the output units.

Lecture notes for Stat 231: Pattern Recognition and Machine Learning 12. Multilayer Perceptrons Multilayer perceptrons can represent any function provided there are a sufficient number of hidden units. But the number of hidden units may be enormous. Also the ability to represent any function may be bad, because of generalization/memorization. Difficult to analyze multilayer perceptrons. They are like “black boxes”. When they are successful, there is often a simpler, more transparent alternative The Neuronal plausibility for multilayer perceptrons is unclear.

Lecture notes for Stat 231: Pattern Recognition and Machine Learning 13. Multilayer Perceptrons Train the multilayer perceptron using training data Define error function for each sample Minimize the error function for each sample by steepest descent: Backpropagation algorithm (propagation of errors).

Lecture notes for Stat 231: Pattern Recognition and Machine Learning Summary Perceptron and Linear Separability. Perceptron rule and convergence proof. Capacity of Perceptrons. Multi-layer Perceptrons. Next Lecture – Support Vector Machines for Linear Separation.