Sensory and Cognitive 40-Hz Event-Related Potentials: Behavioral Correlates, Brain Function, and Clinical Application. In: Başar E, Bullock TH, editors. Brain Dynamics. Berlin: Springer Berlin Heidelberg; 1989. pp. 339-74. [DOI: 10.1007/978-3-642-74557-7_29]

Brain Processes and Phenomenal Consciousness

A hypothesis on the physiological conditions for the occurrence of phenomenal states is presented. It is suggested that the presence of phenomenal states depends on the rate at which neural assemblies are formed. Unconsciousness and various disturbances of phenomenal consciousness occur if the assembly formation rate is below a certain threshold level; if this level is surpassed, phenomenal states necessarily result. A critical production rate of neural assemblies is the necessary and sufficient condition for the occurrence of phenomenal states.

[Neurotheology: neurobiological models of religious experience]

Religions are evolutionary selected social and cultural phenomena. They represent today belief and normative systems on which the main parts of our culture are based. For a long time religions have been seen as mainly originating from a spectrum of religious experiences. These include a broad spectrum of experiences and are astonishingly widespread in the population. The most consistent and transculturally uniform religious experiences are the mystical experiences. Only these (and the prayer experience) have factually been researched in detail neurobiologically. This article presents a review of empirical results and hypothetical approaches to explain mystical religious experiences neurobiologically. Some of the explanatory hypotheses possess logical evidence, some are even supported by neurobiological studies, but all of them have their pitfalls and are at best partially consistent. One important insight from the evidence reviewed here is that there may be a whole array of different neurophysiological conditions which may result in the same core religious mystical experiences.

Event-Related Oscillations and Cognitive Processes in Children

Event-related EEG modulations, which are phase-locked to perceptual, cognitive, and motor processes, are often studied by means of event-related potentials (ERPs), although event-related oscillatory responses in different EEG frequency bands allow a more refined analysis, closer to brain physiology. This article introduces the basics of time-frequency methods, which are typically applied for the analysis of event-related oscillations, focusing on adaptive procedures (e.g., wavelet networks). The potential of these methods is illustrated. Findings about event-related oscillations (gamma responses, theta responses) in children performing an auditory selective attention task are reviewed. Both the neuronal substrates of gamma (30–70 Hz) networks and the ability to synchronize these networks in relation to task-specific processes are available in children and adolescents from 9 to 16 years of age. Developmental changes in the task reactivity of synchronized gamma oscillations may provide evidence for a transition in cognitive processing strategies emerging at the age of 12–13 years. Event-related theta (3–7.5 Hz) activity is enhanced in two latency ranges. The early event-related theta response occurring 0–200 ms after a stimulus may be associated with representations of relevant target features in working memory. The late fronto-central theta response (200 – 450 ms) could be related to the processing of task-irrelevant information. In summary, event-related oscillations can be analyzed using time-frequency methods like wavelet networks. This approach should be used intensively to study neurocognitive development in children.

Deconstructing insight: EEG correlates of insightful problem solving

BACKGROUND

Cognitive insight phenomenon lies at the core of numerous discoveries. Behavioral research indicates four salient features of insightful problem solving: (i) mental impasse, followed by (ii) restructuring of the problem representation, which leads to (iii) a deeper understanding of the problem, and finally culminates in (iv) an "Aha!" feeling of suddenness and obviousness of the solution. However, until now no efforts have been made to investigate the neural mechanisms of these constituent features of insight in a unified framework.

METHODOLOGY/PRINCIPAL FINDINGS

In an electroencephalographic study using verbal remote associate problems, we identified neural correlates of these four features of insightful problem solving. Hints were provided for unsolved problems or after mental impasse. Subjective ratings of the restructuring process and the feeling of suddenness were obtained on trial-by-trial basis. A negative correlation was found between these two ratings indicating that sudden insightful solutions, where restructuring is a key feature, involve automatic, subconscious recombination of information. Electroencephalogram signals were analyzed in the space x time x frequency domain with a nonparametric cluster randomization test. First, we found strong gamma band responses at parieto-occipital regions which we interpreted as (i) an adjustment of selective attention (leading to a mental impasse or to a correct solution depending on the gamma band power level) and (ii) encoding and retrieval processes for the emergence of spontaneous new solutions. Secondly, we observed an increased upper alpha band response in right temporal regions (suggesting active suppression of weakly activated solution relevant information) for initially unsuccessful trials that after hint presentation led to a correct solution. Finally, for trials with high restructuring, decreased alpha power (suggesting greater cortical excitation) was observed in right prefrontal area.

