Biomedical subjects

Walter J Freeman

Publications and source records attributed to Walter J Freeman.

At least 19 recordsLinked to original sources

PubMed2006

A cinematographic hypothesis of cortical dynamics in perception.

The aim of this study was to measure and classify spatial patterns in sensory cortical EEGs relating to conditioned stimuli (CSs) in order to test the hypothesis, based on clinical reports, that cortical dynamics is not continuous but operates in steps that resemble frames in a cinema. Recent advances in the application of the Hilbert Transform to intracranial recordings of the EEG in animals have revealed markers for repetitive phase transitions in neocortex at frame rates in the theta band. The frames were sought in multichannel EEGs that had been recorded from 8x8 high-density arrays that were fixed on primary sensory cortices of rabbits trained to discriminate visual, auditory or somatic conditioned stimuli with reinforcement (CS+) or without (CS-). Localization of frames in EEGs was by use of a new index, H(e)(t), called "pragmatic information". Each spatial pattern was represented by a feature vector from the 64 analytic amplitudes at a maximal value of H(e)(t) from the Hilbert transform and expressed as a 64x1 feature vector specifying a point in 64-space. Classification with respect to CS+/- was by calculation of Euclidean distances of points from centers of gravity of clusters after preprocessing by nonlinear mapping. Stable spatial patterns were found in the form of amplitude modulation (AM) of aperiodic waveforms that included all channels. The impact of a CS on a sensory neocortex reorganized background EEG into two types of sequential patterns of coordinated activity, initially local and modality-specific, later global. The initial stage of phase transitions required 3-7 ms. Large-scale cortical activity then reorganized itself repeatedly and reliably over relatively immense cortical distances within the cycle duration of the center frequency of oscillation. The size, texture, timing, and duration of the AM patterns support the hypothesis that these frames may provide the basis for multisensory percepts (Gestalts).

Animals↗

PubMed2006

Origin, structure, and role of background EEG activity. Part 4: Neural frame simulation.

OBJECTIVE: To develop a method for simulating background EEG based on the premise that the self-organized activity from synaptic interaction among populations of neurons creates sustained fluctuations that can be modeled with the filtered output of a random number generator. METHODS: The logarithm of the amplitude of activity was weighted in accordance with 1/f, the log frequency in both temporal (PSD(T)) and spatial (PSD(X)) power spectral densities. The activity was spatially smoothed by volume conduction. Further deviation from full randomness was by sustained spatial coherence averaging 25% of total power. The departure from the background state to an active state, as seen in the awake EEG, was simulated by adding segments that were 90% correlated while attenuating by 50% the uncorrelated background activity in those segments. Spatial amplitude modulation was imposed on the correlated noise to create signals that simulated AM patterns. RESULTS: The statistical properties of the EEG that were replicated (Freeman, 2004a,b, 2005) included the PSD(T), PSD(X), point spread function (PSF), partitioning of the variance with PCA, and the percentages of correct classification of AM patterns. CONCLUSIONS: The origin of background EEG was traced to self-sustaining mutual excitation among pyramidal cells creating stable noise that was filtered by self-organized criticality to give 1/f(2) PSD, by inhibitory feedback to give oscillations in the classic clinical bands, and by volume conduction to give smoothing. The essential change that identified a frame in EEG was transient synchrony by phase transition among cortical populations in beta and gamma bands of the PSD(T). SIGNIFICANCE: This simulation can provide test data with which to optimize techniques for noninvasively extracting information from the EEG for diagnosis and treatment evaluation of neuropsychiatric disorders and for operation by paraplegics of prosthetic devices.

Brain↗

PubMed2006

Fine spatiotemporal structure of phase in human intracranial EEG.

