Neuronal circuitry controlling the near response.

Experiments in primates have contributed greatly to our understanding of the neural mechanisms involved in the eye movements required to view objects at different distances. Early work focused on the circuitry for generating horizontal vergence eye movements alone. However, vergence eye movements are associated with lens accommodation and are usually accompanied by saccadic eye movements, so more recent work has been directed at understanding the interactions between vergence and accommodation, and between vergence and saccades. A new model explains the neural basis for interactions between vergence and accommodation by a neural network in which pre-motor elements are shared by these two systems. The effects of saccades on vergence eye movements appear to be the result of shared pre-motor circuits as well. Current evidence suggests that pontine omnipause neurons, known to be crucial for the generation of saccades, play an important role in the vergence pre-motor circuitry.

Retinal projections to the pretectum in the pigeon (Columba livia).

The retinal projection to the pretectum in the pigeon has previously been described in detail only by means of anterograde degeneration techniques (Repérant, '73). The present study reinvestigated these retinal projections by using the more sensitive anterograde autoradiographic technique. In general, our results confirm and extend those of Repérant ('73). We have found that three pretectal nuclei--the nucleus lentiformis mesencephali, tectal gray, and the area pretectalis--received heavy retinal input. A fourth pretectal nucleus, pretectalis diffusus, receives a slight retinal input. The nucleus lentiformis mesencephali can be divided into two closely apposed subnuclei that are cytoarchitecturally similar. We have termed them "lentiformis mesencephali, pars medialis" and "lentiformis mesencephali, pars lateralis." The tectal gray can be divided into a rostral, retinorecipient region and a narrow, caudal, nonretinorecipient region. The cytoarchitecture and retinal terminal field in area pretectalis have been described previously by us (Gamlin et al., '84). Pretectalis diffusus is located caudal to the retinorecipient dorsal thalamus and rostral to area pretectalis. Localized retinal injections of 3H proline delineated the number and extent of the retinal representations in the pretectum. Separate retinal representations were present in lentiformis mesencephali, pars medialis, lentiformis mesencephali, pars lateralis, the tectal gray, area pretectalis, and pretectalis diffusus. Only in the lentiformis mesencephali, pars medialis, lentiformis mesencephali, pars lateralis, and the tectal gray could the retinal representations be analyzed. Whereas the retinal representations in the lentiformis mesencephali, pars medialis and the tectal gray are comparable, the retinal representation in the lentiformis mesencephali, pars lateralis is different, being a mirror-image mediolaterally. In this study we introduce a conservative nomenclature for the retino-recipient pretectal nuclei that is consistent with earlier studies, in particular, those of Kuhlenbeck ('39), but has been modified in the light of our findings. We believe that this nomenclature, combined with the detailed cytoarchitectural descriptions provided, should facilitate future studies of the avian pretectum.

Projections of the retinorecipient pretectal nuclei in the pigeon (Columba livia).

We have used anterograde autoradiographic and retrograde HRP techniques to investigate the efferent connections of the retinorecipient pretectal nuclei in the pigeon. In the accompanying paper we identified these nuclei in the pigeon as the nucleus lentiformis mesencephali--pars lateralis and pars medialis, the tectal gray, the area pretectalis, and pretectalis diffusus. Although there are reports of a few of the projections of these nuclei, they had not previously been the subject of a detailed study. We found that different cell types in the lentiformis mesencephali, pars medialis and the lentiformis mesencephali, pars lateralis have descending projections to different targets. These targets include the inferior olive, the cerebellum, the lateral pontine nucleus, the nucleus papillioformis, the nucleus of the basal optic root, the nucleus mesencephalicus profundus, pars ventralis, the nucleus principalis precommissuralis, and the stratum cellulare externum. We found that a few cells in the lentiformis mesencephali project to the medial pontine nucleus, but that a much heavier projection arises from the nucleus laminaris precommissuralis, which is medial to the nucleus lentiformis mesencephali, pars medialis. The tectal gray has predominantly ascending projections to the diencephalon. The nuclei that it projects to are the nucleus intercalatus thalami, the nucleus of the ventral supraoptic decussation, the nucleus posteroventralis, the ventral lateral geniculate nucleus, the nucleus dorsolateralis medialis, and the nucleus dorsolateralis anterior. The tectal gray also projects topographically to layers 4 and 8-13 of the optic tectum. Area pretectalis has both ascending and descending projections. It has ipsilateral ascending projections to the nucleus dorsolateralis anterior, pars magnocellularis, the nucleus lateralis anterior, and the nucleus ventrolateralis thalami. It has ipsilateral descending projections to the central gray, the nucleus of the basal optic root, pars dorsalis, the lateral pontine nucleus, and the deep layers of the optic tectum. It has contralateral projections to the area pretectalis, the nucleus Campi Foreli, the interstitial nucleus of Cajal, the nucleus of Darkschewitsch, the cerebellum, and the Edinger-Westphal nucleus. The efferent projections of pretectalis diffusus are limited. It projects contralaterally to the pretectalis diffusus, and ipsilaterally to the nucleus of the ventral supraoptic decussation, the lateral pons, and the cerebellum.4

