Synchrony among multicortical EEGs 2 Freeman, Gaál & Jörnsten Information transfer and integration among functionally distinct areas of cerebral cortex of oscillatory activity requires some degree of phase synchrony of the trains of action potentials that carry the information prior to the integration. However, propagation delays are obligatory. Delays vary with the lengths and conduction velocities of the axons carrying the information, causing phase dispersion. In order to determine how synchrony is achieved despite dispersion, we recorded EEG signals from multiple electrode arrays on five cortical areas in cats and rabbits, that had been trained to discriminate visual or auditory conditioned stimuli. Analysis by time-lagged correlation, multiple correlation and PCA, showed that maximal correlation was at zero lag and averaged.7, indicating that 50 % of the power in the gamma range among the five areas was at zero lag irrespective of phase or frequency. There were no stimulus-related episodes of transiently increased phase locking among the areas, nor EEG "bursts " of transiently increased amplitude above the sustained level of synchrony. Three operations were identified to account for the sustained correlation. Cortices broadcast their outputs over divergent-convergent axonal

Population activity in cortico-basal ganglia circuits is synchronized at different frequencies according to brain state. However, the structures that are likely to drive the synchronization of activity in these circuits remain unclear. Furthermore, it is not known whether the direction of transmission of activity is fixed or dependent on brain state. We have used the directed transfer function (DTF) to investigate the direction in which coherent activity is effectively driven in cortico-basal ganglia circuits. Local field potentials (LFPs) were simultaneously recorded in the subthalamic nucleus (STN), globus pallidus (GP) and substantia nigra pars reticulata (SNr), together with the ipsilateral frontal electrocorticogram (ECoG) of anaesthetized rats. Directional analysis was performed on recordings made during robust cortical slow-wave activity (SWA) and ‘global activation’. During SWA, there was coherence at ∼1Hzbetween ECoG and basal ganglia LFPs, with much of the coherent activity directed from cortex to basal ganglia. There were similar coherent activities at ∼1Hzwithin the basal ganglia, with more activity directed from SNr to GP and STN, and from STN to GP rather than vice versa. During global activation, peaks in coherent activity were seen at higher frequencies (15–60 Hz), with most coherence also directed from cortex to basal ganglia. Within the basal ganglia, however, coherence was

of primate subgenual cingulate cortex neurons is related to sleep. J Neurophysiol 90: 134–142, 2003; 10.1152/jn.00770.2002. The most frequent type of neuronal response found in the subgenual cingulate cortex (area 25) of the rhesus macaque was a highly significant increase of firing rate when the monkey fell asleep (median rate � 1.6 spikes/s) compared with the awake state (median rate � 0.1 spikes/s). On average, the firing rate of the neurons when awake was 23 % of that when the monkeys were asleep. Neurons were not found in this region with responses related to taste, olfactory, and visual stimuli including faces or related to movement. These results are relevant to understanding the function of this region in humans, in which it has been suggested that activation may be related to disengagement from tasks and to induced sadness, both of which we note lead to a more passive or resting behavior. A decrease in the activation of this area in humans has been observed during the recovery from depression, which we note leads to a more active state of behavior.

Oscillations are present at many levels in the basal ganglia (BG), and can describe regular fluctuations in, for example, gene expression, current flow across the plasma membrane, the firing rate of a single neuron, the activity within and between small networks of neurons, and activity at the level of whole nuclei. Many BG neurons,

Abstract not found

We examined fluctuations in band-limited power (BLP) of local field potential (LFP) signals recorded from multiple electrodes in visual cortex of the monkey during different behavioral states. We asked whether such signals demonstrated coherent fluctuations over time-scales of seconds and minutes, and would thus serve as good candidates for direct comparison with data obtained from functional magnetic resonance imaging (fMRI). We obtained the following results. (i) The BLP of the local field displayed fluctuations at many time-scales, with particularly large amplitude at very low frequencies (<0.1 Hz). (ii) These fluctuations exhibited high coherence between electrode pairs, particularly for BLP signals derived from the gamma (γ) frequency range. (iii) Coherence in the BLP, unlike that in the raw LFP, did not fall off sharply as a function of cortical distance. (iv) The structure and coherence of BLP changes were highly similar under distinctly different behavioral states. These results demonstrate the existence of widespread coherent activity fluctuations in the brain of the awake monkey over very long time-scales. We propose that such signals may make a significant contribution to the high variability observed in the time course of physiological signals, including those measured with functional imaging techniques. The results are discussed in the context of combined fMRI/electrophysiological recordings.

The large-scale neural dynamics underlying higher cognitive processes are characterized by at least three types of stimulusresponse: (i) the resetting of ongoing oscillatory brain activity without concomitant changes in response amplitude (phase alignment response); (ii) the addition of response amplitude to the ongoing brain activity in a time-locked manner (evoked response); and (iii) the addition of response amplitude that is not time-locked (induced response). Recent animal studies identified evoked responses as a characteristic neural response during stimulus perception but leave open the possibility that higher cognition, such as memory, is characterized more predominantly by phase alignment and/or induced responses. Using whole-head single-trial magnetoencephalography data from eight healthy adults, we show that all three types of response are related to the discrimination of old and new stimuli in a visual word recognition memory paradigm. In four subjects, single-trial evoked responses were the single constituents of event-related field old/new differences that have been previously related to familiarity-based and recollection-based recognition memory. While these data show that the oscillatory brain dynamics underlying recognition memory are characterized by a complex mix of three types of stimulus-response, they also clearly implicate evoked responses in higher cognitive processes such as recognition memory.

Phenotypic and genetic correlations between evoked EEG/ERP measures during the response anticipation period of a delayed response task

EEG containing EMG Objective: To evaluate rapid changes in regional EEG synchronization in normal subjects with spatial and temporal resolution exceeding prior art 10fold. Methods: A curvilinear array of 64 electrodes 3 mm apart extending 18.9 cm across the scalp was used to record EEG at 200/s. 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. Results: 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

Oscillations and synchronization are fundamental processes in many biological systems, from prokaryotes to humans on timescales that range from milliseconds to days (Winfree, 2001; Gillette and Sejnowski, 2005). In nervous systems, synchronized oscillations are prevalent in active assemblies of neurons. They have been recorded in a wide variety of species, from invertebrates to humans. Although the function of some of these oscillations appears obvious, such as those related to breathing and locomotion, the functions of others, such as those related to human cognition, remain elusive. The human cortex generates oscillations in many frequency ranges, as evidenced by electroencephalography (EEG). Much progress has been made since the original description of the human EEG (Berger, 1929), and we are beginning to understand the mechanisms underlying network oscillations in the mammalian cortex. Recent experiments have shifted focus