Projection-specific filtering of delta-theta input by layer 5 neurons in mouse secondary visual cortex

Delta- and theta-band oscillations are thought to coordinate cortical activity and to convey top-down signals, but how the different output neurons of a cortical area transform such rhythmic input is less well understood. Layer 5 contains two projection classes, extratelencephalic (ET) and intratelencephalic (IT) neurons, which carry different information to different targets. Here we asked whether they also differ in how they process slow rhythmic input in the delta-theta range. We combined whole-cell recordings in three stimulus regimes in mouse medial secondary visual cortex (V2M) with local field potential (LFP) recordings in awake mice. With small inputs, IT cells summated strongly and acted as low-pass filters, whereas ET cells resonated at low frequencies, forming moderate-resonance (ET1, about 3 Hz) and high-resonance (ET2, about 5 Hz) subgroups. With large inputs, ET cells amplified depolarisation more than hyperpolarisation, at similar resonance frequencies. When driven to fire by a 4 Hz sinusoid at threshold, cells of all classes fired at a similar phase and kept it nearly constant across cycles. In awake mice, visual stimulation increased deep-layer LFP power across about 1-6 Hz, a band that spans the mean ET1 and ET2 resonance frequencies. Thus, the two output classes of layer 5 in V2M filter slow rhythmic input differently, but once firing, they show no detectable difference in spike timing.