Figure 6. The shifting effect of cortical stimulation is short-lived.

A, recordings aligned at stimulus onset (20 selected trials at 4 s inter-stimulus interval) of a MSN–ECoG pair showing that the phase shifting effect of cortical stimulation lasts about 1 s. B, twenty selected 4 s epochs of spontaneous activity of the same MSN–ECoG pair were aligned at a transition from a Down to an Up state to study the inherent variability of slow oscillations. Although traces seem aligned for the initial second, there is no obvious concentration of slow oscillation phase beyond 2 s after the aligning tag. C, to estimate the duration of the phase shifting effect of cortical stimulation, we computed inter-trial instantaneous phase (IIP) distributions following the stimulus and the probability of IIP distributions to be uniform with the Rayleigh test. Rayleigh probabilities were normalized (−1 to 0) and depicted as a function of post-stimulus time, for the MSN–ECoG pair illustrated above (90 trials). IIP distributions were strongly non-uniform for about 1 s after the stimulus, but Rayleigh probability increased steeply following stimulation, indicating a strong trend towards uniformity. The curves obtained with post-stimulus data and with artificially aligned spontaneous data overlapped completely. Inset: the duration of the ‘phase shifting effect’ was established as the time at which the Rayleigh probability increased 50% in the relative scale. There were no differences between phase shifting durations of cortical stimulation and artificially aligning spontaneous activity (n = 6 MSN–ECoG pairs tested with inter-stimulus intervals of 4–6 s, paired t test).