, 2008; O’Connor et al., 2010) (Figure 1D).

Results from imaging studies are therefore in good agreement with electrophysiological measurements. Interestingly, the sparseness of L2/3 neuron firing appears to be modulated by anesthesia, brain state, development, and experience. In the visual cortex, L2/3 pyramidal neurons fire less in awake mice than in anesthetized mice (Haider et al., 2013). In the auditory cortex, the mean firing rate decreases in L2/3 during activated states occurring spontaneously or induced by stimulation of the pedunculopontine tegmental nucleus (Sakata and Harris, 2012). Two-photon calcium imaging in L2/3 mouse visual cortex during development has revealed a switch from dense to sparse network activity after eye opening (Rochefort et al., 2009). A similar imaging approach also showed sparsification of L2/3 barrel cortex activity during early postnatal www.selleckchem.com/products/pf-06463922.html development (Golshani et al., 2009). Furthermore, in the barrel cortex, whisker associative fear learning enhances the sparseness of L2/3 responses to whisker stimulation (Gdalyahu et al., 2012). The very low rates of AP firing in the majority of excitatory

L2/3 neocortical neurons could Selleck SB431542 indicate that many neurons might receive very little synaptic input. However, whole-cell membrane potential recordings from L2/3 excitatory neurons in awake head-restrained mice reveal large-amplitude (∼20 mV) subthreshold membrane potential fluctuations driven by synaptic inputs, even in neurons that fire APs very rarely (Figure 1E) (Petersen et al., 2003; Crochet and Petersen, 2006; Poulet and Petersen, 2008; Crochet et al., 2011). The paucity of spontaneous and evoked APs in the majority of L2/3 excitatory neurons is therefore not due to the absence of excitatory input, but rather because through of the strong impact of inhibition, as we discuss below. An important question that remains to be elucidated is whether the sparse firing of L2/3 pyramidal cells reflects the existence of a small population of highly excitable neurons and/or a high selectivity of L2/3 pyramidal cells for specific

sensory input. In other words, does L2/3 contain a small pool of broadly tuned neurons ready to respond to any stimulus within the receptive field or does it contain a large pool of finely tuned neurons that only respond to a specific parameter of the stimulus and context? Recent studies suggest that L2/3 pyramidal neurons show a certain degree of stimulus selectivity. Selectivity to the direction of a moving stimulus is a well-known feature of neurons in the primary visual cortex. Two-photon calcium imaging studies have revealed that L2/3 neurons in the rodent primary visual cortex show high selectivity for stimulus orientation, even though they are not organized into the orientation pinwheel maps found in cats (Ohki et al., 2005, 2006).