Classical conditioning of the nictitating membrane response of the rabbit

Summary The nictitating membrane response (NMR) of 15 rabbits was conditioned to light and white noise conditional stimuli (CSs) using a periorbital shock unconditional stimulus (US). Unilateral lesions of the inferior olive were then made. Lesions restricted to the medial parts of rostral dorsal accessory olive (DAO) and principal olive (PO) abolished conditioning and prevented subsequent acquisition on either side. Unconditional responses to the US were intact. Lesions in all other parts of the olive did not impair conditioning. The effective lesions were located in that part of the olive which supplies somatosensory information from the face to cerebellar lobule HVI. Lobule HVI is also essential for NMR conditioning. We suggest that this region of the inferior olive is part of a circuit which provides US information to the cerebellar cortex during NMR conditioning.     

Classical conditioning of the nictitating membrane response of the rabbit

Summary We report the connections of cerebellar cortical lobule HVI in the rabbit. We have studied the anterograde and retrograde transport of wheatgerm-agglutinated horseradish peroxidase (WGA-HRP) following its injection into HVI to reveal efferent and afferent connections. All of the cases showed strong anterograde transport to the anterior interpositus nucleus (AIP) — indicating that this is the major efferent target of HVI. Retrogradely labelled cells were found in the inferior olivary, spinal trigeminal, lateral reticular, inferior vestibular and pontine nuclei. Within the olive, the medial part of the rostral dorsal accessory olive (DAO) and the adjacent medial part of the principal olive (PO) were consistently labelled in all cases. This area is known to receive somatosensory information from the face and neck. There was no projection to the hemispheral part of lobule VI from visual parts of the olive within the dorsal cap and medial parts of the medial accessory olive. Likely sources of visual and auditory information to HVI are the dorsolateral basilar pontine nuclei and nucleus reticularis tegmenti pontis, which were densely labelled in all cases. These anatomical findings are consistent whith the suggestion that, during NMR conditioning, information related to the periorbital shock unconditional stimulus (US) may be provided by climbing fibres to HVI and light and white noise conditional stimulus (CS) information may be supplied by pontine mossy fibres.     

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.     

Sagittal organization of the olivocerebellonuclear pathway in the rat. II. Connections with the nucleus interpositus

The organization of the sagittal Zone C of the cerebellar cortex of the rat was studied with respect to its efferent projections and to its inferior olive (IO) afferent connections. Wheat-germ agglutinin conjugated to horseradish peroxidase was used as a tracer. Zone C has been defined as the cortical region projecting to the nucleus interpositus anterior (NIA) and posterior (NIP). The results show that, in spite of some differences, Zone C of the rat is homologous to that of the cat. Three subzones, C1, C2 and C3, were clearly identified. Subzone C1 appears as a longitudinal band of the cerebellar cortex interrupted at the level of lobules VIb,c and part of lobule VII. It is therefore divided into two sagittal segments, one anterior to lobules I to VIa adjacent to Zone B; and one posterior to lobules VII to VIII adjacent to Zone A. Both segments receive climbing fibres from the lateral aspect of the rostral two-thirds and the medial aspect of the caudal one-third of the dorsal accessory olive (DAO). The Purkinje cell axons from subzone C1 project to both the NIA and the NIP where they occupy the medial one-third of the nucleus. Subzone C2 consists of a continuous sagittal band of the cerebellar cortex and lies between Subzones C1 and C3. It receives climbing fibres from the rostral aspect of the medial accessory olive (MAO) and projects to the central aspect of the NIA and to the lateral half of the NIP. Subzone C3, which is lateral to Subzone C2 and medial to Zone D, appears as a sagittal band of cortex interrupted at the level of lobule VI. It receives climbing fibres from the medial aspect of the DAO and projects to the lateral aspect of the NIA. The orientation of the olivocerebellonuclear circuit is fundamentally sagittal not only in the cerebellar cortex but also in the nuclei and, although less sharply, in the inferior olive.     

