Fig. 1
Experimental model and stimulation protocols. a Cortical neuronal network grown over a micro electrode array (MEA). Top left A typical 60-channel MEA produced by multichannel systems (MCS, Reutlingen, Germany). The glass ring is necessary to contain cell culture medium. Bottom left Dissociated cortical neurons over substrate-embedded planar microelectrodes of a MEA. The electrodes, placed in a square 8 × 8 layout (the four corners are missing), are 30 µm wide, with an electrode spacing of 200 µm. Right Zoom of the cultured cortical network showing cells coupled to microelectrodes and randomly developing their neurites and synaptic connections (age of the culture: 21 days in vitro, DIV). b Stimulation regimes. b1 We used trains of electrical pulses whose time intervals followed a 1/f β distribution, with β assuming values from 0 (i.e. white noise stimulation) to ∞ (perfectly regular stimulation). Depicted here are the profiles of the instantaneous stimulus rate (ISR, see text—rescaled for comparison) for each value of β: β = 0 (green trace), β = 0.5 (red trace), β = 1 (blue trace), β = 1.5 (orange trace) and β = ∞ (light blue trace). Autocorrelation functions (b2) and PSD (b3) of irregular stimulation sequences (same color code as in b1). c Adopted experimental protocols. c1 1st experimental protocol (protocol 1 throughout the manuscript): three phases of spontaneous activity [BAS1—1 h, BAS2—30 min and BAS3—1 h] are interleaved by two stimulation sessions [STIM1 and STIM2] lasting 50 min (i.e. 10 min × 5 different β values [0, 0.5, 1, 1.5, ∞], delivered in random order). c2 2nd experimental protocol (protocol 2 throughout manuscript): three phases of spontaneous activity [BAS1—1 h, BAS2—30 min and BAS3—1 h] are interleaved by one session of off-line analysis [ANALYSIS] and two stimulation sessions [STIM1 and STIM2] lasting 60 min (i.e. 10 min × 2 different β values [1 and ∞] × 3 stimulation frequencies [fMBR/2, fMBR, fMFR]. For details see paragraph 2.3 “Generation of stimuli and experimental protocols”)