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    | • | 
    So far, the
    states of the units in one layer have only been 
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    used to
    determine the effective biases of the units in the 
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    layer below. 
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    It would be much
    more powerful to modulate the pair-wise 
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    interactions in
    the layer below. (A good way
    to design a 
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    hierarchical
    system is to allow each level to determine the 
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    objective
    function of the level below.) 
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    – | 
    For
    example, a vertical edge represents a breakdown in 
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    the
    usual correlational structure of the pixels: Horizontal 
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    interpolation
    does not work, so it needs to be turned off, 
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    but
    interpolation parallel to the edge is OK. 
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    To modulate
    pair-wise interactions we need higher-order 
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    Boltzmann
    machines. 
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