Independent roles for the dorsal paraflocculus and vermal lobule VII of the cerebellum in visuomotor coordination

Two distinct areas of cerebellar cortex, vermal lobule VII and the dorsal paraflocculus (DPFl) receive visual input. To help understand the visuomotor functions of these two regions, we compared their afferent and efferent connections using the tracers wheatgerm agglutinin horseradish peroxidase (WGA-HRP) and biotinilated dextran amine (BDA). The sources of both mossy fibre and climbing fibre input to the two areas are different. The main mossy fibre input to lobule VII is from the nucleus reticularis tegmenti pontis (NRTP), which relays visual information from the superior colliculus, while the main mossy fibre input to the DPFl is from the pontine nuclei, relaying information from cortical visual areas. The DPFl and lobule VII both also receive mossy fibre input from several common brainstem regions, but from different subsets of cells. These include visual input from the dorsolateral pons, and vestibular–oculomotor input from the medial vestibular nucleus (MVe) and the nucleus prepositus hypoglossi (Nph). The climbing fibre input to the two cerebellar regions is from different subdivisions of the inferior olivary nuclei. Climbing fibres from the caudal medial accessory olive (cMAO) project to lobule VII, while the rostral MAO (rMAO) and the principal olive (PO) project to the DPFl. The efferent projections from lobule VII and the DPF1 are to all of the recognised oculomotor and visual areas within the deep cerebellar nuclei, but to separate territories. Both regions play a role in eye movement control. The DPFl may also have a role in visually guided reaching.     

Reacquisition of classical conditioning after removal of cerebellar cortex

Summary We trained rabbits with white noise and light conditioned stimuli and a face shock unconditioned stimulus for classical conditioning of the nictitating membrane response and then removed the ipsilateral cerebellar cortex by aspiration. Larsell's hemisphere lobule VI was completely removed in 5 rabbits; the pattern of degeneration in the inferior olive matches the projections to HVI reported in experiments with horseradish peroxidase. All rabbits showed an initial abolition of conditioned responses but then relearned within two days. This indicates that cerebellar cortical lobule HVI normally is involved but is not necessary for classical conditioning.     

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.     

Cerebellar and olivary projections of the external and rostral internal cuneate nuclei in the cat

Summary The cerebellar projection of the external cuneate nucleus and the adjoining rostral part of the internal cuneate nucleus were investigated by means of anterograde transport of tritiated leucine. The cuneocerebellar tract terminates as mossy fiber rosettes in the granular layer. The termination area has a more or less spherical form with its centre at the ipsilateral side. It comprises the anterior and posterior vermes bilaterally and the ipsilateral hemispheral parts of the anterior and simple lobules, the medial aspect of the ansiform lobule and the paramedian lobule. Within this area the mossy fiber terminals are arranged in continuous sagittal strips, some of them clearly separated from one another. The strips were found in the cerebellar modules A-D. Concomitant bilateral projections to several subdivisions of the inferior olive were found. Some of these provide the anatomical substrate for the simultaneous activation of a number of mossy and climbing fiber zones observed in the anterior lobe following stimulation of different forelimb nerves. No evidence was found for a termination of mossy fiber collaterals in the central cerebellar nuclei.Abbreviations subnucleus - bp brachium pontis - CCT cuneocerebellar tract - CE nucleus cuneatus externus - cf climbing fiber - CI nucleus cuneatus internus - cr corpus restiforme - DAO dorsal accessory olive - DCN dorsal column nuclei - DF-SOCP dorsal funiculus spino-olivocerebellar pathway - dl dorsal lamina of the PO - dmcc dorso-medial cell column - DV nucleus vestibularis descendens - E-CCP exteroceptive cuneocerebellar pathway - F nucleus fastigii - FL flocculus - flm fasciculus longitudinalis medialis - G nucleus gracilis - HRP horseradish peroxidase - HVI hemisphere of lobule VI (Roman numerals) - IA nucleus interpositus anterior - IP nucleus interpositus posterior - L nucleus lateralis - MAO medial accessory olive - mf mossy filber - mft mossy fiber terminal - MV nucleus vestibularis medialis - PAR lobus paramedianus - PFLD dorsal paraflocculus - PFLV ventral paraflocculus - PH nucleus prepositus hypoglossi - PO principal olive - P-CCP proprioceptive cuneocerebellar pathway - rV ramus descendens of the nucleus trigeminus - TS nucleus solitarius - Vi pars interpolaris of the nucleus trigeminus - vl ventral lamina of the PO - I-X lobules I to X (Roman numerals) - XII nucleus hypoglossi (Roman numerals)     

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.     

