Effect of age on absolute values of test and retest differences of saturated amplitude (Vmax), semisaturation constant (K) and slope (n) parameters of Naka-Rushton fits to intensity response data of PhNR measured from baseline (BT – Figs. 6A, B, C) and preceding b-wave peak (PT – Figs. 6D, E, F) as well as b-wave (Figs. 6G, H, I)