def get_reconstruction_cost(self, updates, pre_sigmoid_nv):
        """Approximation to the reconstruction error

        Note that this function requires the pre-sigmoid activation as
        input.  To understand why this is so you need to understand a
        bit about how Theano works. Whenever you compile a Theano
        function, the computational graph that you pass as input gets
        optimized for speed and stability.  This is done by changing
        several parts of the subgraphs with others.  One such
        optimization expresses terms of the form log(sigmoid(x)) in
        terms of softplus.  We need this optimization for the
        cross-entropy since sigmoid of numbers larger than 30. (or
        even less then that) turn to 1. and numbers smaller than
        -30. turn to 0 which in terms will force theano to compute
        log(0) and therefore we will get either -inf or NaN as
        cost. If the value is expressed in terms of softplus we do not
        get this undesirable behaviour. This optimization usually
        works fine, but here we have a special case. The sigmoid is
        applied inside the scan op, while the log is
        outside. Therefore Theano will only see log(scan(..)) instead
        of log(sigmoid(..)) and will not apply the wanted
        optimization. We can not go and replace the sigmoid in scan
        with something else also, because this only needs to be done
        on the last step. Therefore the easiest and more efficient way
        is to get also the pre-sigmoid activation as an output of
        scan, and apply both the log and sigmoid outside scan such
        that Theano can catch and optimize the expression.

        """

        cross_entropy = T.mean(
                T.sum(self.input * T.log(T.nnet.sigmoid(pre_sigmoid_nv)) +
                (1 - self.input) * T.log(1 - T.nnet.sigmoid(pre_sigmoid_nv)),
                      axis=1))

        return cross_entropy