Sagittal organization of the olivocerebellonuclear pathway in the rat. I. Connections with the nucleus fastigii and the nucleus vestibularis lateralis

The organization of the afferent and efferent connections of the sagittal Zones A and B of the cerebellar cortex of the rat have been studied using wheat-germ agglutinin conjugated to horseradish peroxidase as a tracer. A single injection of this tracer into the cerebellar cortex allowed us to study, simultaneously, the crossed olivocortical connections (revealed by the retrograde transport) and the direct corticonuclear connections (revealed by the anterograde transport). The results demonstrate that the olivocerebellonuclear pathway is organized in a longitudinal direction so that for a given small injection of the tracer in one lobule of the cortex, a long sagittal band of the retrograde-labelled cells is obtained in the inferior olive, and a long sagittal band of the labelled terminals is obtained in the cerebellar nuclei. Zone A and Zone B have been arbitrarily defined as the cortical regions projecting, respectively, to the nucleus fastigii (NF) and the nucleus vestibularis lateralis (NVL). Zone A of the rat runs parasagitally from lobules I to IX as described in the cat, but in the posterior lobe it extends much more laterally than in the other mammals to include the lobulus paramedianus and crus I regions. The projections of Zone A to the NF recognize a mediolateral as well as a dorsoventral organization. Zone A receives climbing fibres exclusively from the caudal half of the medial accessory olive (MAO) with a further topographical organization in 4 distinct connections. Zone B of the rat is a narrow strip of the cortex lying adjacent to Zone A and extending from lobule I to VI. It receives climbing fibres from the caudolateral half of the dorsal accessory olive (DAO) and projects to the ipsilateral NVL with no other detectable organization. The majority of the labelled terminals end in the dorsal aspect of the NVL, but a non-negligible quantity also end in the ventral aspect.     

The afferent connections of the inferior olivary complex in rats. An anterograde study using autoradiographic and axonal degeneration techniques

Autoradiographic and axonal degeneration techniques were employed to determine the distribution patterns of inferior olivary afferents whose origins were determined using the horseradish peroxidase method.⁷⁰ The Fink-Heimer stain for degenerating axons was used following lesions of the cerebral cortex and spinal cord, while brainstem and cerebellar afferents were mapped by tritiated leucine autoradiography.After unilateral lesions of the mid-thoracic spinal cord, degenerating axons were observed within the subnuclei a and b of the caudolateral medial accessory olive and in the caudolateral dorsal accessory olive. Degeneration after upper cervical cord lesions extended more rostrally and medially within the same olivary subdivisions.Several nuclei within the caudal brainstem projected to the inferior olivary complex. The dorsal column nuclei distributed fibers primarily contralaterally to the lateral part of the dorsal accessory olive and to the caudolateral part of the medial accessory olive; the spinal trigeminal nucleus projected contralaterally to the rostromedial dorsal accessory olive; the medial and inferior vestibular nuclei projected to the ipsilateral subnuclei b, c, and β of the medial accessory olive and to the contralateral dorsomedial cell column; the nucleus prepositus hypoglossi sent fibers to the subnuclei c and β, the dorsal cap and the ventrolateral outgrowth; the lateral reticular nucleus projected to the subnucleus a of the caudolateral medial accessory olive bilaterally; and the reticular formation distributed fibers to the dorsal accessory olive contralaterally and to the β subnucleus ipsilaterally.Study of inferior olivary complex afferents from the deep cerebellar nuclei showed a projection from the fastigial nucleus to the β subnucleus and the ventrolateral outgrowth. The dentate and interpositus nuclei demonstrated topographic connections from these nuclei to the principal olive and accessory olives, respectively. All cerebellar connections were predominantly contralateral.Analysis of mesencephalic and diencephalic areas also demonstrated several inferior olivary complex afferent systems: the caudal pretectum and the superior colliculus projected to the subnucleus c contralaterally and the dorsal lamella of the principal olive ipsilaterally; the nucleus of the optic tract sent fibers to the dorsal cap; the lateral deep mesencephalic nucleus distributed fibers to the ipsilateral dorsal accessory olive and β subnucleus; the medial terminal nucleus of the accessory optic tract projected ipsilaterally to the ventrolateral outgrowth; and several areas including the medial deep mesencephalic nucleus, periaqueductal gray, the nucleus of Darkschewitsch, the subparafascicular nucleus, the rostral red nucleus and the prerubral field all projected ipsilaterally to the principal olive, rostral medial accessory olive, ventrolateral outgrowth and, to a lesser extent, the caudal medial accessory olive, dorsal cap and β subnucleus.Lesions of the frontal cortex produced axonal degeneration primarily ipsilaterally within many olivary subdivisions, especially the medial dorsal accessory olive and the caudomedial medial accessory olive.Although some notable differences in the distribution and laterality of fibers are described, our findings generally corroborate several earlier reports which used different techniques on a variety of species. Inferior olivary afferents from functionally related areas typically demonstrated similar distribution patterns within the subdivisions of the inferior olivary complex. These patterns suggest a functional localization within the inferior olivary complex which may facilitate an understanding of afferents from areas whose functions are not clearly known.     

