(a) Scheme of a whole-cell recording showing biocytin reconstruction of a pyramidal neuron from human temporal cortex. Right: typical voltage responses to depolarizing somatic current injections. (b) Examples of AP traces at increasing instantaneous firing frequencies (frequency is shown in color code in insets) recorded from a subject with IQ = 119 (above panel, red) and a subject with IQ = 69 (lower panel, blue). AP rising phase in shaded area is displayed to the right (c) APs from subjects with higher IQ are better able to maintain their rise speed at increasing frequencies. Average (per neuron and subject) AP rise speed and (d) relative to first AP rise speeds in neurons from subjects with IQ < 100 (red, n subjects = 21, n neurons = 91) and subjects with IQ > 100 (blue, n subjects = 10, n neurons = 38) are displayed against instantaneous firing frequency. Right: data points in shaded area are shown as averaged values for 20–40 Hz (filled squares are group means, open circles are mean rise speeds per subject), *p<0.05. (e) IQ scores positively correlate with the rise speeds of first AP in the train (n subjects 31, n neurons = 129; R2 = 0.17), (f) AP rise speed at 20–40 Hz (same data as right panel in (c), R2 = 0.21) and (g) relative AP rise speeds at 20–40 Hz (same data as right panel in (d), R2 = 0.14). (h) Larger neurons show less slowing of AP rise speed at higher frequencies: relative AP rise speeds at 20–40 Hz for individual neurons are plotted as a function of their TDL (n = 21 neurons, R2 = 0.30). In c,d data are mean per subject ±S.E.M; in e, f, g data are mean ±standard deviation.