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Neurocomputational models of working memory.
During working memory tasks, the firing rates of single neurons recorded in behaving monkeys remain elevated without external cues. Modeling studies have explored different mechanisms that could underlie this selective persistent activity, including recurrent excitation within cell assemblies, synfire chains and single-cell bistability. The models show how sustained activity can be stable in the presence of noise and distractors, how different synaptic and voltage-gated conductances contribute to persistent activity, how neuromodulation could influence its robustness, how completely novel items could be maintained, and how continuous attractor states might be achieved. More work is needed to address the full repertoire of neural dynamics observed during working memory tasks.
Synchrony is stubborn in feedforward cortical networks.
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Synchrony does not promote grouping in temporally structured displays.
It has been proposed that the human visual system can use temporal synchrony to bind image regions into unified objects, as proposed in some neural models. Here we present experimental results from a new dynamic stimulus suggesting that previous evidence for this hypothesis can be explained with the well-established mechanisms of early visual processing, thus obviating the need to posit new synchrony-sensitive grouping mechanisms (see also ref. 5 for a critique of the binding by neural synchrony hypothesis).
High times for memory: cannabis disrupts temporal coordination among hippocampal neurons.
Exogenous cannabinoid receptor agonists impair hippocampus-dependent learning and decrease the power of hippocampal electroencephalographic activity. A new paper shows that cannabinoids desynchronize neuronal assemblies without affecting average firing rates, and that this effect correlates with memory deficits in individuals.
Synchrony and covariation of firing rates in the primary visual cortex during contour grouping.
The visual system imposes structure onto incoming information, by grouping image elements of a single object together, and by segregating them from elements that belong to other objects and the background. One influential theory holds that the code for grouping and segmentation is carried by the synchrony of neuronal discharges on a millisecond time scale. We tested this theory by recording neuronal activity in the primary visual cortex (area V1) of monkeys engaged in a contour-grouping task. We found that synchrony was unrelated to contour grouping. The firing rates of V1 neurons are also correlated across trials. We demonstrate that this rate covariation is mainly determined by fluctuations in visual attention. Moreover, we show that rate covariation depends on perceptual grouping, as it is strongest between neurons that respond to features of the same object.
Cannabinoids reveal importance of spike timing coordination in hippocampal function.
Cannabinoids impair hippocampus-dependent memory in both humans and animals, but the network mechanisms responsible for this effect are unknown. Here we show that the cannabinoids Delta(9)-tetrahydrocannabinol and CP55940 decreased the power of theta, gamma and ripple oscillations in the hippocampus of head-restrained and freely moving rats. These effects were blocked by a CB1 antagonist. The decrease in theta power correlated with memory impairment in a hippocampus-dependent task. By simultaneously recording from large populations of single units, we found that CP55940 severely disrupted the temporal coordination of cell assemblies in short time windows (<100 ms) yet only marginally affected population firing rates of pyramidal cells and interneurons. The decreased power of local field potential oscillations correlated with reduced temporal synchrony but not with firing rate changes. We hypothesize that reduced spike timing coordination and the associated impairment of physiological oscillations are responsible for cannabinoid-induced memory deficits.
Hippocampal noradrenergic neurotransmission in concurrent EEG desynchronization and inhibition of penile erection induced by cocaine in the rat.
We previously reported that cocaine may induce activation of cortical (cEEG) and hippocampal (hEEG) electroencephalographic signals, concurrent with inhibition of penile erection, via an action on the hippocampal formation. The present study further evaluates the role of noradrenergic neurotransmission at the hippocampal formation in this process, using adult, male Sprague-Dawley rats anaesthetized and maintained by chloral hydrate. Unilateral microinjection of cocaine (100 nmoles) into the hippocampal CA1 or CA3 subfield or dentate gyrus elicited significant activation of both cEEG and hEEG activity. At the same time, the intracavernous pressure (ICP), our experimental index for penile erection, underwent a discernible reduction. Co-administration of equimolar doses (250 pmoles) of prazosin, naftopidil, yohimbine or rauwolscine significantly reversed those effects elicited by cocaine on cEEG, hEEG and ICP. Microinjection unilaterally of equimolar doses (5 nmoles) of norepinephrine, phenylephrine or BHT 933 into the hippocampal formation, similar to cocaine, also induced appreciable cEEG and hEEG excitation, with a simultaneous decrease in ICP. We conclude that cocaine may activate cEEG and hEEG and decrease ICP via noradrenergic neurotransmission, possibly engaging at least alpha(1A/D)-, alpha(2B)- and alpha(2C)-adrenoceptors at the hippocampal formation.
Levels of brain wave activity (8-13 Hz) in persons with spinal cord injury.
