Pretectal Region Research Articles

Efferent connections of the centrum medianum of the cat thalamus revealed by autoradiography

Efferent connections of the centrum medianum and parafascicular nucleus of the thalamus (CM-Pf complex) in cats were studied by the method of anterograde axonal transport of tritiated amino acids followed by autoradiography. Projections from CM-Pf ascend to nuclei of the ventral group and nonspecific nuclei of the thalamus, preoptic, dorsal, lateral, and posterior areas of the hypothalamus, and also into the subthalamic region. Descending pathways are formed only by neurons of the caudomedial part of CM-Pf. They project into the pretectal region, superior colliculus, reticular formation, locus coeruleus, region of the ramus communicans, and substantia grisea centralis of the mesencephalon and pons, and also into the nuclei raphe, magnocellular reticular area, and inferior olivary nucleus of the medulla. In agreement with previous observations it was found that the caudomedial part of CM-Pf does not send direct projections into the cortex and striatum.

Morphological and physiological study of afferent projections of the posterolateral thalamic nucleus in rats

It was shown by the method of retrograde axonal transport of horseradish peroxidase that the posterolateral thalamic nucleus (NPL) in rats receives considerable ascending projections from the superior colliculus (SC), the dorsal part of the lateral geniculate body (LGB), and the pretectal region (PT) and smaller projections from n. ventralis posterior (VP) and n. ventralis lateralis (VL) of the thalamus, the ventral part of LGB, the zona incerta, and anterior hypothalamus. The most marked descending projections run into NPL from area 18A of the cortex and the dentate fascia of the hippocampus, whereas inputs from cortical areas 18, 20, 7, 29c, 17, and 36 are less marked. In electrophysiological experiments with peripheral stimulation of visual, auditory, and somatosensory systems, polysensory convergence and interaction between signals from these systems were studied during isolated and simultaneous presentation of heterosensory stimuli. Of 229 neurons tested, 134 (58.5%) responded to at least one of the stimuli mentioned. Among monomodal neurons (53 of 134) there were some cells which responded to visual (77.4%) and somatic (22.6%) stimulation; neurons which responded only to acoustic stimulation were not found in the nucleus. As far as polymodal neurons (81 of 134) responding to two or three sensory stimuli are concerned, the most effective inputs of these units were visual and somatosensory. Interaction between stimuli acting on polymodal neurons was expressed as mutual inhibition or facilitation of responses; opposite effects could be observed on the various components of these responses.

Basic optokinetic-ocular reflex pathways in the frog

Frogs (Rana temporaria) have two midbrain nuclei that receive contralateral retinal afferents, and whose neurons respond to optokinetic stimulation. The basal optic nucleus is composed of direction-selective neurons with different response types. One type is activated exclusively by upward moving optokinetic targets; another type is activated only by downward moving targets. Two other types of basal optic neurons show this vertical preference, but each is also activated by patterns moved horizontally from the nasal to temporal visual field. No activation of these cells was found with patterns moved horizontally from the temporal to nasal visual fields. Rather, cells in a discrete pretectal region have this type of sensitivity: they increase their resting rate with temporal to nasal stimulation and decrease it with nasotemporal stimulation. Oculomotor neurons (antidromically identified) have similar optokinetic sensitivities. As with basal optic neurons, these cells have exclusively upward or downward sensitivity, and some also have nasotemporal sensitivity. An additional type of oculomotor neuron and abducens motoneurons are activated by temporonasal pattern movement. In general, the extraocular motoneurons have similar velocity and pattern size preferences, as have the sensory nuclei. Investigations of the connectivity between the sensory and motor nuclei were primarily restricted to the relation between the pretectum and the abducens. A monosynaptic connection between the pretectum and the abducens is suggested by four points: (1) excitatory postsynaptic potential onset latency in antidromically identified abducens motoneurons, following optic nerve stimulation, is consistent with the interpretation of a disynaptic pathway to the abducens from the retina; (2) pretectal cells, sensitive to optokinetic stimulation, can be activated antidromically from stimulation of the abducens nucleus; (3) horseradish peroxidase injections into the pretectum result in labeling of axons, which terminate in the abducens nucleus; (4) horseradish peroxidase injections into the abducens result in labeling of cells in the pretectal region, where optokinetically sensitive cells are found. In the frog, there seem to be three-neuronal retino-ocular reflexes mediating optokinetic slow phase behavior as there are three-neuronal vestibulo-ocular reflexes that also mediate compensatory spatial behavior. It is suggested that these direct connections act to initiate ocular movements and accelerate the eye, whereas more indirect pathways may act to maintain eye position.

