Lesions which interrupt the ascending limb of the midbrain-limbic circuit in the cat at different levels, or which even destroy it completely, do not prevent electroencephalographic desynchronization at the beginning of periods of deep sleep, nor do they affect the maintenance of desynchronization throughout the sleep episodes. The pontine mechanisms responsible for these electroencephalographic patterns can apparently exert their influence through ascending pathways other than those directly impinging on the hypothalamus and the limbic system.

EEG correlated with anatomical aspects of 198 brain tumours in children gave the following results in hemispheric neoplasms: a rather precise location in 40% of the cases; lateralized in 42%; 14% showed only generalized anomalies and 4% were normal tracings. The findings depended on location and ICP condition, but not on neurological signs nor on histological nature. Basal-midlìne location (88 tumors) displayed 82.4% abnormal EEGs; 50% pointed to the more invaded hemisphere, mainly from the upper brain stem, ventricles and basal ganglia regions. Hypophyseal tumours gave 50% abnormalities with proved compression of the third ventricle, hypothalamus and cerebral peduncles. In posterior fossa tumours (60 cases), abnormalities are frequent (86.7%), but the localizing value of EEG is uncertain due to bisynchronous bursts mainly in the occipital region; 15% were lateralized with correlating asymmetrical location and/or growth of tumours. Differences compared to adult tumours are mentioned as specially influenced by maturational processes of the brain.

In a recent communication (Palestini, Lifschitz & Armengol, 1959), a study of the habituation of primary and secondary cortical photic potentials in cats which had been subjected to a mid-pontine pre-trigeminal transection was reported. E.e.g. recordings of these evoked potentials after the transection showed a definite lengthening of the period of time in which the primary cortical photic potentials would attain habituation. This lengthening was even more striking in the secondary cortical photic potentials, since normally they reach habituation in a very short time.To explain these results it was suggested (Palestini & Lifschitz, 1959) that an inhibitory influence, arising in structures situated below the level of the lesion, was being exerted. The present paper presents results, obtained in preparations with a mid-pontine pre-trigeminal section, on the changes of the photically evoked cortical potentials, which give additional support to this hypothesis. METHODSFifty cats were used in these experiments. After anaesthesia with sodium pentobarbitone, extradural monopolar stainless-steel electrodes were implanted on the suprasylvian gyrus, on the gyrus lateralis and gyrus lateralis anterior.Control recordings were taken from these animals, non-anaesthetized, at least 5 days after implantation. The light stimulus in all experiments consisted of flashes at a rate of one per second from a Grass PS 1 photic stimulator. To ensure relatively constant stimulation of the retina the light source was always held at the same distance from the eyes, the animal was slung in a hammock and head excursions were restrained by an aluminium cone. All cats were dark-adapted at the beginning of each experiment and their pupils dilated with homatropine. Satisfactory isolation from outside sounds was achieved in all experiments. As a further means of ensuring a constant stimulus to the retina, the nictitating membranes were removed and the facial nerves were divided in some animals.Both monopolar (with an indifferent electrode on the frontal sinus) and bipolar electrodes were used and the electrical changes were recorded by a Grass e.e.g. machine on paper or photographed from an oseilloscope trace. In the latter method successive cortical responses were superimposed on the same picture.

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