Figure 1. Response of L4 single RSUs in whisker/barrel cortex of sham and CA rats. A, The rat whisker pad consists of an ordered array of order whiskers; as an example, filled black circles indicate whisker D3 as the PW (see Materials and Methods), whiskers D2 and D4 as AWs in the same row, and whiskers C3 and E3 as AWs in the same column. B, To characterize spatial receptive field properties (relative responses across the whisker array), we recorded from neurons while stimulating the PW and two to four AWs, individually, in either the same column or same row as the PW. C, An example tangential section of the L4 barrel field stained for CO (see Materials and Methods), illustrating an electrolytic lesion to confirm the location of the primary whisker. D, In response to a 1-mm ramp-and-hold whisker deflection (see Materials and Methods), an L4 RSU responds with a robust, transient increase in firing rate. After the whisker deflection offset, the unit responds with a similar, albeit smaller, transient increase in firing rate. The shaded area schematically represents the time window for computing the ON response. E, A representative wave form of a single-unit RS neuron recording. All recorded waveforms are shown in gray. The average wave form is shown in black. F, ISI histogram for all recorded units in sham rats. The ISI histogram in CA rats is essentially identical (not shown). Bin width, 1 ms. G, The first 5 ms of the ISI histogram in F. Note absence of recorded spikes in the absolute refractory period of 1 ms. H, The set of spike time responses of an example sham neuron, presented as a spike time raster plot, illustrates the sparseness of spontaneous activity and relative robustness of whisker evoked responses. I, The peristimulus time histogram (PSTH) of a representative RSU from a sham rat in response to PW deflection (asterisks indicate onset and offset times) indicates a robust, temporally precise response (top). The responses to the four AWs are smaller in amplitude, and in at least one case (AW2), there is no appreciable response. J, An L4 RSU from a CA rat displays a small PW-evoked response similar in magnitude to those evoked by some of the AWs (AW1 and AW2).