Unit activity related to head and eye movements in central thalamus of cats

Neurophysiology of the saccadic system: The reticular formation

This chapter discusses the neurophysiology and function of subcortical circuits and cortical areas involved in saccade generation. While cells within the different nuclei of the brainstem reticular formation shape the temporal details of ipsiversive horizontal and vertical/cyclotorsional saccade components, the cerebellar flocculus, vermis and fastigial nucleus are thought to modulate these saccadic waveforms. Burst neurons in the deep layers of the superior colliculus encode the saccade vector in the contralateral field by a localized population in a motor-error map. The complexity of the saccadic system is evident in the different subclasses of SC cells, ranging from purely visual, to visual-motor, purely motor, and quasi-visual cells. Movement-related activity in all SC cells is dissociated from the retinotopic visual activity. The chapter further discusses neurophysiological findings obtained from the substantia nigra (pars reticulata), the medial thalamus, the frontal eye fields, the supplementary motor area and the parietal lobes, discussing the ever more complex response patterns of their neurons in relation to saccades.

Oculomotor integration in patients with a pulvinar lesion

The pulvinar nucleus of the thalamus, with its connections to visual areas and to frontal and parietal oculomotor cortex, might serve as a nexus for integrating cortical control of voluntary eye movements with reflexive eye movements generated by the superior colliculus. To investigate this hypothesis, we tested five patients with a unilateral lesion of the pulvinar on the oculomotor capture paradigm. In this task, participants have to ignore a distractor item and make a saccade to a target in a visual search display. Results showed that the interference of the distractor was stronger when it was presented contralateral to their lesion compared to when it was presented in the ipsilesional visual field. These findings were confirmed by an additional single case experiment in which we measured saccade trajectory deviations as evoked by a single distractor. These results show that the pulvinar is involved in the successful influence of higher order signals (like our goals and intentions) on the guidance of our eye movements.

Effects of lesions of the human posterior thalamus on ocular fixation during voluntary and visually triggered saccades

OBJECTIVE

To investigate the role of the posterior thalamus in controlling voluntary and visually triggered eye movements and ocular fixation.

METHODS

The latency to initiate saccades to peripheral targets (visually triggered) and in response to verbal commands (voluntary) was measured in three patients with unilateral lesions of the posterior thalamus, in normal controls, and in neurological controls with Parkinson's disease. On half the trials a fixation point offset simultaneously with target onset, and on half it remained visible.

RESULTS

Offset of the fixation point simultaneous with target onset decreased saccade latency for both voluntary and visually triggered eye movements in controls, but only for voluntary saccades in patients with thalamic lesions.

CONCLUSIONS

These findings suggest that separate neural systems control fixation when making voluntary and visually triggered eye movements, and that the thalamus is involved in the control of fixation for visually triggered but not for voluntary saccades.

Brainstem projections to midline and intralaminar thalamic nuclei of the rat

The projections from the brainstem to the midline and intralaminar thalamic nuclei were examined in the rat. Stereotaxic injections of the retrograde tracer cholera toxin beta -subunit (CTb) were made in each of the intralaminar nuclei of the dorsal thalamus: the lateral parafascicular, medial parafascicular, central lateral, paracentral, oval paracentral, and central medial nuclei; in the midline thalamic nuclei-the paraventricular, intermediodorsal, mediodorsal, paratenial, rhomboid, reuniens, and submedius nuclei; and, in the anteroventral, parvicellular part of the ventral posterior, and caudal ventral medial nuclei. The retrograde cell body labeling pattern within the brainstem nuclei was then analyzed. Nearly every thalamic site received a projection from the deep mesencephalic reticular, pedunculopontine tegmental, dorsal raphe, median raphe, laterodorsal tegmental, and locus coeruleus nuclei. Most intralaminar thalamic sites were also innervated by unique combinations of medullary and pontine reticular formation nuclei such as the subnucleus reticularis dorsalis, gigantocellular, dorsal paragigantocellular, lateral, parvicellular, caudal pontine, ventral pontine, and oral pontine reticular nuclei; the dorsomedial tegmental, subpeduncular tegmental, and ventral tegmental areas; and, the central tegmental field. In addition, most intralaminar injections resulted in retrograde cell body labeling in the substantia nigra, nucleus Darkschewitsch, interstitial nucleus of Cajal, and cuneiform nucleus. Details concerning the pathways from the spinal trigeminal, nucleus tractus solitarius, raphe magnus, raphe pallidus, and the rostral and caudal linear raphe nuclei to subsets of midline and intralaminar thalamic sites are discussed in the text. The discussion focuses on brainstem-thalamic pathways that are likely involved in arousal, somatosensory, and visceral functions.

Effects of centrolateral or medial thalamic lesions on motor coordination and spatial orientation in rats

The effects of central lateral (CL) or medial thalamic lesions were evaluated on tests requiring balance and equilibrium (rotorod, stationary beam, and hole-board), and spatial orientation (Morris water maze). Lesions of the medial nuclei impaired rotorod and spatial orientation, while CL lesions impaired only rotorod performance. Neither lesion affected stationary beam and hole-board tests or visuomotor guidance while swimming toward a visible platform. These spatial and sensorimotor deficits cannot be explained by reduced cerebral activation, but instead are attributable to specific projections from the anterior intralaminar nuclei to the basal ganglia and neocortex.

