Possible visual pathways to the cat vestibular nuclei involving the nucleus prepositus hypoglossi

Non-cerebellar afferents from visual relays to the vestibular nuclei (VN) of the cat have been re-evaluated with the use of the horseradish peroxidase technique. From our data it can be concluded that: (1) A monosynaptic projection from the nucleus reticularis tegmenti pontis to the VN can be excluded. (2) Monosynaptic projections from the superior colliculus and some of the pretectal nuclei (nucleus of the optic tract, olivary pretectal nucleus) to the nucleus prepositus hypoglossi may constitute polysynaptic visual afferents to the VN, which would account for the residual visual sensitivity of the VN neurons after cerebellar or inferior olivary lesions.

Non-cerebellar visual afferents to the vestibular nuclei involving the prepositus hypoglossal complex: an autoradiographic study in the rat

Radioactive amino-acids were injected into the nucleus reticularis tegmenti pontis (NRTP) and the pretectum (PT) in the rat. Beside the labeling of the several nuclei which are known to receive afferents of either the NRTP and/or the PT, monosynaptic projections from these two structures to the prepositus hypoglossal complex (PHN) were demonstrated. Pretectal visual inputs to the vestibular nuclei (VN) may thus be conveyed not only by the classical PT-inferior olive-cerebellar route, but also by two other non-cerebellar ones involving the strong efferent projections of the PHN onto the VN. These last two pathways are strong candidates to account for the residual visual sensitivity of VN neurons after cerebellectomy or inferior olive lesions.

Single neuron activity related to natural vestibular stimulation in the cat's visual cortex

This study examines the possibility of a vestibular input to the visual cortex using chronically implanted cats subjected to horizontal sinusoidal rotation in the dark. In areas 17 and 18 the activity of respectively 14% and 11% of units was modified by vestibular stimulation. Both non-specific and specific influences were observed. Specific influences (42% in area 17 and 33% in area 18) were similar to the types of responses recorded in the vestibular nuclei, and were encountered more frequently within the cortex subserving the peripheral visual field. Our results could provide a neurophysiological basis for some psychophysiological observations concerning visuo-vestibular interactions.

Retinotopic organization of extra-retinal saccade-related input to the visual cortex in the cat

Single unit activity of 842 cells has been recorded in cat visual cortex and analyzed with respect to vestibular induced, and spontaneous saccadic eye movements performed in the dark. This study has been done in awake, chronically implanted cats, subsequently placed in "acute" conditions to achieve the precise retinotopic mapping of the cortical areas previously investigated. In areas 17 and 18, respectively, 27% and 24% of the cells tested were influenced by horizontal saccadic eye movements in the dark (E.M. cells). In the Clare-Bishop area, the proportion of E.M. cells was 12%, while only 2% of such cells were found in areas 19 and 21. The distribution of E.M. cells in areas 17 and 18 with respect to retinotopy showed that E.M. cells were more numerous in the cortical zones devoted to the representation of the area centralis (38% in area 17, 27% in area 18) than in the zones subserving the periphery of the visual field (17% and 12%, respectively). Two of the characteristics of E.M. cell activations appear dependent on the retinotopic organization. First, larger number of E.M. cells presenting an asymmetry in their responses to horizontal saccadic eye movements in opposite directions (directional E.M. cells) were encountered in the cortical representation of the peripheral visual field. 53% of E.M. cells recorded in area 17 and 71% in area 18 were directional in the cortex corresponding to the peripheral visual field. This percentage was of 23% and 25% respectively in the cortex devoted to area centralis. Second, E.M. cells were found to have a latency from the onset of the saccade systematically larger than 100 ms (i.e., they discharged at, or after the end of the eye movement) if they were located in the cortical representation of the area centralis, while E.M. cells related to the peripheral visual field displayed a wider range of latencies (0-240 ms). Results obtained in Clare Bishop area, although limited to the representation of the peripheral visual field, were quantitatively and qualitatively similar to those observed in the homologous retinotopic zones of areas 17 and 18. It is concluded that an extra-retinal input related to oculomotor activity is sent to the cat visual cortex and is organized, at least in areas 17 and 18, with respect to the retinotopic representation of the visual field. These data support the hypothesis of a functional duality between central and peripheral vision and are discussed in the context of visual-oculomotor integration.

