Cortical Cell Assemblies Research Articles

EFFECTS OF DISTURBING AUDITORY NOISE ON THE COMPLEXITY OF ELECTROCORTICAL ACTIVITY

The effects of disturbing noise on the nonlinear electrical brain (EEG) dynamics were investigated in a sample of 12 healthy volunteers. The prediction that a high intensity noise increases the dimensional complexity of brain activity as compared to a low intensity noise, was confirmed using a deterministic chaos algorithm. Further, there was a tendency for rest conditions to have a higher dimensional complexity than the condition in which the low-intensity noise was presented. Although a Fourier-analysis of the EEG revealed a difference in the alpha power between the rest condition at the end of the experiment and the noise conditions, no effect of noise intensity on the EEG power spectrum was obtained. The data indicate that the disturbing effect of a loud noise results in the recruitment of additional cortical networks and thus increases the complexity of cortical cell assemblies.

The influence of semantic priming on event-related potentials to painful laser-heat stimuli in humans

In this study we investigated the effects of different semantic primes on the processing of painful stimuli. For prime stimuli, descriptors of three categories were used: somatosensory pain-related, affective pain-related, and neutral adjectives. While subjects ( n=10) processed these primes, a painful laser-heat stimulus was applied. Laser-evoked potentials (LEPs) were recorded and pain intensity ratings were obtained after each single laser stimulus. Painful stimuli applied while subjects processed pain-related primes (affective and somatosensory adjectives) resulted in larger LEP amplitudes at 370 ms post laser stimulus compared to amplitudes of laser-evoked activities while subjects processed neutral primes ( F (2,18)=3.90, P=0.05). It is suggested that pain-related semantic primes might preactivate neural networks subserving pain memory and pain processing. The processing of pain-related primes seems to preactivate cortical cell-assemblies involved in the processing of the succeeding painful laser stimuli.

Enhanced dimensional complexity of the EEG during memory for personal pain in chronic pain patients

Associative connections between cortical cell assemblies representing pain-related memories should be stronger and more extensive in subjects with chronic pain. To test this hypothesis, the dimensional complexity of the electroencephalograph (EEG) was examined during the actual experience as well as during memory for pain. Nine chronic pain patients and nine matched healthy controls participated in the study. During acute pain induction, acute pain recall, personal stress and pain recall, the EEG was recorded from 15 scalp sites. Non-linear analysis, based on the theory of deterministic chaos, revealed higher and more widespread EEG complexity in the patients compared to the healthy controls only during the recall of the personal pain scene. The personal stress scene was rated equally aversive but did not induce more EEG complexity. These more extensive and more readily accessible pain memories may be instrumental for the persistence of chronic pain.

Slow negative brain potentials as reflections of specific modular resources of cognition

The paper summarizes a series of experiments in which the topography and the amplitude of slow event-related brain potentials (slow ERPs) was studied in cognitive tasks which imposed different amounts of load on functionally distinct processing modules. The results suggest that the topography of slow ERPs reflects the relative activation/inactivation of distinct cortical cell assemblies while the absolute amplitude of the negative maximum seems to reflect how much a particular cell assembly is activated at a particular time. Translated into the terminology of cognitive resource theories one could say that tasks which evoke distinct slow wave patterns draw on independent resources. Likewise, the amplitude of a slow wave pattern could be related to the construct of resource allocation, i.e. the larger the amplitude of a slow wave pattern the more resources of a particular type are allocated to a task. These findings are discussed with respect to possible generator mechanisms of slow waves and in relation to possible causes of capacity limitations of the human information processing system.

Topography of brain electrical activity dissociates the retrieval of spatial versus verbal information from episodic long-term memory in humans

Topography and amplitude of slow event-related potentials (ERPs) of the electroencephalogram (EEG) were studied during acquisition and recall of spatial and verbal associations. Subjects learned associations between line drawings and two types of mediators. The latter were either positions in a grid or concrete nouns. In a cued recall test subjects had to decide whether two drawings were linked to each other or not via an associated position or noun. The topography of slow ERPs 1–4 s after stimulus presentation obtained from 18 scalp electrodes dissociated the memory processes: The maximum potential was found over the parietal cortex with spatial and over the left frontal cortex with verbal information. The same topographic pattern emerged during both anticipation learning and cued recall. Moreover, the amplitude at the topographic maximum increased when more associations had to be retrieved. These results are compatible with the idea that memory representations are reactivated in localized cortical cell assemblies specialized for particular codes.

