Extended Data Fig. 2: Reproducible firing patterns in human brain organoids.

a, Raster plot visualization of single-unit spiking (blue dots) measured across the surface of a human brain organoid slice from ref. ²⁷ (HO5), positioned on top of the same Maxwell Biosystems microelectrode array as used for the organoid recordings included in the main Figs. 1–5. The population firing rate is shown by the red solid line. Population bursts are marked by sharp increases in the population rate. Burst peak events are denoted by local maxima (black dots) that exceed 4× r.m.s. fluctuations in the population rate. The shaded gray regions denote the burst duration window as defined by the time interval in which the population rate remains above 10% of its peak value in the burst. b, The instantaneous firing rate of single-unit activity from a after reordering. The backbone units are plotted above the dashed line, while non-rigid units are plotted below the dashed line. In each category, units are ordered based on their median firing rate peak time relative to the burst peak, considered over all bursts in the recording. c, The average burst peak-centered firing rate measured across all burst events for the example recording of which part is shown in a. The burst peak is indicated by the dotted line. The unit order is the same as b. Please note the progressive increase in the firing rate peak time relative to the burst peak, as well as a spread in the active duration for units having their peak activity later in the burst. The average firing rate is normalized per unit to aid in visual clarity. d, The distributions of the log of the average firing rate per unit, separated for backbone and non-rigid units. All units from the four recordings mentioned in ref. ²² are pooled together and all units from the four recordings mentioned in ref. ²⁷ are pooled together (P < 10⁻²⁰ and P = 0.40, respectively, two-sided linear mixed-effect model, n = 4 organoids each). e, The distributions of the average burst-to-burst correlations per unit after average rate normalization, separated for backbone and non-rigid units. All units from the four recordings mentioned in ref. ²² are pooled together and all units from the four recordings mentioned in ref. ²⁷ are pooled together. Please note the significant difference between backbone and non-rigid units present for both sets of recordings (P = 1 × 10⁻¹⁰ and P = 0.0092, respectively, two-sided linear mixed-effect model, n = 4 organoids each). f, The distributions of the pairwise correlations per unit pair after average rate normalization, separated for backbone pairs, backbone and non-rigid combinations and non-rigid pairs. All unit pairs from the four recordings mentioned in ref. ²² are pooled together and all unit pairs from the four recordings mentioned in ref. ²⁷ are pooled together. Please note that only the normalized correlation for backbone pairs are significantly larger than 0 for both sets of recordings (P = 0.0001, 0.145 and 1; P = 7 × 10⁻¹¹, 1 and 1, respectively), one-sided linear mixed-effect model, n = 4 organoids each).