Anterograde axoplasmic transport of peroxidase in the rat visual system after intraocular neurotoxin injection

The effect of intraocular injection of the neurotoxin, kainic acid, on the retinofugal pathway has been examined by anterograde transport of horseradish peroxidase. Although less peroxidase was present in all the primary optic centers after the intraocular injection of kainic acid that in the controls, the area occupied by the retinal terminals remained fairly constant except for the pretectal region where there is a smaller area of peroxidase precipitate. These results suggest that certain ganglion cells are killed by kainic acid but some others survive and their terminals are present in the majority of the terminal fields.

Direct projections to the pretectum and the midbrain reticular formation from auditory relay nuclei in the lower brainstem of the cat

Direct projections to the pretectum and the midbrain reticular formation from auditory relay nuclei in the lower brainstem were examined by the retrograde and anterograde tracer methods in the cat. After horseradish peroxidase (HRP) injection into the pretectomesencephalic reticular region (Pt-MRF), which includes caudoventral regions of the pretectum and rostrodorsal regions of the midbrain reticular formation, labeled neurons were seen in the dorsal nucleus of the lateral lemniscus (DLL), the pericentral (PC) and external (EN) nuclei of the inferior colliculus (IC), the rostral process of IC (RP) and the nucleus of the brachium of IC (NB); no labeled neurons were found in the main laminated portion of the central nucleus of IC. Subsequently, tritiated leucine was injected into DLL, EN, RP or NB for autoradiographic fiber tracing. After injection into DLL or EN, terminal labeling was confined to the ventral portions of the anterior pretectal nucleus. After injection into RP or NB, heavy terminal labeling was observed in the midbrain reticular formation, extending dorsally into the anterior pretectal nucleus. Thus, 3 sectors are distinguishable in Pt-MRF in terms of termination of fibers from the midbrain auditory relay nuclei; the dorsomedial, intermediate or ventrolateral Pt-MRF sector receives fibers arising from DLL, RP or NB, respectively. Fibers from EN terminate only in the dorsal portion (pretectal regions) of the intermediate sector.

Retinopretectal projections in albino and pigmented rabbits: An autoradiographic study

The ipsilateral and contralateral retinal projection was studied in pigmented rabbits and in 3 strains of albino rabbits by anterograde transport of [3H]proline and [3H]fucose combined with autoradiographic techniques. Special attention was paid to the terminals in the pretectal area of both the pigmented and albino strains. On the contralateral side terminal labeling was found in both pigmented and albino rabbits in the nucleus of the optic tract (NOT), the anterior pretectal nucleus (PA), the posterior pretectal nucleus (PP) and the pretectal olivary nucleus (PO). Ipsilaterally labeling was found only in the pigmented strain in small patches in the PP. Ipsilateral projection was not found in the albinos in the pretectal area. The results are in agreement with the findings of Scalia in pigmented rabbits. The absence of ipsilateral labeling in the pretectal region in albinos is in contrast with earlier findings of Giolli and Takahashi et al., in pigmented rabbits but is in agreement with the observations of Takahashi and Oyster. Since no radioactively labeled fibers were found to project to the NOT in either pigmented or albino rabbits, these results do not support the hypothesis of Collewijn that the inverted optokinetic nystagmus in albinos is due to misrouting of the ipsilateral retinal fibers to the NOT.

Overlap of retinal and prestriate cortical pathways in the primate pretectum

Connections between the pretectum and prestriate visual cortex were examined following injection of HRP into area V4 of Macaca nemestrina monkeys. Orthograde label was found in nucleus olivarius (NO) and the sublenticular (SL) regions while retrogradely labeled neurons were present in the nucleus of the optic tract (NOT). Following injection of [3H]proline into one eye, 3 pretectal zones of retinal input were seen: NO, SL and the posterior pretectal region (PPR). These results indicate that NO and SL may be sites for retinal and cortical pathway interaction in primate pretectum.