Association between dendritic lamellar bodies and complex spike synchrony in the olivocerebellar system

Dendritic lamellar bodies have been reported to be associated with dendrodendritic gap junctions. In the present study we investigated this association at both the morphological and electrophysiological level in the olivocerebellar system. Because cerebellar GABAergic terminals are apposed to olivary dendrites coupled by gap junctions, and because lesions of cerebellar nuclei influence the coupling between neurons in the inferior olive, we postulated that if lamellar bodies and gap junctions are related, then the densities of both structures will change together when the cerebellar input is removed. Lesions of the cerebellar nuclei in rats and rabbits resulted in a reduction of the density of lamellar bodies, the number of lamellae per lamellar body, and the density of gap junctions in the inferior olive, whereas the number of olivary neurons was not significantly reduced. The association between lamellar bodies and electrotonic coupling was evaluated electrophysiologically in alert rabbits by comparing the occurrence of complex spike synchrony in different Purkinje cell zones of the flocculus that receive their climbing fibers from olivary subnuclei with different densities of lamellar bodies. The complex spike synchrony of Purkinje cell pairs, that receive their climbing fibers from an olivary subnucleus with a high density of lamellar bodies, was significantly higher than that of Purkinje cells, that receive their climbing fibers from a subnucleus with a low density of lamellar bodies. To investigate whether the complex spike synchrony is related to a possible synchrony between simple spikes, we recorded simultaneously the complex spike and simple spike responses of Purkinje cell pairs during natural visual stimulation. Synchronous simple spike responses did occur, and this synchrony tended to increase as the synchrony between the complex spikes increased. This relation raises the possibility that synchronously activated climbing fibers evoke their effects in part via the simple spike response of Purkinje cells. The present results indicate that dendritic lamellar bodies and dendrodendritic gap junctions can be downregulated concomitantly, and that the density of lamellar bodies in different olivary subdivisions is correlated with the degree of synchrony of their climbing fiber activity. Therefore these data support the hypothesis that dendritic lamellar bodies can be associated with dendrodendritic gap junctions. Considering that the density of dedritic lamellar bodies in the inferior olive is higher than in any other area of the brain, this conclusion implies that electrotonic coupling is important for the function of the olivocerebellar system.

Signal processing in the C2 module of the flocculus and its role in head movement control

The major novel findings described and reviewed in the present study have all been demonstrated in the C2 module, which is formed by the rostral medial accessory olive, posterior interposed nucleus of the cerebellum, and zone C2. We show (1) that expression of dendritic lamellar bodies and dendrodendritic gap junctions in the rostral medial accessory olive are both down regulated by removal of the GABAergic input from the posterior interposed nucleus of the cerebellum to electrotonically coupled olivary dendrites; (2) that the high density of dendritic lamellar bodies in the rostral medial accessory olive can be correlated with a relatively high level of CS synchrony in the C2 zone of the flocculus; and (3) that the C2 zone of the flocculus is involved in head movements and probably gaze control. These results support the hypothesis that dendritic lamellar bodies are associated with dendrodentritic gap junctions, and they suggest that appropriate executions of compensatory head and eye movements require particular levels of complex spike synchrony in the flocculus.

Neuronal activity related to head and eye movements in cat superior colliculus

1. Movement-related discharges were recorded from single cells in the superior colliculus of alert cats while they made eye saccades (with head fixed) or gaze saccades (with head free). 2. Visual and auditory stimuli were used as saccade targets. In addition, saccades were made to the remembered location of targets and spontaneously, in the absence of targets, during intertrial intervals. 3. When the head was still and the cat performed either spontaneous saccades or saccades to remembered targets, only one class of tectal neurone, the saccade-related burst neurone, inevitably discharged prior to all saccades of appropriate amplitude and direction. 4. Neurones with longer lead times and less intense presaccadic discharges were obligately linked only to visually elicited saccades. The discharge of some of these long-lead neurones was also influenced by the spatial position of the visual target (that is, by craniotopic motor error), while that of the saccade-related burst neurones was a function of retinocentric motor error. 5. Most neurones which discharged before head movements also discharged before eye movements and had large, contralateral movement fields.

The ventral lateral geniculate nucleus in the cat: thalamic and commissural connections revealed by the use of WGA-HRP transport

The present investigation was carried out to clarify the topographical details of both the origin and terminal site of the thalamic projections and the commissural connections of the ventral lateral geniculate nucleus (LGNv) in the cat by using bidirectional transport of wheat germ agglutinin conjugated to horseradish peroxidase (WGA-HRP). Thalamic projections: Unilateral injections of WGA-HRP into the LGNv produced orthograde labeling in the intralaminar nuclei bilaterally and in the lateralis posterior (LP) and the pulvinar (Pul) nucleus ipsilaterally. In the intralaminar nuclei the rostral part of the nucleus centralis lateralis (CL) was most densely labeled by orthogradely transported material, particularly in its dorsal and lateral large-celled portion. Other intralaminar nuclei--such as the nucleus paracentralis, centralis medialis, and centralis dorsalis--also were labeled bilaterally with ipsilateral predominance, but no labeling was detected in the caudal portion of the CL and the centromedian and parafascicular nuclei. In the Pul, labeling of terminal ramifications was found to be concentrated in a region just medial to the so-called retinorecipient zone of the Pul as a slim band of labeling inclining dorsoventrally. In the LP, fine labeled fibers were located in the lateral portion of the LP. Commissural connections: Commissural fibers crossed in the dorsal part of the posterior commissure and reached the most caudal part of the contralateral LGNv. Labeling in the contralateral LGNv was concentrated in the dorsomedial part of the medial zone that extends medially to the middle portion of the cerebral peduncle. Origins of the commissural connections arose mostly from the medial zone that roughly corresponds to the commissural terminal zone and partly from aberrant cells dispersed among optic tract fibers. From these results, together with the previous studies, it is concluded that although the cat's LGNv has connections with diverse structures in the central nervous system, the origin and terminal site of the connections are partially segregated within the nucleus, which suggests that the LGNv may contain functional subsystems.