Thalamic projections to area 17 in a prosimian primate, Microcebus murinus

Electrophysiological recording of single neurons was used to describe the representation of visual space in area 17, and the technique of retrograde transport of horseradish peroxidase (HRP) was applied to relate these results to projections from the thalamus in the prosimian primate Microcebus murinus. The visuotopic organization of area 17 was found to resemble that of other primates. On the dorsal surface, the border of area 17 corresponds to the representation of the vertical meridian. Proceeding medially across the surface the location of receptive fields descends along the vertical meridian, while moving caudally receptive fields progress temporally. Most of the dorsolateral surface is devoted to central vision and corresponds to a well developed area centralis. Following HRP injections in striate cortex, columns of labeled cells were found in the dorsal lateral geniculate (dLGN) extending orthogonally across all six layers. These columns run in a general ventrodorsal and caudorostral direction, parallel to a line connecting the cellular discontinuities corresponding to the optic disc. These discontinuities are present in magnocellular layer 1 and parvocellular layers 5 and 6, thus receiving from the nasal retina of the contralateral eye. The representation of the vertical meridian is situated in the ventromedial portion of the dLGN, and the monocular field is represented in the dorsal extremity. Anterior dLGN projects to the calcarine fissure (lower field) and posterior dLGN projects to the ventral surface of the cortex (upper field). Extrageniculate input to area 17 was found to originate from the pulvinar. HRP labeled cells were located in two distinct divisions of this nucleus, the cytoarchitecture of which is described. In addition, projections to occipital cortex were found to arise from the intralaminar nuclei.

Afferent and efferent connections of the parabigeminal nucleus in cat revealed by retrograde axonal transport of horseradish peroxidase

Afferent and efferent connections of the parabigeminal nucleus (PBG) of the cat have been demonstrated by means of horseradish peroxidase (HRP) tracing technique. Following HRP injection in the PBG, labelled cells were observed mainly in the deep layers of the ipsilateral superior colliculus (SC). The other labelled structures were the prepositus hypoglossi complex (PH), the ventral nucleus of the lateral geniculate body (LGV), the locus coeruleus, the cuneiform nucleus, the periaqueductal gray and the dorsomedial hypothalamic area. Efferent projections of the PBG were investigated by HRP injection in SC, LGV, PH, hypothalamus and in some acoustic relays, i.e. medial geniculate body and inferior colliculus. Only the PBG-SC projection appeared to be well systematized. The positive labelling of the PBG following injection of LGV and hypothalamus is discussed in terms of the specificity of the injection. The absence of afferent and efferent connections of the PGB with any acoustic relay tends to exclude this nucleus from the auditory system in contrast to previous suggestions. On the basis of the close reciprocal PBG-SC connections a possible role of the PBG within visuomotor tectal function is proposed.

A new vestibular thalamic area: electrophysiological study of the thalamic reticular nucleus and of the ventral lateral geniculate complex of the cat

Single unit recordings were carried out in the reticularis thalamic nucleus (RT) and the ventral lateral geniculate body (LGv) of chronically prepared alert cats under sinusoidal vestibular stimulation in the horizontal plane. Optokinetic stimulation was also used. Of the 57 recorded neurons, 12 present vestibular modulation in the dark, analogous to Duensing's and Schaefer's (1958) type I response in the vestibular nuclei. Responses of 26 cells are similar to response of type II vestibular neurons and 14 units have a type III response; the 5 remaining cells were activated by vestibular stimulation in the vertical sagittal plane. The majority of these cells does not present detectable direct visual responses, but 50% can be driven by optokinetic stimulation. 74% of types I, II and III neurons show saccadic resonses to vestibular nystagmic saccades in the dark. About 60% present similar saccadic modulations during optokinetic nystagmus and 55% keep this response for spontaneous saccades in the dark or in front of a striped background. The saccadic responses are constant for a given neuron in all cases of stimulation with latencies ranging from 30 msec prior to the beginning of the saccade to 120 msec after its onset. The histological localization of these units falls on one hand into the caudal part of the RT nucleus (type III neurons) above the dorsal lateral geniculate nucleus and on the other hand within the internal subdivision of the LGv and its rostral limit (all other types). The significance of this new, saccadic and vestibular focus in the feline thalamus is discussed in relation with the two previously known vestibular thalamic relays in terms of interrelations between the vestibular and the visual systems.

Discharge properties of neurons in the monkey thalamus tested with angular acceleration, eye movement and visual stimuli

Monkeys were trained to make visually evoked eye movements while undergoing simultaneous head rotation. Single units were recorded in the pregeniculate nucleus (PGN). PGN neurons discharged during each saccade, but there was no change in activity with horizontal head acceleration or with various combinations of head and smooth pursuit eye movements as previously described in the cat. Therefore, the anatomical homology between LGNv and PGN does not appear to have a neurophysiological basis. Neurons in the oral part of VPL or occasionally in VPI discharged as a function of head velocity but not with saccades, smooth pursuit or fixation eye movements, nor after brief light flashes or during smooth pursuit across structured backgrounds. This suggests that VPLo and VPI are only vestibular relay nuclei and not concerned with vestibular/visual or vestibular/oculomotor interactions.