The development and restriction of the ipsilateral retinofugal projection in the chick

Although it is generally believed that the central projections of the retina in birds are entirely crossed, using wheat germ agglutinin-conjugated horseradish peroxidase (WGA-HRP) as an anterograde tracer, we have found that in normal posthatched chicks there is a small ipsilateral retinofugal projection to the diencephalon and midbrain. Most of the ipsilateral fibers appear to be directed to the lateral anterior and dorsolateral anterior nuclei of the thalamus, to the pretectal region, and to the ectomammillary nucleus and the adjoining nucleus externus. Even in the best preparations the numbers of ipsilateral fibers are so small that it is hardly surprising that they have been overlooked in previous axonal degeneration and autoradiographic experiments. A significantly larger ipsilateral retinal projection develops during the second week of incubation. The ipsilaterally directed fibers can be first seen on the fifth day of incubation and their numbers appear to increase until about embryonic day 12. At this stage the projection involves substantially more fibers than at hatching and is also more extensive in its distribution; in fact, in its general organization (but not its size) it closely parallels the normal crossed retinofugal system, contributing fibers to essentially all the primary visual relay nuclei in the diencephalon and midbrain and to much of the optic tectum, where the densest projection is to its caudomedial aspect. During the second week of incubation there is also a small number of retinal fibers, which after crossing in the optic chiasm, recross the midline in the posterior and tectal commissures (and also in the tectal roof plate), before ending in the pretectal region of the ipsilateral side. In addition, there is a markedly aberrant projection from the retina into the contralateral optic nerve. Most of the ipsilateral retinal fibers are eliminated between the twelfth and sixteenth days of incubation, and by day 17 the ipsilateral projection is reduced to its mature form. The progressive reduction in the ipsilateral projection occurs at a time when it is known (from other studies) that there is an appreciable loss of retinal ganglion cells; but whether the reduction is due to neuronal death or to the selective elimination of ipsilateral axon collaterals remains to be determined. The existence of a significant ipsilateral retinofugal component early in development, probably accounts, in part, for the distinctive and persistent ipsilateral projection that occurs if one eye is removed during the first few days of incubation.

Responses of cats to stimulation of the pulvinar complex: A behavioral and anatomical study

In 14 cats there was a total of 23 bipolar stimulating electrodes successfully implanted in the pulvinar (Pul) complex. An aversive response to ipsilateral intracranial stimulation (ICS) beginning with 0.25 mA occurred in all cases and the majority of cats learned to escape the painful stimulus delivered on at least one side by crossing a shuttle box barrier. After this behavior was well established, lesions were made ipsilaterally through the same electrodes. In the postlesion trials the animals no longer responded to the ICS at currents less than 100% above the prelesion intensities. Upon completion of the postlesion trials, the nonlesion side was subjected to ICS, and never showed a nociceptive block due to the contralateral lesion. Then the animal received a second similar lesion through the electrode and was again subjected to a postlesion trial series. In eight cases an auditory negative reinforcer was introduced prior to the noxious ICS, and when associated with avoidance this preferential response was retained after the lesions. Not infrequently the primary pain responses were expressed as, or associated with, affective (savage) behavior, ocular, or other extrapyramidal motor responses. Histological evaluation showed an essentially equal distribution of the electrode tips in the medial, lateral, and inferior pulvinar nuclei, suprageniculate nucleus, and two in the magnocellular part of the medial geniculate body. In brains where the deOlmos-Ingram silver stain was used to trace axonal degeneration, there were always heavy axonal and terminal projections to the reticular nucleus. The fibers then continued through the internal capsule to spread and end in the medial suprasylvian and ectosylvian gyri. Other significant subcortical projections to the pretectal region, central gray, nucleus centromedianus, and the zona incerta were observed.

Brightness and striated pattern discrimination after bilateral removal of the occipital lobe in the rabbit

The purpose of the study was to determine which brain structures were critical for the capacity to perform a gross luminous flux discrimination in the absence of striate cortex. Fourteen monkeys were trained to a criterion of successful performance, set at 90% correct responses in 30 consecutive trials, on a light vs. no-light discrimination test, using specially designed automatic equipment which allowed predetermination of Gellerman schedules.All monkeys were able to relearn the test after complete bilateral excision of striate cortex and partial ablation of areas 18 and 19 (histologically verified). Additional bilateral lesions of the temporal neocortex, or posterior parahippocampal and retrosplenial allocortex, or pulvinars, or superior colliculi did not interfere significantly with the relearned habit. Instead, when the additional damage involved the superior colliculi and medial pretectum, there was a major discrimination deficit, but eventually these monkeys could reach the criterion level. Only when the lateral pretectal region was included in the latter lesion was there a failure to master the test in 6000 trials of postoperative testing. The effective damage caused bilateral severe degeneration of the nucleus of the accessory optic tract.Findings indicate that the mesodiencephalic junction contains critical structures for extrageniculostriate vision in the monkey and suggest that the accessory optic system could be one main source of visual input for the capacity to perform luminous flux discrimination tests in the absence of striate cortex.

