Cerebellar Purkinje cell activity related to the classically conditioned nictitating membrane response

Summary Because of the purported critical role of cerebellar lobule HVI in classical conditioning of the nictitating membrane response of the rabbit, we recorded extracellularly from HVI Purkinje cells (PCs) during differential conditioning. Rabbits were trained using tonal conditioned stimuli (CSs) and stimulation of the periocular region as the unconditioned stimulus (US). Many PCs responded to the US, the most frequently observed response being a burst of simple spikes. PCs in HVI showed a variety of responses to CSs that were related to conditioned responses (CRs). The most frequently observed response was an increase in simple spikes correlated with CRs. The activity of many of these cells antedated CRs by 20–200 ms. A smaller proportion of cells exhibited inhibition of simple spike activity that antedated CRs. The existence of PCs that alter their firing before CRs suggests that they may be causally involved in this behavior, and in this respect they reinforce reports that lesions of HVI or its connections disrupt nictitating membrane CRs. Although complex spike activity was not generally related to the US or to CRs, a few PCs responded in relation to CRs with only complex spikes. In demonstrating CR-related activity in cerebellar PCs, this study supports theories of cerebellar learning such as those of Marr and Albus.     

Cerebellar cortex and eyeblink conditioning: bilateral regulation of conditioned responses

We examined the role of the cerebellum in classical conditioning of the nictitating membrane response (NMR) of rabbits by comparing the effects of unilateral and bilateral cerebellar cortical lesions. Using extended preoperative conditioning to ensure high levels of learning, we confirmed that unilateral lesions of lobules HVI and ansiform lobe impaired conditioned responses (CRs) previously established to an auditory conditioned stimulus, but did not prevent some relearning with post-operative retraining. Bilateral lesions of HVI and ansiform lobe produced similar impairments of CRs, but also prevented subsequent relearning. Unilateral cortical lesions produced significant enhancement of unconditioned response (UR) amplitudes to periorbital electrical stimulation. Bilateral cortical lesions enhanced UR amplitudes to a lesser extent. Because there was no correlation between the degree of CR impairment and UR enhancement across the unilateral and bilateral lesion groups, the suggestion that the lesions impaired CRs due to general effects upon performance, rather than due to losses of learning, is not supported. Both sides of the cerebellar cortex contribute towards learning a unilaterally trained CR. This finding is important for the re-interpretation of unilateral, reversible inactivation studies that have found no involvement of the cerebellar deep nuclei in the acquisition of NMR conditioning. In addition, we found conditioning-dependent modifications of unconditioned responses that were particularly apparent at low intensities of periorbital electrical stimulation. This finding is important for the re-interpretation of studies that have found apparent changes in the UR of conditioned subjects after cerebellar lesions.     

Involvement of cortical neurones in conditioned and unconditioned startle reflex

Postsynaptic potentials in the sensorimotor cortex of unanaesthetized rabbits were recorded simultaneously with electromyogram of the unconditioned startle reflex or the ‘local conditioned startle reflex’. The startle reflex was produced by a loud click in naive animals. The ‘local conditioned startle reflex’ was evoked by a click of a moderate intensity after a conditioning procedure (pairing of the formerly neutral click with direct cortical and hypothalamic stimulation). The latency of the startle reflex and the ‘local conditioned startle reflex’ was from 12 to 17 ms. Postsynaptic potentials or spike discharges after less than 7 ms latency were found in about 20% of neurones in the sensorimotor cortex during both the startle reflex and the ‘local conditioned startle reflex’. Stimulation of subcortical auditory structures evoked EMG responses after 4–8 ms latency. About 25% of sensorimotor cortical neurones responded with postsynaptic potentials and spike discharges within 4 ms after the stimulation of the colliculus inferior.The data support an idea of multiple level organization of the startle reflex and suggest that a pathway for the startle reflex and the ‘local conditioned startle reflex’ may pass through the sensorimotor cortex.     

