Dopaminergic modulation of visual responses in toads II. Influences of apomorphine on retinal ganglion cells and tectal cells

The behavioral studies of Part I have shown in common toads that after systemic administration of the dopamine agonist apomorphine the prey-directed orienting turning movements are suppressed while prey snapping is facilitated. Part II focusses on retinal and tectal single cell responses to moving objects. (1) After systemic administration of apomorphine, the discharge rates of retinal class R2 and R3 ganglion cell fibres – recorded from the retino-tectal projection – speeded up in response to visual objects traversing their excitatory receptive fields. This enhancing effect was independent of the recording site in the retino-tectal map. (2) The diameters of the excitatory receptive fields of R2 and R3 neurons doubled their sizes. Probably, apomorphine enhances the center-dominated excitatory responses at the expense of the strength of the inhibitory surround. (3) The apomorphine-induced effects were fully developed 20–35 min after drug administration. (4) At the same time the discharge rates of T5.1 and T5.2 tectal neurons were reduced under apomorphine. The effect was independent of the recording site in the retino-tectal map. The diameters of the excitatory receptive fields of these tectal neurons were not influenced. (5) To changes in configurational stimulus features, the basic pattern of discrimination was maintained. (6) It is suggested that tectal output to the turn-generating motor network – mediated by T5.1 and T5.2 neurons – is modulated by a pretecto-tectal pathway which involves dopaminergic pretectal cells. (7) The enhanced snapping can be interpreted in terms of a modulation of reticular/hypoglossal structures by dopaminergic preoptic/hypothalamic/solitary systems.     

BDNF increases synapse density in dendrites of developing tectal neurons in vivo

Neuronal connections are established through a series of developmental events that involve close communication between pre- and postsynaptic neurons. In the visual system, BDNF modulates the development of neuronal connectivity by influencing presynaptic retinal ganglion cell (RGC) axons. Increasing BDNF levels in the optic tectum of Xenopus tadpoles significantly increases both axon arborization and synapse density per axon terminal within a few hours of treatment. Here, we have further explored the mechanisms by which BDNF shapes synaptic connectivity by imaging tectal neurons, the postsynaptic partners of RGCs. Individual neurons were co-labeled with DsRed2 and a GFP-tagged postsynaptic density protein (PSD95-GFP) to visualize dendritic morphology and postsynaptic specializations simultaneously in vivo. Immunoelectron microscopy confirmed that PSD95-GFP predominantly localized to ultrastructurally identified synapses. Time-lapse confocal microscopy of individual, double-labeled neurons revealed a coincident, activity-dependent mechanism of synaptogenesis and axon and dendritic arbor growth, which is differentially modulated by BDNF. Microinjection of BDNF into the optic tectum significantly increased synapse number in tectal neuron dendritic arbors within 24 hours, without significantly influencing arbor morphology. BDNF function-blocking antibodies had opposite effects. The BDNF-elicited increase in synapse number complements the previously observed increase in presynaptic sites on RGC axons. These results, together with the timescale of the response by tectal neurons, suggest that the effects of BDNF on dendritic synaptic connectivity are secondary to its effects on presynaptic RGCs. Thus, BDNF influences synaptic connectivity in multiple ways: it enhances axon arbor complexity expanding the synaptic territory of the axon, while simultaneously coordinating synapse formation and stabilization with individual postsynaptic cells.     

Binocular depth perception mechanisms in tongue-projecting salamanders

Tongue-projecting salamanders (Bolitoglossini) combine extreme speed and high precision in prey capture. They possess all requirements for stereoscopic depth perception: frontally oriented eyes, a substantial amount of direct ipsilateral projection in addition to the contralateral one, and binocularly driven neurons. Extracellular recordings were made from retinal afferents in the tectum as well as from the somata of tectal neurons. RF-sizes of afferents and tectal neurons were determined, and the response properties of tectal neurons were tested under monocular and binocular conditions with stimuli of different size and velocity. While RF-sizes and response properties of binocular neurons during binocular and contralateral stimulation were similar, ipsilaterally stimulated neurons exhibited much smaller RFs, lower spike rates and different size preferences.Furthermore, the contralateral retinotectal projection from one eye and the ipsilateral from the other are in register. While retinal afferents are distributed linearly over the tectal surface, most tectal neurons are activated by a retinal area corresponding to the frontal visual field; this results in a magnification of this region. The two monocular receptive fields of binocular neurons exhibit zero disparities (horopter) at distances that coincide with the maximum reach of the tongue. We hypothesize that bolitoglossine salamanders (as well as amphibians in general) make use of two kinds of disparities: (1) between the maps in the left and right tectal hemisphere, coding for the lateral eccentricity of an object, and (2) between the ipsilateral and contralateral retinotectal map, coding for the distance. The presence of substantial direct ipsilateral afferents in bolitoglossine salamanders appears to be the basis for a fast computation of object distance, which is characteristic of these animals.Abbreviations Ax/Ay coordinates of a recorded afference - Nx/Ny coordinates of a recorded neuron - RF receptive field - RFc contralateral receptive field - RFi ipsilateral receptive field - RFx/RFy coordinates of a receptive field center - RGC retinal ganglion cell     