Developmental changes in eyeblink conditioning and simple spike activity in the cerebellar cortex

The activity of neurons in the cerebellum exhibits learning-related changes during eyeblink conditioning in adult mammals. The induction and preservation of learning-related changes in cerebellar neuronal activity in developing rats may be affected by the level of maturity in cerebellar feedback to its brainstem afferents, including the inferior olive. Developmental changes in cerebellar plasticity were examined by recording the activity of Purkinje cells in eye regions of cerebellar cortical lobule HVI (lobulus simplex) in infant rats during eyeblink conditioning. The percentage and amplitude of eyeblink conditioned responses increased as a function of age. Analyses of Purkinje cell simple spike activity revealed developmental increases in the number of units that exhibited stimulus-evoked and learning-related changes in activity. Moreover, the magnitude of these changes exhibited a substantial age-related increase. The results support the view that the emergence of learning-specific cerebellar plasticity and the ontogeny of eyeblink conditioning are influenced by developmental changes in the synaptic interactions within brainstem-cerebellum circuits.     

The role of the cerebellum in classical conditioning of discrete behavioral responses

The cerebellum and its associated circuitry constitutes the entire essential neuronal system for classical conditioning of eye-blink and other discrete responses (e.g. limb flexion) learned with an aversive unconditioned stimulus (US) using the standard delay paradigm where the conditioned stimulus (CS) and the US coterminate. Evidence reviewed here strongly supports the following conclusions. The CS pathway involves sensory relay nuclei projections to the pontine nuclei and its mossy fiber projections to the cerebellar cortex and nuclei. The US pathway involves activation of the inferior olive (dorsal accessory olive for eye blink) and its climbing fiber projections to the cerebellar cortex and nuclei. The conditioned response (CR) pathway involves the cerebellar interpositus nucleus, the superior cerebellar peduncle pathway to the magnocellular red nucleus and rubral projections to premotor and motor nuclei generating the behavioral response. Anatomical data, neuronal unit recordings, electrical stimulation, lesions and methods of reversible inactivation all strongly support the hypothesis that the essential memory trace for the learning of these discrete conditioned responses is formed and stored in the cerebellar interpositus nucleus. Neuronal/synaptic plasticity is also established in the cerebellar cortex in this form of learning but the role of the cortex is less clear. We argue that the cortex plays a key role in normal acquisition and adaptive timing of the conditioned response, under certain circumstances, but it remains unclear exactly what features of conditioning are being encoded in the cerebellar cortex in this basic form of associative learning and memory.     

The projection from nucleus reticularis tegmenti pontis onto the cerebellum in the cat

Summary Following stereotactically performed lesions in nucleus reticularis tegmenti pontis (N.r.t.) degenerating fibers are traced to the contralateral N.r.t., to the pontine nuclei, through brachium pontis to restricted areas of the cerebellar nuclei and to most parts of the cerebellar cortex where they terminate in the granular layer. Most degenerating fragments are found in the contralateral half of the cerebellum with the greatest density in the vermal lobules VI and VIIA and in the flocculus.Following injections of HRP in the various cerebellar lobules labeled cells are mainly present within limited groups in the N.r.t.. Injections in vermal lobules VI-VIII B give rise to labeled cells within circumscribed areas in the dorsal and ventral parts throughout the rostrocaudal extent of the N.r.t.. In cases with injections in lobule IX or the ventral paraflocculus labeled cells are found ventrally in the rostral half of the N.r.t., while following injections in the vermal lobules I-V labeled cells are mainly found in the ventral and caudal part of the N.r.t.. Following injections in the intermediate and lateral parts of the anterior lobe, Crus I and II, the paramedian lobule and the dorsal paraflocculus labeled cells occur within groups in medial and lateral parts throughout the rostrocaudal extent of the N.r.t.. Following injections in the flocculus labeled cells are found in a very distinct group in the dorsal and rostral part of the N.r.t., While an injection in the nodulus (lobule X) gave rise to a smaller group of labeled neurons in the dorsolateral corner in the caudal part of the N.r.t.. Labeled cells within processus tegmentosus lateralis (p.t.l.) are only found following injections in lobules VI-VIIIA, Crus I and II and the dorsal paraflocculus.From what is known about afferents to the N.r.t., it is concluded that no cerebellar lobule gets information from one only of these sources via the N.r.t.. Visual information can probably be mediated from the superior colliculus via the N.r.t. to the flocculus and to a minor extent to the vermal lobules VI-VIII B, and from the pretectum via the N.r.t. to both vermal and lateral parts of the cerebellum.     

