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enable them to rhythmically fire action potentials and to participate in generating network oscillations in some frequency range. These membrane properties can be measured by performing ► intracellular recordings or patch clamp recordings. These neurons are, by virtue of their synapses from and onto other neurons, embedded in local network loops and the balance of net interactions determines the composite frequencies. Indeed, remotely located pacing structures, as is the case for thalamocorti-cal loops, are necessary to entrain alpha and beta frequencies as described (e.g., Steriade 2001) by correlating signals from depth electrodes with cortex recordings; in practice, the net result of cortical generator events in the crown of sulci (cells with dendrites oriented radially to the skull) are most likely to be detected on the scalp of e.g., human or rat skull.

Similarly, impinging volleys via afferent septohippo-campal pathways drive/impose the theta rhythm that arises primarily from the ► hippocampus (Buszaki et al. 1983). In animals, the pharmacology of the hippocampal theta rhythm is well studied. Theta rhythm is largely abolished by the muscarinic acetylcholine receptor antagonist atropine, whereas muscarinic agonists, like carba-chol, induce "theta-like" rhythmic network activity in isolated hippocampal slice preparations. Hence there is a neurochemical basis for oscillatory brain potentials. The study of scopolamine-induced deteriorations in EEG (see ► Dementias: Animal Models and also ► scopolamine) helps in the development of ► Acetylcholinesterase inhibitors and ► Cognitive Enhancers or drugs for ► Dementias and Other Amnestic Disorders.

Neuropharmacology is applied to study the organization of brain rhythms. Systemically administered ► benzodiazepines modify beta-activity in the cortex of rats. EEG effects, when plotted as a function of blood concentration of ► midazolam (PK-PD), follow a sigmoi-dal curve evidencing a relationship of beta waves and the enhancement of the functions of ► GABAa receptors; moreover, the position of curves for e.g., drug-analogs with comparable EEG-effects reflect potency differences (Mandema and Danhof 1992). Experimental conditions can be refined (topic application of drugs) and pharmacology is useful for the understanding of how brain structures presumed to play a role in the generator process do cooperate with other regions to make up the EEG, but also to understand the normal equilibrium state in neural circuitry in general.

Components of the EEG

There is consensus to divide EEG (quasi-sinusoidal) waves into the following frequency bands: delta (1-4 Hz), theta (5-8 Hz), alpha-1 (9-10 Hz), alpha-2 (10-12 Hz), beta-1 (13-17 Hz), beta-2 (18-20 Hz), beta-3 (21-30 Hz), and gamma (31-100 Hz) in accordance with the International Pharmaco EEG Group (IPEG). To study the role of these waves, EEG experiments have been performed in numerous animal species, making use of conditioning paradigms. Repeated walking on a treadmill (see ► operant behavior in both animals and humans and ► instrumental conditioning) was used by Lopes Da Silva to establish empirically behavioral correlates of EEG components. Figure 2 shows examples of performance-related

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Electroencephalography. Fig. 2. Functional correlates of EEG rhythms. Examples of EEG traces from the hippocampal formation for two mammalian species. The animals were conditioned to perform treadmill-walking and to remain alert, resting wakefulness. Note the presence of regular quasi-sinusoidal waves when the animals are moving, which disappear under immobility. Calibration bar 250mV; speed in arbitrary units (remastered own unpublished ink-on-paper recordings from a Siemens ELEMA Mingograf® 16-channel EEG-machine (mfi-TNO); the author acknowledges P Dalenconte, a professional photographer, for image processing).

hippocampal theta in two animal species; 4-5 Hz dominant activity is more synchronized during walking than during rest or eating. The result fits in with the concept that theta rhythm, also named ► RSA (Vanderwolf 1969), is related to voluntary movement. The hippocampal formation is believed to play an important role in memory updating. J O'Keefe, L Nadel and collaborators, have produced evidence that the theta rhythm serves as a reference framework for a higher mental function, namely spatial navigation (http://www.congnitivemap.net/). Interestingly, subpopulations of individual neurons have properties like place-specific firing at a given location in e.g., a foraging task on an ► elevated plus-maze, and the moment of firing displays a phase relationship with hippo-campal electrical field potentials in the theta frequency band suggesting that theta rhythm plays a role in navigation (Lisman 2005) and ► Spatial Learning in Animals. This finding can be extrapolated to ► Spatial Learning in Humans.

Examples of human EEG-components from four mid-line scalp locations in normal quiet resting state with eyes closed, show clearcut alpha spindles (Fig. 3 left panel) and correlates of hypovigilance or psychotropic drug states give changes in slow wave (Fig. 3 middle panel) and beta frequency bands (Fig. 3 right panel). What is known about the functional correlates of the frequency bands in man during waking and/or sleep? In brief, high power in the alpha band can be seen during certain attention tasks and/

or when the subject is awake with the eyes closed and mixed beta frequencies are associated with active thinking and concentration. Gamma frequency activity is associated with higher mental function and can be generated in many different brain regions. EEG segments characterized by ongoing activity in the theta frequencies may indicate that the subject is involved in active exploration of the environment. Paradoxically, sleep stage N1 is associated with low amplitude 4-7 Hz activity (mostly theta frequency band, Iber et al. 2007). EEG segments with a preponderance of large amplitude waves in the delta frequency band (see ► function of delta waves), in particular, during polysomnography recordings (one or few EEG-channels) are scored according to the latest guidelines from the American Academy of Sleep Medicine as sleep stage N3, slow wave sleep (SWS) which makes part of

► NREM sleep and distinguishes from the ► REM sleep stage. The translational value from animal to human (and vice versa) is evident. The EEG montage along with an EMG from a postural (usually neck) muscle and a motion detector and/or a video monitor are used together to discriminate the different stages in animal studies. In drug development, it is well known that e.g., rat sleep-EEG has predictive value for the efficacy of new

► hypnotics and possibly for the development of ► anti-depressants, in particular by studying REM-suppression.

Digital EEGs and related analyzes in the frequency domain are performed more frequently in animal and in a Control Fz ,

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