CONCLUSIONS/SIGNIFICANCE

Our results provide a first account of cognitive insight by dissociating its constituent components and potential neural correlates.

Electrophysiological alterations during hypnosis for ego-enhancement: a preliminary investigation

EEG activity at the midfrontal (Fz) region was recorded during pre- and postbaselines, live hypnotic induction, arm levitation and progressive relaxation (PNR) deepening, and therapeutic ego-enhancing suggestions among 60 college student volunteers, previously screened with the Stanford Hypnotic Susceptibility Scale, Form C. Comparisons across conditions for delta, theta, alpha, and beta activity were made between low, moderate, high, and very high hypnotizable groups. Results indicated (a) significant increases in theta EEGs across the hypnosis process with a peak at PNR and a drop in theta thereafter to termination, with highs showing significantly more dramatic effects than moderates; (b) a similar inverted U-shaped pattern for beta EEGs across hypnosis conditions, with very highs significantly higher in beta power than moderates and lows, and with highs significantly higher than moderates; (c) general profile differences between the highs for theta and the highs and very highs for beta in comparison to the moderates and lows, with peak theta and beta power occurring during ego-enhancing suggestions for more highly hypnotizable participants; (d) a drop in alpha EEGs across the trance process with a return to baseline after hypnosis, with moderates showing significantly lower alpha power; and (e) an increase in delta power across conditions to PNR and then a decrease to post-hypnosis baseline, with moderates significantly lower than highs.

Developmental event-related gamma oscillations: effects of auditory attention

This study describes maturational changes in topographical patterns, stability, and functional reactivity of auditory gamma band (31-63 Hz) responses (GBRs) as brain electrical correlates relevant for cognitive development during childhood. GBRs of 114 healthy children from 9 to 16 years were elicited in an auditory focused attention task requiring motor responding to targets, and analyzed by means of the wavelet transform (WT). The effects of age and task variables (attended side and stimulus type relevance) were examined for GBR power and phase-locking within 120 ms after stimulation. Similar to the spontaneous gamma band power, the power and phase-synchronization of GBRs did not depend on the age. However, the functional reactivity of GBRs at specific locations changed in the course of development. In 9-12-year-old children, GBRs at frontal locations were larger and better synchronized to target than to nontarget stimulus type, and were larger over the left hemisphere (contralateral to the responding hand), thus manifesting sensitivity to external stimulus features and motor task. In 13-16-year-old adolescents, GBRs at parietal sites were enhanced by active attending to the side of stimulation, thus being associated with a maintenance of attentional focus to stimulus location. The results indicate that (i) specific aspects of task-stimulus processing engage distinct spatially localized gamma networks at functionally relevant areas, and (ii) the neuronal substrates of gamma band networks and the ability to synchronize them in relation to task-specific processes are available in all age groups from 9 to 16 years. However, the mode and efficiency with which gamma networks can be entrained depends on the age. This age-dependent reactivity of GBRs to different task variables may reflect a transition in processing strategies emerging at approximately 12-13 years in relation to the maturation of cognitive and executive brain functions.

Oscillatory Neuronal Responses in the Visual Cortex of the Awake Macaque Monkey

An important step in early visual processing is the segmentation of scenes. Features constituting individual objects have to be grouped together and segregated from those of other figures or the background. It has been proposed that this grouping could be achieved by synchronizing the fine temporal structure of responses from neurons excited by an individual figure. In the cat visual cortex evidence has been obtained that responses of feature-selective neurons have a distinctive oscillatory structure and can synchronize both within and across cortical areas, the synchronization depending on stimulus configuration. Here we investigate the generality of oscillatory responses and their synchronization and specifically whether these phenomena occur in extrastriate areas of the visual cortex of the awake behaving primate. We find in the caudal superior temporal sulcus of the macaque monkey (Macaca fascicularis) that adjacent neurons can synchronize their responses, in which case their discharges exhibit an oscillatory temporal structure. During such periods of local synchrony spatially separated cell groups can also synchronize their responses if activated with a single stimulus. These findings resemble those described previously for the cat visual cortex, except that in the awake monkey the oscillatory episodes tend to be of shorter duration and exhibit more variability of oscillation frequency.

