Summary Neocortical assemblies produce complex activity patterns both in response to sensory stimuli, and spontaneously without sensory input. To investigate the structure of these patterns, we recorded from populations of 40–100 neurons in auditory and somatosensory cortices of anesthetized and awake rats using silicon microelectrodes. Population spike time patterns were broadly conserved across multiple sensory stimuli and spontaneous events. Although individual neurons showed timing variations between stimuli, these were not sufficient to disturb a generally conserved sequential organization observed at the population level, lasting for approximately 100ms with spiking reliability decaying progressively after event onset. Preserved constraints were also seen in population firing rate vectors, with vectors evoked by individual stimuli occupying subspaces of a larger but still constrained space outlined by the set of spontaneous events. These results suggest that population spike patterns are drawn from a limited “vocabulary,” sampled widely by spontaneous events but more narrowly by sensory responses.

Even in the absence of sensory stimulation, the neocortex shows complex spontaneous activity patterns, often consisting of alternating ''DOWN'' states of generalized neural silence and ''UP'' states of massive, persistent network activity. To investigate how this spontaneous activity propagates through neuronal assemblies in vivo, we simultaneously recorded populations of 50 -200 cortical neurons in layer V of anesthetized and awake rats. Each neuron displayed a virtually unique spike pattern during UP states, with diversity seen amongst both putative pyramidal cells and interneurons, reflecting a complex but stereotypically organized sequential spread of activation through local cortical networks. Spike timing was most precise during the first Ϸ100 ms after UP state onset, and decayed as UP states progressed. A subset of UP states propagated as traveling waves, but waves passing a given point in either direction initiated similar local sequences, suggesting local networks as the substrate of sequential firing patterns. A search for repeating motifs indicated that their occurrence and structure was predictable from neurons' individual latencies to UP state onset. We suggest that these stereotyped patterns arise from the interplay of intrinsic cellular conductances and local circuit properties.neuronal assembly ͉ repeating sequences ͉ slow oscillations ͉ syntire chains ͉ microcircuits T he neocortex contains a rich variety of neural classes that are connected together in a complex, but stereotyped manner (1-3). Only a fraction of cortical synapses carry ascending information from the thalamus; the majority arises from other cortical cells. It has been suggested that this recurrent connectivity allows the cortex to process information through the sequential activation of neuronal assemblies (4). Consistent with this picture, cortical activity exhibits coordinated dynamics beyond that predicted from common modulation by sensory input (5-7). Even in the absence of sensory stimulation (e.g., during sleep) the cortex shows complex spontaneous activity patterns, which have been suggested to reflect an ''off-line'' mode of information processing (7-12).During sleep and quiet wakefulness, cortical spontaneous activity is dominated by the ''slow oscillation,'' consisting of alternating ''DOWN'' states of generalized neural silence and ''UP'' states of massive, persistent network activity (13). The dynamics of spontaneous UP states show striking similarities to those of sensory-evoked activity (14), suggesting that spontaneous patterns may be a useful experimental model for the flow of activity through cortical circuits. The way spontaneous activity propagates through cortical populations is unclear: whereas in vivo optical imaging results suggest a random and unstructured process (15), in vitro models suggest a more complex picture involving local sequential organization and/or traveling waves (16-21).Here, we study the spatiotemporal structure of spontaneous activity in vivo by recording populations of 50-200 cells in ra...

In the idling brain, neuronal circuits transition between periods of sustained firing (UP state) and quiescence (DOWN state), a pattern the mechanisms of which remain unclear. Here we analyzed spontaneous cortical population activity from anesthetized rats and found that UP and DOWN durations were highly variable and that population rates showed no significant decay during UP periods. We built a network rate model with excitatory (E) and inhibitory (I) populations exhibiting a novel bistable regime between a quiescent and an inhibition-stabilized state of arbitrarily low rate. Fluctuations triggered state transitions, while adaptation in E cells paradoxically caused a marginal decay of E-rate but a marked decay of I-rate in UP periods, a prediction that we validated experimentally. A spiking network implementation further predicted that DOWN-to-UP transitions must be caused by synchronous high-amplitude events. Our findings provide evidence of bistable cortical networks that exhibit non-rhythmic state transitions when the brain rests.

Cortical circuits work through the generation of coordinated, large-scale activity patterns. In sensory systems, the onset of a discrete stimulus usually evokes a temporally organized packet of population activity lasting ∼50-200 ms. The structure of these packets is partially stereotypical, and variation in the exact timing and number of spikes within a packet conveys information about the identity of the stimulus. Similar packets also occur during ongoing stimuli and spontaneously. We suggest that such packets constitute the basic building blocks of cortical coding.

The activity of neural populations is determined not only by sensory inputs but also by internally-generated patterns. During quiet wakefulness, the brain produces spontaneous firing events which can spread over large areas of cortex, and have been suggested to underlie processes such as memory recall and consolidation. Here we demonstrate a different role for spontaneous activity in sensory cortex: gating of sensory inputs. We show that population activity in rat auditory cortex is composed of transient 50-100ms packets of spiking activity which occur irregularly during silence and sustained tone stimuli, but reliably at tone onset. Population activity within these packets had broadly consistent spatiotemporal structure, but the rate and also precise relative timing of action potentials varied between stimuli. Packet frequency varied with cortical state, with desynchronized state activity consistent with superposition of multiple overlapping packets. We suggest that such packets reflect the sporadic opening of a “gate” that allows auditory cortex to broadcast a representation of external sounds to other brain regions.