Rescue of Retinal Function by BDNF in a Mouse Model of Glaucoma

Vision loss in glaucoma is caused by progressive dysfunction of retinal ganglion cells (RGCs) and optic nerve atrophy. Here, we investigated the effectiveness of BDNF treatment to preserve vision in a glaucoma experimental model. As an established experimental model, we used the DBA/2J mouse, which develops chronic intraocular pressure (IOP) elevation that mimics primary open-angle glaucoma (POAG). IOP was measured at different ages in DBA/2J mice. Visual function was monitored using the steady-state Pattern Electroretinogram (P-ERG) and visual cortical evoked potentials (VEP). RGC alterations were assessed using Brn3 immunolabeling, and confocal microscope analysis. Human recombinant BDNF was dissolved in physiological solution (0.9% NaCl); the effects of repeated intravitreal injections and topical eye BDNF applications were independently evaluated in DBA/2J mice with ocular hypertension. BDNF level was measured in retinal homogenate by ELISA and western blot. We found a progressive decline of P-ERG and VEP responses in DBA/2J mice between 4 and 7 months of age, in relationship with the development of ocular hypertension and the reduction of Brn3 immunopositive RGCs. Conversely, repeated intravitreal injections (BDNF concentration = 2 µg/µl, volume = 1 µl, for each injection; 1 injection every four days, three injections over two weeks) and topical eye application of BDNF eye-drops (12 µg/µl, 5 µl eye-drop every 48 h for two weeks) were able to rescue visual responses in 7 month DBA/2J mice. In particular, BDNF topical eye treatment recovered P-ERG and VEP impairment increasing the number of Brn3 immunopositive RGCs. We showed that BDNF effects were independent of IOP reduction. Thus, topical eye treatment with BDNF represents a promisingly safe and feasible strategy to preserve visual function and diminish RGC vulnerability to ocular hypertension.     

A role for retinal brain-derived neurotrophic factor in ocular dominance plasticity

Visual deprivation is a classical tool to study the plasticity of visual cortical connections. After eyelid closure in young animals (monocular deprivation, MD), visual cortical neurons become dominated by the open eye, a phenomenon known as ocular dominance (OD) plasticity . It is commonly held that the molecular mediators of OD plasticity are cortically derived and that the retina is immune to the effects of MD . Recently, it has been reported that visual deprivation induces neurochemical, structural, and functional changes in the retina , but whether these retinal changes contribute to the effects of MD in the cortex is unknown. Here, we provide evidence that brain-derived neurotrophic factor (BDNF) produced in the retina influences OD plasticity. We found a reduction of BDNF expression in the deprived retina of young rats. We compensated this BDNF imbalance between the two eyes by either injecting exogenous BDNF in the deprived eye or reducing endogenous BDNF expression in the nondeprived eye. Both treatments were effective in counteracting the OD shift induced by MD. Retinal BDNF could also influence OD distribution in normal animals. These results show for the first time that OD plasticity is modulated by BDNF produced in the retina.     

Self-motion-induced eye movements: effects on visual acuity and navigation

Self-motion disturbs the stability of retinal images by inducing optic flow. Objects of interest need to be fixated or tracked, yet these eye movements can infringe on the experienced retinal flow that is important for visual navigation. Separating the components of optic flow caused by an eye movement from those due to self-motion, as well as using optic flow for visual navigation while simultaneously maintaining visual acuity on near targets, represent key challenges for the visual system. Here we summarize recent advances in our understanding of how the visuomotor and vestibulomotor systems function and interact, given the complex task of compensating for instabilities of retinal images, which typically vary as a function of retinal location and differ for each eye.     

