The active process in which the brain combines sensory inputs and the prior experience of what might be out there, to form a coherent interpretation of the incoming information is called perception. The neural basis of this process remain largely unknown. Although neurophysiologist have been years characterizing the response properties of sensory neurons as stimulus parameters vary, it has been only recently that studies have turned the attention to the other part of the process, namely the representation of the internal expectations and priors which modulate and bias the activity at early stages of the sensory pathways (e.g. primary sensory cortices). Understanding the mechanisms which give rise to the integration of feed-forward sensory inputs, ongoing activity generated by the dynamics of the local circuits and the so called top-down inputs into percepts is the goal of our lab. To achieve this, we combine neuronal population cortical recordings in rats with quantitative analysis and mathematical modeling. We are particularly interested in (1) the role of spontaneous activity and (2) the role of neuronal noise and synchrony in perception and decision making.