Sources of ascending afferent projections to the midbrain central tegmental area and centrum medianum thalami in cats

After microinjections of horseradish peroxidase into the central tegmental area of the midbrain and centrum medianum thalami in cats, labeled neurons were found in the nucleus of the tractus solitarius, gracile and cuneate nuclei, spinal nuclei of the trigeminal nerve, the external nucleus and nucleus of the brachium of the inferior colliculus, the medial pretectal region, nucleus of the posterior commissure and stratum intermediale of the superior colliculus, and reticular structures of the medulla and pons. Comparison of the location of the sources of ascending afferent projections in the central tegmental area of the midbrain and centrum medianum thalami showed that the reticular formation receives mainly visceral projections through the nucleus of the tractus solitarius, whereas the centrum medianum thalami is innervated mainly by the system of sensory somatic nuclei.

Efferent connections of the ventral medulla oblongata in the rat

The efferent connections of the ventral medulla oblongata have been analyzed in the rat using the anterograde autoradiographic method and the HRP technique. Fibers originating from the nucleus interfascicularis hypoglossi (B1 serotonergic cell group) and nucleus reticularis gigantocellularis, pars a (B3 serotonergic cell group) innervate the intermediolateral cell column, ventral horn and intermediate gray matter of the spinal cord. Some fibers innervate the hypoglossal, dorsal motor vagal, and medial solitary nuclei. Ascending fibers project through the medullary and pontine reticular formation, providing inputs to the Kölliker-Fuse, lateral parabrachial, laterodorsal tegmental, subcoeruleus and locus coeruleus nuclei. In the midbrain, the fibers ascend in the central tegmental field and then divide into several fiber bundles. Some course medially to innervate the central gray matter. Others diverge laterally to innervate the external nucleus of the inferior colliculus and cuneiform nucleus as well as the deep layers of the contralateral superior colliculus. Still others course dorsally through the ventral pretectal region to reach the thalamus (laterodorsal, paraventri-cular, paracentral, and centrolateral thalamic nuclei). The remaining fibers innervate the hypothalamus (dorsal hypothalamic area, paraventricular nucleus, perifornical area, supraoptic nucleus, retrochiasmatic area, and median eminence). Some of these continue through the lateral preoptic region, shift medially as they course through the area of the nucleus of the diagonal band, septofimbrial nucleus, and medial septum, and arch around the genu of the corpus callosum to innervate the hippocampal formation.

Electrophysiological study of the nucleus of the optic tract that transfers optic signals to the nucleus reticularis tegmenti pontis—the visual mossy fiber pathway to the cerebellar flocculus

Neuronal activities (n = 43) in the pretectal region in rabbits were recorded. They were orthodromically activated from the optic chiasm (latency, 1.86 ± 0.35msec) and antidromically from the ipsilateral nucleus reticularis tegmenti pontis (Nrt) (latency, 0.97 ± 0.22sec). Thirty-one (72%) neurons were in the nucleus of the optic tract (NOT), four (9%) in the anterior pretectal nucleus (PA) and seven (16%) in the border between NOT and PA. These findings demonstrate that the NOT is involved in the visual mossy fiber pathways to the flocculus and may contribute to optokinetic eye movements.

Olivary afferents from the pretectal nuclei in the cat.

The organization of the projection from the pretectal region to the inferior olive in the cat was studied by means of retrograde protein tracing and experimental degeneration. Small injections of horseradish peroxidase (HRP) were made into various parts of the inferior olive from a ventral approach. The number of nerve cells in the pretectal nuclei retrogradely labelled with HRP was counted and put in relation to the site of injection. Labelled cells were only found in the posterior pretectal nucleus (NPP), the nucleus of the optic tract (NOT) and the anterior pretectal nucleus (NPA). Most labelled cells were found in NPP and NOT in cases in which the rostral or caudal levels of the principal olive were labelled by the injection. NAP labelling occurred in one case with a very rostral injection of the inferior olive. Unilateral electrolytical destruction of the pretectal region produced terminal degeneration in the ipsilateral inferior olive. The heaviest ipsilateral degeneration was found in the upper half of the principal and dorsal accessory olives, and caudally in the ventrolateral outgrowth, the dorsal cap and nucleus beta with the adjacent part of the medial accessory olive. Some functional implications of the findings are discussed.