Anatomy,neurophysiology and functional aspects of the nucleus isthmi in salamanders of the family Plethodontidae

Summary Tongue-projecting plethodontid salamanders have massive direct ipsilateral retinal afferents to the tectum opticum as well as a large and well developed nucleus isthmi. Retrograde staining revealed two subnuclei: A ventral one projecting to the contralateral tectal hemisphere and a dorsal one projecting back to the ipsilateral side. The isthmic nuclei show a retinotopic organization, which is in register with that of the tectum. Electrophysiological recordings from nucleus-isthmi neurons revealed response properties that are very similar to those found in tectal neurons. Thus, there is no substantial processing of tectal neural activity in the nucleus isthmi. Measurements of peak latencies after electrical and light stimulation suggest the continuous coexistence of 4 representations of the visual field in the tectum mediated by (1) the contralateral and (2) the ipsilateral direct retinal afferents, (3) the uncrossed and (4) the crossed isthmo-tectal projection. (1) and (2) originate at the same moment in the retina and arrive simultaneously in the tectum. It is assumed that in plethodontid salamanders with massive ipsilateral retino-tectal projections depth perception based on disparity cues is achieved by comparison of these images.Representations mediated by (3) and (4) arriving in the tectum at the same time as (1) and (2) originate 10–30 ms earlier in the retina. It is hypothesized that these time differences between (1)/(2) and (3)/(4) are used to calculate three-dimensional trajectories of fast-moving prey objects.Abbreviations EL edge length - FDA fluoresceine dextranamine - RDA tetramethylrhodamine dextranamine - RF receptive field     

Features of the Retinotopic Representation in the Visual Wulst of a Laterally Eyed Bird,the Zebra Finch (Taeniopygia guttata)

The visual wulst of the zebra finch comprises at least two retinotopic maps of the contralateral eye. As yet, it is not known how much of the visual field is represented in the wulst neuronal maps, how the organization of the maps is related to the retinal architecture, and how information from the ipsilateral eye is involved in the activation of the wulst. Here, we have used autofluorescent flavoprotein imaging and classical anatomical methods to investigate such characteristics of the most posterior map of the multiple retinotopic representations. We found that the visual wulst can be activated by visual stimuli from a large part of the visual field of the contralateral eye. Horizontally, the visual field representation extended from -5° beyond the beak tip up to +125° laterally. Vertically, a small strip from -10° below to about +25° above the horizon activated the visual wulst. Although retinal ganglion cells had a much higher density around the fovea and along a strip extending from the fovea towards the beak tip, these areas were not overrepresented in the wulst map. The wulst area activated from the foveal region of the ipsilateral eye, overlapped substantially with the middle of the three contralaterally activated regions in the visual wulst, and partially with the other two. Visual wulst activity evoked by stimulation of the frontal visual field was stronger with contralateral than with binocular stimulation. This confirms earlier electrophysiological studies indicating an inhibitory influence of the activation of the ipsilateral eye on wulst activity elicited by stimulating the contralateral eye. The lack of a foveal overrepresentation suggests that identification of objects may not be the primary task of the zebra finch visual wulst. Instead, this brain area may be involved in the processing of visual information necessary for spatial orientation.     

Effects of chronic iTBS‐rTMS and enriched environment on visual cortex early critical period and visual pattern discrimination in dark‐reared rats

Early cortical critical period resembles a state of enhanced neuronal plasticity enabling the establishment of specific neuronal connections during first sensory experience. Visual performance with regard to pattern discrimination is impaired if the cortex is deprived from visual input during the critical period. We wondered how unspecific activation of the visual cortex before closure of the critical period using repetitive transcranial magnetic stimulation (rTMS) could affect the critical period and the visual performance of the experimental animals. Would it cause premature closure of the plastic state and thus worsen experience‐dependent visual performance, or would it be able to preserve plasticity? Effects of intermittent theta‐burst stimulation (iTBS) were compared with those of an enriched environment (EE) during dark‐rearing (DR) from birth. Rats dark‐reared in a standard cage showed poor improvement in a visual pattern discrimination task, while rats housed in EE or treated with iTBS showed a performance indistinguishable from rats reared in normal light/dark cycle. The behavioral effects were accompanied by correlated changes in the expression of brain‐derived neurotrophic factor (BDNF) and atypical PKC (PKCζ/PKMζ), two factors controlling stabilization of synaptic potentiation. It appears that not only nonvisual sensory activity and exercise but also cortical activation induced by rTMS has the potential to alleviate the effects of DR on cortical development, most likely due to stimulation of BDNF synthesis and release. As we showed previously, iTBS reduced the expression of parvalbumin in inhibitory cortical interneurons, indicating that modulation of the activity of fast‐spiking interneurons contributes to the observed effects of iTBS. © 2015 Wiley Periodicals, Inc. Develop Neurobiol 76: 19–33, 2016     

