Oscillatory Patterning Research Articles

Surface pattern formation induced by oscillatory loading of frontally polymerized gels

Frontal polymerization (FP) is a rapid, energy-efficient technique for the manufacturing of polymeric materials and composites. It has also emerged as a way to rapidly alter the shape of partially cured polymeric materials through mechanical deformation. The first objective of this paper is to introduce a coupled thermo-chemo-mechanical theory capable of describing within the framework of nonlinear kinematics the evolution of deformation and temperature fields during frontal polymerization of gels. At the heart of the proposed theory is the introduction of eigenstrains corresponding to the phase transformation prompted by the frontal curing process. Another objective is to introduce a novel bio-inspired oscillatory loading-induced patterning technique in which we form thermoset polymeric materials with periodic surface topography patterns by applying oscillatory uniaxial loads to partially cured gels during FP. After a detailed presentation of the experimental methodology, the theoretical predictions are compared with experimental results.

The impact of cathodal tDCS on the GABAergic system in the epileptogenic zone: A multimodal imaging study.

ObjectivesWe aimed to investigate the antiepileptic effects of cathodal transcranial direct current stimulation (c-tDCS) and mechanisms of action based on its effects on the neurotransmitters responsible for the abnormal synchrony patterns seen in pharmacoresistant epilepsy. This is the first study to test the impact of neurostimulation on epileptiform interictal discharges (IEDs) and to measure brain metabolites in the epileptogenic zone (EZ) and control regions simultaneously in patients with pharmacoresistant epilepsy.MethodsThis is a hypothesis-driven pilot prospective single-blinded repeated measure design study in patients diagnosed with pharmacoresistant epilepsy of temporal lobe onset. We included seven patients who underwent two sessions of c-tDCS (sham followed by real). The real tDCS session was 20 min in duration and had a current intensity of 1.5 mA delivered via two surface electrodes that had dimensions of 3 × 4 cm. The cathode electrode was placed at FT7 in the center whereas the anode at Oz in the center. After each session, we performed electroencephalographic recording to count epileptiform IEDs over 30 min. We also performed magnetic resonance spectroscopy (MRS) to measure brain metabolite concentrations in the two areas of interest (EZ and occipital region), namely, gamma-aminobutyric acid (GABA), glutamate (Glx), and glutathione. We focused on a homogenous sample where the EZ and antiepileptic medications are shared among patients.ResultsReal tDCS decreased the number of epileptiform IEDs per min (from 9.46 ± 2.68 after sham tDCS to 5.37 ± 3.38 after real tDCS), p = 0.018, as compared to sham tDCS. GABA was decreased in the EZ after real c-tDCS stimulation as compared to sham tDCS (from 0.129 ± 0.019 to 0.096 ± 0.018, p = 0.02). The reduction in EZ GABA correlated with the reduction in the frequency of epileptiform IED per min (rho: 0.9, p = 0.003).ConclusionThese results provide a window into the antiepileptic mechanisms of action of tDCS, based on local and remote changes in GABA and neural oscillatory patterning responsible for the generation of interictal epileptiform discharges.

Intracranial recordings in humans reveal specific hippocampal spectral and dorsal vs. ventral connectivity signatures during visual, attention and memory tasks

Invasive brain recordings using many electrodes across a wide range of tasks provide a unique opportunity to study the role of oscillatory patterning and functional connectivity. We used large-scale recordings (stereo EEG) within and beyond the human hippocampus to investigate the role of distinct frequency oscillations during real-time execution of visual, attention and memory tasks in eight epileptic patients. We found that activity patterns in the hippocampus showed task and frequency dependent properties. Importantly, we found distinct connectivity signatures, in particular concerning parietal-hippocampal connectivity, thus revealing large scale synchronization of networks involved in memory tasks. Comparing the power per frequency band, across tasks and hippocampal regions (anterior/posterior) we confirmed a main effect of frequency band (p = 0.002). Gamma band activity was higher for visuo-spatial memory tasks in the anterior hippocampus. Further, we found that alpha and beta band activity in posterior hippocampus had larger modulation for high memory load visual tasks (p = 0.004). Three functional connectivity task related networks were identified: (dorsal) parietal-hippocampus (visual attention and memory), ventral stream- hippocampus and hippocampal-frontal connections (mainly tasks involving face recognition or object based search). These findings support the critical role of oscillatory patterning in the hippocampus during visual and memory tasks and suggests the presence of task related spectral and functional connectivity signatures. These results show that the use of large scale human intracranial recordings can validate the role of oscillatory and functional connectivity patterns across a broad range of cognitive domains.