A second ascending visual pathway from the optic tectum to the telencephalon in the pigeon (Columba livia).

Previous studies in the pigeon (Karten and Revzin: Brain Res. 2:368-377, '66; Karten and Hodos: J. Comp. Neurol. 140:35-52, '70) have described an ascending tectofugal visual pathway from the optic tectum to the ectostriatum by way of the nucleus rotundus of the thalamus. This present study used anterograde autoradiographic and retrograde horseradish peroxidase pathway-tracing techniques to investigate another ascending tectofugal pathway in the pigeon. Injections of 3H-proline/leucine confirmed a previous report that the optic tectum projects to the nucleus dorsolateralis posterior of the thalamus (DLP). This projection is predominantly ipsilateral and is confined to a large-celled caudal region of the nucleus (DLPc); the rostral region of the nucleus (DLPr) is not tectorecipient. Injections of horseradish peroxidase in DLPc labeled cells predominantly ipsilaterally in layers 8-15 of the optic tectum. Injections of 3H-proline/leucine placed in the DLPc labeled a discrete region of the ipsilateral telencephalon. Similar injections of DLPr labeled a contiguous, but more rostral, region of the neostriatum intermedium. Nissl- and silver-stained material indicated that the region in which DLP terminates is cytoarchitecturally distinct from ventromedial ectostriatal core and belt. Injections of horseradish peroxidase at various locations in the neostriatal DLP terminal field demonstrated a rostrocaudal ordering of the DLP projection upon the neostriatum intermedium. Single-unit recording demonstrated that cells in DLPc respond to whole-field illumination at the same latency as cells in the nucleus rotundus, indicating that the tecto-DLPc-neostriatal pathway transmits visual information to the telencephalon. We suggest that comparable pathways may exist in both reptiles and mammals.

The Edinger-Westphal nucleus: sources of input influencing accommodation, pupilloconstriction, and choroidal blood flow.

This study used neuroanatomical techniques to investigate sources of afferents to the Edinger-Westphal nucleus (EW) of the pigeon. The EW contains the parasympathetic preganglionic neurons that, by way of the oculomotor nerve, project to the ciliary ganglion (Narayanan and Narayanan, '76; Lyman and Mugnaini, '80). The ciliary ganglion, in turn, innervates the internal musculature of the eye; the ciliary body, the iris sphincter muscle, and the smooth muscle of choroidal blood vessels (Marwitt et al., '71; Pilar and Tuttle, '82). In the bird, the neurons in the ciliary ganglion that innervate the iris sphincter muscle and the ciliary body receive input specifically from cells in the lateral EW (EWl), whereas those that innervate choroidal blood vessels receive input from cells in the medial EW (EWm) (Reiner et al., '83). Thus neurons in the EWl mediate pupilloconstriction and accommodation, whereas neurons in the EWm modulate choroidal blood flow. To study the afferents to EW, injections of horseradish peroxidase (HRP) were placed in this nucleus. These injections resulted in labeled cells in the area pretectalis, a retinorecipient pretectal nucleus and the suprachiasmatic nucleus, a retinorecipient hypothalamic nucleus. We have previously identified both these areas as being sources of afferents to EW (Gamlin et al., '82, '84). In addition, these HRP injections into EW resulted in labeled cells in the medial mesencephalic reticular formation (MRF) lateral and ventral to the oculomotor nucleus and in a localized area of the rostral lateral mesencephalic reticular formation (LRF) dorsolateral to nucleus subpretectalis. Injections of tritiated amino acids into the MRF labeled the entire EW, while such injections into the LRF labeled only the lateral EW. Both of these projections were predominantly contralateral. This study has identified the sources of two previously undocumented inputs to the avian EW. Both sources of input, the MRF and rostral LRF, receive afferents from visuomotor areas of the telencephalon and visual structures in the midbrain. The MRF input to EW could have either direct or modulatory influences on pupil diameter, accommodation, and choroidal blood flow. The LRF input to EW could play a role in controlling accommodation and possibly the pupillary near response.