Acute effect of nicotine on non-smokers: II. MLRs and 40-Hz responses

This paper is the second in a series of three investigating the role of cholinergic mechanisms in the auditory system by assessing the acute effects of nicotine, an acetylcholinomimetic drug, on aggregate responses within the auditory pathway. In a single-blind procedure, auditory responses were obtained from 20 normal-hearing, non-smokers (10 male) under two conditions (nicotine, placebo). The effects of nicotine on central, mesogenous responses of the auditory system (middle latency and 40-Hz responses) are described in this second paper. Results indicated that transdermal administration of nicotine to non-smokers does significantly affect the central, neural transmission of acoustic information. Na-Pa amplitude and Nb latency of the middle latency response and latency measures of the 40-Hz response were acutely altered by the presence of nicotine.

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.

Acute effect of nicotine on non-smokers: III. LLRs and EEGs

This paper is the last in a series of three investigating the role of cholinergic mechanisms in the auditory system by assessing the acute effects of nicotine, an acetylcholinomimetic drug, on aggregate responses within the auditory pathway. In a single-blind procedure, auditory responses were obtained from 20 normal-hearing, non-smokers (10 male) under two conditions (nicotine, placebo). The effects of nicotine on long-latency responses of the auditory system and on electroencephalograms are described in this paper. Results indicated that transdermal administration of nicotine to non-smokers significantly affects the afferent and efferent transmission of acoustic information, as well as enhancing cortical activation. Long-latency response amplitudes and electroencephalogram activity (dominant power and frequencies) were altered by acute doses of transdermal nicotine.

An electrocortical comparison of executed and rejected shots in skilled marksmen

Electroencephalographic (EEG) activity during the preshot period was investigated in seven skilled marksmen. Specifically, alpha and beta spectral power were obtained for the 4-s period prior to the execution or rejection of shots. Rejected shots were defined as those that resulted in the marksman's self-initiated decision to withdraw their rifle from the target rather than execute the shot. EEG activity during the preparatory period was contrasted between the executed and rejected shots to better understand the involved attentional processes associated with the preshot state. Results for rejected compared with executed shots revealed a progressive increase in alpha and beta power for rejected compared with executed shots, which increased across the preparatory period. Furthermore, increased spectral power was found in the left compared with the right hemisphere for both executed and rejected shots, and in the different regions of the scalp. Therefore, the decision to reject a shot seems to be characterized by inappropriate allocation of the neural resources associated with task execution.

Electroencephalographic imaging of higher brain function

High temporal resolution is necessary to resolve the rapidly changing patterns of brain activity that underlie mental function. Electroencephalography (EEG) provides temporal resolution in the millisecond range. However, traditional EEG technology and practice provide insufficient spatial detail to identify relationships between brain electrical events and structures and functions visualized by magnetic resonance imaging or positron emission tomography. Recent advances help to overcome this problem by recording EEGs from more electrodes, by registering EEG data with anatomical images, and by correcting the distortion caused by volume conduction of EEG signals through the skull and scalp. In addition, statistical measurements of sub-second interdependences between EEG time-series recorded from different locations can help to generate hypotheses about the instantaneous functional networks that form between different cortical regions during perception, thought and action. Example applications are presented from studies of language, attention and working memory. Along with its unique ability to monitor brain function as people perform everyday activities in the real world, these advances make modern EEG an invaluable complement to other functional neuroimaging modalities.

