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Current theories of the neural basis of sensory awareness suggest that neocortex is constantly comparing expected with observed sensory information. This comparison arises through the integration of ascending inputs from the sensory periphery and descending cortical inputs from the same or other sensory modalities. The importance of this integrative process for awareness is suggested by its selective loss upon anesthetic-induced hypnosis and during slow-wave sleep, but how this integration and its disruption by anesthetics occur within a cortical column is unclear. Using electrophysiological and imaging techniques in rodents in vivo and in brain slices, we show that extrastriate visual cortex provides descending input to primary auditory cortex that modulates responses to auditory stimuli, and that the integration of these information streams is disrupted by general anesthetics.

Space and time are two fundamental properties of our physical and psychological realms. While much is known about the integration of information across space within the visual system, little is known about the integration of information over time. Using two complementary methods of functional magnetic resonance imaging (fMRI) and intracranial electroencephalography (iEEG), I will present evidences that the brain uses similar strategies for integrating information over space and over time. It is well established that neurons along visual cortical pathways have increasingly large spatial receptive fields. This is a basic organizing principle of the visual system: neurons in higher-level visual areas receive input from low level neurons with smaller receptive fields and thereby accumulate information over space. Drawing an analogy with the spatial receptive field (SRF), we defined the temporal receptive window (TRW) of a neuron as the length of time prior to a response during which sensory information may affect that response. As with SRFs, the topographical organization of the TRWs is distributed and hierarchical. The accumulation of information over time is distributed in the sense that each brain area has the capacity to accumulate information over time. The processing is hierarchical because the capacity of each TRW increases from early sensory areas to higher order perceptual and cognitive areas. Early sensory cortices such as the primary auditory or visual cortex have relatively short TRWs (up to hundreds of milliseconds), while the TRWs in higher order areas can accumulate information over many minutes.