Some cortical and subcortical fiber projections to the accessory optic nuclei in the cat

The terminal nuclei of the accessory optic system receive a dense fiber projection from the contralateral retina and have recently been implicated in visuo-vestibular aspects of oculomotor function. The present anatomical study, based on anterograde tracer experiments with 3H-labeled amino acids or horseradish peroxidase, provides evidence for a diverse set of non-retinal inputs to the accessory optic nuclei arising in both the neocortex and the brainstem. Corticofugal fiber projections to the terminal nuclei originate in several areas of the visual cortex; those from the medial Clare-Bishop area may be especially dense. Subcortical inputs to the terminal nuclei include fiber projections arising in ( a) the pretectal region; ( b) the ventral mesencephalon, probably including the substantia nigra; and ( c) more dorsal or caudal parts of the mesencephalic reticular formation. The source of the pretectal projection to the terminal nuclei appears to be the nucleus of the optic tract or the underlying posterior pretectal nucleus. Fiber projections from the ventral mesencephalic tegmentum were labeled by injections infiltrating the substantia nigra, but these deposits all involved, at least marginally, either interstitial cells of the transpeduncular limb of the accessory optic tract or the perinigral tegmentum. The part of the midbrain reticular formation giving rise to an accessory optic projection was not precisely delimited, but it appears to lie outside the substantia nigra proper. Although these afferent projections to the accessory optic nuclei appear individually to be much weaker than the input from the contralateral eye, their presence clearly suggests that transmission of visual information through the terminal nuclei may involve a complex modulation of retinal input by non-retinal pathways.

Central projections of the frontal organ of Rana pipiens, as demonstrated by the anterograde transport of horseradish peroxidase.

The central projections of the frontal organ of Rana pipiens are more widespread, and more similar to those of the epiphysis, than previously realized. Fibers labeled with horseradish peroxidase were traced to the amygdala, the epiphysis, the pretectal region, and several nuclear areas of the mesencephalic and diencephalic central gray. When peroxidase reaction product was carefully distinguished from neuromelanin by means of polarization microscopy, no unequivocally labeled cell bodies off centrifugal fibers could be found.

The pretectal region of the cat: a structural and topographical study with stereotaxic coordinates.

Using Nissl and myelin staining methods, the structure of the pretectal region has been analyzed in the cat. From this study and a review of the literature, three groups of cell populations have been identified, each made up of several parts or nuclei: 1) Nuclei of the posterior commissure, including dorsal, lateral, and ventral divisions; 2) A dorsolateral group of nuclei, which includes the pars lateralis and pars medialis of the nucleus of the optic tract, and the nucleus olivaris; 3) An intermediate nuclear group (pretectal nuclei, "sensu stricto") composed of the nuclei posterior, subopticus, anterior (with two portions, compacta and reticularis), and medialis. Stereotaxic coordinates of this region were also determined and plotted both on coronal and horizontal sections. The combined use of several spatial planes has proven to be of great value in studying transitional regions such as the pretectal region.

Autoradiographic evidence for a projection from the pretectal nucleus of the optic tract to the dorsal lateral geniculate complex in the cat

A pathway from the pretectal region to the dorsal division of the lateral geniculate body has been studied by autoradiography in adult cats following injections of tritium-labeled amino acids into the meso-diencephalic border zone. The findings suggest that the pretecto-geniculate connection arises in the ipsilateral nucleus of the optic tract (NOT) and terminates in the principal dorsal layers of the geniculate body (A and A1), in lamina C and most massively in the medial interlaminar nucleus. In the A layers and lamina C, the pretectal fibers form a relatively even matting with partial interruptions occurring at the interlaminar leaflets. The ventral C layers do not appear to receive a substantial pretectal input as they were only weakly labeled, and partly in a perforant fiber pattern, after injections confined to the pretectum. Topographic ordering of the projection was evident at least along the medial to lateral axis of the NOT, fibers from the medial part terminating caudally, and those from the lateral part rostrally, within the lateral geniculate complex. The laminar pattern of termination correlates closely with the distribution of retinal Y-cell input and acetylcholinesterase activity in the geniculate body but contrasts sharply with the pattern of termination of tecto-geniculate fibers. Of potential importance in considering the functional significance of the pretecto-geniculate pathway is the fact that the NOT has been implicated in the mechanism of optokinetic nystagmus and the fact that the pathway terminates within the layers of the lateral geniculate body known to provide the main geniculo-cortical input to layer IV of the striate cortex.