A reflux-and-growth mechanism explains oscillatory patterning of lateral root branching sites.

Modular, repetitive structures are a key component of complex multicellular body plans across the tree of life. Typically, these structures are prepatterned by temporal oscillations in gene expression or signaling. Although a clock-and-wavefront mechanism was identified and plant leaf phyllotaxis arises from a Turing-type patterning for vertebrate somitogenesis and arthropod segmentation, the mechanism underlying lateral root patterning has remained elusive. To resolve this enigma, we combined computational modeling with in planta experiments. Intriguingly, auxin oscillations automatically emerge in our model from the interplay between a reflux-loop-generated auxin loading zone and stem-cell-driven growth dynamics generating periodic cell-size variations. In contrast to the clock-and-wavefront mechanism and Turing patterning, the uncovered mechanism predicts both frequency and spacing of lateral-root-forming sites to positively correlate with root meristem growth. We validate this prediction experimentally. Combined, our model and experimental results support that a reflux-and-growth patterning mechanism underlies lateral root priming.

Protein gradients on the nucleoid position the carbon-fixing organelles of cyanobacteria

Carboxysomes are protein-based bacterial organelles encapsulating key enzymes of the Calvin-Benson-Bassham cycle. Previous work has implicated a ParA-like protein (hereafter McdA) as important for spatially organizing carboxysomes along the longitudinal axis of the model cyanobacterium Synechococcus elongatus PCC 7942. Yet, how self-organization of McdA emerges and contributes to carboxysome positioning is unknown. Here, we identify a small protein, termed McdB that localizes to carboxysomes and drives emergent oscillatory patterning of McdA on the nucleoid. Our results demonstrate that McdB directly stimulates McdA ATPase activity and its release from DNA, driving carboxysome-dependent depletion of McdA locally on the nucleoid and promoting directed motion of carboxysomes towards increased concentrations of McdA. We propose that McdA and McdB are a previously unknown class of self-organizing proteins that utilize a Brownian-ratchet mechanism to position carboxysomes in cyanobacteria, rather than a cytoskeletal system. These results have broader implications for understanding spatial organization of protein mega-complexes and organelles in bacteria.

Oscillatory motor patterning is impaired in neurofibromatosis type 1: a behavioural, EEG and fMRI study

BackgroundNeurofibromatosis type1 (NF1) is associated with a broad range of behavioural deficits, and an imbalance between excitatory and inhibitory neurotransmission has been postulated in this disorder. Inhibition is involved in the control of frequency and stability of motor rhythms. Therefore, we aimed to explore the link between behavioural motor control, brain rhythms and brain activity, as assessed by EEG and fMRI in NF1.MethodsWe studied a cohort of 21 participants with NF1 and 20 age- and gender-matched healthy controls, with a finger-tapping task requiring pacing at distinct frequencies during EEG and fMRI scans.ResultsWe found that task performance was significantly different between NF1 and controls, the latter showing higher tapping time precision. The time-frequency patterns at the beta sub-band (20–26 Hz) mirrored the behavioural modulations, with similar cyclic synchronization/desynchronization patterns for both groups. fMRI results showed a higher recruitment of the extrapyramidal motor system (putamen, cerebellum and red nucleus) in the control group during the fastest pacing condition.ConclusionsThe present study demonstrated impaired precision in rhythmic pacing behaviour in NF1 as compared with controls. We found a decreased recruitment of the cerebellum, a structure where inhibitory interneurons are essential regulators of rhythmic synchronization, and in deep brain regions pivotally involved in motor pacing. Our findings shed light into the neural underpinnings of motor timing deficits in NF1.