Interconnections between the primate cerebellum and midbrain near-response regions.

The goal of this study was to determine the pattern of the connections between the midbrain and cerebellum that may play a role in the modulation of the near-response in the macaque. Injection of the retrograde tracer wheat germ agglutinin conjugated horseradish peroxidase (WGA-HRP) into the physiologically identified midbrain near-response region, which includes the supraoculomotor area, labelled cells throughout the deep cerebellar nuclei. However, labelled cells were particularly concentrated in the ventrolateral corner of the contralateral posterior interposed nucleus and in the contralateral and, to a lesser extent, the ipsilateral fastigial nuclei. Subsequently, injections of WGA-HRP were used to define the midbrain terminations of the deep cerebellar nuclei. Fastigial nucleus injections labelled terminals in a band along the border between the oculomotor nucleus and the supraoculomotor area that included the Edinger-Westphal nucleus. Injections of the posterior interposed nucleus labelled terminals in the portion of the supraoculomotor area dorsal to the fastigial projection and did not involve the Edinger-Westphal nucleus. In both cases, the terminal label was primarily found contralaterally. In contrast, retrogradely labelled cells were primarily found ipsilaterally within the supraoculomotor area following cerebellar injections. Retrogradely labelled cells projecting to the deep nuclei were also found bilaterally in the anteromedian nucleus, along with sparse terminal label. Taken as a whole, these results demonstrate the presence of a highly specific pattern of labelling in the supraoculomotor area, which may indicate that the posterior interposed nucleus and the fastigial nucleus play different roles in the control of the near-response. Alternatively, these projections may subserve other functions, such as modulating the pupillary light reflex. The fact that the projection from the deep nuclei is primarily contralateral, while the supraoculomotor projection to the deep nuclei is primarily ipsilateral, suggests that this may not be a simple feedback system, but may instead be involved in balancing the gains in the two eyes. In sum, physiological experiments have indicated the presence of near-response neurons in the midbrain supraoculomotor area and have indicated that the cerebellum may play a role in modulating the components of the near-response, as well as activity in the intrinsic eye muscles. The present experiments suggest a pattern of connections that might subserve this cerebellar modulation.

The neural substrate for the pupillary light reflex in the pigeon (Columba livia).