OBJECTIVE: To transfer to the clinic for humans the technology and theory for high-resolution EEG analysis that have been developed in the laboratory with animals. METHODS: EEGs were recorded at high spatial resolution from a 1 x 1 cm 8 x 8 electrode array on the right inferior temporal gyrus of a patient undergoing preoperative monitoring for epilepsy surgery. Cosines were fitted to EEG segments to measure frequency and phase and compute location, size, latency, phase velocity, duration, and recurrence rate of radially symmetric spatial patterns called phase cones. The Hilbert transform was also used to get high temporal resolution. RESULTS: In the awake state, the power spectral density (PSD) showed power-law decrease in log power with log frequency at 1/falpha, alpha approximately 2, but with peaks in the standard empirical ranges. The phase in beta and gamma ranges had spatial gradients in conic form. Resetting of these stable spatial patterns of phase cones was spatially coincident at intermittent discontinuities ('phase slip') recurring at theta rates. Cones had half power diameters from 2 to 50+ mm; their durations had power-law distributions with values ranging from 6 to 300+ ms depending on length of the analysis window. In slow wave sleep PSD decreased at 1/falpha, alpha approximately 3,with loss of beta-gamma spectral peaks and diminished or absent oscillations and spatiotemporal phase structure. CONCLUSIONS: Spatiotemporal structures in awake human and rabbit EEG showed striking similarities. The only clear differences were ascribable to differing scales of measurement. These fine spatiotemporal structures of EEG were diminished or lost in slow wave sleep. SIGNIFICANCE: The fine structure indicates that neocortical stability is sustained at self-organized criticality; that synaptic input in the awake state drives neocortex away from criticality causing beta-gamma oscillations in re-stabilizing 'neural avalanches'; and that diminished input in slow wave sleep allows return toward criticality but with some added risk of instability and seizure.

Adult↗

PubMed2005

Phase transitions in the neuropercolation model of neural populations with mixed local and non-local interactions.

We model the dynamical behavior of the neuropil, the densely interconnected neural tissue in the cortex, using neuropercolation approach. Neuropercolation generalizes phase transitions modeled by percolation theory of random graphs, motivated by properties of neurons and neural populations. The generalization includes (1) a noisy component in the percolation rule, (2) a novel depression function in addition to the usual arousal function, (3) non-local interactions among nodes arranged on a multi-dimensional lattice. This paper investigates the role of non-local (axonal) connections in generating and modulating phase transitions of collective activity in the neuropil. We derived a relationship between critical values of the noise level and non-locality parameter to control the onset of phase transitions. Finally, we propose a potential interpretation of ontogenetic development of the neuropil maintaining a dynamical state at the edge of criticality.

Algorithms↗

PubMed2005

A field-theoretic approach to understanding scale-free neocortical dynamics.

A mesoscopic field-theoretic approach is compared with neural network and brain imaging approaches to understanding brain dynamics. Analysis of high spatiotemporal resolution rabbit electroencephalogram (EEG) reveals neural fields in the form of spatial patterns in amplitude (AM) and phase (PM) modulation of gamma and beta carrier waves that serve to classify EEGs from trials with differing conditioned stimuli (CS+/-). Paleocortex exemplified by olfactory EEG has one AM-PM pattern at a time that forms by an input-dependent phase transition. Neocortex shows multiple overlapping AM-PM patterns before and during presentation of CSs. Modeling suggests that neocortex is stabilized in a scale-free state of self-organized criticality, enabling cooperative domains to form virtually instantaneously by phase transitions ranging in size from a few hypercolumns to an entire hemisphere. Self-organized local domains precede formation of global domains that supervene and contribute global modulations to local domains. This mechanism is proposed to explain Gestalt formation in perception.

Animals↗

PubMed2005

Origin, structure, and role of background EEG activity. Part 3. Neural frame classification.