Protein Gradients on the Nucleoid Position the Carbon-Fixing Organelles of Cyanobacteria

Carboxysomes are protein-based bacterial organelles that encapsulate a key enzyme of the Calvin-Benson-Bassham cycle. Previous work has implicated a ParA-like protein (hereafter McdA) as important for spatially organizing carboxysomes along the longitudinal axis of the model cyanobacterium Synechococcus elongatus PCC 7942. Yet, how self-organization of McdA emerges and contributes to carboxysome positioning is unknown. Here, we show that a small protein, termed McdB, localizes to carboxysomes through interactions with carboxysome shell proteins to drive emergent oscillatory patterning of McdA on the nucleoid. Our results demonstrate that McdB directly interacts to stimulate McdA ATPase activity, and indicate that carboxysome-dependent McdA depletion zone formation on the nucleoid is required for directed motion of carboxysomes towards increased concentrations of McdA. We propose that McdA and McdB are a new class of self-organizing proteins that follow a Brownian-ratchet mechanism, challenging the cytoskeletal model of organelle transport, for equidistant positioning of carboxysomes in cyanobacteria. These results have broader implications for understanding spatial organization of protein mega-complexes and organelles in bacteria more broadly.

Gamma Responses Correlate with Temporal Expectation in Monkey Primary Visual Cortex

Gamma oscillations have been linked to selective attention. Here, we investigate the effects of expecting a behaviorally relevant event (a change in the fixation point) on the oscillatory patterning of the local field potential and spiking responses in V1. Three protocols were used. In the first protocol, fixation point change occurred at a fixed time point, enabling predictions on task timing. In the second, fixation point change occurred in trial blocks either early or late in the trial, allowing us to compare responses during epochs of low and high expectation. Finally, we used a cue to indicate the upcoming fixation point change. All protocols led to an increase in gamma oscillations associated with alpha suppression when the monkeys expected an event in time. These effects were spatially widespread, since comparable results were observed for both central and peripheral visual representations in V1. Our findings indicate that expectations associated with perceptual decisions, motor responses, or upcoming reward may have a strong effect on the primary visual cortex, causing global, spatially nonselective modulation of gamma activity.

Synchronization hubs may arise from strong rhythmic inhibition during gamma oscillations in primary visual cortex

Parallel multiunit recordings from V1 in anesthetized cat were collected during the presentation of random sequences of drifting sinusoidal gratings at 12 fixed orientations while gamma oscillations were present. In agreement with the seminal work [1], most units were orientation selective to varying degrees and synchronization was evident in spike train crosscorrelograms computed between units with similar preferred orientations, particularly during the presentation of optimal stimuli. Interestingly, a subset of units, which we refer to as synchronization hubs, were additionally found to synchronize with units having differing preferred orientations which was consistent with a previous study [2]. Moreover, oscillatory patterning in spike train autocorrelograms was also found to be strongest in units denoted as synchronization hubs, and synchronization hubs also tended to have narrower tuning curves relative to other units. We used simplified computational models of small networks of V1 neurons to demonstrate that neurons subject to a sufficiently strong level of inhibitory input can function as synchronization hubs. Neurons were endowed either with integrate-and-fire or conductance-based dynamics and each neuron received a combination of excitatory (AMPA) synaptic inputs that were Poisson-distributed and inhibitory (GABA) inputs that were coherent at a gamma-frequency range. If the strength of rhythmic inhibition was increased for a subset of neurons in the network, and excitation was increased simultaneously to maintain a fixed firing rate, then these neurons produced stronger oscillatory patterning in their discharge probabilities. The oscillations in turn synchronized these neurons with other neurons in the network. Importantly, the strength of synchronization increased with neurons of differing orientation preferences even though no direct synaptic coupling existed between the hubs and the other neurons. Enhanced levels of inhibition account for the emergence of synchronization hubs in the following way: Inhibitory inputs exhibiting a gamma rhythm determine a time window within which a cell is likely to discharge. Increased levels of inhibition narrow down this window further simultaneously leading to (i) even stronger oscillatory patterning of the neuron's activity and (ii) enhanced synchronization with other neurons. This enables synchronization even between cells with differing orientation preferences. Additionally, the same increased levels of inhibition may be responsible for the narrow tuning curves of hub neurons. In conclusion, synchronization hubs may be the cells that interact most strongly with the network of inhibitory interneurons during gamma oscillations in primary visual cortex.

