© 2020 IOP Publishing Ltd. The work experimentally demonstrates the fundamental difference between the constriction in helium and other inert gases (argon, neon). Despite the visual similarity, during the constriction of the positive column of a glow discharge in helium, only a compression of the emission of spectral lines is observed. In the constricted helium discharge there is no formation of a thin channel of current which is evident from the bremsstrahlung measurements. This phenomenon can be called the optical constriction. Unlike neon and argon, where the current cord is formed because of losses of charged particles in the volume due to the dissociative recombination, the discharge in helium is controlled by the ambipolar diffusion. Moreover, while in neon and argon constriction arises in times of hundreds of microseconds - units of milliseconds after the ionization balance establishment, constricted discharge in helium forms in times about 2 min after the establishment of the thermal balance between the discharge tube and the environment. It was shown that in helium the main mechanism of constriction is the inhomogeneous heating of the neutral gas which leads to the redistribution of the neutrals over the volume, in contrast to argon and neon, where thermal effects are almost insignificant and constriction is due to the peculiarities of electron kinetics. The absence of constriction of the emission of spectral lines during the cooling of the tube walls is demonstrated. The interpretation of the observed phenomena is given on the basis of a simple qualitative model. Experiments in helium were performed at reduced pressure pR = 200 torr cm and reduced currents up to 150 mA cm-1.