The central cervical nucleus in the cat. III

Summary Injections of ³H-leucine were made into the region of the central cervical nucleus (CCN) in the C1–C4 segments of the spinal cord in 19 adult cats. In the cerebellum labelled mossy fibre terminals were found bilaterally concentrated in lobule I and adjoining parts of lobule II of the anterior lobe. In addition, a fair number of terminals were found in the basal parts of lobules III–VIII. The terminals were mainly found in the vermal zone. Only a minor proportion was observed in the intermediate zone of the anterior lobe and only occasionally were terminals seen in the hemispheral parts. No labelled climbing fibres were observed. Axons probably derived from the CCN were found to ascend in the brain stem, mostly contralateral to the injection site. They passed close to the vestibular nuclear complex and some fibres appeared to terminate in the nucleus x of Brodal and Pompeiano. The majority of axons entered the cerebellum via the superior cerebellar peduncle, a few via the restiform body. The findings suggest that lobules I–II are important recipients of information from the neck.Abbreviations b.c. brachium conjunctivum - c. r. restiform body - DV descending (inferior) vestibular nucleus - fl. flocculus - 1. pmd. ant. pars anterior of paramedian lobule - 1. pmd, copul. pars copularis of paramedian lobule - 1. pmd. post. pars posterior of paramedian lobule - LRN lateral reticular nucleus - LV lateral vestibular nucleus - MV medial vestibular nucleus - N.d. dentate (lateral) nucleus - N.f. fastigial (medial) nucleus - N.i. nucleus interpositus - N. mes. V mesencephalic nucleus and tract of the trigeminal nerve - N.p. pontine nuclei - N.pr. V principal nucleus of the trigeminal nerve - N. V motor trigeminal nucleus - n. VII facial nerve - N. XII hypoglossal nucleus - 01. i. inferior olive - 01. s. superior olive - pfl. d. dorsal paraflocculus - pfl. v. ventral paraflocculus - SV superior vestibular nucleus - Tr. V spinal tract of the trigeminal nerve - x nucleus x of Brodal and Pompeiano (1957) - I-X cerebellar lobules I-X according to Larsell (1953)     

Further studies on the fiber connections of the central cervical nucleus in the cat

Summary The course and cerebellar termination of the axons of the cells in the central cervical nucleus (CCN) was studied in five cats after injections of wheat germ agglutinin-horseradish peroxidase conjugate (WGA-HRP) into the C1-4 segments involving the CCN. In two cats a hemisection was performed ipsilateral to and above the injections in order to prevent transport of WGA-HRP from cerebellar-projecting neurons in lamina VI, just dorsal to the CCN. Labeled axons were found in the brain stem contralateral to the injections just lateral to the spinal trigeminal tract and the vestibular nuclei, and in the reticular formation. Some fibers terminated in the vestibular nuclei (especially group x), the reticular formation, and the inferior olive. The axons entered the cerebellum via the superior cerebellar peduncle. In the cerebellum mossy fiber terminals were found bilaterally in the deep vermal parts of lobules I–VIII. Only in cases without lesions were terminals found in the paramedian lobule ipsilateral to the injection, suggesting that neurons in lamina VI, but not in the CCN, project to the paramedian lobule. In the brain stem retrogradely labeled neurons (possible afferents to the CCN) were found in the vestibular (lateral, inferior and medial) nuclei, the reticular formation, and the trigeminal (spinal and mesencephalic) nuclei. The cerebellar distribution of mossy fiber terminals suggests that spinocerebellar fibers from lower parts of the spinal cord passing through the injection area do not transport WGA-HRP to their terminals.     

Classical conditioning of the nictitating membrane response of the rabbit

The classically conditioned nictitating membrane response (NMR) of the rabbit, a simple form of associative motor learning, is crucially dependent upon the cerebellum. Discrete unilateral lesions of the cerebellar nuclei were made in 20 rabbits. Lesions of the anterior interpositus nucleus (IA) abolished NMR conditioning to light and white noise stimuli on the side of the lesion without affecting unconditional responses. Lesions of the posterior interpositus nucleus, fastigial and dentate nuclei were without effect upon NMR conditioning.     