Memory processes, brain oscillations and EEG synchronization

This article tries to integrate results in memory research from divergent disciplines such as cognitive psychology, neuroanatomy, and neurophysiology. The integrating link is seen in more recent findings that provide strong arguments for the assumption that oscillations are a basic form of communication between cortical cell assemblies. It is assumed that synchronous oscillations of large cell assemblies — termed type 1 synchronization — reflect a resting state or possibly even a state of functional inhibition. On the other hand, during mental activity, when different neuronal networks may start to oscillate with different frequencies, each network may still oscillate synchronously (this is termed type 2 synchronization), but as a consequence, the large scale type 1 oscillation disappears. It is argued that these different types of synchronization can be observed in the scalp EEG by calculating event-related power changes within comparatively narrow but individually adjusted frequency bands. Experimental findings are discussed which support the hypothesis that short-term (episodic) memory demands lead to a synchronization (increase in band power) in the theta band, whereas long-term (semantic) memory demands lead to a task-specific desynchronization (decrease or suppression of power) in the upper alpha band. Based on these and other findings, a new memory model is proposed that is described on three levels: cognitive, anatomical and neurophysiological. It is suggested that short-term (episodic) memory processes are reflected by oscillations in an anterior limbic system, whereas long-term (semantic) memory processes are reflected by oscillations in a posterior-thalamic system. Oscillations in these frequency bands possibly provide the basis for encoding, accessing, and retrieving cortical codes that are stored in the form of widely distributed but intensely interconnected cell assemblies.

Age increases brain complexity

This study investigated age-related changes in the human brain function using both traditional EEG analysis (power spectra) and the correlation dimension, a measure reflecting the complexity of EEG dynamics and, probably, the complexity of neurophysiological processes generating the EEG. Assuming that the accumulation of individual experience is determined by the formation of functionally related groups of neurons showing a repetitive synchronous activation (cell assemblies), an increase in the number of such independently oscillating cortical cell assemblies can be expected, despite a decline of some metabolic and memory functions with normal ageing. Thus, the ‘wisdom of old age’ may find its neurophysiological basis in greater complexity of brain dynamics compared to young ages. The experimental hypothesis was that EEG dimension steadily increases with age. In order to test this hypothesis the resting EEGs of 5 age groups from 7 to 60 were analysed. The results confirm the hypothesis: after a jump in the brain dynamics complexity during puberty a linear increase with age is observed. During maturation (7–25 years), the maximum gain in complexity occurs over the frontal associative cortex.

The Concept of Transcortical Cell Assemblies: a Key to the Understanding of Cortical Lateralization and Interhemispheric Interaction

PULVERMÜLLER, F AND MOHR, B. Transcortical cell assemblies: a key to the understanding of cortical lateralization and interhemispheric interaction. NEUROSCI BEHAV REV 20(4) 557–566, 1996.—According to Hebb, elements of higher cognitive processes, such as concepts, words and mental images, are realized in the brain as cortical cell assemblies, i.e. large and strongly connected neuron populations that form functional units. Neurons belonging to such assemblies may be scattered over wide cortical areas, and some cell assemblies may even comprise neurons of both hemispheres (transcortical assemblies). If full activation (ignition) of an assembly leads to fast circulation of neuronal activity in the assembly, this process should be visible in high-frequency cortical responses. Some evidence will be reviewed that cell assembly ignition indeed leads to changes in high-frequency cortical responses which can be recorded in the EEG and MEG. Within the cell assembly-framework, the question of cortical laterality translates into the question of how neurons of transcortical assemblies are balanced between the hemispheres. This approach allows for different degrees of laterality. Recent evidence is summarized that the degree of laterality indeed differs between language units. For example, the cortical representation of certain words appears to be strongly lateralized to the left hemisphere while those of others are less lateralized. If neurons of both hemispheres are part of one assembly bihemispheric processing should lead to a processing advantage compared to processing in the dominant hemisphere alone. The latter appears to be the case for lexical processing, as revealed by recent behavioral studies. In conclusion, the cell assembly-framework suggests a more fine-grained description of the issue of cortical laterality: it is not appropriate to ask whether “modules” supporting higher cortical functions are located either in the left or right hemisphere. Rather, it appears fruitful to ask how the neurons of transcortical cell assemblies are balanced between the hemispheres. Copyright © 1996 Elsevier Science Ltd.

