(A) Intracellularly recorded ‘generator’ membrane potential trace of a V1 simple cell (black) with spikes clipped off (see Materials and methods). Orange vertical lines show spike times, orange horizontal line shows estimated firing threshold (). Red marks show response baseline (), signal variance (), and noise magnitude (σ) estimated from the generator potential responses to multiple cycles of the moving grating stimulus. (B) Illustration of estimating the FRNL from electrophysiological data. The firing rate of a cell as a function of the membrane potential (top panel, black line, gray shaded area shows s.e.m.) is obtained as the normalised ratio of the probability distributions of the generator potential at spike times and at all times (bottom panel, orange and gray histograms, respectively). A threshold-power-law function was fitted to the firing rate function and the threshold linear fit was used to estimate the firing threshold (vertical orange line). Threshold-power-law fits with different levels of the FRNL exponent (red traces, , ) were largely consistent with the firing rate estimated from the data. (C) Normalised thresholds of four V1 simple cells as a function of the noise-to-signal ratio of their membrane potential responses (blue circles). Normalised thresholds and noise-to-signal ratios were computed from the original cellular parameters (panels A-B) as in Figure 6. Shaded area shows robust decodability regime of RU (range of normalised thresholds within which > 90% of maximal linear decoding performance is achieved across all values between 1 and 2, cf. Figure 6E, Figure 7—figure supplement 2). Inset: same as main panel but black dots correspond to normalised threshold values and noise-to-signal ratios obtained by shuffling the experimentally measured parameters across recorded neurons ( = −46.8, –42.4, –47.3, –36.9 mV; = 7.15, 9.1, 4.48, 7.83 mV; = 1.7, 2.83, 2.39, 3.21; and = −44.9, –38.6, –44.9, –35.1 mV). (D) Same as panel (C) but recorded data is compared with the prediction of maximal information transmission on the normalised threshold.