Electrophysiological analysis of the tecto-olivo-cerebellar (lobulus simplex) projection in the rat

An internal model architecture for novelty detection: implications for cerebellar and collicular roles in sensory processing

The cerebellum is thought to implement internal models for sensory prediction, but details of the underlying circuitry are currently obscure. We therefore investigated a specific example of internal-model based sensory prediction, namely detection of whisker contacts during whisking. Inputs from the vibrissae in rats can be affected by signals generated by whisker movement, a phenomenon also observable in whisking robots. Robot novelty-detection can be improved by adaptive noise-cancellation, in which an adaptive filter learns a forward model of the whisker plant that allows the sensory effects of whisking to be predicted and thus subtracted from the noisy sensory input. However, the forward model only uses information from an efference copy of the whisking commands. Here we show that the addition of sensory information from the whiskers allows the adaptive filter to learn a more complex internal model that performs more robustly than the forward model, particularly when the whisking-induced interference has a periodic structure. We then propose a neural equivalent of the circuitry required for adaptive novelty-detection in the robot, in which the role of the adaptive filter is carried out by the cerebellum, with the comparison of its output (an estimate of the self-induced interference) and the original vibrissal signal occurring in the superior colliculus, a structure noted for its central role in novelty detection. This proposal makes a specific prediction concerning the whisker-related functions of a region in cerebellar cortical zone A₂ that in rats receives climbing fibre input from the superior colliculus (via the inferior olive). This region has not been observed in non-whisking animals such as cats and primates, and its functional role in vibrissal processing has hitherto remained mysterious. Further investigation of this system may throw light on how cerebellar-based internal models could be used in broader sensory, motor and cognitive contexts.

Zonal organization of the climbing fiber projection to the flocculus and nodulus of the rabbit: a combined axonal tracing and acetylcholinesterase histochemical study

The localization and termination of olivocerebellar fibers in the flocculus and nodulus of the rabbit were studied with anterograde axonal transport methods [wheatgerm agglutinin-horseradish peroxidase (WGA-HRP) and tritiated leucine] and correlated with the compartments in the white matter of these lobules delineated with acetylcholinesterase histochemistry (Tan et al. J. Comp. Neurol., 1995, this issue). Olivocerebellar fibers originating from the caudal dorsal cap travel through floccular compartments FC2 and FC4 to terminate as climbing fibers in floccular zones FZII and FZIV. Fibers from the rostral dorsal cap and the ventrolateral outgrowth traverse compartments FC1 and FC3, which are interleaved with compartments FC2 and FC4, and terminate in zones FZI and FZIII. Fibers from the rostral pole of the medial accessory olive traverse the C2 compartment and terminate in the C2 zone. FZI-III extend into the adjoining folium (folium p) of the ventral paraflocculus. The C2 zone continues across folium p into other folia of the ventral paraflocculus and into the dorsal paraflocculus. Four compartments and five zones were distinguished in the nodulus. Medial compartment XC1 contains olivocerebellar fibers from the caudal dorsal cap and subnucleus beta that terminate in the XZI zone. Olivocerebellar fibers from the rostral dorsal cap and the ventrolateral outgrowth occupy XC2 and terminate in XZII. The XC4 compartment contains fibers from both the caudal dorsal cap and from the rostral dorsal cap and the ventrolateral outgrowth. The latter terminate in a central portion of the XZIV zone. The dorsomedial cell column projects to the XZIII zone, which is present only in the dorsal part of the nodulus. The rostral medial accessory olive projects to the XZV zone, which occupies the lateral border of the nodulus. These results confirm and extend the conclusions of Katayama and Nisimaru ([1988] Neurosci. Res. 5:424-438) on the zonal pattern in the olivo-nodular projection in the rabbit. Additional observations were made on the presence of a lateral A zone (Buisseret-Delmas [1988] Neurosci. Res. 5:475-493) in the hemisphere of lobules VI and VII. Retrograde labeling of the nucleo-olivary tract of Legendre and Courville ([1987] Neuroscience 21:877-891) was observed after WGA-HRP injections into the inferior olive including the rostral dorsal cap and the ventrolateral outgrowth. The anatomical and functional implications of these observations are discussed.

The tectorecipient zone in the inferior olivary nucleus in the rat

Tecto-olivary and olivocerebellar projections in the rat were investigated in order to identify the tectorecipient zone in the inferior olivary nucleus and to determine whether inferior olivary neurons projecting to the cerebellar tecto-olivo-recipient zones (lobule VII, crus II, lobulus simplex, and paramedian lobule) originate in the different regions within the tectorecipient zone. An electrophysiological method and an axonal transport technique of wheat-germ agglutinin-conjugated horseradish peroxidase were used. The tectorecipient zone was identified in the caudomedial region of the medial accessory olive. Neurons projecting to lobule VII originated in the caudomedial region of the tectorecipient zone, but those to crus II, lobulus simplex, and paramedian lobule originated in its rostrolateral region. These observations suggest that there are two independent tecto-olivo-cerebellar systems: 1) superior colliculus--the medial region of the tectorecipient zone--lobule VII--the caudomedial region of the fastigial nucleus; and 2) superior colliculus--the rostralateral region of the tectorecipient region--crus II, lobulus simplex, and paramedian lobule--the dorsolateral protuberance of the fastigial nucleus.

Electrophysiological analysis of the trigemino-olivo-cerebellar (crura I and II, lobulus simplex) projection in the rat

In albino rats the whisker area was electrically stimulated while climbing fiber responses were surveyed in the cerebellar hemisphere on the ipsilateral side. They were identified both deep in the intercrural sulcus, and in the posterior superior fissure. Histological examination has revealed that the response areas extend longitudinally from the dorsal surface of crus II to the ventral surface of crus I in the intercrural sulcus, and from the rostral surface of crus I to the caudal surface of lobulus simplex in the posterior superior fissure. These are supposed to be transmitted through direct trigemino-olivary projections.

Electrophysiological analysis of the trigemino-tecto-olivo-cerebellar (crus II) projection in the rat

In albino rats the whisker area was electrically stimulated while climbing fiber responses were surveyed in the cerebellar cortex of the tecto-olivo-recipient zone in the medial region of crus II. Climbing fiber responses were evoked by the vibrissal stimulation only in the medial portion of the zone. Based both on lesion experiments of the superior colliculus and on recording multiunit potentials in the inferior olive it is suggested that the trigeminal climbing fiber responses are evoked via the superior colliculus.