  Since their discovery in the mid-1990s, ultrafast laser filaments in gases
have been described as products of a dynamic balance between Kerr self-focusing
and defocusing by free electric charges that are generated via multi-photon
ionization on the beam axis. This established paradigm has been recently
challenged by a suggestion that the Kerr effect saturates and even changes sign
at high intensity of light, and that this sign reversal, not free-charge
defocusing, is the dominant mechanism responsible for the extended propagation
of laser filaments. We report qualitative tests of the new theory based on
electrical and optical measurements of plasma density in femtosecond laser
filaments in air and argon. Our results consistently support the established
paradigm.
