  By studying the optical conductivity of BSLCO and YCBCO, we show that the
metal-to-insulator transition (MIT) in these hole-doped cuprates is driven by
the opening of a small gap at low T in the far infrared. Its width is
consistent with the observations of Angle-Resolved Photoemission Spectroscopy
in other cuprates, along the nodal line of the k-space. The gap forms as the
Drude term turns into a far-infrared absorption, whose peak frequency can be
approximately predicted on the basis of a Mott-like transition. Another band in
the mid infrared softens with doping but is less sensitive to the MIT.