Rippling the cortex with high-frequency (>100 Hz) alternating current stimulation

A wide range of non-invasive brain stimulation techniques are currently available which can induce lasting changes in the excitability of the stimulated cortex (Ziemann et al. 2008). Regular repetitive transcranial magnetic stimulation (rTMS) and constant transcranial direct current stimulation (c-tDCS) are well-established stimulation modalities that have now been used for more than 10 years. Other stimulation techniques have only recently been introduced, such as continuous transcranial alternating current stimulation (c-tACS), oscillatory tDCS (o-tDCS) or patterned rTMS protocols. Currently, c-tDCS uses weak currents up to 2 mA which induce currents in the cortex that are far below the threshold for inducing action potentials. The tissue current is still strong enough to polarise the membrane potential of cortical neurons, resulting in lasting shifts in the resting membrane potential and associated changes in postsynaptic spiking activity. These effects are thought to mediate the after-effects of c-tDCS. A different mechanism is emphasized when c-tACS is used to manipulate cortical plasticity. Here the assumption is that the oscillatory current interacts with and shapes intrinsic neural oscillations in the stimulated cortex. Indeed recent studies support this notion showing that c-tACS can interact with brain function in a frequency specific manner (Kanai et al. 2008; Pogosyan et al. 2009). In an article in this issue of The Journal of Physiology, Moliadze et al. (2010) significantly add to this line of research. In healthy volunteers, 10 min of c-tACS of the human motor hand area (M1hand) at 140 Hz but not at 80 Hz enhanced regional corticospinal excitability during and for at least an hour after c-tACS. At 250 Hz, c-tACS also induced an increase in corticospinal excitability but to a lesser extent and with a delayed onset of facilitation. The increase in corticospinal excitability (measured by motor evoked potential amplitude in a hand muscle) was paralleled by a relative decrease in short latency intracortical inhibition (SICI), an electrophysiological marker of GABAA receptor mediated inhibition. Although Moliadze et al. (2010) are cautious to draw firm conclusions, they favour the hypothesis that tACS at 140 Hz targets cortical ripples and the resulting after-effects are due to an interaction between externally applied high frequency oscillation in the ripple range and intrinsic cortical ripple activity in M1hand. This is a possible scenario as ripple oscillations in the frequency range from 80 to 200 Hz have been demonstrated in the cat cortex, yet they are mainly expressed during non-REM sleep (Grenier et al. 2001). Furthermore, pulsed stimulation in the ripple range (130 Hz) of afferents to the subthalamic nucleus is therapeutically effective in Parkinsonian rodents (Gradinaru et al. 2009) and is being used for deep brain stimulation in human patients. The perspective that transcranial stimulation can be tuned to specifically target cortical oscillatory in specific frequency bands is intriguing. Oscillatory patterning of neuronal activity in different frequency bands supports temporal coding of different aspects of cortical processing (Singer, 2009). Therefore, transcranial stimulation protocols that can efficiently manipulate specific oscillatory activity might be more efficient and specific with respect to shaping specific brain functions than conventional regular rTMS or c-tDCS. At first glance, it seems straightforward to postulate that transcranial stimulation protocols should mimic as close as possible the temporal pattern of the intrinsic cortical oscillations that one wishes to modulate with transcranial stimulation. However, a number of general questions need to be clarified.

Prevalence of chronic kidney disease in South Asian and Black minorities: findings from a population based screening study in London, UK

Sensorimotor transformations in the mammalian superior colliculus (SC) are mediated by large sets of distributed neurons. For such distributed coding systems, stimulus superposition poses problems attributable to the merging of...

Short- and Long-Term Effects of Cholinergic Modulation on Gamma Oscillations and Response Synchronization in the Visual Cortex

Neurons can engage in synchronized oscillatory activity in the gamma-frequency range when responding to sensory stimuli. Both the oscillatory patterning and the synchronization of responses are enhanced with arousal and attention or when the electroencephalogram is activated by electrical stimulation of the mesencephalic reticular formation. Here we show with intracortical application of cholinergic antagonists that the enhancement of gamma oscillations and response synchronization is mediated by acetylcholine and muscarinic receptors. We demonstrate further that coapplication of cholinergic agonists with synchrony-inducing light stimuli causes a lasting increase in the probability that the stimulated cells engage in gamma oscillations and response synchronization. These changes develop slowly over tens of minutes and then persist for many hours. Thus, cholinergic modulation plays a crucial role both in the fast, state-dependent facilitation of gamma oscillations and response synchronization and in use-dependent long-term modifications of cortical dynamics that favor synchronization of responses in the gamma-frequency range.