Temporal binding, binocular rivalry, and consciousness

Cognitive functions like perception, memory, language, or consciousness are based on highly parallel and distributed information processing by the brain. One of the major unresolved questions is how information can be integrated and how coherent representational states can be established in the distributed neuronal systems subserving these functions. It has been suggested that this so-called "binding problem" may be solved in the temporal domain. The hypothesis is that synchronization of neuronal discharges can serve for the integration of distributed neurons into cell assemblies and that this process may underlie the selection of perceptually and behaviorally relevant information. As we intend to show here, this temporal binding hypothesis has implications for the search of the neural correlate of consciousness. We review experimental results, mainly obtained in the visual system, which support the notion of temporal binding. In particular, we discuss recent experiments on the neural mechanisms of binocular rivalry which suggest that appropriate synchronization among cortical neurons may be one of the necessary conditions for the buildup of perceptual states and awareness of sensory stimuli.

Psychophysiological correlates of hypnosis and hypnotic susceptibility

This article reviews and summarizes electroencephalographic (EEG)-based research on physiological and cognitive indicators of hypnotic responding and hypnotic susceptibility, with special attention to the author's programmatic research in this area. Evidence that differences in attention levels may account for hypnotic depth and individual differences in hypnotizability is provided with traditional EEG rhythms, event-related potentials, and 40-Hz EEG activity. The alteration of stimulus perception may be a secondary effect with respect to allocation of attentional resources. In both nonhypnosis and hypnosis conditions, high hypnotizables appeared to show greater task-related EEG hemispheric shifts than did low hypnotizables. Findings concerning cognitive and physiological correlates of hypnotic analgesia are discussed with respect to hemispheric functioning in the apparent control of focused and sustained attention. The conclusion is that although a definitive EEG-based signature for hypnosis and hypnotizability is not yet established, there are a number of promising leads.

Early gamma response is sensory in origin: a conclusion based on cross-comparison of results from multiple experimental paradigms

The study investigates the functional correlates of the early, time-locked gamma response. The study utilized a unique experimental strategy which involved the utilization of a series of experimental paradigms to which all subjects (n = 20) were exposed to in the same recording session. These paradigms induced an increasingly complex configuration of processes for their respective task performance and also required different levels of attention allocation. In their order of administration, the paradigms were single stimulus (SS), mismatch negativity (MMN), evoked potential (EP), easy oddball (OB-EZ) and hard oddball (OB-HD). Auditory stimuli were used in the study (10 ms r/f time, 50 ms duration, 65 dB SPL) with the standards as 1000 Hz or 1900 Hz and deviants as 2000 Hz. The early gamma showed a frontocentral topography. The difference between Fz and Pz recording sites were statistically significant. A comparative analysis of the gamma responses showed that the gamma that was obtained at the early time-window of 0-150 ms as a time-locked activity occurred irrespective of experimental paradigm; the early gamma did not vary with the degree of task complexity or with attentional allocation. It was concluded from these findings that the early gamma is basically a sensory phenomenon. Various studies have previously shown that under perceptual/cognitive tasks, gamma response is obtained as a non-phase-locked activity in the late time-windows. These studies concluded that the gamma response is basically perceptual/cognitive in function. However, in these studies the early sensory gamma was also present in the data. Collectively taken, these findings may lead to the conclusion that the gamma response is a multifunctional phenomenon, with the early portion representing sensory and the late portion perceptual/cognitive processing.

40 Hz auditory steady-state response and EEG spectral edge frequency during sufentanil anaesthesia

PURPOSE

The auditory steady-state evoked response (ASSR) is an evoked potential which provides a sensitive measure of the effects of general anaesthetics on the brain. We used pharmacokinetic-pharmacodynamic (PK-PD) modelling to compare the effects of sufentanil on the amplitude of the ASSR with its effect on spectral edge frequency (SEF) of the electroencephalogram.

METHODS

Nine patients scheduled for elective cardiac surgery participated. Midazolam (70 micrograms.kg-1 i.m.) was given 60 min before entering the operating room. Anaesthesia was induced with 5 micrograms.kg-1 sufentanil at a rate of 0.83 microgram.kg-1.min-1. The ASSR, SEF and plasma sufentanil concentrations were measured for 30 min after induction of anaesthesia before surgery. The half-life between the central and effect site compartments (t1/2Keo), the 50% inhibitory concentration (IC50) and the slope factor (gamma) were computed.

