No two roses smell exactly alike, but our brain accurately bundles these variations into a single percept `rose'. We found that ensembles of rat olfactory bulb neurons decorrelate complex mixtures that vary by as little as a single missing component, whereas olfactory (piriform) cortical neural ensembles perform pattern completion in response to an absent component, essentially filling in the missing information and allowing perceptual stability. This piriform cortical ensemble activity predicts olfactory perception.The need for perceptual discrimination must be balanced with the need for perceptual stability. Without an ability to ignore some differences between input patterns, nearly all experiences would be unique, with each presentation of a similar stimulus being devoid of previously acquired associations and meaning. Computational modeling and experimental data suggest that some cortical circuits balance discrimination and stability through the network emergent functions of pattern separation and pattern completion 1-5 . Simply put, pattern separation allows partially overlapping input patterns to be decorrelated and processed as being distinct. Pattern completion is a memory-based phenomenon 6 that allows degraded input patterns to be compared to existing templates and, if they are sufficiently close to those templates, `completed' and processed as a match. These processes have been examined in some detail in the hippocampal formation, where slight changes in the spatial distribution of visual cues can be completed to promote stability in hippocampal neuronal place fields and presumably stability in spatial maps and perception. With further change or degradation in the visual spatial patterns, hippocampal place fields shift, presumably along with spatial perception. In olfaction, the need for pattern separation and completion is particularly intense, as most natural odors derive from odorant mixtures, evoking complex spatiotemporal patterns of olfactory sensory neuron and olfactory bulb activity 7 . Given this complexity, it is rare for a given stimulus to always have the exact same components in the exact same proportions, yet it is possible for a noisy or degraded stimulus to reliably evoke a stable percept. On the other hand, if the component makeup changes sufficiently, discrimination ensues. Evidence for pattern completion would be consistent with the view of olfactory perception as an object-oriented sense, where sensation , from which components were removed (for example, 10c-1 (10 components with 1 removed), 10c-2 (10 components with 2 removed), etc.) or replaced (10cR1, 10cR2, etc). This core mixture and its subsets were repeatedly presented during testing, and, given the speed at which cortical units become familiarized to odor mixtures even under anesthesia 13 , it was assumed they were familiar to the rats. We examined aPCX single units responding to the odorant mixtures (two typical examples are shown in Fig. 1a). These units responded to several different mixture combinations, and a...
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