Transient direct connection of vestibular mossy fibers to the vestibulocerebellar Purkinje cells in early postnatal development of kittens

Postnatal development of mossy fiber afferents from the vestibular and the visual system to the vestibulocerebellum was studied electrophysiologically and morphologically. In kittens anesthetized with pentobarbital sodium and N2O plus halothane, extracellular simple and complex spikes of Purkinje cells were recorded in the flocculus, nodulus and uvula. In the flocculus, stimulation of the VIIIth, but not the optic nerve, evoked simple spike responses with a latency of 16 ms at the day of birth which decreased to 5 ms by day 15 (short latency group). On the other hand, another group of simple spike responses with much longer latencies (50-80 ms) began to be elicited on day 7 via both the optic and VIIIth nerves. The latency decreased to 24 ms by day 15 and 10 ms on day 30. These latencies further shortened with development to the adult latency value (3-5 ms). Simple spike responses of the short latency group were also evoked in the nodulus and uvula from the VIIIth nerve with a slightly longer latency than that in the flocculus (23 ms on day 3 and 12 ms on day 17). Because of the immaturity of granule cells in early postnatal days, short latency simple spike responses from the VIIIth nerve suggested the direct synaptic connection of vestibular mossy fibers with Purkinje cells. Horseradish peroxidase was injected into the white matter of the flocculus, nodulus and uvula in slice preparations. Mossy fibers labeled with horseradish peroxidase showed fine branches extending to reach Purkinje cell somata from mossy swellings in the internal granular layer during days 2-20. Electron microscopy showed that the labeled mossy fibers made intimate contacts with Purkinje cell somata and the terminals contained many round synaptic vesicles. Pre and postsynaptic densities were occasionally found. After day 20, direct mossy fiber connections with Purkinje cells could not be observed. During days 7-20, these direct connections, as well as mossy fiber-granule cell connections could be observed. It was demonstrated that during early postnatal development, vestibular mossy fibers temporarily make direct contact with Purkinje cells, through which impulses could be transmitted to elicit simple spikes in Purkinje cells.     

Cerebellar afferent projections from the vestibular nuclei in the cat: An experimental study with the method of retrograde axonal transport of horseradish peroxidase

Summary Details of cerebellar afferent projections from the vestibular nuclei were investigated by the method of retrograde axonal transport of horseradish peroxidase (HRP) in the cat. The distribution of labeled cells in the vestibular nuclei following HRP injections in various parts of the cerebellum indicates that vestibular neurons in the medial and descending nuclei and cell groups f and x project bilaterally to the entire cerebellar vermis, the flocculus, the fastigial nucleus and the anterior and posterior interpositus nuclei. In addition, labeled cells (giant, medium and small) were consistently found bilaterally in the superior and lateral vestibular nuclei following HRP injections in the nodulus, flocculus, fastigial nucleus, and following large injections in the vermis. No labeled cells were observed in cases of HRP injections in crus I and II, the paramedian lobule, paraflocculus and lateral cerebellar nuclei. The present findings indicate that secondary vestibulocerebellar fibers project to larger areas in the cerebellum and originate from more subdivisions and cell groups of the vestibular nuclear complex than previously known.List of Abbreviations B.c. superior cerebellar peduncle (brachium conjunctivum) - D descending (inferior) vestibular nucleus - f cell group f in descending vestibular nucleus - g group rich in glia cells, caudal to the medial vestibular nucleus - HIX hemispheral lobule IX - HVIIA cr. Ia, p; cr. IIa, p anterior and posterior folia of crus I and II of the ansiform lobule - HVIIB, HVIIIA, B sublobules A and B of hemispheral lobules VII and VIII - i.c. nucleus intercalatus (Staderini) - L lateral vestibular nucleus (Deiters) - l small-celled lateral group of lateral vestibular nucleus - M medial (triangular or dorsal) vestibular nucleus - N. cu. e. accessory cuneate nucleus - N. f. c. cuneate nucleus - N. mes. V mesencephalic nucleus of trigeminal nerve - N.tr. s. nucleus of solitary tract - N. VII facial nerve - pfl. d. dorsal paraflocculus - pfl. v. ventral paraflocculus - S superior vestibular nucleus (Bechterew) - Sv. cell group probably representing the nucleus supravestibularis - Tr. s. solitary tract - x small-celled group x, lateral to the descending vestibular nucleus - y small-celled groupy, lateral to the lateral vestibular nucleus (Deiters) - z cell group dorsal to the caudal part of the descending vestibular nucleus - I–VI vermian lobules I–VI - V, VI, XII cranial motor nerve nuclei - VIIA, B; VIIIA, B anterior and posterior sublobules of lobules VII and VIII - IX uvula - X nodulus; dorsal motor nucleus of vagus nerve On leave from the Laboratory of Neurobiology and Department of Anatomy, Faculty of Science, Mahidol University, Bangkok, Thailand, under the Felllowship Program of the Norwegian Agency for International Development (NORAD)     