Features of neuronal synchrony in mouse visual cortex.

Synchronization of neuronal discharges has been hypothesized to play a role in defining cell assemblies representing particular constellations of stimulus features. In many systems and species, synchronization is accompanied by an oscillatory response modulation at frequencies in the gamma-band. The cellular mechanisms underlying these phenomena of synchronization and oscillatory patterning have been studied mainly in vitro due to the difficulty in designing a direct in vivo assay. With the prospect of using conditional genetic manipulations of cortical network components, our objective was to test whether the mouse would meet the criteria to provide a model system for the study of gamma-band synchrony. Multi-unit and local field potential recordings were made from the primary visual cortex of anesthetized C57BL/6J mice. Neuronal responses evoked by moving gratings, bars, and random dot patterns were analyzed with respect to neuronal synchrony and temporal patterning. Oscillations at gamma-frequencies were readily evoked with all types of stimuli used. Oscillation and synchronization strength were largest for gratings and decreased when the noise level was increased in random-dot patterns. The center peak width of cross-correlograms was smallest for bars and increased with noise, yielding a significant difference between coherent random dot patterns versus patterns with 70% noise. Field potential analysis typically revealed increases of power in the gamma-band during response periods. Our findings are compatible with a role for neuronal synchrony in mediating perceptual binding and suggest the usefulness of the mouse model for testing hypotheses concerning both the mechanisms and the functional role of temporal patterning.

Feed-forward synchronization: propagation of temporal patterns along the retinothalamocortical pathway.

Visual responses in the cortex and lateral geniculate nucleus (LGN) are often associated with synchronous oscillatory patterning. In this short review, we examine the possible relationships between subcortical and cortical synchronization mechanisms. Our results obtained from simultaneous multi-unit recordings show strong synchronization of oscillatory responses between retina, LGN and cortex, indicating that cortical neurons can be synchronized by oscillatory activity relayed through the LGN. This feed-forward synchronization mechanism operating in the 60 to 120 Hz frequency range was observed mostly for static stimuli. In response to moving stimuli, by contrast, cortical synchronization was independent of oscillatory inputs from the LGN, with oscillation frequency in the range of 30 to 60 Hz. The functional implications of synchronization of activity from parallel channels are discussed, in particular its significance for signal transmission and cortical integration processes.

On the Correlation between Synchronized Oscillatory Activities and Consciousness

Recent experiments have shown that the amplitudes of cortical gamma band oscillatory activities that occur during anesthesia are often greater than amplitudes of similar activities that occur without anesthesia. This result is apparently at odds with the hypothesis that synchronized oscillatory activities constitute the neural correlate of consciousness. We argue that while synchronization and oscillatory patterning are necessary conditions for consciousness, they are not sufficient. Based on the results of a binocular rivalry study of Fries et al. (1997), we propose that the degrees of oscillatory strength and synchronization of neuronal activities determine the degree of awareness those activities produce. On the other hand, the overal firing rates of neurons in cortical sensory areas are not correlated with the degree of awareness the activities of those neurons produce. The results of the experiment of Fries et al. (1997) appear to conflict with the results of another binocular rivalry experiment, in which monkeys were trained to pull a lever in order to report which stimulus object was being perceived (Leopold & Logothetis, 1996). In the latter experiment, it was demonstrated that the firing rates of neurons in striate cortex did not change during perceptual alterations, while 90% of neurons in inferior and superior temporal cortices changed their firing rate when the perceived image changed. This result led to the conclusion that activities in temporal cortex are correlated with visual awareness, but those in striate cortex are not. We argue that activities in temporal cortex contribute little, if anything, to perceptual awareness, and that their primary function is computational. Thus the correlation between the firing rates of neurons in these areas and the responses of the monkeys is due to the recognition of a particular stimulus object, which in turn is due to the computations made there.

