SUMMARY1. In rats under Nembutal anaesthesia the inferior olive region has been reversibly inactivated by applying a cooling probe to the ventral surface of the medulla. Simple and complex spike activity has been recorded from Purkinje cells of the cerebellar cortex.2. Following cooling of the inferior olive of one side we have observed a remarkable increase of the simple spike activity in all the twenty-two Purkinje cells, showing a disappearance of the complex spike activity.3. In some rats two Purkinje cells were recorded simultaneously from each side of the cerebellar cortex. Following cooling of the left inferior olive the effect on the Purkinje cell was observed only or predominantly on the contralateral cerebellar cortex.4. In a group of animals the inferior olive has been destroyed by 3-acetylpyridine 4-221 days before the recording session. Cooling of the inferior olive region was not accompanied by any significant and consistent increase in the spike activity of presumed Purkinje cells of the contralateral cerebellar cortex.5. These results indicate that the remarkable increase of the simple spike frequency following cooling of the inferior olive region is due specifically to the suppression of the activity of the olivocerebellar neurones.6. Only a small amount of the simple spike frequency increase is attributable to the removal of the post-climbing fibre pause.7. In some lesioned rats recording was made from Purkinje cells, which showed complex spikes due to the few surviving inferior olive cells. In these Purkinje cells cooling of the inferior olive region was accompanied by a disappearance of the complex spike and by a small increase ofthe simple spike frequency of discharge. Such an increase is mainly attributable to the removal of the post-climbing fibre pause.8. These results suggest that a given Purkinje cell is not only under the inhibitory influence of its own climbing fibre, but also of other olivocerebellar neurones, probably through climbing fibre collaterals to the cerebellar cortical interneurones.9. It is suggested that one role of the olivocerebellar system is to exert a powerful tonic inhibitory action on the Purkinje cells and consequently to exert a significant control on the excitability of the subcerebellar centres.

Fear conditioning involves learning that a previously neutral stimulus (CS) predicts an aversive unconditioned stimulus (US). Lesions of the cerebellar vermis may affect fear memory without altering baseline motor/autonomic responses to the frightening stimuli. Reversible inactivation of the vermis during the consolidation period impairs retention of fear memory. In patients with medial cerebellar lesions conditioned bradycardia is impaired. In humans, cerebellar areas around the vermis are activated during mental recall of emotional personal episodes, if a loved partner receives a pain stimulus, and during learning of a CS-US association. Moreover, patients with cerebellar stroke may fail to show overt emotional changes. In such patients, however, the activity of several areas, including ventromedial prefrontal cortex, anterior cingulate, pulvinar and insular cortex, is significantly increased relative to healthy subjects when exposed to frightening stimuli. Therefore, other structures may serve to maintain fear response after cerebellar damage. These data indicate that the vermis is involved in the formation of fear memory traces. We suggest that the vermis is not only involved in regulating the autonomic/motor responses, but that it also participates in forming new CS-US associations thus eliciting appropriate responses to new stimuli or situations. In other words, the cerebellum may translate an emotional state elaborated elsewhere into autonomic and motor responses.

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Spontaneous saccadic eye movements were recorded in seven head-restrained pigmented rats by means of a phase detection search coil system, both in the light and in the dark. In an illuminated environment, all the rats made numerous spontaneous saccades with an average amplitude of 13.2 deg (+/- 2.2 SD) and a maximal amplitude of 35 deg. In the dark, mean saccadic amplitude was significantly reduced to 9.2 deg (+/- 2.0 SD). Saccadic peak velocity increased linearly as a function of saccadic size, with no saturation at high amplitude values. In the light, peak velocity increase was 32.7 deg/s/deg (+/- 3.5 SD). This value is higher than that described in many other species including man and is similar to that of the monkey. Also saccadic duration increased linearly as a function of size at a rate of 1 ms/deg, which is closer to that of monkey than to that of other species including man. Both peak velocity and duration were not significantly different in the dark from those measured in the light. In the light, following a saccadic gaze shift, the rats were able to maintain a steady eye position for long periods, also at large orbital eccentricities. In the dark, on the contrary, the eye presented a drift towards the central position in the orbit. Such a drift had an exponential-like time course with a time constant of 1567 ms (+/- 829 SD), a value which is much shorter than that of cat and primates. This indicates that in the absence of a visual input, the rat has a poor gaze holding ability compared to other species.

SUMMARY1. In rats under sodium pentobarbitone anaesthesia the inferior olive region has been reversibly inactivated by applying a cooling probe to the ventral surface of the medulla. Unitary activity has been recorded from the fastigial, interpositus and Deiters nuclei.2. Identification of units was based on the presence of a dye spot, left by the recording micropipette. In the Deiters nucleus, an additional criterion ofidentification was the antidromic activation from spinal cord stimulation.3. Following cooling of the inferior olive of one side, we have observed suppression of the activity of all the fourteen Deiters neurones and of seventeen out of twenty neurones recorded from the intracerebellar nuclei.4. In two out of seven Deiters neurones tested the antidromic invasion elicited by spinal cord stimulation was suppressed.5. In rats, whose inferior olive was previously destroyed, cooling ofthe inferior olive region was not followed by the powerful depression of spike activity seen in the vestibular and cerebellar nuclei cells in the intact rats.6. These results indicate that the olivocerebellar system is very important in regulating the level of excitability of the subcerebellar structures and therefore in controlling both postural mechanisms and the processing of information relating to sensorimotor integration. INTRODUCTIONThe inferior olive projects to the entire cerebellar cortex (Brodal, 1940) and it is likely to be the only source of climbing fibres (Eccles, Ito & Szentaigothai, 1967;Armstrong, 1974;Montarolo, Raschi & Strata, 1980). Its important role in cerebellar function and consequently in motor control is widely accepted (Armstrong, 1978). Evidence for such an assumption is provided by the fact that in rats an inferior olive lesion induces motor disorders, which are as severe as those following cerebellectomy (Llinas, Walton, Hillman & Sotelo, 1975).It has been recently suggested that one function of the inferior olive is to exert a powerful tonic control on the excitability of the Purkinje cells ofthe cerebellar cortex and consequently ofthe neurones ofthe intracerebellar and Deiters nuclei (Montarolo,