The neural substrate of the pupillary light reflex in the pigeon was investigated using anatomical, stimulation, and lesion techniques. In birds, as in mammals, the sphincter pupillae muscle (which constricts the iris) is innervated by cells in the ciliary ganglion (Pilar and Tuttle, '82). These cells are in turn innervated by cells in the Edinger-Westphal nucleus (EW) (Cowan and Wenger, '68; Narayanan and Narayanan, '76; Lyman and Mugnaini, '80). The efferent link of the pupillary light reflex must therefore involve cells in EW. To study the central course of this reflex pathway, injections of horseradish peroxidase (HRP) were placed in EW. These injections labeled cells in a number of regions including a contralateral pretectal nucleus, area pretectalis (AP). Only a limited number of cells in AP project to EW. Injections of tritiated amino acids into AP labeled a discrete region of the contralateral EW. This projection is confined to a dorsolateral region of caudal EW and overlies the somata of approximately 100 cells. Tritiated proline was injected into the eye, and the results confirmed an earlier report (Reperant, '73) that AP receives retinal input from the contralateral eye. Immunohistochemical studies demonstrated fibers in AP that stained positively for substance-P-like, enkephalin-like and tyrosine-hydroxylase-like immunoreactivity. Injections of HRP were placed in AP to examine the retinal ganglion cells mediating the reflex. Cells with an average diameter of approximately 14 microns (5-25 microns range) were labeled and averaged approximately 6 microns greater in diameter than the retinal ganglion cells (mean = 7.3 microns) labeled by an optic chiasm injection. The cells labeled by AP injections were distributed unevenly throughout the retina with a higher concentration in the central and temporal retina and a paucity in the red field and fovea. Our results demonstrate that AP receives input from a distinct subpopulation of large retinal ganglion cells that comprises a very small percentage of the total population of retinal ganglion cells. Unilateral lesions of AP abolished the pupillary light reflex in the eye contralateral to the lesion; stimulation of AP elicited pupilloconstriction in the eye contralateral to the stimulation site. These results delineate the central course of the pupillary light reflex pathway in the pigeon and identify the retinal ganglion cells that subserve this reflex. They show that, at every point in the pathway, only a few cells mediate this simple reflex.(ABSTRACT TRUNCATED AT 400 WORDS)

Projection of the nucleus pretectalis to a retinorecipient tectal layer in the pigeon (Columba livia).

The avian optic tectum is composed of at least 15 separate laminae that are distinguishable on the basis of their morphological features and patterns of afferent and efferent connectivity. Layer 5b, a major retinorecipient layer, exhibits dense, dust-like, neuropeptide Y-positive (NPY+) immunoreactive labeling, whereas sparse, larger caliber NPY+ fibers are found in laminae 4 and 7. Anterograde and retrograde labeling techniques, immunohistochemistry, and retinal lesion studies were used to determine the source of this tectal NPY+ labeling. NPY+ was not detectable in cells of the optic tectum or in retinal ganglion cells, and retinal ablation did not diminish the abundance of tectal NPY+ fibers. Neurons of two nuclei previously shown to be sources of tectal input, the nucleus pretectalis (PT) and the intergeniculate leaflet (IGL; Brecha, 1978), were found to be NPY+. Unilateral injection of retrograde tracers into the tectum resulted in bilateral labeling of neurons within PT, and injections of anterograde tracer into PT confirmed that this nucleus projected bilaterally to layer 5b of the optic tectum. Unilateral lesions of PT nearly eliminated NPY+ fibers in the ipsilateral layer 5b and significantly reduced them in the contralateral layer 5b. Bilateral lesions of PT eliminated NPY+ fibers bilaterally in layer 5b. However, these PT lesions had little effect on the NPY+ fibers in layers 4 and 7. Combined retrograde and immunohistochemical studies showed that NPY+ neurons of the IGL project to the optic tectum, and anterograde studies demonstrated that IGL projects to layers 4 and 7. The NPY+ projection to laminae 5b from PT is one of many inputs, which include cholinergic afferents from the nucleus isthmi parvicellularis, terminals from retinal ganglion cells, and dendrites of layer 13 neurons (Karten et al., 1993). The NPY+ input to layer 5b may modulate visual information flow from retinal input to various tectal neurons, including those in layer 13.

Pupil responses to stimulus color, structure and light flux increments in the rhesus monkey.

Visual stimuli that isolate pupil color and pupil grating responses in human vision have been used to investigate the properties, of stimulus-specific pupil responses in the rhesus monkey. We measured and compared pupil responses to light flux increments, isoluminant chromatic stimuli, and gratings of equal and lower space-averaged luminance. The parameters investigated were luminance contrast and chromatic saturation. The results demonstrate clearly the existence of pupil color, pupil grating and pupil light reflex responses in the rhesus monkey. Comparison of pupil color and pupil grating responses of equivalent amplitude reveals similar onset response latencies. However, both are approximately 40 ms longer than the corresponding pupil light reflex latency. In general these pupil responses are qualitatively similar to those observed in humans. However, when compared to equivalent human data, pupil onset response latencies are some 80-100 ms shorter and the pupil shows more rapid recovery from constriction.

