Outline. Self-Organizing Maps (SOM) US Hebbian Learning, Cntd. The learning rule is Hebbian like:

Transcription

1 Self-Organzng Maps (SOM) Turgay İBRİKÇİ, PhD. Outlne Introducton Structures of SOM SOM Archtecture Neghborhoods SOM Algorthm Examples Summary 1 2 Unsupervsed Hebban Learnng US Hebban Learnng, Cntd 3 A lnear unt: V = W j X j The learnng rule s Hebban lke: ΔW j = ηv j ( X j j VW ) The change n weght depends on the product of the neuron s output and nput, wth a term that makes the weghts decrease 4 Such a net converges nto a weght that maxmzes the average on V 2 Ths means that the weght ponts at the frst prncpal component of the data The network learns a feature of the data wthout any pror knowledge Ths s called feature extracton Unsupervsed Compettve Learnng Smple Compettve Learnng Algorthm In Hebban networks, all neurons can fre at the same tme Compettve learnng means that only a sngle neuron from each group fres at each tme step Output unts compete wth one another. These are wnner takes all-wta unts (grandmother cells) - Intalze weghts of samples from the nput Leaky learnng: also update the weghts of the losers (but wth a smaller h) Arrange neurons n a geometrcal way: update also neghbors Turn on nput patterns gradually Conscence mechansm Add nose to nput patterns 5 6 1

2 Smple Compettve Learnng Network Actvaton N nputs unts P output neurons P x N weghts h = 1, 2... P N = W X j = 1 j j x1 x2 WP1 W11 W12 W22 Y1 Y2 The unt wth the hghest feld h fres * s the wnner unt Geometrcally s closest to the current nput W * The wnnng unt s weght s updated to be even closer to the current nput Y = 1or 0 xn WPN YP 7 8 Learnng Introducton of SOM Startng wth small random weghts, at each step: 1. a new nput s presented to the network 2. all felds are calculated to fnd a wnner 3. s updated to be closer to the nput W * SOM s the neuron network Development n 1980s by Teuvo Kohonen n fnland SOM s the Unsupervsed Learnng SOM look lkes Grd (Smple for lookng) Usng SOM for Clusterng 9 10 Some Hstorcal Notes Prncples of Self Organzaton Local orderng (von der Malsbyrg, 1973) (Amar, 1980) a matematcal analyss elucdates the dynamc stablty of a cortcal map Self-organzng feature map (SOM), (Kohonen 1982) (Erwn, 1992) a convex neghbourhood functon should be used (~ Gaussan) The relatonshp between the SOM and prncpal curves s dcussed (Rtter, 1992 & Cherkassky and Muler, 1995) 1. Modfcatons n synaptc weghts tend to self amplfy 2. Lmtaton of resources lead to competton among synapses 3. Modfcatons n synaptc weghts tend to cooperate 4. Order and structure n actvaton patterns represent redundant nformaton that s transformed nto knowledge by the network

3 13 Determne the wnner (the neuron of whch the weght has the smallest dstance to the nput ) Move the weght w of the wnnng neuron towards the nput w Before learnng w After learnng 14 Impose a topologcal order onto the compettve neurons (e.g., rectangular map) Let neghbors of the wnner share the prze (The postcode lottery prncple) After learnng, neurons wth smlar weghts tend to cluster on the map Another vew of 15 Concept of the SOM Structures of SOM Input space Input layer Reduced feature space Map layer X X X X Ba s 1 lnear X X X X X X Sr Mn s 2 X X X X X X X X X X X X X X X X 17 Cluster centers (code s) Place of these code s n the reduced space Clusterng and orderng of the cluster centers n a two dmensonal grd 18 Rectangular X X X X X X X X X X X X X X X X X Hexagonal 3

4 SOM Archtecture SOM Archtecture Two layers of unts Input: n unts (length of tranng s) Output: m unts (number of categores) Input unts fully connected wth weghts to output unts neghborhood wnnng node Intralayer (lateral) connectons Wthn output layer Defned accordng to some topology, no weghts for those connectons Adaptve process changes weghts to more closely nputs nput (n-dmensonal) SOM Archtecture (2) Neghborhoods Based on the lattce structure, each neuron has a neghborhood of adjacent neurons. Member of neghbors -lnear has 2 nearest neghbors - rectangular has 8 nearest neghbors - hexagonal has 6 nearest neghbors. When a neuron wns a competton, the weghts of the neurons n ts neghborhood are adjusted so the learnng rule s appled locally. Set of Neghborhoods Wnnng node SOM Algorthm Compettve process Intalze weghts randomly set weghts for each node n the map Compettve process fnd the wnnng node Cooperatve process determne the neghborhood of the wnnng neuron at ths tme step. Weght adjustment process nput matrx x = [x 1, x 2, x 3,, x m ] weght matrx w = [ w 1, w 2, w 3,, w n ] dstant d = x w, = { 1,2,3, } wnnng node d = mnmum x w weghts of the wnnng neuron are strengthen and weghts n the neghborng neurons are suppressed

5 Cooperatve process Weght adjustment process The topologcal neghborhood s centered on the wnnng neuron and falls off as the dstance to neghborng neuron ncreases. weght adjustment formula w j (new) = w j (current) + w j w( new) = w( n) + η( n)[ x w( n)] j j j SOM Learnng Algorthm Pseudocode Example 1. Randomly ntalse all weghts and topology (1d or 2d) 2. Select nput x = [x 1, x 2, x 3,, x n ] from tranng set 3. Compare x wth weghts w j for each neuron j to 2 d j = ( w j x ) 4. Determne wnner and fd fnd unt j wth h the mnmum dstance 5. Update wnner so that t becomes more lke x, together wth the wnner s neghbours for unts wthn the radus accordng to wj( n+ 1) = wj( n) + η( n)[ x wj( n 6. Adjust parameters: learnng rate & neghbourhood functon 7. Repeat from (2) untl? )] Form a self-organzng map (SOM) to cluster a set of four s We wll look for 2 clusters for ths set. 27 Note that: Learnng rate generally decreases wth tme: 0 < η( n) η( n 1) 1 28 Trace Trace (2) Input d 1 = d 2 = Input d 1 = d 2 = Weght Weght

6 Trace (3) Trace (4) Input d 1 = d 2 = Input d 1 = d 2 = Weght Weght Trace of Weghts Fnal clusters Intal Epoch Epoch Epoch Epoch Epoch Epoch Epoch fnal cluster assgnment of the nputs: Input Vectors Fnal weght Input d 1 d 2 cluster Analyss Input Vectors Cluster = Cluster Cluster

7 Example for Cluster the color

8 Examples of Applcatons Summary-I 43 Kohonen (1984). Speech recognton - a map of phonemes n the Fnsh language Optcal character recognton - clusterng of letters of dfferent fonts Angelol etal (1988) travellng salesman problem (an optmzaton problem) Kohonen (1990) learnng quantzaton (pattern classfcaton problem) Rtter & Kohonen (1989) semantc maps 44 Unsupervsed learnng s very common US learnng requres redundancy n the stmul Self organzaton s a basc property of the bran s computatonal structure The SOM uses an unsupervsed clusterng process. Propertes of the fnal output map correspond to statstcally related nput data. Summary-II Questons & Answers Almost usng SOM for Clusterng the colors SOMs are based on competton (wnner takes all-wta unts) cooperaton synaptc adaptaton SOMs conserve topologcal relatonshps between the stmul Artfcal SOMs have many applcatons n computatonal neuroscence