STUDY DESIGN: Brain wave activity in people with spinal cord injury (SCI) was compared to brain wave activity in able-bodied controls. OBJECTIVES: To investigate whether a spinal injury results in changes in levels of brain wave activity in the 8-13 Hz spectrum of the electroencephalography (EEG). SETTING: Sydney, Australia. METHODS: Monopolar, multichannel EEG assessment was completed for 20 persons with SCI and 20 able-bodied, sex- and age-matched controls. A total of 14 channels of EEG were measured across the entire scalp for all participants. Comparisons between the able-bodied and SCI participants were made across the frontal, central, parietal, occipital and temporal regions. Comparisons were also made for impairment level, that is, between participants with tetraplegia and paraplegia. RESULTS: Compared to the able-bodied controls, consistently reduced brain wave activity (measured by magnitude and peak amplitude) in the 8-13 Hz component of the EEG occurred in persons with SCI across all regions and sites, and differences were larger in the central, parietal and occipital sites. The SCI group also had consistently lower frequencies than the able-bodied controls. Furthermore, the subgroup of SCI participants with tetraplegia generally had significantly reduced brain activity (magnitude and peak amplitude) compared with the paraplegic subgroup and able-bodied controls. CONCLUSIONS: The findings of this research enhance our understanding of changes in brain wave activity that could be associated with deafferentation that occurs following SCI, as well as provide essential data on the potential of SCI persons to use a 'hands free' environmental control system that is based upon 8-13 Hz brain activity.
The electrophysiologic assessment of hemispheric asymmetries during speech.
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Hemispheric-synchronisation during anaesthesia: a double-blind randomised trial using audiotapes for intra-operative nociception control.
The possible antinociceptive effect of hemispheric-synchronised sounds, classical music and blank tape were investigated in patients undergoing surgery under general anaesthesia. The study was performed on 76 patients, ASA 1 or 2, aged 18-75 years using a double-blind randomised design. Each of the three tapes was allocated to the patients according to a computer-generated random number table. General anaesthesia was standardised and consisted of propofol, nitrous oxide 66%/oxygen 33%, isoflurane and fentanyl. Patients breathed spontaneously through a laryngeal mask and the end-tidal isoflurane concentration was maintained near to its minimum alveolar concentration value of 1.2%. Fentanyl was given intravenously sufficient to keep the intra-operative heart rate and arterial blood pressure within 20% of pre-operative baseline values and the fentanyl requirements were used as a measure of nociception control. Patients to whom hemispheric-synchronised sounds were played under general anaesthesia required significantly less fentanyl compared with patients listening to classical music or blank tape (mean values: 28 microgram, 124 microgram and 126 microg, respectively) (p < 0.001). This difference remained significant when regression analysis was used to control for the effects of age and sex.
Sleep in seasonal affective disorder patients in forced desynchrony: an explorative study.
The majority of winter-type seasonal affective disorder (SAD) patients complain of hypersomnia and daytime drowsiness. As human sleep is regulated by the interaction of circadian, ultradian and homeostatic processes, sleep disturbances may be caused by either one of these factors. The present study focuses on homeostatic and ultradian aspects of sleep regulation in SAD. Sleep was recorded polysomnographically in seven SAD patients and matched controls subjected to a 120-h forced desynchrony protocol. In time isolation, subjects were exposed to six 20-h days, each comprising a 6.5-h period for sleep. Patients participated while being depressed, while remitted after light therapy and in summer. Controls were studied in winter and in summer. In each condition, the data of each subject were averaged across all recordings. Thus, the influence of the effects of the circadian pacemaker on sleep was excluded mathematically. The comparison of patients with controls and with themselves in the various conditions revealed no abnormalities in homeostatic parameters: sleep stage variables, relative power spectra and time courses of power in various frequency bands across the first three non-rapid eye movement-rapid eye movement (NREM-REM) cycles showed no differences. The data suggest that homeostatic processes are not involved in the disturbance of sleep in SAD.
Nocturnal sleep spindle activity in blind subjects.
To determine spindle activity during sleep in congenitally blind subjects, electroencephalogram sleep recordings were done on two or three consecutive nights in five subjects aged in their 20s and 30s. The number per minute and the duration of sleep spindles, scored visually, were compared with the data of sighted persons of comparable ages. The results indicated that the total number of sleep spindles a night was ranged from 117 to 585, and the number per minute ranged 0.52 to 2.06 during stage 2. Both values were much less than the values of sighted persons.
Effects of microdialysis application of monoamines on the EEG and behavioural states in the cat mesopontine tegmentum.
The peri-locus coeruleus alpha (peri-LCalpha) of the mediodorsal pontine tegmentum contains cholinergic and non-cholinergic neurons, and is critically implicated in the regulation of both wakefulness and paradoxical sleep (PS). The peri-LCalpha receives dense monoaminergic (adrenergic, noradrenergic, serotonergic, dopaminergic and histaminergic) afferent projections, but little is known about their exact roles in the control of sleep-wake cycles. We have therefore examined the in vivo effects of microdialysis application of monoamines to the peri-LCalpha and adjacent cholinergic and non-cholinergic tegmental structures on behavioural states and the electroencephalogram (EEG) in freely moving cats. Norepinephrine, epinephrine and dopamine selectively inhibited PS and induced PS without atonia when applied to the caudal part of the peri-LCalpha, which mainly contains non-cholinergic descending neurons, whereas histamine and serotonin had no effect at this site. In the rostral part of the peri-LCalpha and the adjacent X area (nucleus tegmenti pedunculopontinus, pars compacta), which contain many ascending cholinergic neurons, norepinephrine and epinephrine suppressed PS with a significant increase in waking and a decrease in slow-wave sleep, as expressed by a marked decrease in the power of the cortical and hippocampal delta (0.5-2.5 Hz) and cortical alpha (8-14 Hz) bands, and an increase in the cortical gamma (30-60 Hz) band. At these sites, histamine had similar waking and EEG-desynchronizing effects, but never suppressed PS, while dopamine and serotonin had no effect. These findings indicate a special importance of the adrenergic, noradrenergic and dopaminergic systems in the inhibitory or permissive mechanisms of PS, and of the adrenergic, noradrenergic and histaminergic systems in the control of behavioural and EEG arousal.
