Pascal Fries

DYNAMIC PREDICTIONS: OSCILLATIONS AND SYNCHRONY IN TOP-DOWN PROCESSING

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.

Is synchronized neuronal gamma activity relevant for selective attention

Today, much evidence exists that sensory feature binding is accomplished by phase synchronization of induced neuronal gamma activity (30-80 Hz). Recent studies furthermore suggest that phase synchronization of induced gamma activity may represent a general mechanism enabling transient associations of neural assemblies and thus may play a central role in cortical information processing. Here, we describe findings indicating that synchronized gamma activity is moreover specifically involved in selective attention. While feature binding appears to depend primarily on induced gamma synchronization, attentional processes seem to involve both induced and evoked gamma oscillations. Yet it is still an open question, as to which top-down and bottom-up processes are associated with attentional modulation of gamma activity. A possible mechanism to project influences from attentional control structures to areas concerned with stimulus representation and vice versa, may be neuronal synchronization and the resulting firing rate changes of coincidence-detecting neurons in target areas.

Empirical mode decomposition: a method for analyzing neural data

Almost all processes that are quantified in neurobiology are stochastic and nonstationary. Conventional methods that characterize these processes to provide a meaningful and precise description of complex neurobiological phenomenon may be insufficient. Here, we report on the use of the data-driven empirical mode decomposition (EMD) method to study neuronal activity in visual cortical area V4 of macaque monkeys performing a visual spatial attention task. We found that local field potentials were resolved by the EMD into the sum of a set of intrinsic components with different degrees of oscillatory content. High-frequency components were identified as gamma band (35-90Hz) oscillations, whereas low-frequency components in single-trial recordings contributed to the average visual evoked potential (AVEP). Comparison with Fourier analysis showed that EMD may offer better temporal and frequency resolution. The EMD, coupled with instantaneous frequency analysis, may prove to be a vital technique for the analysis of neural data.

Assessing Neuronal Coherence with Single-Unit, Multi-Unit, and Local Field Potentials

The purpose of this study was to obtain a better understanding of neuronal responses to correlated input, in particular focusing on the aspect of synchronization of neuronal activity. The first aim was to obtain an analytical expression for the coherence between the output spike train and correlated input and for the coherence between output spike trains of neurons with correlated input. For Poisson neurons, we could derive that the peak of the coherence between the correlated input and multi-unit activity increases proportionally with the square root of the number of neurons in the multi-unit recording. The coherence between two typical multi-unit recordings (2 to 10 single units) with partially correlated input increases proportionally with the number of units in the multi-unit recordings. The second aim of this study was to investigate to what extent the amplitude and signal-to-noise ratio of the coherence between input and output varied for single-unit versus multi-unit activity and how they are affected by the duration of the recording. The same problem was addressed for the coherence between two single-unit spike series and between two multi-unit spike series. The analytical results for the Poisson neuron and numerical simulations for the conductance-based leaky integrate-and-fire neuron and for the conductance-based Hodgkin-Huxley neuron show that the expectation value of the coherence function does not increase for a longer duration of the recording. The only effect of a longer duration of the spike recording is a reduction of the noise in the coherence function. The results of analytical derivations and computer simulations for model neurons show that the coherence for multi-unit activity is larger than that for single-unit activity. This is in agreement with the results of experimental data obtained from monkey visual cortex (V4). Finally, we show that multitaper techniques greatly contribute to a more accurate estimate of the coherence by reducing the bias and variance in the coherence estimate.

Empirical mode decomposition of field potentials from macaque V4 in visual spatial attention

Empirical mode decomposition (EMD) has recently been introduced as a local and fully data-driven technique for the analysis of non-stationary time-series. It allows the frequency and amplitude of a time-series to be evaluated with excellent time resolution. In this article we consider the application of EMD to the analysis of neuronal activity in visual cortical area V4 of a macaque monkey performing a visual spatial attention task. We show that, by virtue of EMD, field potentials can be resolved into a sum of intrinsic components with different degrees of oscillatory content. Low-frequency components in single-trial recordings contribute to the average visual evoked potential (AVEP), whereas high-frequency components do not, but are identified as gamma-band (30–90 Hz) oscillations. The magnitude of time-varying gamma activity is shown to be enhanced when the monkey attends to a visual stimulus as compared to when it is not attending to the same stimulus. Comparison with Fourier analysis shows that EMD may offer better temporal and frequency resolution. These results support the idea that the magnitude of gamma activity reflects the modulation of V4 neurons by visual spatial attention. EMD, coupled with instantaneous frequency analysis, is demonstrated to be a useful technique for the analysis of neurobiological time-series.

