Figure 1. The oddball paradigm. A. A schematic spectrographic representation of the three basic sequences used in this study. In each trial, either f1 or f2 are presented pseudo-randomly according to their probability of occurrence. B. The average LFP responses in a typical recording site to the two frequencies of the paradigm in each of the sequences (f1 = 13.3 kHz, black, and f2 = 19.2 kHz, gray). The level was 30 dB attenuation (,70dB SPL). Error bars: 6 s.e.m., shaded interval: stimulus. C. MUA Responses at the same site. The raster plots show 25 presentations for each of the two frequencies, corresponding to 5% of the 500 tone presentations in the sequence. For the standard and equal-probability conditions, which had more than 25 presentations in a sequence, the 25 presentations were selected so that they represent the spike count distribution of all responses in the time window shown. The line graphs represent PSTHs smoothed by a 10 ms Hamming window. doi:10.1371/journal.pone.0023369.g001 Fig. 2B. The CSI was essentially always positive, demonstrating the robustness of the SSA in rat auditory cortex. Controls for SSA The main goal of this study was to elucidate the effects of the standard tone on the deviant responses. We therefore used several control sequences in which the two tested tones had the same probability of occurrence as the deviant in the oddball sequences, while the other tone presentations in the sequence, the ‘context’, had different frequency compositions. Figure 3A describes the control sequences. The first control consisted of tones of 20 different frequencies, including f1 and f2. Each frequency was presented with an equal probability of 5% in a pseudo-random order, so f1 and f2 were as rare as the deviants in the oddball sequences, but so were all the Deviance Detection in Rat Auditory Cortex Figure 2. SSA indices for Df = 44%. A. Scatter plots of SI2 vs. SI1 for all sets with Df = 44%. B. Histograms of CSI. The numbers near the zero line indicate the number of CSIs smaller and greater than 0. The labels A – D mark the bins corresponding to the data in Figure 4. doi:10.1371/journal.pone.0023369.g002 other frequencies. The 20 frequencies were equally distributed on a logarithmic scale that spanned about twice Df, with f1 and f2 symmetrically positioned (the 5th and 16th frequencies among the 20). Thus, the frequencies were densely packed in a relatively narrow frequency band. For Df = 44% the frequency ratio between adjacent frequencies was 3.37%. This sequence will be referred to here as the narrowband diverse sequence, or ‘Diverse- narrow’ in short. The second control was similar to the diverse-narrow sequence, but the frequency interval between adjacent tones was Df (the frequency separation between the test frequencies f1 and f2). With intervals of Df = 44%, a series of 20 frequencies would span far more than the audible frequency range of the rat. Therefore, we used tones of only 12 different frequencies for this control. Frequencies f1 and f2 were each presented with a probability of 5% to match that of a deviant in the oddball sequences, but the other 10 frequencies were presented with a probability of 9% each. This distribution resulted in an asymmetry between f1 and f2 on the one hand and the other frequencies in the sequence on the other hand, but the number of different frequencies was large enough to mask this asymmetry [16]: in humans, it is sufficient to ‘distribute’ the probability of the standard among only 4 different stimuli in order to remove the contribution of deviance to the responses, and we observed similar results in rats in preliminary experiments (data not shown). This sequence will be called the broadband diverse sequence or ‘Diverse-broad’ in short. In terms of tone probability and lack of regularity the Diverse- narrow and Diverse-broad controls are comparable. However, the responses to the oddball sequences were expected to depend on frequency difference between the standard and the deviant. The two controls made it possible to study the effects of frequency separation on SSA. The third control condition was based on the ‘Deviant-alone’ control of the MMN literature [15], and there were two such sequences, one for each of the two main frequencies. In the ‘Deviant-alone f1’ sequence, tones of frequency f1 were randomly PLoS ONE | www.plosone.org 3 August 2011 | Volume 6 | Issue 8 | e23369