OBJECTIVE: To show that cortical responses to conditioned stimuli (CS) include intermittently induced spatial patterns of amplitude modulation (AM) of beta-gamma oscillation called frames. METHODS: EEGs were recorded from 8x8 high-density arrays fixed on primary sensory cortices of rabbits trained to discriminate CS with reinforcement (CS+) from those without (CS-). EEG frames were located with a pragmatic information index, H(e). The spatial patterns of the first 3 frames on each of 37-40 trials were measured by the square of 64 analytic amplitudes from the Hilbert transform to give points in 64-space. The questions were asked: Did the frames from CS+ trials and CS- trials differ within each sequential group? Did the 3 frames differ from each other (form 3 clusters of points)? RESULTS: EEG frames that were identified by high H(e) had AM patterns that could be classified with respect to CS+ and CS- well above chance levels. Two stages of correct frame classification occurred on each trial: 40-130 ms after CS onset with a gamma carrier frequency, and 450-550 ms with a beta carrier frequency. Peak power in the beta frames was double that in gamma frames, and mean pattern surface area of beta frames was nearly 4-fold greater. CONCLUSIONS: Under the impact of a CS on a sensory neocortex, the background EEG activity reorganized in sequential frames of coordinated activity, first local and modality-specific, thereafter global. SIGNIFICANCE: The size, texture and duration of these AM patterns indicate that spatial patterns of human beta frames may be accessible with high-density scalp arrays for correlation with phenomenological reports by human subjects.

Acoustic Stimulation↗

PubMed2005

Ndn, volume transmission, and self-organization in brain dynamics.

Fields of neural activity are seen in synchronized oscillations that are detected at mesoscopic scales in syntheses of multicellular recordings of action potentials and electroencephalograms (EEGs) over broad areas of cerebral cortex. The waves often have large-scale, highly textured spatial patterns of cortical activity, formed in the context of associative learning under classical and operant conditioning in rabbits. The patterns show spatial amplitude modulation of shared oscillations of carrier waves in the beta and gamma ranges of the EEG, with recurrence at frame rates in the alpha and theta ranges. The frames also show spatial phase modulation that is inconsistent with driving of the oscillations by focal pacemakers. The hypothesis is developed that the synchronization manifests continuous distributions of activity in cortical neuropil that modulate firings of selected neural networks embedded in the neuropil. Five interactive agencies have been postulated to explain the mechanism for the field synchrony: electric fields; magnetic fields; electromagnetic fields (radio waves); diffusion chemical gradients; and order parameters that control self-organization of large populations of neurons by widespread synaptic interaction constituting negative and positive feedback. Only the last interactive agency fits the data. The points are emphasized that these field patterns in frames require interactive neural dynamics that is modulated in respect to global operations mediating arousal, attention, selective emotional stance, wake, sleep, learn, habituate, dishabituate, etc., and that these operations require differing but complementary fields that form by massive parallel feed-forward architectures of brainstem neuromodulatory nuclei. An example is given using histamine of the neural discharges of brainstem nuclei that do not require fine spatiotemporal texturing of their firing; they operate by nonsynaptic release of neuromodulators that effect changes in background state, such that textured patterns of cortical activity can form and update in flexible adaptations of brains to their environments. These systems instantiate volume transmission by nonsynaptic diffusion transmission, in concert with the self-organization of the textured neural activity that supports cognition.

Animals↗

PubMed2004

Origin, structure, and role of background EEG activity. Part 2. Analytic phase.

OBJECTIVE: To explain spontaneous EEG through measurements of spatiotemporal patterns of phase among beta-gamma oscillations. METHODS: High-density 8 x 8 intracranial arrays were fixed over sensory cortices of rabbits. EEGs were spatially low pass filtered, temporally bandpass filtered and segmented in overlapping windows stepped at 2 ms. Phase was measured with the cosine as the temporal basis function, using both Fourier and Hilbert transforms to compensate for their respective limitations. Spatial patterns in 2D phase surfaces were measured with the geometric form of the cone as the spatial basis function. RESULTS: Two fundamental state variables were measured at each digitizing step in the 64 EEGs: the rate of change in phase with time (frequency) and the rate of change in phase with distance (gradient). The parameters of location, diameter, duration, and phase velocity of the cone of phase were derived from these two state variables. Parameter distributions including recurrence intervals extending into the low theta range were fractal; the mean values varied with window duration and interelectrode distance. CONCLUSIONS: The formation of spatial amplitude patterns began with state transitions that were documented by phase discontinuities and phase cones. The multiplicity of overlapping cones indicated that sensory neocortices maintained a scale-free state of self-organized criticality (SOC) in each hemisphere as the basis for its rapid integration of sensory input with prior learning stored in cortical synaptic webs. Further evidence came from the fractal properties of the phase parameters and the self-similarity of phase patterns in the ms/mm to m/s ranges. SIGNIFICANCE: These EEG data suggest that neocortical dynamics is analogous to the dynamics of self-stabilizing systems, such as a sand pile that maintains its critical angle by avalanches, and a pan of boiling water that maintains its critical temperature by bubbles that release heat. Beta-gamma oscillations stem from the ability of neocortex to maintain its stability under continuous sensory bombardment. Modeling implies that the critical parameter of neocortex (analogous to angle of repose or temperature) is the mean firing rates of neurons that are homeostatically regulated by refractory periods everywhere at all times in cortex. The advantage of SOC in perception may be the ability it gives neocortex to generate instantaneous global state transitions (avalanches, bubbles) large enough to include the multiple sensory areas that are necessary to form gestalts (multisensory percepts).

