The paper analyzes possible mechanism of manifestation of second-order nonlinear effects, in particular, the effect of optical rectification, upon propagation of femtosecond laser pulses in air under self-action conditions. With a Ti:Sapphire laser as an example, it is shown that the laser pulse propagation under such conditions is accompanied by generation of a video pulse, whose shape is determined by the derivative of the spatiotemporal shape of the laser field envelope.