RESULTS

The amplitude of the ASSR increased during the first three minutes of infusion of sufentanil by up to 40%. This was followed by a rapid decrease between the fourth and fifth minutes to 16% of baseline. The SEF decreased progressively during the first five minutes of infusion to 18% of baseline. Both measures subsequently showed modest recovery. The parameters gamma, IC50 and t1/2Keo for ASSR were (mean +/- SD) 6.0 +/- 3.7, 2.1 +/- 1.2 ng.ml-1 and 7.3 +/- 2.4 min. For SEF the values were 5.9 +/- 5.2, 1.4 +/- 0.7 ng.ml-1 (P < 0.05 compared with ASSR) and 6.8 +/- 2.4 min.

CONCLUSION

The sensitivity of ASSR to sufentanil is less than that of the SEF.

The string measure of the ERP: what does it measure?

The event-related brain potential (ERP) has been investigated extensively inan effort to understand the neurophysiological bases of intelligence. Measures derived from the ERP have been used as indices of intelligence, particularly the string measure of the complexity of the ERP. However, the string measure has been criticised for being non-specific and for being dependent on ERP amplitude. These criticisms were tested by investigating relationships between ERP string measure, ERP amplitude measures, and the ERP power spectrum. It was found that the string measure was non-specific in that it indexes both low and high frequency event-related activity; the string measure is also dependent on ERP amplitude. The string measure is therefore not a valid measure of the ERP. It was concluded that the string measure should be abandoned; human intelligence cannot map in a simple way onto gross measures of scalp-recorded electrocortical activity.

The transient 40-Hz response, mismatch negativity, and attentional processes in humans

1. Recent experimental studies on the neurophysiological basis of auditory selective attention and sensory memory forming the sensory-data basis for tuning the selective-attention system in humans are reviewed. 2. The results demonstrate that the transient 40-Hz response is enhanced by selective attention, attenuated in the course of long-term stimulation, but is not affected by changes in auditory stimuli. 3. Therefore, the 40-Hz response seems to be closely related to selective and sustained attention, whereas it does not seem to be associated with passive attention, as it does not reflect the detection of changes in auditory stimuli. 4. Changes in auditory stimulation are registered by pre-attentive sensory memory, indexed by the mismatch negativity (MMN), a change-specific component of the event-related potentials (ERPs). By this time, the transient 40-Hz response has already terminated. The magnitude of stimulus change is reflected in MMN latency. These latency changes predict changes in attentive reaction time (RT). 5. Thus, the pre-attentive memory mechanism seems to govern attentive detection of changes in the auditory environment. 6. It is concluded that the transient 40-Hz response is related to active attention and MMN is related to passive attention.

Global and serial neurons form A hierarchically arranged interface proposed to underlie memory and cognition

It is hypothesized that the cholinergic and monoaminergic neurons of the brain from a global network. What is meant by a global network is that these neurons operate as a unified whole, generating widespread patterns of activity in concert with particular electroencephalographic states, moods and cognitive gestalts. Apart from cholinergic and monoaminergic global systems, most other mammalian neurons relay sensory information about the external and internal milieu to serially ordered loci. These "serial" neurons are neurochemically distinct from global neurons and commonly use small molecule amino acid neurotransmitters such as glutamate or aspartate. Viewing the circuitry of the mammalian brain within the global-serial dichotomy leads to a number of novel interpretations and predictions. Global systems seem to be capable of transforming incoming sensory data into cognitive-related activity patterns. A comparative examination of global and serial systems anatomy, development and physiology reveals how global systems might turn sensation into mentation. An important step in this process is the permanent encoding of memory. Global neurons are particularly plastic, as are the neurons receiving global inputs. Global afferents appear to be capable of reorganizing synapses on recipient serial cells, thus leading to enhanced responding to a signal, in a particular context and state of arousal.

