a–f, Experiments in ex vivo brain preparation after cortex removal. a, Top, ex vivo brain preparation. Bottom, spontaneous SWRs recorded in the claustrum (<150 Hz) (top trace); HP: 70–150-Hz filtered LFP, showing ripples (bottom trace). b, Local pressure injection of 20 μM TTX into the claustrum and post hoc assessment of injection with Evans blue (transverse section at the bottom, red). c, Injection of TTX into the claustrum (shading) silences sharp-wave activity in the claustrum, but also (indirectly) in the DVR. d, Analysis of four experiments as in c. The filled circles represent mean ± s.e.m. Claustrum: *P = 0.029, T = 26 (two-sided Mann–Whitney rank-sum test); DVR: *P = 0.029, T = 26 (two-sided Mann–Whitney rank-sum test). e, Top, average trace and s.d. (shading) from 3,842 sharp waves recorded from the claustrum of an ex vivo forebrain (alignment on trough). Bottom, HPI (>70 Hz) aligned on sharp-wave trough, showing ripple alignment. f, Top, simultaneous recordings from ipsilateral claustrum and DVR in an ex vivo preparation. Bottom, cross-correlation between simultaneous recordings in ipsilateral claustrum and DVR, showing that the claustrum precedes the DVR by around 100 ms. g, Peristimulus time histogram for multi-unit activity in the cortex, in response to activation of ipsilateral claustrum in an intact ex vivo forebrain. The experiment was carried out in normal ACSF at room temperature in the presence of 30 μM serotonin to suppress spontaneous SWRs in the claustrum and 50 μM carbachol to raise cortex excitability. The claustrum stimulus consisted of a single 50-μs electrical pulse, delivered with a bipolar electrode. Cortex multi-unit activity was recorded with a glass micropipette. h, Change in cortical firing rate (FR) measured in a 200-ms bin after the claustrum stimulus versus a 200-ms bin before the stimulus (as in g, on each side of t = 0). The control column plots the firing-rate ratio measured in the experiment in g, and the GBZ + CGP column plots the results of the same experiment after addition of the GABA receptor antagonists gabazine (GBZ; 5 μM) and CGP52432 (CGP; 2 μM); n = 4 ex vivo brains from 3 animals each. The control experiment shows that stimulation of the claustrum has an immediate and reliable inhibitory effect on the cortex (#: significantly different from baseline, P = 0.017, t 3 = 4.8 (two-sided paired t-test)). The stimulation experiment in GABA receptor antagonists shows that stimulation of the claustrum now slightly excites the cortex (**: significantly different from control, P = 2.0 × 10−3, t 6 = −5.22 (two-sided Student’s t-test)), suggesting that projections from the claustrum both activate and inhibit cortical neurons, probably via direct excitatory projections and indirect inhibitory ones through interneurons (see rodent experiments in a previous study39). Short horizontal lines indicate mean.