Figure 7.

Models fit to the desynchronized state are more linear than to the synchronized state. a, Evolution of states for a segment of 135 s of continuous spontaneous activity in which desynchronization was induced with electrical stimulation of the PPT (pink vertical bars). Data were subdivided into 45 adjacent, nonoverlapping 3 s intervals. For each interval, the model was fit, and a phase diagram was obtained. The location of the fixed point along the v-axis of the phase diagram is plotted for each interval (dark blue dots) and closely follows the mean firing rate (green trace). After each PPT stimulation, cortical activity undergoes a sharp transition to the desynchronized state, followed by a gradual decay over tens of seconds back to the synchronized state. b, Sample phase diagrams corresponding to shaded 3 s intervals in a. Before PPT stimulation, the cortex was in a synchronized state, with a highly nonlinear phase diagram (i). Immediately after PPT stimulation, near-linear dynamics are seen (ii) that gradually become more nonlinear as the cortical state returns to synchronization (iii, iv). c, Degree of synchronization versus degree of nonlinearity for nonoverlapping 3 s spontaneous activity periods of one recording. Degree of synchronization is computed from the power spectrum; degree of nonlinearity measures the nonlinearity of the vector field corresponding to each model fit (see Materials and Methods). d, A similar correlation can be seen in data pooled from all recording sessions. deg, Degree; synch, synchronized; desynch, desynchronized.