Evidence from retractor bulbi EMG for linearized motor control of conditioned nictitating membrane responses

Classical conditioning of nictitating membrane (NM) responses in rabbits is a robust model learning system, and experimental evidence indicates that conditioned responses (CRs) are controlled by the cerebellum. It is unknown whether cerebellar control signals deal directly with the complex nonlinearities of the plant (blink-related muscles and peripheral tissues) or whether the plant is linearized to ensure a simple relation between cerebellar neuronal firing and CR profile. To study this question, the retractor bulbi muscle EMG was recorded with implanted electrodes during NM conditioning. Pooled activity in accessory abducens motoneurons was estimated from spike trains extracted from the EMG traces, and its temporal profile was found to have an approximately Gaussian shape with peak amplitude linearly related to CR amplitude. The relation between motoneuron activity and CR profiles was accurately fitted by a first-order linear filter, with each spike input producing an exponentially decaying impulse response with time constant of order 0.1 s. Application of this first-order plant model to CR data from other laboratories suggested that, in these cases also, motoneuron activity had a Gaussian profile, with time-of-peak close to unconditioned stimulus (US) onset and SD proportional to the interval between conditioned stimulus and US onsets. These results suggest that for conditioned NM responses the cerebellum is presented with a simplified "virtual" plant that is a linearized version of the underlying nonlinear biological system. Analysis of a detailed plant model suggests that one method for linearising the plant would be appropriate recruitment of motor units.     

Optokinetic response of simple spikes of Purkinje cells in the cerebellar flocculus and nodulus of the pigmented rabbit

Summary Under anesthesia with N₂O (70%) and halothane (2–4%), Purkinje cell activities were extracellularly recorded in the flocculus and nodulus of immobilized pigmented rabbits. Large field (60° × 60°) optokinetic stimulation (OKS) was delivered to the central visual field of one eye with a constant velocity (0.1–4.0 °/S) at 0°, 45°, 90° or 135° to the horizontal plane of the eye. Most of the Purkinje cells in the flocculus and the nodulus showed significant simple spike modulations to OKS delivered to either eye. As a whole, the preferred directions of simple spike responses in the flocculus had the same orientation as those of complex spike responses. However, the preferred directions and amplitudes of modulation of simple spike responses did not necessarily correlate with those of complex spike responses in individual flocculus Purkinje cells. On the other hand, the preferred directions of simple and complex spike responses were not necessarily in the same orientation in the nodulus. The optimum velocity for simple spike responses was in the range 0.1–2.0°/s for Purkinje cells in both the flocculus and the nodulus. The amplitude and time to peak of the simple spike responses of nodulus Purkinje cells were significantly smaller and longer, respectively, than those of flocculus Purkinje cells. In both the flocculus and the nodulus, Purkinje cells whose simple spikes preferred the horizontal orientation (H cells) and the vertical orientation (V cells) showed clustering. In particular, zonal organization was noted in the flocculus. H cells were localized in a dorso-ventral zone in the rostral one third of the flocculus, and V cells were in two distinct zones rostral and caudal to the H cell zone. The locations of H and V cells in the flocculus correspond to the H zone and V zones, respectively, determined on the basis of the preferred directions of complex spike responses to OKS. This indicates that the same subdivisions of the flocculus are supplied with optokinetic signals with the same orientation selectivity through both mossy and climbing fibers, and suggest that such subdivisions of the flocculus are functional units which control horizontal and vertical components of optokinetic eye movements. The present results indicate that the flocculus and the nodulus are supplied with distinct optokinetic signals through mossy fibers and play different roles in controlling optokinetic eye movements.     