Representation of a species-specific vocalization in the primary auditory cortex of the common marmoset: temporal and spectral characteristics.

1. The temporal and spectral characteristics of neural representations of a behaviorally important species-specific vocalization were studied in neuronal populations of the primary auditory cortex (A1) of barbiturate-anesthetized adult common marmosets (Callithrix jacchus), using both natural and synthetic vocalizations. The natural vocalizations used in electrophysiological experiments were recorded from the animals under study or from their conspecifics. These calls were frequently produced in vocal exchanges between members of our marmoset colony and are part of the well-defined and highly stereotyped vocal repertoire of this species. 2. The spectrotemporal discharge pattern of spatially distributed neuron populations in cortical field A1 was found to be correlated with the spectrotemporal acoustic pattern of a complex natural vocalization. However, the A1 discharge pattern was not a faithful replication of the acoustic parameters of a vocalization stimulus, but had been transformed into a more abstract representation than that in the auditory periphery. 3. Subpopulations of A1 neurons were found to respond selectively to natural vocalizations as compared with synthetic variations that had the same spectral but different temporal characteristics. A subpopulation responding selectively to a given monkey's call shared some but not all of its neuronal memberships with other individual-call-specific neuronal subpopulations. 4. In the time domain, responses of individual A1 units were phase-locked to the envelope of a portion of a complex vocalization, which was centered around a unit's characteristic frequency (CF). As a whole, discharges of A1 neuronal populations were phase-locked to discrete stimulus events but not to their rapidly changing spectral contents. The consequence was a reduction in temporal complexity and an increase in cross-population response synchronization. 5. In the frequency domain, major features of the stimulus spectrum were reflected in rate-CF profiles. The spectral features of a natural call were equally or more strongly represented by a subpopulation of A1 neurons that responded selectively to that call as compared with the entire responding A1 population. 6. Neuronal responses to a complex call were distributed very widely across cortical field A1. At the same time, the responses evoked by a vocalization scattered in discrete cortical patches were strongly synchronized to stimulus events and to each other. As a result, at any given time during the course of a vocalization, a coherent representation of the integrated spectrotemporal characteristics of a particular vocalization was present in a specific neuronal population. 7. These results suggest that the representation of behaviorally important and spectrotemporally complex species-specific vocalizations in A1 is 1) temporally integrated and 2) spectrally distributed in nature, and that the representation is carried by spatially dispersed and synchronized cortical cell assemblies that correspond to each individual's vocalizations in a specific and abstracted way.

Exploring memory functions by means of brain electrical topography: a review.

A series of experiments is reviewed which explored whether the functional brain state of long-term memory retrieval is correlated with specific changes in slow, DC-like event-related brain potentials. The main results are: (1) Retrieving associations from long-term memory is accompanied by a slow negative shift of 5-10 microV which prevails about as long as the retrieval process lasts, i.e., in our experiments, for a period of several seconds: (2) When different types of representations have to be reactivated in memory the slow negative wave shows a clearly distinct topography. The maximum was found in a verbal condition over the left frontal, in a spatial condition over the parietal, and in a color condition over the right occipital to temporal cortex. All these conditions were completely equivalent with respect to the established associative structure, the learning procedure, and the performance criterion. (3) The amplitude of the topographic maximum increases with the number of representations which have to be reactivated. This effect is not due to a non-specific increase of effort but specifically related to the number of activated episodic memory contents which had been experimentally established. In contrast, the reactivation of a priori given semantic association did not become manifest in a specific slow wave effect. These findings are compatible with the idea that memory retrieval implies a reactivation of those cortical cell assemblies in the cortex in which the constituting features of a mnestic entity had originally been processed during perception and learning. The results are also discussed with respect to the possible advantages of EEG and MEG recordings for a cognitive psychophysiology in comparison to other brain imaging techniques as PET or fMRI.