Distributed gamma band responses in the brain studied in cortex, reticular formation, hippocampus and cerebellum

The transient evoked responses of auditory cortex, reticular formation, hippocampus and cerebellar cortex to auditory stimulation have been analysed in the gamma frequency band on cats with chronically implanted electrodes. We found gamma band transient responses consisting of wave packets with 3-4 oscillations in all of the studied brain structures in the first 100 ms of the poststimulus period. The responses were strongly time-locked to the stimulation time point. The observation that the gamma band responses exist simultaneously in various brain structures supports the tentative proposal of our group on the "diffuse gamma response system" of the brain, which seems to be an important, universal operator in brain function. Furthermore, it shows that in search for generalized approaches to brain phenomena it is important to analyse the simultaneous behavior of different brain structures.

Rhythmically firing (20-50 Hz) neurons in monkey primary somatosensory cortex: activity patterns during initiation of vibratory-cued hand movements

The activity patterns of rhythmically firing neurons in monkey primary somatosensory cortex (SI) were studied during trained wrist movements that were performed in response to palmar vibration. Of 1,222 neurons extracellularly recorded in SI, 129 cells (approximately 11%) discharged rhythmically (at approximately 30 Hz) during maintained wrist position. During the initiation of vibratory-cued movements, neuronal activity usually decreased at approximately 25 ms after vibration onset followed by an additional decrease in activity at approximately 60 ms prior to movement onset. Rhythmically firing neurons are not likely to be integrate-and-fire neurons because, during activity changes, their rhythmic firing pattern was disrupted rather than modulated. The activity pattern of rhythmically firing neurons was complimentary to that of quickly adapting SI neurons recorded during the performance of this task (Nelson et al., 1991). Moreover, disruptions of rhythmic activity of individual SI neurons were similar to those reported previously for local field potential (LFP) oscillations in sensorimotor cortex during trained movements (Sanes and Donoghue, 1993). However, rhythmic activity of SI neurons did not wax and wane like LFP oscillations (Murthy and Fetz, 1992; Sanes and Donoghue, 1993). It has been suggested that fast (20-50 Hz) cortical oscillations may be initiated by inhibitory interneurons (Cowan and Wilson, 1994; Llinas et al., 1991; Stern and Wilson, 1994). We suggest that rhythmically firing neurons may tonically inhibit quickly adapting neurons and release them from the inhibition at go-cue onsets and prior to voluntary movements. It is possible that rhythmically active neurons may evoke intermittent oscillations in other cortical neurons and thus regulate cortical population oscillations.

Attention-related electroencephalographic and event-related potential predictors of responsiveness to suggested posthypnotic amnesia

Higher frequency electroencephalographic (EEG) activity around 40 Hz has been shown to play a role in cognitive functions such as attention. Furthermore, event-related brain potential (ERP) components such as N1 and P1 are sensitive to selective attention. In the present study, 40-Hz EEG measures and early ERP components were employed to relate selective attention to hypnotic response. Participants were 20 low hypnotizable individuals, half assigned as simulators, and 21 high hypnotizable individuals. Each of these groups was subsequently divided into two groups based on recognition amnesia scores. The four groups differed in 40-Hz (36-44 Hz) EEG spectral amplitude recorded during preinduction resting conditions but not in EEG amplitude postinduction. The groups also differed in N1 amplitudes recorded during hypnosis. Regression analysis revealed that these effects only distinguish the high hypnotizable participants who experienced recognition amnesia from all other groups. The findings support the role of selective attention in hypnotic responsiveness, and the utility of subdividing high hypnotizable individuals is discussed.

Event related dynamics of 40 Hz electroencephalogram during visual discrimination task

Visual priming tasks were presented to human subjects in two conditions: first, the warning stimulus contained information on the target stimulus, second, the warning stimulus only cued that a target was to come. The subjects had to respond as fast as possible to target stimuli with a certain button press. In both conditions 40 Hz EEG power showed reliably event related dynamics. Considering the differences between both conditions, 40 Hz power and motor response time effect sizes were found to be correlated 200 ms following warning stimuli. These results demonstrate that dynamics of 40 Hz activity are related to stimulus evaluation in sensorimotor processing during a visual discrimination task.