Cerebellar afferent projections from the perihypoglossal nuclei: An experimental study with the method of retrograde axonal transport of horseradish peroxidase

Summary Details of cerebellar afferent projections from the perihypoglossal nuclei were studied in the cat by means of retrograde axonal transport of horseradish peroxidase (HRP). Labeled cells were observed bilaterally (with a preponderance ipsilaterally) in nuclei intercalatus and praepositus hypoglossi following injections in various folia of the entire vermis, paraflocculus, flocculus, fastigial nucleus, and the nucleus interpositus anterior and posterior. Relatively high densities of labeled cells were found in nucleus intercalatus following injections in the anterior part of the vermis, whereas labeled cells in nucleus praepositus hypoglossi were found more frequently following injections in the posterior part of the vermis. Labeled cells in the nucleus of Roller were found only following injections in the anterior lobe vermis, posterior vermal lobules VI and VII, in the flocculus and in the nucleus interpositus anterior. No labeled cells could be detected in the three subdivisions of the perihypoglossal nuclei following HRP injections in crus I, crus II, paramedian lobule, and lateral cerebellar nucleus. The distribution of the HRP positive cells indicated the presence of a topographically organized projection from certain regions of the perihypoglossal nuclei to different parts of the cerebellum. The afferent and efferent connections of the perihypoglossal nuclei in relation to a functional role in eye and head movements are discussed.Abbreviations in Figures a,b,c sublobules of lobules V, VI and VII - f.apm. ansoparamedian fissure - f.icul. intraculminate fissure - f.in.cr. intercrural fissure - f.pc. preculminate fissure - f.pfl. parafloccular fissure - f.p.l. posterolateral fissure - f.ppd. prepyramidal fissure - f.pr. fissura prima - f.prc. precentral fissure - f.prc.a. precentral fissure a - f.p.s. posterior superior fissure - f.sec. fissura secunda - fl. flocculus - g.n. VII genu of facial nerve - HII-HVI, HIX hemispheral lobules II–VI, IX - HVIIA cr.Ia,p; cr.IIa,p anterior and posterior folia of crus I and II of the ansiform lobule - HVIIB, HVIIIA,B sublobules A and B of hemispheral lobules VII and VIII - ic nucleus intercalatus - l.ans. ansiform lobule - N.f. nucleus fastigii - Nfc nucleus cuneatus - Nfg nucleus gracilis - N.i.a. nucleus interpositus anterior - N.i.p. nucleus interpositus posterior - N.l. nucleus lateralis - pfl.d. dorsal paraflocculus - pfl.v. ventral paraflocculus - Ph nucleus praepositus hypoglossi - Ro nucleus of Roller - S solitary tract - s.int.cr.1,2 intracrural sulcus 1 and 2 - SL lateral nucleus of the solitary tract - SM medial nucleus of the solitary tract - VIN inferior vestibular nucleus - VLD lateral vestibular nucleus, dorsal division - VMN medial vestibular nucleus - I-VI vermian lobules I–VI - VI nucleus of abducent nerve - VIIA,B; VIIIA,B anterior and posterior sublobules of lobules VII and VIII - IX uvula - X dorsal motor nucleus of vagus nerve; nodulus - XII nucleus of hypoglossal nerve Parts of this paper were presented at the Symposium Control of Gaze by Brain Stem Neurons, Paris, July 13–15, 1977On leave from the Laboratory of Neurobiology, Faculty of Science, Mahidol University, Bangkok, Thailand, under NORAD Fellowship Program from the Norwegian Agency for International Development     

