Ranjbar-Slamloo, YadollahArabzadeh, Ehsan2021-07-020022-3077http://hdl.handle.net/1885/238512Supra-granular layers of sensory cortex are known to exhibit sparse firing. In rodent vibrissal cortex, a small fraction of neurons in layer 2 and 3 (L2/3) respond to whisker stimulation. Here, we combined whole-cell recording and two-photon imaging in anesthetized mice and quantified the synaptic response and spiking profile of L2/3 neurons. Previous literature has shown that neurons across layers of vibrissal cortex are tuned to the velocity of whisker movement. We therefore used a broad range of stimuli that included the standard range of velocities (0-1.2 degree/ms) and extended to a "sharp" high-velocity deflection (3.8 degree/ms). Consistent with previous literature, whole-cell recording revealed a sparse response to the standard range of velocities: although all recorded cells showed tuning to velocity in their postsynaptic potentials, only a small fraction produced stimulus-evoked spikes. In contrast, the sharp stimulus evoked reliable spiking in the majority of neurons. The action-potential threshold of spikes evoked by the sharp stimulus was significantly lower than that of the spontaneous spikes. Juxta-cellular recordings confirmed that application of sharp stimulus to single or multiple whiskers produced temporally precise spiking with minimal trial-to-trial spike-count variability (Fano factors equal or close to the theoretical minimum). Two-photon imaging further confirmed that most neurons that were not responsive to the standard deflections responded to the sharp stimulus. Altogether, our results indicate that sparseness in L2/3 cortex depends on the choice of stimulus: strong single- or multi-whisker stimulation can induce the transition from sparse to "dense" population response.This work was supported by the Australian Research Council (ARC) Discovery Project DP130101364, Future Fellowship FT20100357, and the ARC Centre of Excellence for Integrative Brain Function CE140100007application/pdfen-AU© 2017 the American Physiological Society201710.1152/jn.00815.20162020-11-23