Cognition, Gamma Oscillations and Neuronal Synchrony

Neuroanatomical and electrophysiological evidence emphasizes the distributed nature of brain processes. There is no single convergence center in the brain for the representation of the final results of cognitive operations and for the coordination of executive functions. This raises the question how the numerous, simultaneously occurring subprocesses are bound together in order to permit coherent interpretations ofpolymodal sensory signals and coordination of adapted behaviour. At a smaller scale the same problems arise in sensory processing and motor coordination because representations of perceptual objects and movements appear to consist of ensembles of widely distributed interacting neurons rather than of individual, object-specific cells and movement specific command neurons (1). Following the discovery of stimulus-induced synchronization and oscillatory patterning of sensory responses in the visual cortex (2) it has been suggested that binding of distributed responses into coherent representations is achieved by precise synchronization of neuronal responses. Since then numerous experiments in anesthetized and awake animals have shown that this synchronization results from cooperative interactions among selectively coupled cortical neurons and changes in a context-dependent way. Evidence indicates that synchronization correlates with feature binding in sensory processing, with shifts in attention and with the preparation and execution of movements, suggesting that it serves as a general mechanism for the definition of functional relations among distributed neuronal responses. A conspicuous feature of these processing related synchronization phenomena is that they very often go along with an oscillatory patterning of the synchronized neuronal discharges in the betaand gamma-frequency range (20 60 Hz), in particular when synchronization is established over larger distances. This suggests that oscillatory patterning supports the establishment of precise temporal relations among neuronal responses. Fortunately, these synchronization phenomena are sufficiently robust so that epochs of synchronized oscillatory activity can be detected in EEG and MEG recordings that average over large populations of neurons. This permitted examination of hypotheses derived from animal experiments in human subjects. Evidence accumulated over the past years suggests close relations between the occurrence and interareal coherence of gamma oscillations on the one hand and a variety of cognitive processes on the other. These processes include feature binding, sensory-motor coordination, target selection by attention, short-term memory and associative learning, suggesting again that synchronization processes in the gamma frequency range play an important and rather general role in the coordination of distributed processes in the brain.

High-frequency oscillations (20 to 120 Hz) and their role in visual processing.

Oscillatory firing of neurons in response to visual stimuli has been observed to occur with different frequencies at multiple levels of the visual system. In the cat retina, oscillatory firing patterns occur with frequencies in the range of 60 to 120 Hz (omega-oscillations). These millisecond-precise temporal patterns are transmitted reliably to the cortex and may provide a feed-forward mechanism of response synchronization. In the cortex, visual responses often show oscillatory patterning with frequencies between 20 and 60 Hz (gamma-oscillations), which are not phase locked to the stimulus onset and therefore do not show up in regularly averaged evoked potentials. Gamma-oscillatory responses synchronize with millisecond precision over long distances and are mediated by the reciprocal corticocortical connectivity. Modulatory systems like the ascending reticular activating system facilitate synchronization and increase the strength of gamma-oscillations. During states of such functional cortical activation, the dominant frequency of the EEG is shifted from lower frequencies in the delta-/theta-range to higher frequencies in the gamma-range. Therefore, functional states indicate different degrees of temporal precision with which large neuronal populations interact. Response synchronization also depends on relations of global stimulus features. This suggests that millisecond-precise neuronal interactions serve as a fundamental mechanism for visual information processing.

Precisely Synchronized Oscillatory Firing Patterns Require Electroencephalographic Activation