Animals↗

PubMed2004

Origin, structure, and role of background EEG activity. Part 1. Analytic amplitude.

OBJECTIVE: To explain the neural mechanisms of spontaneous EEG by measuring the spatiotemporal patterns of synchrony among beta-gamma oscillations during perception. METHODS: EEGs were measured from 8 x 8 (5.6 x 5.6 mm2) arrays fixed on the surfaces of primary sensory areas in rabbits that were trained to discriminate visual, auditory or tactile conditioned stimuli (CSs) eliciting conditioned responses (CRs). EEG preprocessing was by (i) bandpass filtering to extract the beta-gamma range (deleting theta-alpha); (ii) low-pass spatial filtering (not high-pass Laplacians used for localization), (iii) spatial averaging (not time averaging used for evoked potentials), and (iv) close spacing of 64 electrodes for simultaneous recording in each area (not sampling single signals from several areas); (v) novel algorithms were devised to measure synchrony and spatial pattern stability by calculating variances among patterns in 64-space derived from the 8 x 8 arrays (not by fitting equivalent dipoles). These methodological differences are crucial for the proposed new perspective on EEG. RESULTS: Spatial patterns of beta-gamma EEG emerged following sudden jumps in cortical activity called 'state transitions'. Each transition began with an abrupt phase re-setting to a new value on every channel, followed sequentially by re-synchronization, spatial pattern stabilization, and a dramatic increase in pattern amplitude. State transitions recurred at varying intervals in the theta range. A novel parameter was devised to estimate the perceptual information in the beta-gamma EEG, which disclosed 2-4 patterns with high information content in the CS-CR interval on each trial; each began with a state transition and lasted approximately 0.1 s. CONCLUSIONS: The function of each primary sensory neocortex was discontinuous; discrete spatial patterns occurred in frames like those in cinema. The frames before and after the CS-CR interval had low content. SIGNIFICANCE: Derivation and interpretation of unit data in studies of perception might benefit from using multichannel EEG recordings to define distinctive epochs that are demarcated by state transitions of neocortical dynamics in the CS-CR intervals, particularly in consideration of the possibility that EEG may reveal recurring episodes of exchange and sharing of perceptual information among multiple sensory cortices. Simultaneously recorded, multichannel beta-gamma EEG might assist in the interpretation of images derived by fMRI, since high beta-gamma EEG amplitudes imply high rates of energy utilization. The spatial pattern intermittency provides a tag to distinguish gamma bursts from contaminating EMG activity in scalp recording in order to establish beta-gamma recording as a standard clinical tool. Finally, EEG cannot fail to have a major impact on brain theory.

Acoustic Stimulation↗

PubMed2003

Aperiodic phase re-setting in scalp EEG of beta-gamma oscillations by state transitions at alpha-theta rates.