Involvement of cerebral cortical structures in the classical conditioning of eyelid responses in rabbits

The classical conditioning of the eyelid motor system in alert behaving rabbits has been used to study the expression of Fos in the hippocampus, and in the occipital, parietal, piriform and temporal cortices. Animals were classically conditioned with both delay and trace conditioning paradigms. As conditioned stimulus, both short and long (20 and 100 ms) tones (600 Hz, 90 dB) or short, weak (20 ms, 1 kg/cm²) air puffs were used. The unconditioned stimulus was always a long, strong (100 ms, 3 kg/cm²) air puff that started 250–270 ms after the onset of the conditioned stimulus. The expression of Fos was significantly increased after both delayed and trace conditioning in the hippocampus, and in the parietal and piriform cortices contralateral to the unconditioned stimulus presentation side, compared with equivalent ipsilateral structures in conditioned animals, or with Fos production in the same contralateral structures in pseudo-conditioned and control animals. Fos expression in some cortical sites was specific to tone versus air puff stimuli when used as conditioned stimulus. Thus, Fos expression was significantly increased in the contralateral temporal lobe when tones were used as conditioned stimulus, for both delayed and trace conditioning paradigms, but not when animals were conditioned to short, weak air puffs.

The present results indicate a specific Fos activation in several cerebral cortical structures during associative eyelid conditioning.     

Sensory stimulus evokes inhibition rather than excitation in cerebellar Purkinje cells in vivo in mice

Cerebellar Purkinje cells (PC) response precisely to tactile stimulus via granule cells, however, the interaction between sensory evoked synaptic input and the resulting pattern of output spikes in cerebellar cortex is unclear. In this study, we used electrophysiological recording and pharmacological methods to investigate the cerebellar PC in response to natural stimulus on ipsilateral whisker pad in urethane-anesthetized mice. We found that air-puff stimulus on ipsilateral whisker pad evoked neither complex spikes nor simple spike firing, but indeed evoked a strong GABA(A) receptor-mediated inhibition in PCs in cerebellar cortex folium Crus II. Field potential recordings from both molecular layer and PC layer showed that air-puff stimulus evoked a sequence of parallel fiber volley followed by a GABA(A) receptor-mediated inhibition, which completely blocked by AMPA receptor antagonist, NBQX. Cell-attached recordings showed that air-puff stimulus evoked a pause of simple spike firing, GABA(A) receptor antagonist abolished the pause, revealed the tactile stimulus-evoked spike firing in PCs. These results indicated that natural stimulus of whisker pad neither evoked complex spikes, nor fired simple spikes, but induced inhibition in PCs, suggesting that the interneuron network are rapid activated and involved in controlling the spread of sensory information processing in mouse cerebellar cortex folium Crus II.     

The role of the pedunculopontine tegmental nucleus in relation to conditioned motor performance in the cat I. Context-dependent and reinforcement-related single unit activity

The activity of the pedunculopontine tegmental nucleus (PPTg) neurons was recorded in three unrestrained cats operantly conditioned to perform a lever-release movement. The movement had to be initiated either rapidly after a (click) stimulus in a simple reaction-time paradigm or had to be delayed after the same stimulus in trials identified by a tone cue. Successful trials were rewarded by a food pellet. A total of 107 neurons were recorded with microelectrodes. Brief spike neurons (mean duration: 0.7 ms) and broad spike neurons (mean duration: 2 ms) presumed to be cholinergic were detected. Of the 73 neurons localized in the PPTg area, 53 had brief spikes and 20 broad spikes. Changes in activity most commonly occurred very early after the stimulus or during the reinforcement process. Most neurons with brief spikes exhibited very early excitation after the stimulus and reinforcement-related activity. These neurons had a mean activity of 23.7 impulses/s in the period preceding the stimulus. The onset of activation after the stimulus had a latency of 8.6±6.9 ms (mean±SD), with a range of 4–35 ms. In trials where the movement had to be delayed after the stimulus, the early activation disappeared or was considerably reduced, showing that it was context-dependent. A small proportion of neurons with brief spikes initially decreased activity after the stimulus, but with a latency >9 ms. All the neurons with broad spikes, except one, had reinforcement-related activity. Half of them showed exclusively reinforcement-related activity, the other half also early activation after the stimulus. These neurons were about half as active in the period preceding the stimulus occurrence than the neurons with brief spikes. The early context-dependent activation is discussed in relation to the excitatory projection of PPTg neurons on the subthalamic nucleus. The reinforcement-related activity, preferentially evidenced in broad spike neurons presumed to be cholinergic, is speculated to be associated with cholinergic projection of PPTg neurons to the dopaminergic neurons of the substantia nigra. Finally, the role of PPTg in the ongoing control of motor performance and reinforcement processes is discussed in relation to the basal ganglia circuitry. Received: 14 July 1997 / Accepted: 3 March 1998     

