Fig。 3。

Fig。 3。 Closed-loop feedback TES stimulation aborts SW episodes。 (A) Mean duration of SW episodes in each of the nine rats tested and the group mean (and SD)。 (B) Percent time spent in SW episodes in a given recording session。Shown are results from inpidual rats and group。 Mean percent change refers to group data。

We first demonstrated the effect of TES on cortical excitability。 Local field potentials (LFPs)and multiple-unit activity (MUA) were recorded by chronically implanted tripolar electrodes (Fig。1A) and placed in the deep and superficial layers of the frontal and parietal cortical areas (21)。TES was applied either between the left and righttemporal electrodes, placed directly on the skull,or between these bitemporal electrodes, against a frontal midline electrode (Fig。 1B and fig。 S1) (18)。 TES-induced artifacts were reduced by using tripolar electrodes and deriving current source density (CSD) of the recorded traces online (Fig。 1, C and D)。 TES sinusoid trains at 1 Hz induced significant rate modulation of multiple unit firing patterns both in the absence and presence ofspike-and-wave (SW) episodes (P < 0。01; 61 of 103 cortical sites; n = 3 rats; Rayleigh test) (Fig。 2)。In addition to affecting the firing rates of neurons,TES at 1 Hz also strongly modulated the spike amplitude of SW patterns (Fig。 2, C and D, and fig。 S2) but had no effect on the duration of SW episodes [time (t) = 8。62 s, 21,902 s of TES epochs;t = 9。14 s, 21,775 s of control epochs; n = 9 rats;P > 0。05; t test for dependent samples]。 Additional control experiments demonstrated that TES,at the intensities used, neither induced arousal effects when applied during sleep nor affected overt behavior during waking, as demonstrated by the lack of TES-induced head movements(figs。 S3 and S4)。

We next sought to examine how brain activity–triggered stimulation affects SWpatterns。 Because SW patterns are strongly periodic events, involving reverberatory activity of the thalamocortical loop (14), we applied Gaussian waveforms of 50-ms TES after the detected spike components。SW-triggered TES shortened the duration of SW episodes in an intensity-dependent manner (Fig。 3)。Group analysis of closed-loop TES stimulation showed that the mean duration of the SW episodes was significantly shorter in nine of the nine rats  (P

< 0。01; two sample t test)。 In addition,the percent time spent in SW episodes in a given session was significantly reduced in seven of the nine rats (P < 0。01)。 Overall, feedback TES stimulation lead to a >60% decrease of both the duration of SWepisodes and the fraction of session time spent in SW across animals (Fig。 4)。The percent decrease of time spent in SW episodes in a given session was largely a consequence of the decreased duration of Swepisodes because there was a significant correlation between the mean duration and the fraction of time in SWepisodes [correlation coefficient (r) =0。60; P < 0。001; Pearson’s correlation test] (fig。S5)。 This relationship thus shows that TES did not simply fragment or delay SWepisodes。 The number of SW episodes per unit time was not significantly different between control and TES sessions (P > 0。05; two sample t test), indicating that TES-induced reduction of SW episodes did not lead to a rebound or long-term compensatory increase of their probability of occurrence。

These findings show that brain pattern–triggered feedback TES of cortical neurons caninterfere with thalamocortical reverberation during SW episodes and effectively reduce their duration。SWpatterns—the hallmark of generalized petit mal epilepsy—arise from complex interactions between thalamic and neocortical neurons(14, 22–24)。 During the wave component of the SW cycle, both neocortical and thalamic neurons are largely silent (22–25)。 We hypothesize that cortical excitatory feedback during this silent period, brought about with TES, quenched the ongoing rhythm by recruiting subsets of thalamic cells, which in turn became refractory during the duty phase of the native SW cycle, as shown through tandem optogenetic activation of the thalamus and neocortex in mice (fig。 S6)。Brain activity–timed feedback of TES appears critical because 1-Hz sinusoid trains did not affect the duration of SWepisodes。