Enhancing cognitive performance with repetitive transcranial magnetic stimulation at human individual alpha frequency.
We applied rapid-rate repetitive transcranial magnetic stimulation (rTMS) at individual alpha frequency (IAF) to improve cognitive performance by influencing the dynamics of alpha desynchronization. Previous research indicates that a large upper alpha power in a reference interval preceding a task is related to both large suppression of upper alpha power during the task and good performance. Here, we tested the hypothesis that rTMS at individual upper alpha frequency (IAF + 1 Hz) can enhance alpha power in the reference interval, and can thus improve task performance. Repetitive TMS was delivered to the mesial frontal (Fz) and right parietal (P6) cortex, and as sham condition with 90 degrees-tilted coil (P6 position). The behavioural effect was assessed in a mental rotation task. Further control conditions were rTMS at a lower IAF (IAF - 3 Hz) and at 20 Hz. The results indicate that rTMS at IAF + 1 Hz can enhance task performance and, concomitantly, the extent of task-related alpha desynchronization. This provides further evidence for the functional relevance of oscillatory neuronal activity in the alpha band for the implementation of cognitive performance.
The integration of cortical and behavioural dynamics during initial learning of a motor task.
Here we test the hypothesis that frequency and topographically specific changes in the strength of functional cortico-cortical coupling occur during the acquisition of a completely new task. To this end we studied the behavioural and cortical dynamics of a bimanual multifrequency coordination pattern during which one hand moved at twice the frequency of the other hand. This pattern represents a noninherent assignment and necessitates training before appropriate interlimb decoupling takes place. Results showed that acquisition of the multifrequency task was associated with an improved behavioural output that matched specific changes in the electroencephalogram dynamics. In particular, practice of the coordination pattern was accompanied by a decrease in coherence between the primary sensorimotor regions, and over the midline area in the alpha and beta bands, respectively, along with an increase in functional interhemispheric coupling between the prefrontal areas in the gamma band. These data suggest that the strength of cortico-cortical connectivity is adaptively modified across regions and across frequencies during early learning as the functional couplings are created and optimized for the purpose of movement execution.
Histaminergic facilitation of electrocorticographic activation: role of basal forebrain, thalamus, and neocortex.
The neuromodulator histamine plays an important role in the regulation of behavioural state and the neocortical electrocorticogram (ECoG). With the present experiments, we characterized the anatomical targets that mediate the cortical-activating effects of histamine. Urethane-anaesthetized rats displayed continuous large-amplitude, low-frequency oscillations with a maximal spectral power in the delta (0.5-3.9 Hz) frequency band. Electrical (100 Hz) stimulation of the pontine-tegmentum suppressed slow, large-amplitude oscillations and induced ECoG activation. Application of histamine (1 mm) into the basal forebrain cholinergic complex by reverse microdialysis enhanced ECoG activation elicited by tegmental stimulation without changing resting ECoG activity. Ventrolateral or central thalamic application of histamine had no effect on resting ECoG activity, and ventrolateral thalamic application produced only a slight enhancement of brainstem-induced activation. Neocortical application of histamine in close proximity (< 500 micro m) to the recording electrode reduced low-frequency delta power in the resting ECoG without affecting stimulation-induced ECoG activation. These data suggest that, under the present experimental conditions, histamine facilitates ECoG activation primarily by potentiating the excitatory influence of brainstem fibers at the level of the basal forebrain. Histamine release in some parts of the thalamus results in a minor enhancement of ECoG activation, and cortical histamine release produces a small but consistent suppression of slow delta oscillations in the resting ECoG. These concurrent subcortical and cortical actions probably permit histamine to effectively modulate cortical activation and excitability across different behavioural states.
Changes in rolandic mu rhythm during observation of a precision grip.
We recorded 128-channel EEG from 16 participants while they observed, imitated, and self-initiated the precision grip of a manipulandum. Mu rhythm amplitudes were significantly lower during observation of a precision grip than during observation of a simple hand extension without object interaction. Scalp topographies for subtractions of observation, imitation, and execution conditions from the control condition showed a high degree of congruence, supporting the notion of a human observation-execution matching system. Surface Laplacian transformations suggest that the decrease in mu amplitude during precision grip observation reflects desynchronization of mu rhythm generators in the sensorimotor cortex. These results support the hypothesis that sensorimotor cortex is a neural substrate involved in the representation of both self- and other-generated actions and show the mu rhythm is sensitive to subtle changes in observed motor behavior.