Dissociaton of conditioned eye and limb responses in the cerebellar interpositus

Thompson and colleagues have demonstrated that the lateral interpositus nucleus of the cerebellum is the essential locus for the classical conditioning of the somatic eyeblink response. Preliminary studies reported that lesioning the cerebellar interpositus nucleus ipsilateral to the side of training also appears to abolish conditioned limb flexion responses. Previous studies have suggested that the interpositus nucleus is somatotopically organized with the eye being represented laterally and the hindlimb medially. Presently, we employed a double dissociation paradigm to examine the effects of muscimol (a GABA(A) agonist) injections on eyeblink versus limb flexion conditioned responses in the ipsilateral cerebellar interpositus nucleus of New Zealand white rabbits. For eyeblink conditioning, the conditioned stimulus (CS) was a 14-V lamp bulb and the unconditioned stimulus (US) was a 3-psi corneal airpuff to the left eye. For limb flexion conditioning, the CS was a 1-kHz, 85-95 dB SPL tone and the US was a 3- to 5-mA shock to the upper left hindlimb. Upon training on both responses to a 60-100% criterion, the rabbits were then tested on eyeblink and limb flexion responses after injections of muscimol (0.1-0.3 mul of a 0.01- to 1.0-M solution) into either the lateral (eyeblink) or medial (limb flexion) interpositus nucleus. We have been able to successfully decrease or abolish the percent conditioned responses (CRs) of both the eyeblink and limb flexion conditioning selectively without affecting the other. These results thus lend further support for the notion of the existence of a somatotopic map in the interpositus nucleus for learning.     

Cerebellar posterior interpositus nucleus as an enhancer of classically conditioned eyelid responses in alert cats

Cerebellar posterior interpositus neurons were recorded in cats during delayed and trace conditioning of eyeblinks. Type A neurons increased their firing in the time interval between conditioned and unconditioned stimulus presentations for both paradigms, while type B neurons decreased it. The discharge of different type A neurons recorded across successive conditioning sessions increased, with slopes of 0.061-0.078 spikes/s/trial. Both types of neurons modified their firing several trials in advance of the appearance of eyelid conditioned responses, but for each conditioned stimulus presentation their response started after conditioned response onset. Interpositus microstimulation evoked eyelid responses similar in amplitude and profiles to conditioned responses, and microinjection of muscimol decreased conditioned response amplitude. It is proposed that the interpositus nucleus is an enhancer, but not the initiator, of eyelid conditioned responses.     

Dopaminergic and non-dopaminergic neurons in the ventral tegmental area of the rat project, respectively, to the cerebellar cortex and deep cerebellar nuclei.