Reduction in choroidal blood flow occurs in chicks wearing goggles that induce eye growth toward myopia.

Goggles that degrade the retinal image produce axial enlargement of the ocular globe and large myopic refractive errors. Many authors have assumed that visual image degradation itself leads to myopia. Hodos and co-authors have shown, however, that goggled eyes in chicks are considerably warmer than normal. Such temperature changes may either underlie or be a consequence of alterations in choroidal blood flow (CBF). Since alterations in CBF could affect eye growth, we explored the effect of monocular goggling on CBF in chicks. Plastic goggles were glued over one eye in four-day old chicks and the goggles were left in place for 12 or 14 days. Fourteen days after the goggling, CBF was measured using laser Doppler velocimetry. Three groups of chicks were studied: 1) chicks with goggles for 14 days; 2) chicks with goggles for 12 days followed by no goggles for the two days; 3) age matched non-goggled chicks. A -scan ultrasonography confirmed that the visual deprivation produced vitreous chamber elongation in the goggled eye and that the degree of elongation for the goggled eye was the same for the two goggled groups. The results were: 1) blood flow in non-goggled chicks was similar in both eyes; 2) blood flow was significantly reduced in the goggled eye in chicks wearing goggles for 14 days- 37% of control; and 3) blood flow was still significantly reduced in the goggled eye in chicks whose goggles were removed two days before measurement- 51% of control. These results show that CBF is reduced by goggles that result in myopic eye growth.(ABSTRACT TRUNCATED AT 250 WORDS)

Central projections of intrinsically photosensitive retinal ganglion cells in the macaque monkey.

Circadian rhythms generated by the suprachiasmatic nucleus (SCN) are entrained to the environmental light/dark cycle via intrinsically photosensitive retinal ganglion cells (ipRGCs) expressing the photopigment melanopsin and the neuropeptide pituitary adenylate cyclase-activating polypeptide (PACAP). The ipRGCs regulate other nonimage-forming visual functions such as the pupillary light reflex, masking behavior, and light-induced melatonin suppression. To evaluate whether PACAP-immunoreactive retinal projections are useful as a marker for central projection of ipRGCs in the monkey brain, we characterized the occurrence of PACAP in melanopsin-expressing ipRGCs and in the retinal target areas in the brain visualized by the anterograde tracer cholera toxin subunit B (CtB) in combination with PACAP staining. In the retina, PACAP and melanopsin were found to be costored in 99% of melanopsin-expressing cells characterized as inner and outer stratifying melanopsin RGCs. Two macaque monkeys were anesthetized and received a unilateral intravitreal injection of CtB. Bilateral retinal projections containing colocalized CtB and PACAP immunostaining were identified in the SCN, the lateral geniculate complex including the pregeniculate nucleus, the pretectal olivary nucleus, the nucleus of the optic tract, the brachium of the superior colliculus, and the superior colliculus. In conclusion, PACAP-immunoreactive projections with colocalized CtB represent retinal projections of ipRGCs in the macaque monkey, supporting previous retrograde tracer studies demonstrating that melanopsin-containing retinal projections reach areas in the primate brain involved in both image- and nonimage-forming visual processing.

Subcortical neural circuits for ocular accommodation and vergence in primates.

Our current knowledge of the neural bases of vergence and accommodation has increased significantly over the past few years. The behavior of medial rectus motoneurons during vergence, which has been reported by a number of investigators, is described. The behavior of Edinger-Westphal neurons during accommodation is also described, as are the characteristics of midbrain near-response neurons in the supraoculomotor area. Evidence that some of these near-response neurons provide the vergence input to medial rectus motoneurons and possibly the accommodation input to Edinger-Westphal neurons is reviewed. Anatomical studies have shown that the midbrain near-response region receives input from two deep cerebellar nuclei, the posterior interposed and the fastigial nucleus. Single-unit recording in the posterior interposed nucleus has revealed cells that increase their activity during the far-response, and the behavior of these neurons is reviewed. In addition, studies of a precerebellar nucleus, the nucleus reticularis tegmenti pontis, have revealed some cells that increase their activity during the near-response and others that do so during the far-response. The behavior of these neurons is reviewed. This review documents the great strides that are occurring in our understanding of the anatomy and physiology of the neural pathways controlling vergence and accommodation in the primate.

