Examples, neuroscience analogy Perceptrons, mlps: How they work



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Outline

  • Examples, neuroscience analogy

  • Perceptrons, MLPs: How they work

  • How the networks learn from examples

  • Backpropagation algorithm

  • Learning parameters

  • Overfitting



Examples in Medical Pattern Recognition

  • Diagnosis

  • Protein Structure Prediction

  • Diagnosis of Giant Cell Arteritis

  • Diagnosis of Myocardial Infarction

  • Interpretation of ECGs

  • Interpretation of PET scans, Chest X-rays

  • Prognosis

  • Prognosis of Breast Cancer

  • Outcomes After Spinal Cord Injury



Biological Analogy





Perceptrons



Perceptrons



AND



XOR



XOR



XOR



Linear Separation





Abdominal Pain



Multilayered Perceptrons



Activation Functions...

  • Linear

  • Threshold or step function

  • Logistic, sigmoid, “squash”

  • Hyperbolic tangent



Neural Network Model



“Combined logistic models”



Hidden Units and Backpropagation



Minimizing the Error



Gradient descent



Overfitting



Overfitting



Overfitting in Neural Nets



Parameter Estimation

  • Logistic regression

  • It models “just” one function

    • Maximum likelihood
    • Fast
    • Optimizations
      • Fisher
      • Newton-Raphson


What do you want? Insight versus prediction

  • Insight into the model

  • Explain importance of each variable

  • Assess model fit to existing data



Model Selection Finding influential variables

  • Logistic

  • Forward

  • Backward

  • Stepwise

  • Arbitrary

  • All combinations

  • Relative risk



Small sets: Cross-validation

  • Several training and test set pairs are created so that the union of all test sets corresponds exactly to the original set

  • Results from the different models are pooled and overall performance is estimated

  • “Leave-n-out”

  • Jackknife



ECG Interpretation



Thyroid Diseases



Time Series



Time Series



Evaluation



Evaluation: Area Under ROCs



ROC Analysis: Variations





























Dijagnoza

  • Dijagnoza

  • Struktura proteina

  • Diajagnoza arteritisa gigantskih ćelija

  • Infarkta miokarda

  • Interpretacije EKG-a

  • Interpretacija PET scena, rentgenskog snimka

  • Prognoza

  • Karcinoma dojke

  • Ishoda posle povrede kičmene moždine



Primeri, analogno neuronaukama

  • Primeri, analogno neuronaukama

  • Perceptrons, MLPs: kako oni rade?

  • Kako mreža uči iz primera

  • Backpropagation algoritam

  • Parametri učenja

  • Overfitting












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