Agrammatism: Behavioral Description and Neurobiological Explanation

Abstract Subjects with brain damage resulting in agrammatic aphasia frequently omit or substitute function items (function words and inflectional affixes). However, they show only mild deficits in using meaningful content words. Agrammatics' performance on comprehension tests reveals a rather complex pattern. They usually understand active sentences correctly, but perform on chance level on passives. The same contrast is observed for more complex sentence types, such as subject vs. object clefts or relatives. This complex comprehension pattern suggests that agrammatism is a syntactic disturbance that selectively affects particular sentence structures. Nevertheless, it is possible that both the production and comprehension patterns go back to an access problem for grammatical morphemes. If an agrammatic aphasic comprehends only 50% of these morphemes, she or he may well be problem-free in understanding the resulting "pruned" active sentences in which some of the grammatical morphemes are missing. In contrast, she or he is likely to arrive at the wrong meaning when confronted with "pruned" passives. This hypothesis raises the question of how fully competent speakers interpret ungrammatical "pruned" strings derived from well-formed sentences by systematically deleting function items. Experimental data demonstrate that competent speakers approximate the agrammatic comprehension pattern when being presented with "pruned" strings. This argues that agrammatism can, indeed, be viewed as a deficit in processing function words and inflectional affixes, which may manifest itself in production and/or comprehension tasks. Assuming that cortical cell assemblies with distinct topographies correspond to content words and grammatical morphemes, it is possible to explain agrammatism in terms of lesions to these neuronal networks. This neurobiological model can explain additional aspects of agrammatics' performance.

Words and pseudowords elicit distinct patterns of 30-Hz EEG responses in humans

Meaningful words, such as moon, and physically similar but meaningless pseudowords, such as noom, were presented visually in a lexical decision task. The EEG was recorded from 17 scalp electrodes. Significant differences between both stimulus classes were observed in evoked spectral responses of the ‘γ-band’ ∼30 Hz. A hemisphere by wordness interaction demonstrated that 30-Hz spectral power over the left hemisphere was reduced after pseudowords only. These results indicate that γ-band responses reflect the different cognitive processes induced by words and pseudowords. A possible explanation is the following. Synchronous activation of large cortical cell assemblies takes place after word presentation but not after presentation of pseudowords.

Cortical Cell Assemblies: A Possible Mechanism for Motor Programs

The concept of a motor program has been used to interpret a diverse range of empirical findings related to preparation and initiation of voluntary movement. In the absence of an underlying mechanism, its exploratory power has been limited to that of an analogy with running a stored computer program. We argue that the theory of cortical cell assemblies suggests a possible neural mechanism for motor programming. According to this view, a motor program may be conceptualized as a cell assembly, which is stored in the form of strengthened synaptic connections between cortical pyramidal neurons. These connections determine which combinations of corticospinal neurons are activated when the cell assembly is ignited. The dynamics of cell assembly ignition are considered in relation to the problem of serial order. These considerations lead to a plausible neural mechanism for the programming of movements and movement sequences that is compatible with the effects of precue information and sequence length on reaction times. Anatomical and physiological guidelines for future quantitative models of cortical cell assemblies are suggested. By taking into account the parallel re-entrant loops between the cerebral cortex and basal ganglia, the theory of cortical cell assemblies suggests a mechanism for motor plans that involve longer sequences. The suggested model is compared with other existing neural network models for motor programming.

Cortical plasticity and memory

The roles of extrinsically modulated, plastic Hebb-like synapses and dynamic cortical cell assemblies underlying cortical plasticity in learning and memory operations are described. From our understanding of the distributed form of representation of learned behaviors in somatosensory and auditory cortical fields, and given new findings about the nature and distribution of responses representing learned and remembered stimuli in the inferior temporal cortex, a hypothetical picture of the cortical engram representing learned behaviors and memories is posited.

Corticostriatal interactions in neuromotor programming

Considerable evidence suggests that the neostriatum contributes to certain aspects of motor programming. However, the neostriatum does not appear to be a site at which the motor programmes are stored. A likely site for the storage of motor programmes is the cerebral cortex, especially areas which receive input from the neostriatum via the globus pallidus and thalamus. A motor programme may be conceptualized as a cell assembly, which is stored in the form of strengthened synaptic connections between cortical pyramidal neurones. These connections constrain the combinations of movements and the sequence that will result when the cell assembly is activated. This model for a motor programme may be used to explain, in terms of the dynamical behaviour of cortical cell assemblies, how a specific goal can consistently be attained by patterns of muscular activity that are highly variable. The contribution of the corticostriatal interactions may be ensure that cortical activity is attracted towards a state which corresponds to the goal of the movement.

Formation of Cortical Cell Assemblies

Formation of Cortical Cell Assemblies