Neuronal activities underlying the electroencephalogram and evoked potentials of sleeping and waking: Implications for information processing

Abstract

The low amplitude, high frequency waves of the electroencephalogram (EEG) indicative of wakefulness, are produced by a summation of potentials of thalamocortical neurons, which fire in a “tonic mode” of depolarization. In this mode, the transfer of information from the peripheral sense organs to the sensory cortex is facilitated, due to a tonic lowering of the discharge threshold of thalamocortical neurons. The transfer decreases during drowsiness when thalamocortical units are more hyperpolarized and have higher thresholds. In this state, neurons fire synchronously in a “burst mode”, which is expressed in EEG spindling. During slow wave sleep sensory blocking reaches a maximum, when thalamocortical cells are yet more deeply hyperpolarized. During slow that what still passes to the cortex allows a shallow, subconscious, evaluation. The collective burst firing is more irregular, which results in large and slow EEG waves. In contrast, during rapid eye movement (REM) sleep the depolarized tonic mode of firing commonly associated with waking, is again reached. Similar to EEG-patterns, the architecture of evoked potentials is dependent on the state of alertness. During waking, components in event related potentials (ERP) are moderate in amplitude, while during slow wave sleep larger waves are visible. This is caused by more synchronized unit responses with sharper phases of excitations and inhibitions, which results from increased hyperpolarizations. In contrast, visual ERPs belonging to REM sleep closely resemble those of wakefulness. In analyzing unit responses of thalamorcortical neurons, it appeared that neuronal excitations are expressed in negative components of the ERP, while inhibitory neuronal, activities are associated with positivity. Transient phenomena in the EEG, such as ERP waves, spindles and spike-wave discharges, are the expression of synaptic potentials in superficial cortical layers, where numerous synapses of afferent thalamocortical fibers are localized on the apical dendrites of deeper lying pyramidal neurons. It is suggested that the morphology of these EEG components is primarily due to the discharge characteristics of thalamocortical relay cells, whereby excitations underly negative waves and inhibitions positive waves. The notion of a general correspondence between thalamocortical neuronal activities and the polarity of transients in the cortical surface EEG, allows prudent speculations regarding components of ERPs. Two examples are given: the contingent negative variations (CNV) and the P300 of an ERP which can be elicited by an infrequent stimulus. The EEG negativity in the CNV, regarded as a readiness potential, is interpreted as the expression of a general neuronal activation, while the positivity of the second EEG phenomenon may be considered as associated with inhibitory processes related to specific processes of stimulus recognition.