An area for vergence eye movement in primate frontal cortex.

To view objects at different distances, humans rely on vergence eye movements to appropriately converge or diverge the eyes and on ocular accommodation to focus the object. Despite the importance of these coordinated eye movements (the 'near response') very little is known about the role of the cerebral cortex in their control. As near-response neurons exist within the nucleus reticularis tegmenti pontis, which receives input from the frontal eye field region of frontal cortex, and this cortical region is known to be involved in saccadic and smooth-pursuit eye movements, we propose that a nearby region might play a role in vergence and ocular accommodation. Here we provide evidence from rhesus monkeys that a region of frontal cortex located immediately anterior to the saccade-related frontal eye field region is involved in vergence and ocular accommodation, and in the sensorimotor transformations required for these eye movements. We conclude that the macaque frontal cortex is involved in the control of all voluntary eye movements, and suggest that the definition of the frontal eye fields should be expanded to include this region.

The pupillary light reflex pathway of the primate.

BACKGROUND: Many studies of the pupillary light reflex pathway in mammals have indicated that the pretectum is important for this reflex. However, no single retinorecipient pretectal nucleus has been unequivocally identified as being involved in the light reflex pathway. In this study, anatomical studies in the rhesus monkey were carried out to identify the relevant retinorecipient pretectal nucleus and to better define the central pathway of this reflex. METHODS: An injection of Wheatgerm Agglutinin/Horseradish peroxidase, a neuroanatomical tracer, was placed under physiological guidance into the Edinger-Westphal nucleus. Intravitreal injection of the same tracer in another animal was used to define the pretectal retinal terminal fields. RESULTS: Following injection of tracer in the Edinger-Westphal nucleus, retrogradely labeled cells were found in only one retinorecipient nucleus, the pretectal olivary nucleus. Most labeled cells were located contralateral to the injection site. A few labeled cells were located ipsilaterally. Intravitreal injection of tracer resulted in anterograde labeling of all the retinorecipient pretectal nuclei, including the pretectal olivary nucleus. The retinal terminal field in the pretectal olivary nucleus coincided with the location of the cells that were retrogradely labeled by the injection of tracer into the Edinger-Westphal nucleus. CONCLUSIONS: These results demonstrate that there is a direct projection from the pretectum to the Edinger-Westphal nucleus, that it arises from only one retinorecipient pretectal nucleus, the pretectal olivary nucleus, and that cells in the pretectal olivary nucleus almost all appear to project to the contralateral Edinger-Westphal nucleus.

Single-unit activity in the primate nucleus reticularis tegmenti pontis related to vergence and ocular accommodation.

1. In the present study we used single-unit recording techniques in alert rhesus monkeys to investigate a precerebellar nucleus, the nucleus reticularis tegmenti pontis (NRTP), for neurons related to vergence and ocular accommodation. 2. In the medial NRTP, we identified 32 cells with activity that linearly increased with increases in the amplitude of the near response and 33 cells with activity that linearly increased with increases in the amplitude of the far response. These near and far response neurons were often encountered close to neurons displaying saccade-related activity, but their activity was related neither to saccadic nor to smooth pursuit eye movements. Micro-stimulation at the site of near or far response neurons often produced changes in vergence angle and accommodation. 3. The NRTP is known to receive cortical afferents and to have reciprocal connections with the cerebellum; therefore it is likely that the near and far response neurons in the medial NRTP form part of a cerebropontocerebellar pathway modulating or controlling vergence and ocular accommodation.

Dynamic properties of medial rectus motoneurons during vergence eye movements.

