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Natural acoustic stimuli contain slow temporal fluctuations, and the modulation of ongoing slow-wave activity by bottom-up and top-down factors plays essential roles in auditory cortical processing. However, the spatiotemporal pattern of intrinsic slow-wave activity across the auditory cortical modality is unknown. Using in vivo voltage-sensitive dye imaging in anesthetized guinea pigs, we measured spectral tuning to acoustic stimuli across several core and belt auditory cortical areas, and then recorded spontaneous activity across this defined network. We found that phase coherence in spontaneous slow-wave (delta-theta band) activity was highest between regions of core and belt areas that had similar frequency tuning, even if they were distant. Further, core and belt regions with high phase coherence were phase shifted. Interestingly, phase shifts observed during spontaneous activity paralleled latency differences for evoked activity. Our findings suggest that the circuits underlying this intrinsic source of slow-wave activity support coordinated changes in excitability between functionally matched but distributed regions of the auditory cortical network.

The significance of the mismatch negativity (MMN), an event-related potential measured in humans which indexes novelty in the auditory environment, has motivated a search for a cellular correlate of this process. A leading candidate is stimulus-specific adaptation (SSA) in auditory cortex units, which shares several characteristics with the MMN. Whether auditory cortex responses encode sensory novelty, a defining property of the MMN, however, has not been resolved. To evaluate this key issue, we used several variations of the auditory oddball paradigm from the human literature and examined psychophysical and pharmacological properties of multiunit activity in the auditory cortex of awake rodents. We found converging evidence dissociating SSA from sensory novelty and the MMN. First, during an oddball paradigm with frequency deviants, neuronal responses showed clear SSA but failed to encode novelty in a manner analogous to the human MMN. Second, oddball paradigms using intensity or duration deviants revealed a pattern of unit responses that showed sensory adaptation, but again without any measurable novelty correlates aligning to the human MMN. Finally NMDA antagonists, which are known to disrupt the MMN, suppressed the magnitude of multiunit responses in a nonspecific manner, leaving the process of SSA intact. Together, our results suggest that auditory novelty detection as indexed by the MMN is dissociable from SSA at the level of activity encoded by auditory cortex neurons. Further, the NMDA sensitivity reported for the MMN, which models the disruption of MMN observed in schizophrenia, may occur at a mechanistic locus outside of SSA.

In the adult visual cortex, multiple feature maps exist and have characteristic spatial relationships with one another. The relationships can be reproduced by "dimension-reduction" computational models, suggesting that the principles of continuity and coverage may underlie cortical map organization. However, the mechanisms responsible for establishing these relationships are unknown. We explored whether removing one feature map during development causes a coordinated reorganization of the remaining maps or whether the remaining maps are unaffected. We removed the ocular dominance map by monocular enucleation in newborn ferrets, so that single eye stimulation drove the cortex in a more spatially uniform manner in adult monocular animals compared with normal animals. Maps of orientation, spatial frequency, and retinotopy formed in monocular ferrets, but their structures and spatial relationships differed from those in normal ferrets. The wavelength of the orientation map increased, so that the average orientation gradient across the cortex decreased. The decrease in the orientation gradient in monocular animals was most prominent in the high gradient regions of the spatial frequency map, indicating a coordinated reorganization between these two maps. In monocular animals, the orthogonal relationship between the orientation and spatial frequency maps was preserved, and the orthogonal relationship between the orientation and retinotopic maps became more pronounced. These results were consistent with detailed predictions of a dimension-reduction model of cortical organization. Thus, the number of feature maps in a cortical area influences the relationships between them, and inputs to the cortex have a significant role in generating these relationships.