Characterization of state transitions in spatially distributed, chaotic, nonlinear, dynamical systems in cerebral cortex

The neurons of cerebral cortex are largely autonomous and generate activity that is manifested in trains of microscopic axonal action potentials. The neurons interact by sparse but numerous synaptic connections to generate macroscopic dendritic activity patterns that are observed in electroencephalographic (EEG) waves. The macroscopic patterns are constructed by the populations and they shape the output of cortical neurons in parallel arrays. Sensory cortexes receive sensory information in the form of microscopic action potentials, which induce state transitions in population dynamics. Each state transition transforms sensory information to perceptual meaning. The EEG reflects both kinds of activity. The sensory input is accessed by time ensemble averaging, whereas the perceptual output is found by spatial ensemble averaging. Spatial phase gradients in the EEG are useful for identifying EEG segments in a sequence of state transitions in response to sensory input. The rapidity and flexibility with which they take place give strong reason to postulate that the mechanism for the construction of these sequences of patterns is a dynamical system operating in a chaotic domain.

Event-related prolongation of induced EEG rhythmicities in experiments with a cognitive task

Since we consider evoked potentials (EPs) and event-related potentials (ERPs) as an ensemble of induced rhythmicities we investigated "sensory" evoked potentials (EPs) and "cognitive" event-related potentials (ERPs) with respect to their frequency components (around 5 Hz and around 10 Hz). "Sensory" EP sessions consisted in standard EP measurements with tone bursts of 2000 Hz frequency. Prior to the subsequent "cognitive" sessions, the subjects were instructed to expect frequent 2000 Hz tones and rare 1950 Hz tones, the rare tones thus being difficult to detect. A further instruction was to count the number of the "rare" stimuli. Not a single tone of different frequency, however, was presented. The single trials were then analyzed with respect to their 5 Hz and 10 Hz components. Time-locking was investigated using a modified single sweep wave identification (SSWI) method. Depending on features of the prestimulus EEG, it was observed that theta (4-7 Hz) and alpha (7-13 Hz) responses were prolonged in the case of "cognitive" sessions. We believe that the prolonged endogenous ERP components are induced rhythmicities triggered by additional information processing, when a high degree of uncertainty has to be resolved during our experimental conditions. For the analysis and visualization of these phenomena, the combination of digital filtering and SSWI showed relevant advantages in comparison to other methods.

Electrophysiological evidence of lateralized disturbances in children with learning disabilities

This study used an electrophysiological measurement operation to investigate lateralized processing deficits associated with academic learning-disability subtypes. Fast frequency EEG activity in the 36-44 hertz (Hz) band was recorded from reading-disabled (RLD), arithmetic-disabled (ALD), and nondisabled control children engaged in verbal and nonverbal cognitive tasks. The control group, but neither LD group, exhibited a task-dependent shift in lateralization of 40 Hz EEG; the RLD subjects generated proportionately less left-hemisphere 40 Hz activity than control or ALD subjects during the verbal task; and the ALD subjects generated proportionately less right-hemisphere activity than control or RLD subjects during the nonverbal task. These results indicate that lateralized processing deficits are associated with different types of disabilities, and provide external validation of learning-disability classifications based on academic performance patterns.

A compound P300-40 Hz response of the cat hippocampus

In companion reports (Başar-Eroglu et al., in press a,b) it was shown that the hippocampal P300 response had a frequency content with prominent theta enhancement, especially near CA3 region. Moreover, frequency analysis revealed a marked component in the 40 Hz frequency range. In the present study, a detailed account about "40 Hz response accompanied by the N200-P300 compound response" is given. These results were obtained by use of digital adaptive filtering method described in previous studies (Başar et al., 1987, Başar, 1980). The physiological interpretation of the visual 40 Hz component gained tremendous importance since the discovery of Gray and Singer (1987, 1989) of 40 Hz oscillations in the visual cortex. Our analysis shows clearly that the 40 Hz response component is not only a cortical response pattern. Moreover, the 40 Hz response is accompanied by a P300 response and we discuss that it is also a universal building block of brain responsiveness. The 40 Hz response should be correlated with multimodal sensory and cognitive function.