1. An early study by Keller reported that medial rectus motoneurons display a step change in firing rate during accommodative vergence movements. However, a later study by Mays and Porter reported gradual changes in firing rate during symmetrical vergence movements. Furthermore, subsequent inspection of the activity of individual medial rectus motoneurons during vergence movements indicated transient changes in their firing rate that had not been noted by Mays and Porter. For conjugate eye movements, in addition to a position signal, motoneurons display an eye velocity signal that compensates for the characteristics of the oculomotor plant. This suggested that the transient change in firing rate seen during vergence movements represented a velocity signal. Therefore the present study used single-unit recording techniques in alert rhesus monkeys to examine the dynamic behavior of medial rectus motoneurons during vergence eye movements. 2. The relationship between firing rate and eye velocity was first studied for vergence responses to step changes in binocular disparity and accommodative demand. Inspection of single trials showed that medial rectus motoneurons display transient changes in firing rate during vergence eye movements. To better visualize the dynamic signal during vergence movements, an expected firing rate (eye position multiplied by position sensitivity of the cell plus its baseline firing rate) was subtracted from the actual firing rate to yield a difference firing rate, which was displayed along with the eye velocity trace for individual trials. During all smooth symmetrical vergence movements, the profile of the difference firing rate very closely resembled the velocity profile. 3. To quantify the relationship between eye velocity and firing rate, two approaches were taken. In one, peak eye velocity was plotted against the difference firing rate. This plot yielded a measure of the velocity sensitivity of the cell (prv). In the other, a scatter plot was produced in which horizontal eye velocity throughout the vergence eye movement was plotted against the difference firing rate. This plot yielded a second measure of the velocity sensitivity of the cell (rv). 4. The behavior of 10 cells was studied during both sinusoidal vergence tracking and conjugate smooth pursuit over a range of frequencies from 0.125 to 1.0 Hz. This enabled the frequency sensitivity of the medial rectus motoneurons to be assessed for both types of movements. Both vergence velocity sensitivity and smooth pursuit velocity sensitivity decreased with increasing frequency. This is similar to a finding by Fuchs and co-workers for lateral rectus motoneurons during smooth pursuit eye movements.(ABSTRACT TRUNCATED AT 400 WORDS)

Neurons in the posterior interposed nucleus of the cerebellum related to vergence and accommodation. I. Steady-state characteristics.

The present study used single-unit recording and electrical microstimulation techniques in alert, trained rhesus monkeys to examine the involvement of the posterior interposed nucleus (IP) of the cerebellum in vergence and accommodative eye movements. Neurons related to vergence and ocular accommodation were encountered within a circumscribed region of the IP and their activity during changes in viewing distance was characterized. The activity of these neurons increased with decreases in vergence angle and accommodation (the far-response) but none showed changes in activity during changes in conjugate eye position and we therefore term them "far-response neurons." Far-response neurons were found within a restricted region of the IP that extended approximately 1 mm rostrocaudally and mediolaterally and 2 mm dorsal to the fourth ventricle. Microstimulation of this far-response region of the IP with low currents (<30 microA) often elicited divergence and accommodation for far. The behavior of 37 IP far-response neurons was examined during normal binocular viewing, during monocular viewing (blur cue alone), and during binocular viewing with accommodation open-loop (disparity cue alone). The activity of all cells was modulated under all three conditions. However, the change in activity of some of these neurons was significantly different under these three viewing conditions. The behavior of 70 IP far-response neurons was compared during normal binocular viewing and during viewing in which the accommodative response was significantly dissociated from the vergence response. The data from these two conditions was pooled and multiple regression analyses for each neuron generated two coefficients expressing the activity of the neuron relative to the vergence and to accommodative response respectively. On the basis of these coefficients, the overall activity of the neurons were classified as follows: 34 positively correlated with divergence, 11 positively correlated with far accommodation, 14 positively correlated with divergence and far accommodation, 9 positively correlated with divergence and accommodation, and 2 positively correlated with convergence and far accommodation. The results of this study demonstrate the involvement of a specific region of the posterior interposed nucleus of the cerebellum in vergence and accommodation. IP far-response neurons are active for vergence and accommodation irrespective of whether or not these eye movements are elicited by blur or disparity cues. The data in the present study strongly suggest that this cerebellar region is a far-response region that is involved in vergence as well as accommodative eye movements.

Characteristics of near response cells projecting to the oculomotor nucleus.