We evaluated the rapid changes in regional scalp EEG synchronization in normal subjects with spatial and temporal resolution exceeding prior art 10-fold with a high spatial density array and the Hilbert transform. A curvilinear array of 64 electrodes 3 mm apart extending 18.9 cm across the scalp was used to record EEG at 200/sec. Analytic amplitude (AA) and phase (AP) were calculated at each time step for the 64 traces in the analog pass band of 0.5-120 Hz. AP differences approximated the AP derivative (instantaneous frequency). The AP from unfiltered EEG revealed no reproducible patterns. Filtering was necessary in the beta and gamma ranges according to a technique that optimized the correlation of the AP differences with the activity band pass filtered in the alpha range. The sizes of temporal AP differences were usually within +/-0.5 radian from the average step corresponding to the center frequency of the pass band. Large AP differences were often synchronized over distances of 6 to 19 cm. An optimal pass band to detect and measure these recurring jumps in AP in the beta and gamma ranges was found by maximizing the alpha peak in the cospectrum of the correlation between unfiltered EEG and the band pass AP differences. Synchronized AP jumps recurred in clusters (CAP) at alpha and theta rates in resting subjects and with EMG. Cortex functions by serial changes in state. The Hilbert transform of EEG from high-density arrays can visualize these state transitions with high temporal and spatial resolution and should be useful in relating EEG to cognition.

Alpha Rhythm↗

PubMed2003

Spatial spectra of scalp EEG and EMG from awake humans.

OBJECTIVE: Evaluate spectral scaling properties of scalp electroencephalogram (EEG) and electromyogram (EMG), optimal spacing of electrodes, and strategies for mitigating EMG. METHODS: EEG was recorded referentially from 9 subjects with a 64 channel linear array (electrodes 3mm apart) placed parasagittally or transversely on forehead or occiput, at rest with eyes open or closed, or with deliberate EMG. Temporal (PSD(t)) and spatial (PSD(x)) power spectral densities were calculated with one-dimensional fast Fourier transform (FFT) for comparison with earlier analyses of intracranial EEG. RESULTS: Scaling of PSD(t) from scalp resembled that from pia: near-linear decrease in log power with increasing log frequency (1/f(alpha)). Scalp PSD(x) decreased non-linearly and more rapidly than PSD(x) from pia. Peaks in PSD(t) (especially 4-12Hz) and PSD(x) (especially 0.1-0.4 cycles/cm) revealed departures from 1/f(alpha). EMG power in PSD(t) was more "white" than 1/f(alpha). CONCLUSIONS: Smearing by dura-skull-scalp distorts PSD(x) more than PSD(t) of scalp EEG from 1/f(alpha) scaling at the pia. Spatial spectral peaks suggest that optimal scalp electrode spacing might be approximately 1cm to capture non-local EEG components having the texture of gyri. Mitigation of EMG by filtering is unsatisfactory. A criterion for measuring EMG may support biofeedback for training subjects to reduce their EMG. SIGNIFICANCE: High-density recording and log-log spectral display of EEG provide a foundation for holist studies of global human brain function, as an alternative to network approaches that decompose EEG into localized, modular signals for correlation and coherence.

Brain↗

PubMed2003

Neurodynamic models of brain in psychiatry.

The history of brain theory is described in terms of three kinds of theory of perception. The most widely used kind sees perception as dependent on passive inflow from the environment of information that is used to make and process representations of objects and events. A second kind views perception as an active search for information that is inherent in the environment and is extracted by tuned resonances in brain circuits. A third kind holds that perception works by the creation of information through chaotic dynamics by forming hypotheses about the environment, through which learning takes place. Experimental evidence for creative dynamics in brains is briefly sketched. The explanation is offered that brains, being finite systems, work this way in order to cope with the infinite complexity of the world. All that brains can know is the hypotheses they construct and the results of testing them by acting into the environment, and learning by assimilation from the sensory consequences of their actions. The process is described as intentionality. It works through the action-perception-assimilation cycle. The cost of this solution to the problem of infinite complexity by hypothesis testing is the progressive isolation of individuals, as they accumulate their unique experiences through which their personalities form. Socialization and the acquisition of shared knowledge requires the emergence of new personality structure by self-organization through chaotic dissolution of existing the structure, as a prelude to the creation of new traits, habits, and values. Dissolution works in a crisis situation by regression to earlier stages of development, from which a fresh start can be made. A state of malleability emerges in the depth of crisis, in which compassionate companions through loving care can invite cooperative actions. Joint actions support the growth of a new lifestyle based on trust. Socialization requires neurochemical mechanisms of affiliation and bonding that evolved through the requirements of parental care of altricial offspring in mammalian reproduction. These mechanisms are invoked by means of behavioral techniques from cultural evolution. The dynamics, neural mechanisms, behavioral signs, methods of induction, and therapeutic utility of dissolution should be known by therapists. Lack of recognition and understanding may cause failure to use brief windows of opportunity to instill long-term relief of psychic pain by restructuring intentionality in distressed patients.