Autoradiographic analysis of retinal projections in the flatfish Scophthalmus maximus L

The organization of retinal projections were examined in the flatfish Scophthalmus maximus using the autoradiographic technique. Overall 15 different contralateral primary optic centers were identified at hypothalamic, thalamo-pretectal, tectal and tegmento-mesencephalic levels. A weak ipsilateral retinal contingent was also observed. Elsewhere, the retinal projections of the migrating eye in comparison to those of the non-migrating eye were observed to be similar.     

Acetylcholinesterase activity in the normal and retino-deprived optic tectum of the quail

Summary Acetylcholinesterase localization has been studied by electron microscopic histochemistry in the quail optic tectum. Ultrastructural analysis reveals that the different neuronal types in the tectum possess the metabolic pathways for AChE synthesis to different degrees. From the site of synthesis in cell bodies the enzyme spreads towards areas of neuropil. In the neuropil of AChE-rich areas a balance seems to exist between enzyme stored in dendrites (and sometimes axon terminals) and enzyme released into the extracellular spaces. Precise identification of cholinergic synapses by means of AChE localization is in most cases impossible, due to extensive spread of the enzyme through the extracellular compartments of the neuropil.Unilateral ocular ablation causes disappearance of the stratum opticum and decrease in thickness of the superficial tectal layers in the contralateral optic tectum, but only minor modifications in AChE localization. This finding is in agreement with biochemical results which show equivalence of the relative concentration of AChE in the right and left optic tectum 1 or 2 months after ablation of the right eye. The experimental evidence suggests that cholinergic mechanisms are not related to the discharge of retinal afferents on receptive tectal neurons, but more likely to intrinsic neural circuits which might be involved in the modulation of tectal activity.     

Miniaturization in plethodontid salamanders (Gaudata: Plethodontidae) and its consequences for the brain and visual system

In seven species of plethodontid salamanders (Desmognathus ochrophaeus, Eurycea bislineata, Plethodon cinereus, Batrachoseps attenuatus, Hydromantes italicus, Thorius narisovalis and Bolitoglossa subpalmata), absolute and relative volumes of the eye, the brain, major regions of the brain, and regions containing the major visual and visuomotor centres (i.e. thalamus, praetectum, tectum and tegmentum mesencephali), and the density and number of neurons in these regions were determined. The seven species range from moderately large to extremely small in body size and from the smallest to the largest genome sizes found in terrestrial salamanders. The following processes were observed in miniaturized salamanders with intermediate to large genome and cell sizes (Batrachoseps, Thorius) as compared to small and medium-sized salamanders with small genome and cell sizes: (1) increase in the relative size of the brain, from 3.9 to 12.4% of head volume; (2) reduction in relative size of the ventricles from 10.9 to 5.8% of brain volume; (3) increase in relative volume of those brain regions containing the major visual and visuomotor centres from 29.2 to 37% of brain volume; (4) increase in volume of grey matter relative to white matter, from 33.2 to 44.4% of midbrain volume; (5) increase in volume of tectal relative to tegmental grey matter, from 54.8 to 76.8% of total midbrain volume; (6) increase in neuron packing density in the regions containing the visual centres, from 16 to 31.5%. Because of these compensatory processes, Thorius, the smallest species with a head 1/27 and a brain 1/9 the size of that of the largest one, Hydromantes, has 1/3 as many central visual neurons (58 000 vs. 187 000). Some of these processes found in miniaturized salamanders, such as increase in tectal cell density, also occur in large salamanders with very large genome and cell sizes, viz. in Bolitoglossa (25%) and Hydromantes (29%). Thus, increase in genome size and cell size seem to pose functional problems similar to miniaturization; both cases involve an increase in cell size relative to overall organismal structure.     