It has been suggested recently that dopamine in the cerebellum not only acts as a precursor for noradrenaline in afferent fibers supplied by locus coeruleus neurons, but also subserves an independent transmitter role in a separate neural system. The present study was initiated to investigate the possible sources for dopaminergic innervation of the cerebellum. Employing anterograde and retrograde axonal tracing with cholera toxin and a combination of fluorescent retrograde axonal tracing with Fluoro-Gold and tyrosine hydroxylase immunofluorescence histochemistry, we found in the rat that the ventral tegmental area, containing the A10 dopaminergic cell group, sends projection fibers to the cerebellum bilaterally with a slight contralateral predominance. The projections from the ventral tegmental area to the cerebellum were segregated into the dopaminergic one to the cerebellar cortex and the non-dopaminergic one to the deep cerebellar nuclei. Dopaminergic fibers projecting from the ventral tegmental area to the cerebellar cortex terminated mainly in the granular layer, additionally in the Purkinje cell layer, but not at all in the molecular layer. They were distributed predominantly in the crus I ansiform lobule and paraflocculus, and to a lesser extent in the crus II ansiform lobule. On the other hand, non-dopaminergic fibers projecting from the ventral tegmental area to the deep cerebellar nuclei were seen to terminate mainly in the lateral nucleus, to a lesser extent in the interpositus nucleus, but not at all in the medial nucleus. The ventral tegmental area was also observed to receive projection fibers from the lateral and interpositus cerebellar nuclei bilaterally with a contralateral predominance. The projections from the ventral tegmental area to the cerebellum revealed in the present study might exert limbic influences upon the cerebro-cerebellar loops subserving the execution and co-ordination of voluntary movements.     

Responses of guinea pig primary vestibular neurons to clicks

Responses of single neurons in the vestibular nerve to high-intensity clicks were studied by extracellular recording in anaesthetised guinea pigs. One hundred and two neurons in the posterior division of the superior branch or in the inferior branch of the vestibular nerve were activated at short latency by intense clicks. The latency of activation was short (median 0.9 ms) and the threshold was high: the click intensity for evoking the response of these cells was around 60 dB above the auditory brainstem response threshold. Animals were tilted and rotated to identify physiologically the sensory region of the labyrinth from which the activated neurons originated. Seventeen neurons responded to static tilt as well as clicks. These results show that vestibular receptors, probably the otoliths, respond to clicks at intensities corresponding to those used in a new clinical test of the vestibulo-collic pathway.     

Response characteristics and vestibular receptor convergence of frog cerebellar Purkinje cells. A natural stimulation study

1. The horizontal sinusoidal frequency response and the problem of vestibular receptor convergence in Purkinje cells (P-cells) of the auriculum, dorsal rim and corpus cerebelli were studied in curarized frogs with natural stimulation. 2. Primarily "simple" but also "complex" spikes were evoked by sinusoidal stimulation of the horizontal canals. P-cell "simple" spike activity could be grouped into types I-IV. Type I and II responses were directionally sensitive and thus were evoked at the stimulus frequency. Type III (and IV) cells, on the other hand, had response waveforms double that of the input frequency, with peak increases (or decreases) in discharge inphase with head velocity in the mid-frequency range. Except in the cerebellar midline regions where type III response waveforms were symmetrical, ipsilateral sinusoidal responses were larger in magnitude than those evoked during contralateral rotation. Despite the differences in magnitudes, ipsi- and contralateral response phase angles for one cell were approximately equal. "Complex" spikes were evoked with ipsi (type I) or contralateral (type II) horizontal rotation. Generally only 1-2 spikes were evoked per cycle with short (0-60 degrees) or long (120-150 degrees) phase-lags following acceleration. 3. A Bode analysis of type I "simple" spike activity in yaw indicates a slightly greater phase-lag and a 10-15 fold smaller P-cell gain in the range 0.05-0.5 Hz when compared to peripheral horizontal canal neurons. 4. Stimulation of the vertical canals and otolith organs also evoked "simple" and, to a lesser extent, "complex" P-cell spikes. "Simple" spikes were in most cases (85%) evoked by stimulation of several canal and/or otolithic receptors thus demonstrating a high degree of receptor convergence. "Complex" spikes, however, were only evoked by stimulation of one canal or otolith receptor. 5. Otolithic input to P-cells, examined statically and with low level constant velocity rolls, was mainly phasic or phasic-tonic in nature.     