Brain↗

PubMed2003

Evidence from human scalp electroencephalograms of global chaotic itinerancy.

My objective of this study was to find evidence of chaotic itinerancy in human brains by means of noninvasive recording of the electroencephalogram (EEG) from the scalp of normal subjects. My premise was that chaotic itinerancy occurs in sequences of cortical states marked by state transitions that appear as temporal discontinuities in neural activity patterns. I based my study on unprecedented advances in spatial and temporal resolution of the phase of oscillations in scalp EEG. The spatial resolution was enhanced by use of a high-density curvilinear array of 64 electrodes, 189 mm in length, with 3 mm spacing. The temporal resolution was advanced to the limit provided by the digitizing step, here 5 ms, by use of the Hilbert transform. The numerical derivative of the analytic phase revealed plateaus in phase that lasted on the order of 0.1 s and repeated at rates in the theta (3-7 Hz) or alpha (7-12 Hz) ranges. The plateaus were bracketed by sudden jumps in phase that usually took place within 1 to 2 digitizing steps. The jumps were commonly synchronized in each cerebral hemisphere over distances of up to 189 mm, irrespective of the orientation of the array. The jumps were usually not synchronized across the midline separating the hemisphere or across the sulcus between the frontal and parietal lobes. I believe that the widespread synchrony of the jumps in analytic phase manifest a metastable cortical state in accord with the theory of self-organized criticality. The jumps appear to be subcritical bifurcations. They reflect the aperiodic evolution of brain states through sequences of attractors that on access support the experience of remembering.

Brain↗

PubMed2003

Basic principles of the KIV model and its application to the navigation problem.

EEG measurements indicate the presence of common-mode, coherent oscillations shared by multiple cortical areas. In previous studies the KIII model has been introduced, which interprets the experimental observations as nonlinear, spatially distributed dynamical oscillations of locally coupled neural populations. KIII can account for the fast and robust classification and pattern recognition in sensory cortices. In order to describe selection of action, planning, and spatial orientation functions, in this paper we expand KIII into the KIV model. KIV approximates the operation of the corticostriatal-hippocampal system. KIV consists of three KI, eight KII and three KIII components, including sensory and cortical systems, as well as the hippocampus, amygdala, and the septum. KIV implements various types of dynamic neural activities. The neural activity patterns determine the emergence of global spatial encoding to implement the orientation function of a simulated animal. Our results indicate the mechanisms, which we believe support the generation of cognitive maps in the hippocampus based on the sensory input-based destabilization of cortical spatio-temporal patterns. In this paper, we describe the conceptual design of the KIV model. We outline the biological background and motivation of the basic principles that are applied to design the KIV computational model. We use the KIV model to explain how the hippocampal neural circuitry functions are constructed and controlled by the corticostriatal-hippocampal loops, supplemented with specific subcortical units. In the second part, we implement these principles using the example of the hippocampal formation as a KIII unit. We demonstrate the learning and navigation principles using the Evolving Multi-module Mobile Agent (EMMA) in 2D software environment.

Animals↗

PubMed2003

The wave packet: an action potential for the 21st century.