Anterior and medial thalamic lesions,discriminative avoidance learning,and cingulate cortical neuronal activity in rabbits

Summary Four groups of male albino rabbits were trained to perform a conditioned response (CR, stepping in an activity wheel) to an acoustic (pure tone) conditional stimulus (CS+). A 1.5–2.0 mA shock unconditional stimulus (US) delivered through the grid floor of the wheel was administered 5 s after CS + onset, but stepping during the CS-US interval prevented the US. The rabbits were also trained to ignore a second tone (a negative conditional stimulus or CS-) of different auditory frequency than the CS+, that was presented in an irregular order on half of the conditioning trials but never followed by the US. One group had bilateral electrolytic lesions in the medial dorsal (MD) thalamic nucleus, a second group had combined bilateral lesions in the MD and the anterior thalamic nuclei, and a third group had no lesions. The fourth group was composed of rabbits with combined lesions that resulted in only partial damage in the anterior and MD nuclei. In all rabbits, multi-unit activity and field potentials were recorded from the cingulate cortical projection targets of the MD and anterior nuclei. The average rate of acquisition in rabbits with MD and partial lesions was not significantly different from that in controls, but the asymptotic performance in rabbits with lesions was significantly impaired, relative to that in controls. None of the rabbits that had the combined MD and anterior thalamic lesions reached the acquisition criterion. The average proportion of trials in which these rabbits performed avoidance responses during their final training sessions was 0.3, compared to 0.8 in controls. The unconditioned response was not significantly affected by the lesions, nor was there any indication that the lesions impaired the sensory processing of the CSs. These results and the massive traininginduced neuronal discharges shown in past studies to occur in the limbic thalamic neurons indicate that these neurons are importantly involved in the circuitry that mediates discriminative avoidance conditioning in rabbits. The training-induced neuronal activity in cingulate cortex was dramatically attenuated in rabbits with lesions. Differences in the degree of this attenuation between lesion conditions and with respect to training stages were discussed in relation to a theoretical working model of limbic thalamic and cingulate cortical associative functions.     

Cerebellar afferents from the trigeminal sensory nuclei in the cat

Summary The cerebellar afferent projection from the trigeminal sensory nuclei (TSN) was studied by means of retrograde axonal transport of horseradish peroxidase (HRP). The projection is almost exclusively ipsilateral. Three cortical regions, viz., the intermediate-lateral part of lobulus simplex with the adjacent area of lobule V, the rostralmost folia of the paramedian lobule with the surrounding parts of crus I and II, and lobule IX, especially its rostral two folia, are the main targets for the cerebellar afferent fibres. A few fibres reach also the other cerebellar regions, as shown in Fig. 3.Most of the cerebellar afferent fibres originate in the nucleus interpolaris with nucleus oralis as the second most important region. The projection from the principal nucleus is moderate and reaches primarily the area of the crura bordering on the paramedian lobule and lobule IX. The projections from the nucleus caudalis and nucleus mesencephalicus are scanty. The fibres from the latter reach only the vermal region.The findings are discussed in relation to previous anatomical and physiological observations.On leave from the Laboratory of Neurobiology and Department of Anatomy, Faculty of Science, Mahidol University, Bangkok, Thailand, under the Fellowship Program of the Norwegian Agency for International Development (NORAD)     

Olivary projections to the cerebellar nuclei in the cat

Summary Projections from the inferior olive to the cerebellar nuclei have been studied in the cat using Nauta's silver technique. 1. Numerous degenerating terminals occur after lesions in the inferior olivary complex in the medial nuclei of both sides; the degeneration is considerably less in the subnucl. medialis parvicellularis of both sides. In the interpositus nuclei of both sides degenerating terminals are also abundant, especially in their dorsal and lateral parts. In the lateral nucleus a number of degenerating terminals are seen on both sides restricted to dorsal and lateral parts of the nuclei. Degeneration is scanty in the subnucl. lateralis parvicellularis. 2. Projection of the olivocerebellar fibers to the cerebellar nuclei is always bilateral. The fibers originate in the inferior olive and ascend mainly through the contralateral, however, some also through the ipsilateral restiform body. The olivocerebellar fibers that have crossed in the medulla terminate in the medial and the interpositus nuclei of both sides. It is suggested that the degenerating terminals found in the cerebellar nuclei are derived from collaterals of the olivocerebellar tract fibers.