The main data of the paper, showing the differences in the responses to periodic and random sequences, become thus an important special case of a more general finding. In this paper, we compared responses to oddball sequences in which the deviant tones occurred randomly to ones in which the deviant tones occurred periodically, as well as to sequences that are intermediate in their find more complexity. The main result of this paper is the demonstration that the neural responses were sensitive to this difference. In all cases, responses in the Random condition tended to be the same or larger than the responses in the Periodic condition,

although the details varied as a function of deviant probability. The larger responses to Random sequences were found with a number of measures of neural activity, including membrane potential responses of single neurons, but also LFPs, which are usually attributed to summed synaptic activity, and in MUA that reflects the output of multiple neighboring neurons selleck chemicals llc in the network. Previous studies (Anderson et al., 2009; Malmierca et al., 2009;

Taaseh et al., 2011; Ulanovsky et al., 2003) used oddball sequences similar to the ones we used here in the Random condition. These studies demonstrated, in a number of animal models and at different levels of the auditory pathway, that stimuli elicited a larger response when they were rare than when they were frequent. The responses to Random sequences described here reproduce such data, with the further information that a similar contrast between the responses to common and rare tones can be found also at the Casein kinase 1 level of the membrane potential responses of neurons in auditory cortex. To the best of our knowledge, the contrast between Periodic and Random sequences has not been studied before in animal models. The closest sequences to those we used here are the roving sequences in (Reches and Gutfreund, 2008), in which a stimulus

changed exactly every ten presentations. These are therefore Periodic sequences, but the overall probability of each of the two stimuli in these sequences was 50%. Reches and Gutfreund observed differences between the responses to the first and to the last stimulus of each successive group of ten presentations and used them as a replacement for bona fide oddball sequences. However, roving sequences with equiprobable tones elicit different responses than oddball sequences, as recently shown in the auditory thalamus of the gerbil (Bäuerle et al., 2011). In these experiments, the contrast between first and last stimulus in a sequence of successive identical stimuli was substantially smaller than the difference between the responses to the same tone when common and when rare in an oddball sequence. In contrast with these studies, we used Periodic sequences that had a probability imbalance between the two stimuli. Remarkably, we observed that Random sequences evoked as a rule stronger responses than Periodic sequences.