Spike timing-dependent LTP/LTD mediates visual experience-dependent plasticity in a developing retinotectal system

Sensory experience plays an instructive role in the development of the nervous system. Here we showed that visual experience can induce persistent modification of developing retinotectal circuits via spike timing-dependent plasticity (STDP). Pairing light stimuli with spiking of the tectal cell induced persistent enhancement or reduction of light-evoked responses, with a dependence on the relative timing between light stimulus and postsynaptic spiking similar to that for STDP. Using precisely timed sequential three-bar stimulation to mimic a moving bar, we showed that spike timing-dependent LTP/LTD can account for the asymmetric modification of the tectal cell receptive field induced by moving bar. Furthermore, selective inhibition of signaling mediated by brain-derived neurotrophic factor and nitric oxide, which are respectively required for light-induced LTP and LTD, interfered with moving bar-induced temporally specific changes in the tectal cell responses. Together, these findings suggest that STDP can mediate sensory experience-dependent circuit refinement in the developing nervous system.     

Retinotectal plasticity in Xenopus: Anomalous ipsilateral projection following late larval eye removal

The left eye was removed from Stage 56 Xenopus tadpoles. Two to 9 months after metamorphosis, electrophysiologic analysis showed that the surviving (right) eye mediated a normal visual field projection to the left (contralateral) optic tectum. In addition, a peripheral region of the same retina innervated the entire right (ipsilateral) tectum. Primary evidence that indicates this anomalous ipsilateral projection was due to direct retina-to-tectum innervation comes from singleunit analysis, latency measurements, and tectal lesion studies. Thus, the peripheral retina simultaneously connected in much different patterns to the two optic tecta, solely on the basis of the presence (in the left tectum) or absence (in the right tectum) of central retinal fibers. This documents a role for fiber-fiber interaction (such as repulsion or competition) acting in combination with fiber-tectum interactions in the formation of the retinotectal map.     

The role of visual experience in the formation of binocular projections in frogs

Many parts of the visual system contain topographic maps of the visual field. In such structures, the binocular portion of the visual field is generally represented by overlapping, matching projections relayed from the two eyes. One of the developmental factors which helps to bring the maps from the two eyes into register is visual input. The role of visual input is especially dramatic in the frog, Xenopus laevis. In tadpoles of this species, the eyes initially face laterally and have essentially no binocular overlap. At metamorphosis, the eyes begin to move rostrodorsally; eventually, their visual fields have a 170 degree region of binocular overlap. Despite this major change in binocular overlap, the maps from the ipsilateral and contralateral eyes to the optic tectum normally remain in register throughout development. This coordination of the two projections is disrupted by visual deprivation. In dark-reared Xenopus, the contralateral projection is nearly normal but the ipsilateral map is highly disorganized. The impact of visual input on the ipsilateral map also is shown by the effect of early rotation of one eye. Examination of the tectal lobe contralateral to the rotated eye reveals that both the contralateral and the ipsilateral maps to that tectum are rotated, even though the ipsilateral map originates from the normal eye. Thus, the ipsilateral map has changed orientation to remain in register with the contralateral map. Similarly, the two maps on the other tectal lobe are in register; in this case, both projections are normally oriented even though the ipsilateral map is from the rotated eye. The discovery that the ipsilateral eye's map reaches the tectum indirectly, via a relay in the nucleus isthmi, has made it possible to study the anatomical changes underlying visually dependent plasticity. Retrograde and anterograde tracing with horseradish peroxidase have shown that eye rotation causes isthmotectal axons to follow abnormal trajectories. An axon's route first goes toward the tectal site where it normally would arborize but then changes direction to reach a new tectal site. Such rearrangements bring the isthmotectal axons into proximity with retinotectal axons which have the same receptive fields. Anterograde horseradish peroxidase filling has also been used to study the trajectories and arborizations of developing isthmotectal axons. The results show that the axons enter the tectum before the onset of eye migration but do not begin to branch profusely until eye movement begins to create a zone of binocular space.(ABSTRACT TRUNCATED AT 400 WORDS)     

New findings on the organization of retinal projections in the Scyliorhinus canicula shark. Radioautographic study

The retinal projections of the shark Scyliorhinus canicula were re-examined using the radioautographic method following the intraocular injection of tritiated tracers. New primary visual centers were identified. Overall 12 distinct sites of termination of contralateral optic endings were found distributed within five levels of the brain (hypothalamus, thalamus, pretectum, tectum and mesencephalic tegmentum). Furthermore the presence of a small ipsilateral retinal projection was demonstrated attaining the hypothalamic thalamo:pretectal and tectal levels. These new data broaden our understanding of the organization of the primary visual system in Scyliorhinus as defined previously using the degeneration technique.     

