D. W. F. Schwarz

Projection of the vestibular nerve to the area 3a arm field in the squirrel monkey (Saimiri Sciureus)

SummaryA projection of the vestibular nerve to the anterior bank of the central sulcus was identified. The zone is small and located within the arm field. Surface positive potentials and negative field potentials in deeper cortical layers were evoked within this field by isolated stimulation of the vestibular nerve. Field potentials after isolated stimulation of the facial and auditory nerves were recorded from distinct cortical locations clearly separate from the vestibular field. The tracks of electrodes which recorded the vestibular negative field potentials were histologically located within area 3a. This cytoarchitectonic area extends from the fundus of the central sulcus onto the cortical surface anterior to this sulcus.

Analysis of human vestibulo-ocular reflex during active head movements.

The human vestibulo-ocular reflex (VOR) was investigated during active head movements utilizing spectral analysis techniques in order to extract phase and gain characteristics for the most natural stimulus conditions. Three different experimental conditions were examined: 1) head rotation in darkness to obtain data permitting a comparison with that mode of VOR analysis which has been mose frequently employed in the past; 2) head rotation while fixating a stationary target light in order to quantify natural compensatory eye movements; and 3) head rotation while fixating a target light which moved with the head as a fast method for the quantification of visuo-vestibular interaction. High frequency head rotation in darkness yielded gains not significantly different from unity-unlike previously reported results for passive rotation (Benson, 1970; Keller, 1978). Possible mechanisms which might explain these results are discussed.

Nucleus ventroposterior inferior (VPI) as the vestibular thalamic relay in the rhesus monkey I. Field potential investigation

SummaryIn order to investigate the thalamic relay of the vestibulo-cortical pathway, field potentials were recorded in the rhesus thalamus under pentobarbital anesthesia. Short latency responses (2.5 msec on the average) upon stimulation in isolation of the vestibular nerve were recorded in the inferior ventroposterior nucleus (VPI). These potentials were abolished after transection of the vestibular nerve but were not affected by total cerebellectomy. Projection of VPI neurons to the primary vestibular cortex was demonstrated by antidromic stimulation. Field potentials with latencies of those observed in the vestibular cortex (about 5 msec) in response to vestibular nerve stimulation were recorded in other areas of the thalamus (ventrobasal, ventrolateral, posterior group, including magnocellular medial geniculate nuclei). Thus, the VPI rather than the other nuclei with long latency responses is likely to be the thalamic relay in the vestibulo-cortical path. The close topographical relationship between vestibular and somatic areas in the cortex is parallelled in the thalamus, the VPI being closely related to VPL and VPM nuclei.

Neuronal responses to eye muscle stretch in cerebellar lobule VI of the cat.

SummaryExtraocular proprioceptive input to cerebellar vermis, lobule VI, was investigated in cats under N2O analgesia by recording neuronal responses to eye muscle stretch. Both optic tracts were transected and the periorbital skin and conjunctiva were locally anaesthetized. Eye rotation within the physiological range was achieved by applying a pull of predetermined length and tension to each of the eight musculi recti at their insertion to the globe. Within lobule VI, only small patches of cortex receive stretch receptor afferents. The information made available by these afferents corresponds to a change of eye position. Minimal responses were dependent upon angular deflections of a few degrees. Maximal response amplitudes were obtained within the physiological range of angular deflections and angular velocities for the units tested. Most cells responded to stretch of more than one muscle. Three types of convergence were found: (1) neurons responding according to a certain direction of a conjugated movement of both eyes, (2) neurons responding to movements in either direction of one plane, (3) more complicated response patterns.

Vestibular responses in the Rhesus monkey ventroposterior thalamus. II. Vestibulo-proprioceptive convergence at thalamic neurons

SummaryThe vestibular thalamic relay in the Rhesus ventrobasal complex, identified in a previous field potential study (part I, Deecke et al., 1974), has now been investigated with neuronal recordings in the thalamus in order to clarify its functional role. In part I, short latency responses (2.5 msec) were found in the corner between VPL, VPM and VPI nuclei, largely including dorsal portions of the VPI nucleus. Field potentials of somewhat longer latency (4–5 msec) were recorded in VPL and in other thalamic nuclei, including the posterior nuclear group.Neuronal responses were recorded in thalamic nuclei of awake flaxedilized Rhesus monkeys. Cells not responding to vestibular stimulation (round window polarisation of either labyrinth) were ignored. The great majority (80%) of those neurons responding to labyrinth polarisation showed convergence with deep somatic (proprioceptive) input from joints and muscles of vertebral column and limbs. 60% of these bimodal neurons responded to movement of cervical joints. Very few vestibularly responsive cells received cutaneous (6.6%), non-optokinetic visual or auditory (2.6% each) input. Proprioceptive fields tended to be large, frequently involving more than one joint, and could be even bilateral. For a few cells the pattern of vestibulo-proprioceptive convergence could be fitted to a coordinated body position that might occur during normal locomotion. 78% of the cells responded to polarisation of both labyrinths, indicating strong bilateral projection.