Ocular dominance in extrastriate cortex of strabismic amblyopic cats.

Ocular dominance in extrastriate visual cortex of cats with behaviorally defined strabismic amblyopia was studied using extracellular recording techniques. In area 18, the amblyopic eye drove about as many cells as the normal one. In area posteromedial lateral suprasylvian area (PMLS), about 60% of the cells responded exclusively to stimulation of the normal eye and 30% to stimulation of the amblyopic eye. In area 21a more than 75% of the cells were monocularly driven by the non-amblyopic eye while only 5% were monocularly driven by the amblyopic eye. These findings suggest that ventral pathways (area 21a) are more affected in amblyopia than dorsal pathways (area PMLS).

When neurons form memories

Although long-term memory is central among our cognitive functions, the search for a direct neurophysiological correlate to it has proven difficult. The formation of new memories depends on the hippocampus and adjacent cortex, but the final storage is thought to be in a widely distributed neocortical network. Recent experiments, using simultaneous recordings from hundreds of sites in monkey neocortex, have revealed the activation of such a distributed network -- probably reflecting the consolidation of long-term memory storage.

Temporal dynamics of attention-modulated neuronal synchronization in macaque V4

It was recently observedthat neurons in area V4 exhibitedenhancedgamma band(35 –90 Hz) synchronization when monkeys attend edto a visual stimulus as comparedto when they were not attending to the same stimulus (Science 291 (2001) 1560). Spike-triggered averaging of local 8eldpotentials (LFPs) was usedto show attentional mod ulation in an early periodfrom 50 to 150 ms after stimulus onset (Science 291 (2001) 1560). In this work we further studied the 8ne temporal structure in the same data by focusing only on the LFPs without reference to the concurrent spike trains. With the methodof ad aptive multivariate autoregressive (AMVAR) modeling, we discovered that attentional modulation of gamma power (∼65 Hz) in V4 can be as brief as about 25 ms. Gamma coherence between two V4 recording sites revealed similar attention e<ects, as well as a secondpeak around45 Hz. Directional in=uences between two V4 populations revealedthat one can play a more d ominant role than another. These results implicate gamma oscillation as a possible agent in carrying out attention-biasedcompetition among visual stimuli in favor of those that are behaviorally relevant. The AMVAR methodwas instrumental in revealing the dynamics of gamma frequency synchronization with high temporal and frequency resolution.

Conditions of perceptual selection and suppression during interocular rivalry in strabismic and normal cats.

Presenting the two eyes with incongruent stimuli leads to the phenomenon of interocular rivalry. At any given time, one of the stimuli is perceptually suppressed in order to avoid double vision. In squinting subjects, rivalry occurs permanently also for congruent stimuli because of developmental rearrangement of cortical circuitry. In this study, we have investigated the dynamics and stimulus dependence of rivalry in six esotropic, four exotropic and three non-strabismic cats. As an indicator for perception, we used optokinetic nystagmus that was induced by moving gratings. The esotropic cats were tested for their visual acuity by means of a jumping stand procedure. The results show that one eye can dominate perception even if both eyes have equal visual acuity and are presented with stimuli of equal contrast. Strong eye dominance asymmetry was found in all but one of the tested cats. Notably, all three of the normal cats showed a clear asymmetry in perceptual selection. Measurements with varying contrast and velocity of the stimuli revealed that the influence of these parameters on perceptual selection was independent of the presence of strabismus. In all cats, the time during which a given eye dominated perception increased with the contrast and decreases with the velocity of the stimulus presented to this eye.

Response: The birth of a memory

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