1. Previous work has shown neurons just dorsal and lateral to the oculomotor nucleus that increase their firing rate with increases in the angle of ocular convergence. It has been suggested that the output of these midbrain near response cells might provide the vergence command needed by the medial rectus motoneurons. However, lens accommodation ordinarily accompanies convergence, and a subsequent study showed that only about one-half of these midbrain near response cells carried a signal related exclusively to vergence. One hypothesis suggested by this finding is that this subgroup of neurons might have a unique role in providing a "pure" vergence signal to the medial rectus motoneurons. 2. In the present study extracellular recordings were made from midbrain near response cells in monkeys while eye position and lens accommodation were measured. The monkeys viewed targets through an optical system that allowed the accommodative and ocular vergence demands to be manipulated independently. This approach was used to produce a partial dissociation of accommodative and vergence responses, so that an accommodative and vergence coefficient could be determined for each cell, by the use of the following equation FR = R0 + kda x AR + kdv x CR where FR is the firing rate of the near response cell, R0 is the predicted firing rate for a distant target, kda is the (dissociated) accommodation coefficient, AR is the accommodative response, kdv is the (dissociated) vergence coefficient, and CR is the convergence response. 3. The vergence and accommodation coefficients were determined for a large number of midbrain near response cells, including a subset that could be antidromically activated from the medial rectus subdivisions of the oculomotor nucleus. Some near response neurons were found with signals related exclusively to convergence (i.e., kdv greater than 0 and kda = 0), whereas several others had signals related exclusively to lens accommodation (i.e., kda greater than 0 and kdv = 0). The majority of the near response cells had signals related to both responses (i.e., kda not equal to 0 and kdv not equal to 0). Furthermore, the vergence and accommodation coefficients of near response cells appeared to be continuously distributed. Some cells had negative accommodation or vergence coefficients. 4. The 17 near response cells that could be antidromically activated from the oculomotor nucleus presumably provide vergence signals to the medial rectus motoneurons. Although all had positive vergence coefficients, only four of these cells carried signals that were related exclusively to vergence.(ABSTRACT TRUNCATED AT 400 WORDS)

Antidromic identification of midbrain near response cells projecting to the oculomotor nucleus.

Medial rectus motoneurons carry both conjugate and vergence eye position signals. Abducens internuclear neurons, whose axons travel in the medial longitudinal fasciculus, provide these motoneurons with the major signal for conjugate eye movements but not for vergence eye movements. A vergence signal appropriate for these motoneurons is seen on the near response cells that are found in the mesencephalic reticular formation within 2 mm of the oculomotor nucleus. The goal of the present study was to determine if midbrain near response cells project to the medial rectus subdivision of the oculomotor nucleus. Near response cells were recorded in two trained rhesus monkeys with ocular search coils. A stimulating electrode was positioned within the medial rectus subdivision of the oculomotor nucleus. Twenty-eight near response cells were found that could be driven by single pulse microstimulation of the ipsilateral medial rectus subdivision. In all cases, antidromic activation was confirmed by collision testing. Attempts to antidromically activate midbrain near response cells from the contralateral medial rectus subdivision were unsuccessful. Most antidromically activated cells had a steady state firing rate proportional to vergence angle. One cell also showed burst activity during the vergence eye movements. Divergence cells were not antidromically activated.

Behavior of luminance neurons in the pretectal olivary nucleus during the pupillary near response.

In cats and monkeys, extrastriate visual areas that have been reported to be involved in the near triad of pupilloconstriction, convergence, and accommodation have well-defined projections to the pretectal olivary nucleus (PON), the retinorecipient pretectal nucleus mediating the pupillary light reflex in mammals. We have therefore used alert, behaving primates to investigate the possibility that PON neurons are involved in the pupillary near response in addition to the pupillary light reflex. Single-unit recording revealed that PON luminance neurons significantly increased their firing rate with increases in retinal illumination and the resultant pupilloconstriction. In contrast, their activity did not significantly increase during pupilloconstriction elicited by near viewing. Thus the behavior of PON luminance neurons is appropriate for their participation in the pupillary light reflex, but is inappropriate for any proposed role in the pupillary near response. This result strongly suggests that neurons in the primate PON are solely related to the pupillary light reflex and that the cortical projections to this pretectal nucleus are related to this reflex and do not play a role in the pupillary near response.