Role of the medial prefrontal cortex in the development of conditioned bradycardia in rabbits with lesions of the cerebellar vermis

The effects on the expression of conditioned bradycardia of pairing an early (fourth postnatal day) cerebellar vermal lesion with a lesion of the medial prefrontal cortex (mPFC) were studied in adult New Zealand rabbits. In the conditioning procedure, an auditory stimulus (5 s, 1000 Hz) served as a conditioning stimulus (CS) and a train of electrical impulses applied to the ear (500 ms, 100 Hz, 1.5 mA) was used as the unconditioned stimulus (US). Heart rate (HR) responses exhibited by rabbits with the early double lesion (PFCBs) during orientation (CS-alone) and conditioning (CS–US paired) were analyzed and compared with those shown by unoperated controls as well as by a group of animals in which a cerebellar lesion alone had been performed on the fourth postnatal day (CBs). In all the experimental groups vermal lesions were localized in the cortex of lobules V–VII and the underlying white matter. As for mPFC ablation, the lesioned area involved the agranular precentral region (Brodmann’s area 8), the anterior cingulate cortex (Brodmann’s area 24) and the prelimbic area (Brodmann’s area 32). All the experimental animals had a normal baseline HR as well as a marked orientation response, both comparable with those exhibited by controls. In contrast, while CB rabbits showed an increase in the amplitude of the conditioned bradycardic response when compared with controls, the HR conditioned response of PFCB animals was comparable to that exhibited by controls. These results suggest that, since the double lesion produces a conditioned bradycardia similar to that of the controls, the increase in the amplitude of this response observed after early cerebellar removal may depend on the mPFC which, in the absence of specific cerebellar circuits, is unable to produce a properly calibrated HR conditioned response. Received: 9 September 1998 / Accepted: 29 May 1999     

Effects of early cerebellar removal on the classically conditioned bradycardia of adult rabbits

The magnitude of classically conditioned bradycardia was studied in 18-day-old and adult rabbits in which the cerebellar vermis had been surgically removed on either the 5th or 18th postnatal day. In the conditioning procedure, an auditory stimulus (5 s, 1000 Hz) served as conditioned stimulus (CS) and a train of electric impulses applied to the ear (100 Hz, 500 ms, 1.5 mA) was employed as the unconditioned stimulus (US). Heart rate (HR) responses developed in the operated animals during the CS-alone (orientation), and CS-US paired presentations (conditioning) were analyzed and compared with those developed in control animals. In all the experimental groups, lesions were localized to the cortex of lobules IV–VII and the underlying white matter, sparing the deep cerebellar nuclei. None of the lesioned animals showed any behavioral or somatomotor deficit. All the operated animals exhibited a normal baseline HR and a marked orienting response, both comparable with those of controls. In contrast, while the animals tested at 18 days showed a normal pattern of conditioned bradycardia, at the age of 3 months the HR conditioned response differed significantly from that observed in control rabbits: the animals that received the earliest cerebellar lesion showed a conditioned bradycardia greater than that of controls, the rabbits lesioned on the 18th postnatal day exhibited a reduced bradycardic response. These results suggest that the timing of cerebellar vermis removal, at early stages of development, represents a crucial factor in the organization of the bradycardic response in the adult.     

Cerebellar cortex and eyeblink conditioning: A reexamination

Summary We examined the effects of cerebellar cortical lesions upon conditioned nictitating membrane responses in rabbits. Using extended postoperative conditioning and unpaired presentations of the conditioned stimuli (CSs), we confirmed that combined lesions of lobules HVI and ansiform lobe abolished conditioned responses (CRs) established to light and white noise CSs. Extended retraining enabled some slight recovery of CR frequencies. Less extensive cortical lesions produced initial abolition of CRs but allowed more complete recoveries. Although CR frequencies and amplitudes were profoundly depressed by cortical lesions, unconditioned response (UR) amplitudes to periorbital electrical stimulation were enhanced. The dissociation of lesion effects upon conditioned and unconditioned responses is consistent with the suggestion that cerebellar cortical mechanisms are important for the learning and execution of eyeblink conditioning.