Neuronal response synchronization with millisecond precision has been proposed to serve feature binding in vision and should therefore, like visual experience, depend on central states. Here we test this hypothesis by examining the occurrence and strength of response synchronization in areas 17 and 18 of anesthetized cats as a function of central states. These were assessed from the frequency content of the electroencephalogram, low power in the delta and high power in the gamma frequency ranges (here 20-70 Hz) being considered as a signature of activated states. We evaluated both spontaneous state changes and transitions induced by electrical stimulation of the mesencephalic reticular formation. During states of low central activation, visual responses were robust but lacked signs of precise synchronization. At intermediate levels of activation, responses became synchronized and exhibited an oscillatory patterning in the range of 70-105 Hz. At higher levels of activation, a different pattern of response synchronization and oscillatory modulation appeared, oscillation frequency now being in the range of 20-65 Hz. The strength of response synchronization and oscillatory modulation in the 20-65 Hz range increased with further activation and was associated with a decrease in oscillation frequency. We propose that the oscillatory patterning in the 70-105 Hz range is attributable to oscillatory retinothalamic input and that a minimal level of activation is necessary for cortical neurons to follow this oscillatory pattern. In contrast, the synchronization of responses at oscillation frequencies in the 20-65 Hz range appears to result from intracortical synchronizing mechanisms, which become progressively more effective as central activation increases. Surprisingly, enhanced synchronization and oscillatory modulation in the gamma frequency range were not associated with consistent increases in response amplitude, excluding a simple relation between central activation and neuronal discharge rate. The fact that intracortical synchronizing mechanisms are particularly effective during states of central activation supports the hypothesis that precise synchronization of responses plays a role in sensory processing.

Patterns of Synchronization in the Superior Colliculus of Anesthetized Cats

Sensorimotor transformations in the mammalian superior colliculus (SC) are mediated by large sets of distributed neurons. For such distributed coding systems, stimulus superposition poses problems attributable to the merging of neural populations coding for different stimuli. Such superposition problems could be overcome by synchronization of neuronal discharges, because it allows the selection of a subset of distributed responses for further joint processing. To assess the putative role of such a temporal binding mechanism in the SC, we have applied correlation analysis to visually evoked collicular activity. We performed recordings of single-unit and multiunit activity in the SC of anesthetized and paralyzed cats with multiple electrodes. Autocorrelation analysis revealed that collicular neurons often discharged in broad (20-100 msec) bursts or with an oscillatory patterning in the alpha- and beta-frequency range. Significantly modulated cross-correlograms were observed in 50% (128 of 258) of the collicular multiunit recording pairs, and for these pairs significant correlations occurred in 44% of the stimulation epochs. For the single-unit pairs, significant interactions were observed in 14 of 48 cases studied (29%). Collicular cross-correlograms were often oscillatory, and these oscillations covered a broad frequency range of up to 100 Hz, with a predominance of oscillation frequencies in the alpha- and beta-range. In the majority of the significant correlograms (64%) the phase lag of the center peak was <5 msec. The probability of collicular synchronization increased with the overlap of the receptive fields and the proximity of the recording sites. Correlations were also observed between cells in the superficial and deep SC layers. Collicular synchronization required activation of the respective cells with a single coherent stimulus and broke down when the neurons were activated with two different stimuli. These data are consistent with the notion that collicular synchrony could define assemblies of functionally related cells.

Synchronization of Visual Responses between the Cortex, Lateral Geniculate Nucleus, and Retina in the Anesthetized Cat

Synchronization of spatially distributed responses in the cortex is often associated with periodic activity. Recently, synchronous oscillatory patterning was described for visual responses in retinal ganglion cells that is reliably transmitted by the lateral geniculate nucleus (LGN), raising the question of whether oscillatory inputs contribute to synchronous oscillatory responses in the cortex. We have made simultaneous multi-unit recordings from visual areas 17 and 18 as well as the LGN and the retina to examine the interactions between subcortical and cortical synchronization mechanisms. Strong correlations of oscillatory responses were observed between retina, LGN, and cortex, indicating that cortical neurons can become synchronized by oscillatory activity relayed through the LGN. This feedforward synchronization occurred with oscillation frequencies in the range of 60-120 Hz and was most pronounced for responses to stationary flashed stimuli and more frequent for cells in area 18 than in area 17. In response to moving stimuli, by contrast, subcortical and cortical oscillations dissociated, proving the existence of independent subcortical and cortical mechanisms. Subcortical oscillations maintained their high frequencies but became transient. Cortical oscillations were now dominated by a cortical synchronizing mechanism operating in the 30-60 Hz frequency range. When the cortical mechanism dominated, LGN responses could become phase-locked to the cortical oscillations via corticothalamic feedback. In summary, synchronization of cortical responses can result from two independent but interacting mechanisms. First, a transient feedforward synchronization to high-frequency retinal oscillations, and second, an intracortical mechanism, which operates in a lower frequency range and induces more sustained synchronization.