Human visuo-vestibular interaction as a basis for quantitative clinical diagnostics.

Visuo-vestibular interaction during randomized and sinusoidal head oscillations (0.5-5.0 Hz) was measured by power spectral analysis. It was shown that visual eye movement programmes can adjust the vestibulo-ocular reflex (VOR) gain at frequencies exceeding the dynamic range of visual tracking: above 3 Hz, gains exceeded unity during attempted fixation of a target moving with the subject whereas unity gains prevailed during fixation of an earth-fixed target. At low frequencies, fixation suppression was more efficient (-10 dB) when sinusoidal stimuli rather than randomized oscillations (-3 dB) were employed. Identical results were obtained when the fixation target moved with a total visual surround or against an earth-fixed visual background. Therefore, peripheral vision is normally not important for visual suppression of the VOR, which is dominated by foveal visual tracking at low frequencies.

Response of the human vestibulo-ocular reflex following long-term 2x magnified visual input

SummaryThis study examines the contribution of predictive motor programming to the adjustment of vestibulo-ocular reflex (VOR) gains after exposure to spectacles with a 2x magnification. When fully adapted, subjects exhibited two-fold gain increases with a 3 Hz sinewave stimulus with both an imaginary earth-fixed and imaginary moving target. Before complete adaptation was achieved, quick phases embedded in the slow component were observed intermittently which compensated for insufficient VOR gain. At 0.5 Hz in the same state of full adaptation during fixation of an imaginary earth-fixed target subjects exhibited a gain increase of only approximately 75% indicating that the contribution of VOR adjustment is not sufficient for perfect visual stabilization at lower frequencies. Over the range of random stimulation (0.5–5 Hz), the VOR failed to exhibit complete adaptation. The degree of adaptation derived with a VOR-cancellation task was less overall than that with a task requiring perfect compensatory eye movements. These findings indicate that central motor programmes are required in the adaptive process to achieve visual stability.

Vestibular and auditory cortical projection in the guinea pig (cavia porcellus)

SummaryIsolated electrical stimulation of the vestibular nerve in the guineapig yielded surface positive evoked potentials within the rostral portion of the SI forelimb field. The locus of negative field potential reversal in deeper cortical layers was small. The vestibular field is distinct from those of the auditory and facial nerves. Comparative aspects of vestibular cortical location are discussed. The auditory field corresponds with that of other rodents.

Fine structure of the medial and descending vestibular nuclei in normal rats and after unilateral transection of the vestibular nerve.

Cells and neuropil are of similar structure in the descending and medial vestibular nuclei. Two cell types were found: small neurons and larger cells. Three types of axon terminals have been defined: small and large terminals containing spherical vesicles (SV) and terminals with elongated vesicles (EV). Small SV terminals contact perikarya and dendrites, whereas large SV terminals contact fine dendritic branches. EV terminals contact neurons at perikarya and large as well as small dendritic profiles. SV terminals can be presynaptic to EV terminals. Vestibular nerve transection resulted in degeneration of small SV terminals which were found from the first to fifth postoperative day in the ipsilateral nuclei. Glycogen granules were found in various types of axon terminals bilaterally in the vestibular nuclei 3-5 days after right vestibular transection. They were not observed in normal animals or in degenerating vestibular afferent terminals.

Vestibular evoked potentials in thalamus and basal ganglia of the squirrel monkey (Saimiri sciureus).

In anesthetized squirrel monkeys vestibular representation in the thalamus and basal ganglia was determined by field potential recording using peripheral electrical vestibular nerve stimulation. Vestibular thalamic regions were investigated for cortical connections. Two relatively large thalamic areas, nucleus ventralis posterolateralis, VPL and the posterior nuclear group (Po) received vestibular inputs with short latencies suggesting direct connections with the vestibular nuclei. Antidromic stimulation of the area 3 a vestibular field did not produce responses in any of the vestibular thalamic fields. The vestibular regions in VPL and Po can be antidromically invaded from SI and the anterior parietal lobe respectively. In the striatum vestibular fields were found in the suprathalamic portion of the nucleus caudatus and dorsomedially in the putamen.