Cross-modality comparisons of averaged evoked potentials, their relation to vigilance and stimulus parameters in cats

Abstract

1. 1. Experiments were performed on 8 cats with 34 electrodes chronically implanted in cortical and subcortical sensory specific and non-specific structures. Cross-modality comparisons of averaged evoked potentials (AEPs) to visual, auditory and electrical stimulation were made in awake and anaesthetized cats. Changes in vigilance and stimulus intensity were correlated with the distribution and amplitude of the AEPs. 2. 2. Three groups of AEPs have been distinguished: (a) potentials in sensory specific structures to corresponding or “appropriate” modality stimuli; (b) potentials caused by various stimuli in unspecific structures such as the brain-stem reticular formation; and (c) potentials in the sensory specific structures caused by “inappropriate” stimuli. 3. 3. The AEPs in unspecific structures showed considerable differences in their form, amplitude and latency for each modality of stimulation, thus suggesting the existence of subsystems in these structures differentially responsive to different stimuli. 4. 4. AEPs to inappropriate stimuli were observed in specific sensory structures. Such responses were evoked by clicks in the sensorimotor and visual cortices and in the superior colliculus and by electrical stimulation in the visual and auditory cortices, and in the inferior and superior colliculi. However, light flashes did not evoke clear AEPs in sensorimotor cortex. 5. 5. Increase of AEP amplitude with increase of stimulus intensity (electrical or auditory stimulation) was found in quiet and relaxed cats. 6. 6. The amplitudes of AEPs to stimuli of intensity sufficient to produce widespread responses were smallest in excited cats, larger in alert and largest in relaxed animals. 7. 7. The increase of the AEP amplitude on transition from the excited to the alert state was not a passive reflection of EEG changes from desynchronized to more synchronized patterns. There was a clear difference between the AEP amplitude in alert and excited cats which did not correspond to basic changes in the background EEG.