Classical theories of sensory processing view the brain as a passive, stimulus-driven device. By contrast, more recent approaches emphasize the constructive nature of perception, viewing it as an active and highly selective process. Indeed, there is ample evidence that the processing of stimuli is controlled by top-down influences that strongly shape the intrinsic dynamics of thalamocortical networks and constantly create predictions about forthcoming sensory events. We discuss recent experiments indicating that such predictions might be embodied in the temporal structure of both stimulus-evoked and ongoing activity, and that synchronous oscillations are particularly important in this process. Coherence among subthreshold membrane potential fluctuations could be exploited to express selective functional relationships during states of expectancy or attention, and these dynamic patterns could allow the grouping and selection of distributed neuronal responses for further processing.

A fundamental step in visual pattern recognition is the establishment of relations between spatially separate features. Recently, we have shown that neurons in the cat visual cortex have oscillatory responses in the range 40-60 Hz (refs 1, 2) which occur in synchrony for cells in a functional column and are tightly correlated with a local oscillatory field potential. This led us to hypothesize that the synchronization of oscillatory responses of spatially distributed, feature selective cells might be a way to establish relations between features in different parts of the visual field. In support of this hypothesis, we demonstrate here that neurons in spatially separate columns can synchronize their oscillatory responses. The synchronization has, on average, no phase difference, depends on the spatial separation and the orientation preference of the cells and is influenced by global stimulus properties.

temporal relations requires the joint evaluation of responses from more than one neuron, only experiments that permit simultaneous measurements of responses Wolf Singer* Max-Planck-Institute for Brain Research Deutschordenstrasse 46 60528 Frankfurt from multiple units are considered. These include multi-Bair, W., and Koch, C. (1996). Temporal precision of spike trains in extrastriate cortex of the behaving monkey. Neural Comput. 8, 44-66.

Converging evidence from electrophysiological, physiological and anatomical studies suggests that abnormalities in the synchronized oscillatory activity of neurons may have a central role in the pathophysiology of schizophrenia. Neural oscillations are a fundamental mechanism for the establishment of precise temporal relationships between neuronal responses that are in turn relevant for memory, perception and consciousness. In patients with schizophrenia, the synchronization of beta- and gamma-band activity is abnormal, suggesting a crucial role for dysfunctional oscillations in the generation of the cognitive deficits and other symptoms of the disorder. Dysfunctional oscillations may arise owing to anomalies in the brain's rhythm-generating networks of GABA (gamma-aminobutyric acid) interneurons and in cortico-cortical connections.

Following the discovery of context-dependent synchronization of oscillatory neuronal responses in the visual system, novel methods of time series analysis have been developed for the examination of task- and performance-related oscillatory activity and its synchronization. Studies employing these advanced techniques revealed that synchronization of oscillatory responses in the beta- and gamma-band is involved in a variety of cognitive functions, such as perceptual grouping, attention-dependent stimulus selection, routing of signals across distributed cortical networks, sensory-motor integration, working memory, and perceptual awareness. Here, we review evidence that certain brain disorders, such as schizophrenia, epilepsy, autism, Alzheimer's disease, and Parkinson's are associated with abnormal neural synchronization. The data suggest close correlations between abnormalities in neuronal synchronization and cognitive dysfunctions, emphasizing the importance of temporal coordination. Thus, focused search for abnormalities in temporal patterning may be of considerable clinical relevance.

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In areas 17 and 18 of the cat visual cortex the firing probability of neurons, in response to the presentation of optimally aligned light bars within their receptive field, oscillates with a peak frequency near 40 Hz. The neuronal firing pattern is tightly correlated with the phase and amplitude of an oscillatory local field potential recorded through the same electrode. The amplitude of the local field-potential oscillations are maximal in response to stimuli that match the orientation and direction preference of the local cluster of neurons. Single and multiunit recordings from the dorsal lateral geniculate nucleus of the thalamus showed no evidence of oscillations of the neuronal firing probability in the range of 20-70 Hz. The results demonstrate that local neuronal populations in the visual cortex engage in stimulus-specific synchronous oscillations resulting from an intracortical mechanism. The oscillatory responses may provide a general mechanism by which activity patterns in spatially separate regions of the cortex are temporally coordinated.The mechanism by which populations of neurons in the cerebral cortex temporally coordinate their activity patterns in response to specific sensory stimuli constitutes a basic unresolved question in sensory physiology. In the vertebrate olfactory system the spatiotemporal coordination of neuronal activity is achieved by the synchronization of oscillatory responses having a frequency in the range of 40-80 Hz (1-5). Evidence suggesting that a similar mechanism for the synchronization of activity may operate in the neocortex has come from field potential recordings in awake animals. It has been demonstrated that oscillatory activity in the high betafrequency range (20-50 Hz) occurs in sensory areas of the neocortex when the animals direct their attention to meaningful stimuli (6-11).Previously we have discovered, from recordings in area 17 of awake kittens, that neuronal responses recorded during periods of behavioral attention exhibit a rhythmic firing pattern that is tightly correlated with an oscillatory local field potential (LFP) having a frequency near 40 Hz (12). Thus, we sought to determine whether the oscillatory responses could also be recorded under varying conditions of anesthesia that would permit a more quantitative analysis of both their stimulus specificity as well as their temporal properties. Here, we extend our previous observations (13) and report that local groups of neurons, within functional columns ofthe visual cortex, engage in stimulus-specific oscillatory responses having a frequency near 40 Hz. This periodic neuronal activity is tightly correlated to the simultaneously recorded LFP, which in the majority of recordings has a similar orientation and direction preference as the local cluster of neurons. No comparable oscillations of firing probability were found for the thalamic input to visual cortex, indicating that the generation of oscillatory responses is a cortical phenomenon. The results suggest the hypothesis that the temporal pat...

Brain processing depends on the interactions between neuronal groups. Those interactions are governed by the pattern of anatomical connections and by yet unknown mechanisms that modulate the effective strength of a given connection. We found that the mutual influence among neuronal groups depends on the phase relation between rhythmic activities within the groups. Phase relations supporting interactions between the groups preceded those interactions by a few milliseconds, consistent with a mechanistic role. These effects were specific in time, frequency, and space, and we therefore propose that the pattern of synchronization flexibly determines the pattern of neuronal interactions.