The Relationship and Underlying Neuronal Activities of Cortical and Thalamic Visual Evoked Potentials of Sleeping and Waking

Flash evoked thalamic field potentials recorded from the lateral geniculate nucleus are compared with corresponding potentials recorded from the surface of the visual cortex. The high similarities in waveforms and temporal characteristics between these potentials, together with an identical modulation by the state of vigilance, supports the assumption that cortical evoked potentials are primarily the reflections of activities originating from thalamocortical fibers. The cortical potentials follow the thalamic response with a delay of 2 msec and have a less detailed character. The difference in amplitudes of the potentials and their reversed polarity can be ascribed to the laminated ’open field’ structure of the cortex and the ’closed field’ structure of the thalamic nucleus. When the underlying neuronal activities of thalamic evoked potentials are studied, it can be inferred that positive waves are associated with neuronal excitations and negative waves with neuronal inhibitions. The state dependent modulation of evoked potentials, whereby larger waves are visible during sleep than during wakefulness, is caused by the effects on the underlying neuronal responses. Intrathalamic inhibitions increase with lower levels of vigilance: the ’tonic’ firing mode of wakefulness is replaced by the ’burst’ firing mode of slow wave sleep. In the latter situation a stimulus causes strong burst-pause synchronizations with large amplitudes in evoked potentials. It is not the amount of evoked activity, being higher during wakefulness than during sleep, but the high synchronizations which seem responsible for the large components in evoked potentials during sleep. Thalamic activities are conveyed to the cortical surface forming field potentials which are measured as cortical evoked potentials by electroencephalographic surface electrodes. In these potentials negative components are presumably caused by excitatory postsynaptic potentials evoked by corticothalamic volleys.