Spontaneous and evoked dynamics of simulated BOLD signals.

<p><b>A:</b> The BOLD activity was simulated in response to a stimulus application for 800 trials. <i>Top</i>: evolution of the trial-averaged BOLD activity relative change, with respect to the spontaneous period (before the stimulus onset), averaged across brain regions. <i>Bottom</i>: BOLD trial-by-trial variance relative change, with respect to the spontaneous period, averaged across brain regions. Shaded areas represent 95% confidence intervals. Vertical dotted lines indicate the onset and offset of the stimulus. <b>B:</b> Relation between the amount of mean BOLD change and the amount of variance change of BOLD signals, averaged over the period between <i>t</i> = 5s and <i>t</i> = 23s, for the brain regions receiving directly (<i>black</i>) and indirectly (<i>gray</i>) the external input. <b>C:</b> Averaged change in power spectral density of the summed E-I synaptic activity (ΔPSD) separately for those brain regions that show an increase (<i>red</i>) or a decrease (<i>blue</i>) of variance in the simulated BOLD signal. The shaded area indicates the response function of the BOLD model, given by the amplitude of the BOLD signal in response to cosine inputs of different frequencies, i.e. zi = cos(ωt). <b>D:</b> Autocovariance of the BOLD signals of individual brain regions in the spontaneous condition (<i>light blue</i>) and in the stimulated condition (<i>light red</i>). The autocovariance averaged across brain regions is shown in <i>blue</i> for the spontaneous condition and in <i>red</i> for the stimulated condition. In A–C the brain regions receiving the external input are: r/lLOCC, r/lMT, r/lPCAL, r/lST.