Regulation of pathologic retinal angiogenesis in mice and inhibition of VEGF-VEGFR2 binding by soluble heparan sulfate

Development of the retinal vascular network is strictly confined within the neuronal retina, allowing the intraocular media to be optically transparent. However, in retinal ischemia, pro-angiogenic factors (including vascular endothelial growth factor-A, VEGF-A) induce aberrant guidance of retinal vessels into the vitreous. Here, we show that the soluble heparan sulfate level in murine intraocular fluid is high particularly during ocular development. When the eyes of young mice with retinal ischemia were treated with heparan sulfate-degrading enzyme, the subsequent aberrant angiogenesis was greatly enhanced compared to PBS-injected contralateral eyes; however, increased angiogenesis was completely antagonized by simultaneous injection of heparin. Intraocular injection of heparan sulfate or heparin alone in these eyes resulted in reduced neovascularization. In cell cultures, the porcine ocular fluid suppressed the dose-dependent proliferation of human umbilical vein endothelial cells (HUVECs) mediated by VEGF-A. Ocular fluid and heparin also inhibited the migration and tube formation by these cells. The binding of VEGF-A and HUVECs was reduced under a high concentration of heparin or ocular fluid compared to lower concentrations of heparin. In vitro assays demonstrated that the ocular fluid or soluble heparan sulfate or heparin inhibited the binding of VEGF-A and immobilized heparin or VEGF receptor 2 but not VEGF receptor 1. The recognition that the high concentration of soluble heparan sulfate in the ocular fluid allows it to serve as an endogenous inhibitor of aberrant retinal vascular growth provides a platform for modulating heparan sulfate/heparin levels to regulate angiogenesis.     

Therapeutic benefit of radial optic neurotomy in a rat model of glaucoma

Radial optic neurotomy (RON) has been proposed as a surgical treatment to alleviate the neurovascular compression and to improve the venous outflow in patients with central retinal vein occlusion. Glaucoma is characterized by specific visual field defects due to the loss of retinal ganglion cells and damage to the optic nerve head (ONH). One of the clinical hallmarks of glaucomatous neuropathy is the excavation of the ONH. The aim of this work was to analyze the effect of RON in an experimental model of glaucoma in rats induced by intracameral injections of chondroitin sulfate (CS). For this purpose, Wistar rats were bilaterally injected with vehicle or CS in the eye anterior chamber, once a week, for 10 weeks. At 3 or 6 weeks of a treatment with vehicle or CS, RON was performed by a single incision in the edge of the neuro-retinal ring at the nasal hemisphere of the optic disk in one eye, while the contralateral eye was submitted to a sham procedure. Electroretinograms (ERGs) were registered under scotopic conditions and visual evoked potentials (VEPs) were registered with skull-implanted electrodes. Retinal and optic nerve morphology was examined by optical microscopy. RON did not affect the ocular hypertension induced by CS. In eyes injected with CS, a significant decrease of retinal (ERG a- and b-wave amplitude) and visual pathway (VEP N2-P2 component amplitude) function was observed, whereas RON reduced these functional alterations in hypertensive eyes. Moreover, a significant loss of cells in the ganglion cell layer, and Thy-1-, NeuN- and Brn3a- positive cells was observed in eyes injected with CS, whereas RON significantly preserved these parameters. In addition, RON preserved the optic nerve structure in eyes with chronic ocular hypertension. These results indicate that RON reduces functional and histological alterations induced by experimental chronic ocular hypertension.     

Influence of the motor cortex on lateral geniculate responses evoked in the cat by contralateral superior colliculus stimulation

It is shown that in nembutal anesthetized cats, a single stimulation of motor cortex (MC) causes a response in lateral geniculate nucleus (LGN). The development of this response had a conditioning effect on the LGN response evoked by stimulation of the contralateral superior colliculus (SC), markedly inhibiting it. The degree of this inhibition depended on the time interval between the cortical conditioning stimulation and the tectal test stimulation. A single conditioning MC stimulation did not noticeably change the LGN responses evoked by a light stimulus, but markedly inhibited visual responses from deep SC layers (those regions which on stimulation gave rise to LGN responses). From the results, it is suggested that the MC monitors the execution of tectal influences on LGN function at the tectal level rather than the geniculate level, and it is precisely by this means that it regulates saccadic suppression of LGN function, in the realization of which, as presumed earlier, the SC takes part.A. I. Karaev Institute of Physiology, Azerbaijan Academy of Sciences, Baku. Translated from Neirofiziologiya, Vol. 24, No. 4, July–August 1992.