Plasticity and interaction after ablations of visual or somatosensory motor cortex or retina in neonatal rats

The results obtained following eye enucleation and dorsal occipital cortical (DOC) lesion in normal adult animals are in general agreement with those reported in previous work. The details, however, vary somewhat. A DOC projection to the claustrum and reticularis thalami is reported here for the first time in the rat. Six experiments were performed: (1) DOC lesions followed by eye enucleations. (2) Eye enucleations followed by DOC lesions. (3) Eye enucleations followed by sensorimotor cortical (SMC) lesions. (4) SMC lesions followed by eye enucleations. (5) DOC lesions followed by SMC lesions. (6) SMC lesions followed by DOC lesions. The results obtained from the experimental animals demonstrate a certain degree of plasticity and interaction between the DOC and retinal efferents to the superior colliculus, lateral geniculate nucleus, the pretectal region and the superior colliculus. Thus, following a neonatal left dorsal occipital cortical lesion, the retinal efferents from the right eye produce a denser projection to the contralateral lateralis posterior (LP), nucleus lateralis of the optic tract (NLOT), and the caudal part of the lateral division of the lateral geniculate nucleus pars ventralis (VGN). Following neonatal left eye enucleation, increased ipsilateral projection from the right DOC was noted in the superficial part of the stratum griseum superficiale (SGS), NLOT, pretectal nucleus (PT) and the adjoining stratum griseum intermediate (SGI) of the superior colliculus, lateral geniculate nucleus, pars dorsalis (DGN), and both the medial and lateral halves of the caudal portion of VGN. Following a neonatal DOC lesion a denser projection from the remaining ipsilateral SMC was seen in the medial pretectal nucleus (PTM), PT and deep pretectal nucleus (PTP). Following a neonatal SMC lesion a denser projection from the ipsilateral DOC was seen in the SGI, PT, nucleus medialis of the optic tract (NMOT) and PTM. There was no change in the retinal efferents following neonatal SMC lesions. Nor was there any change in the SMC efferents following neonatal eye enucleations.

Pathways mediating optokinetic responses of vestibular nucleus neurons in the rat.

1. The effects of various brain lesions on the responses of vestibular nuclear neurons (Vn) of the horizontal semicircular canal system to optokinetic stimulations were studied to elucidate the optokinetic path from the retina to the vestibular nuclei. A previous study performed in intact rats served as a control [2]. 2. It was shown that the pretectal region including the n. of the optic tract is the first central relay in the optokinetic path; it receives its functionally effective input from the contralateral eye. Unilateral lesions of this area rendered all Vn responses unidirectional when tested with binocular stimulation. Lesions of other visual centers such as the superior colliculi or visual cortices had no influence on the optokinetic response properties of Vn. 3. The area of the n. reticularis tegmenti pontis (NRTP) proved to be an important link between pretectum and vestibular nuclei: Unilateral lesions produced effects similar to those described for pretectal lesions. Pretectal axons to NRTP descend lateral to the MLF and tectospinal tract. 4. It was demonstrated that the vestibular commissure plays the crucial role in mediating the mirror image optokinetic effects to Vn on the opposite side and assures the bidirectionality of the responses to binocular stimulation. 5. Cerebellectomy did not significantly affect the Vn responses to the optokinetic stimuli presented in this study. 6. Electrical stimulation of the pretectum excited type II and inhibited type I Vn ipsilaterally and had the opposite effect on Vn located on the opposite side. NRTP stimulation excited type II and inhibited type I ipsilaterally; latency analysis of these effects suggested that the pretectal stimuli excited opsilateral NRTP neurons which, in turn, excited ipsilateral type II Vn. Ipsilateral type I inhibition as well as the concurrent contralateral type II inhibition and type I excitation are produced by the inhibitory action of type II on type I and the commisural system. 7. Systemic application of picrotoxin abolished all optokinetic responses of Vn except the type II activation. This finding further supports the hypothesis described above. 8. Unilateral pretectal or NRTP lesions abolished OKN to surround motion in the direction of the lesion.