The expression 'memory curve' is used for functions showing the dependences of breakdown time delays td on the afterglow period tau at various other preset experimental parameters. The time delay values are averages of series of more than 100 measurement cycles each of the electrical breakdown time delays in nitrogen and neon filled diodes at 40 mbar for various preset tau values. It was established that the slopes of curves of td against tau , the memory curves, do not depend on the voltage applied to the diode within the range of 15% to 25% above the static breakdown voltage, indicating that these slopes measure the time variation of metastable concentrations C within the diode, i.e. dC/d tau , in the afterglow period.