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What do P letters mean in neural network layer scheme?

In Wikipedia article about MNIST database it is said, that lowest error rate is of "committee of 35 convolutional networks" with the scheme:

1-20-P-40-P-150-10

What does this scheme mean?

Numbers are probably neuron numbers. But what does 1 mean then?

What do P letters mean?

Solution

  • In this particular scheme, 'P' means 'pooling' layer.

    So, basic structure is following:

    1. One grayscale input image
    2. 20 images after convolution layer (20 different filters)
    3. Pooling layer
    4. 40 outputs from next convolution
    5. Pooling layer
    6. 150... can be either 150 small convolution outputs or just fully-connected 150 neurons
    7. 10 output fully-connected neurons

    That's why 1-20-P-40-P-150-10. Not best notation, but still pretty clear if you familiar with CNN.

    You can read more details about internal structure of CNN in base article of Yann LeCun "Gradient-Based Learning Applied to Document Recognition".