def _update_evens(self, activations, clamped=None):
        """
        Updates just the even layers of the network.

        Parameters
        ----------
        See all of the descriptions for _update_evens.
        """
        evens = xrange(0, len(activations), 2)

        self._update_activations(activations, evens)
        if clamped is not None:
            self._apply_clamping(activations, clamped)

        evens_copy = [(i, activations[i]) for i in evens]
        self.apply_postact_corruption(activations, evens)

        return evens_copy