Sensing and perceiving involve enormous numbers of widely distributed dendritic and action potentials in cortex, before, during and after stimulus arrival but with differing spatiotemporal patterns. Stimulus-activated receptors drive cortical neurons directly (olfactory) or indirectly through thalamocortical relays. The driven activity induces hemisphere-wide, self-organized patterns of neural activity called wave packets. Three levels of brain function are hypothesized to mediate transition from sensation and perception. Microscopic activity expressed by action potentials is sensory. Macroscopic activity of the whole forebrain expressed by behavior is perceptual. Mesoscopic activity bridges the gap by the formation of wave packets. They form when sensory input destabilizes the primary receiving areas by local state transitions. The sensory-driven action potentials condense into mesoscopic wave packets like molecules forming raindrops from vapor. The condensation disks sustain 2D spatial patterns of phase and amplitude of carrier waves in the beta and gamma EEG. The AM patterns correlate not with features but with the context and value of sensory stimuli for the subjects, in a word, their meaning. The wave packets from all sensory areas are broadly transmitted through the forebrain. They induce the formation of macroscopic patterns that coalesce like scintillating pools over much and perhaps all of each hemisphere. The prediction is made for clinical testing that wave packets are precursor to states of awareness. They are not by themselves accessible to experience, as may be the macroscopic states initiated by global state transitions.

Cerebral Cortex↗

PubMed2003

Analysis of evoked and emergent patterns of stimulus-related auditory cortical activity.

Cortical activity contains both evoked patterns and emergent patterns of stimulus-related activity. Here we compared evoked and emergent patterns in the primary auditory cortex, field AI, of the gerbil by studying the differential effects of diluting spatial information about the patterns on their geometric dissimilarity by randomly removing channels from the recording data. This identified the sets of most relevant channels for the discrimination of stimuli in both types of patterns. In the evoked patterns the sets of most discriminative channels were found to be focally organized at locations corresponding to the thalamically relayed input into the cortical tonotopic map. In the emergent patterns the sets of most discriminative channels were broadly distributed and held no apparent relationship to the tonotopic map. The results indicate the coexistence in the same neuronal tissue of a topographic mapping principle for the evoked activity and a holographic mapping principle for the emergent activity.

Acoustic Stimulation↗

PubMed2002

Detection and classification of nonlinear dynamic switching events.

A method is proposed for detecting chaotic switching events. Switching events are classified by the time of the event and by parameter value. Classifications are based on the density of localized dynamics about a test trajectory. This method is shown to be successful in tracking short-time parameter modulation and hyperchaotic key shifting used in otherwise secure communications.

Journal Article↗

PubMed2002

Fine temporal resolution of analytic phase reveals episodic synchronization by state transitions in gamma EEGs.

The analytic signal given by the Hilbert transform applied to an electroencephalographic (EEG) trace is a vector of instantaneous amplitude and phase at the temporal resolution of the digitizing interval (here 2 ms). The transform was applied after band-pass filtering for extracting the gamma band (20-80 Hz in rabbits) to time series from up to 64 EEG channels recorded simultaneously from high-density arrays giving spatial "windows" of 4 x 4 to 6 x 6 mm onto the visual, auditory, or somatosensory cortical surface. The time series of the analytic phase revealed phase locking for brief time segments in spatial patterns of nonzero phase values from multiple EEG that was punctuated by episodic phase decoherence. The derivative of the analytic phase revealed spikes occurring not quite simultaneously (within +/- 4 ms) across arrays aperiodically at mean rates in and below the theta range (3-7 Hz). Two measures of global synchronization over a group of channels were derived from analytic phase differences between pairs of channels on the same area of cortex. One was a synchronization index expressing phase locking. The other was a decoherence index estimating the variance in phase among multiple channels. Spectral analyses of the indices indicated that decoherence events recurred aperiodically at rates in and below the theta range of the EEGs. The results provide support for the hypothesis that neurons in mesoscopic neighborhoods in sensory cortices self-organize their activity by synaptic interactions into wave packets that have spatial patterns of amplitude (AM) and phase (PM) modulation of their spatially coherent carrier waves in the gamma range and that form and dissolve aperiodically at rates in and below the theta range. Each AM pattern is formed by a nonlinear state transition in the cortical dynamics, as shown by spikes in the derivative. Phase locking within each PM pattern is not at zero phase lag but over a fixed distribution of phase values that is consistent with the radially symmetric phase gradients already reported called "phase cones" detected by Fourier-based methods. The insight is suggested that sensory cortices are bistable comparably to cardiac dynamics, with a diastolic state that accepts sensory input and an abrupt transition to a systolic state that transmits perceptual output. Further support for this inference will require improvements in methods for temporal resolution of the times of onset of spatial patterns of phase modulation.

Animals↗