Nucleus Of The Optic Tract Research Articles

The afferent connections of the inferior olivary complex in rats: a study using the retrograde transport of horseradish peroxidase.

AbstractRetrograde transport of horseradish peroxidase (HRP) was used to define the origin of afferents to the inferior olivary complex (IOC) in rats. Using both ventral and dorsal surgical approaches to the brainstem, HRP was injected into the IOC through a micropipette affixed to the tip of a 1‐μl Hamilton syringe. After a 2‐day postoperative survival, animals were sacrificed by transcardiac perfusion with a 1% paraformaldehyde‐1.25% gluteraldehyde solution, and brains were processed according to the DeOlmos protocol (1977), using o‐dianisidine as the chromogen.Labeled cells were found at many levels of the nervous system extending from lumbar spinal cord to cerebral cortex. This wide‐ranging input from numerous regions clearly underscores the complexity of the IOC and its apparent involvement in several functions.Within the spinal cord, labeled neurons were identified from cervical to lumbar but not at sacral levels. These neurons were found contralaterally in the neck region of the dorsal horn and in the medial portions of the intermediate gray.In the caudal brainstem, reactive cells in the dorsal column nuclei, the spinal trigeminal nucleus, and the subnucleus y of the vestibular complex were observed primarily contralateral to the injection sites. Labeling within the gigantocellular, magnocellular, ventral, and lateral reticular nuclei and the nucleus prepositus hypoglossi was primarily ipsilateral. Reactive neurons in the medial and inferior vestibular nuclei were predominantly ipsilateral or contralateral to HRP injections into the caudal or rostral IOC, respectively.The dentate and interposed nuclei of the cerebellum contained small, lightly labeled neurons primarily contralateral to the injection site, while the fastigial nuclei contained a few relatively large, heavily labeled cells bilateral to caudal olivary injections.Ipsilaterally labeled mesencephalic regions included the periaqueductal gray, interstitial nucleus of Cajal, rostromedial red nucleus, ventral tegmental area, medial terminal nucleus of the accessory optic tract, nucleus of the optic tract, and the lateral deep mesencephalic nucleus. The caudal part of the pretectum and small cells of the stratum profundum of the superior colliculus were labeled predominantly contralateral to the injection.In the caudal diencephalon labeled neurons were most numerous within the nucleus of Darkschewitsch and the subparafascicular nucleus, primarily ipsilateral to olivary injections. Scattered reactive neurons were also found within the ipsilateral zone incerta. With the exception of the zona incerta, all labeled mesencephalic and diencephalic nuclei had some bilateral representation of labeled cells.No labeled neurons were identified within the basal ganglia, while numerous reactive cells were found bilaterally within layer V of the frontal and parietal cerebral cortex.

Effects of cholinomimetics on glucose utilization in rat brain optic systems

Physostigmine and oxotremorine effects on local cerebral glucose utilization (LCGU) in rat brain optic systems were examined by the [ 14C]2-deoxy-D-glucose procedure. Both drugs significantly stimulated LCGU in the superior colliculus (superficial layer) and nucleus of the optic tract, but not in the visual cortex. Physostigmine, but not oxotremorine, stimulated LCGU in components of the accessory optic system. The effects in the accessory optic system apparently result from nicotinic actions of physostigmine.

Flash-induced nystagmus (FIN) and the vestibular system in the rabbit.

Flash-induced nystagmus (FIN) was found more often in albino than in pigmented rabbits. Incremental and decremental phase mainly showed exponential change in SPV, but linear change was seen as well with apparent time constant of 5-40 sec. FIN was unaltered by rendering the stimulated eye akinetic. Lesioning the nucleus of the optic tract (NOT) on one side abolished FIN and OKN responses from the other side. Bilateral labyrinthectomy led to a diminished response and after-responses were lost. Algebraic addition was shown if FIN was combined with vestibular reflex eye movements. The gain of these reflexes was enhanced. FIN may be specified as a type of nystagmus that is evoked by monocular essentially open-loop stimulation of the crossed ('subcortical') optic pathway. Direction-selective elements in the retina may play a specific role in that their predominant orientation gives rise to abduction nystagmus during on/off stimulation by flashes.

Projections of the medial terminal nucleus of the accessory optic system upon pretectal nuclei in the pigmented rat.

The projections of the medial terminal nucleus (MTN) of the accessory optic system (AOS) upon pretectal nuclei have been studied in pigmented rats by means of (i) the anterograde transport of 3H-leucine with the use of light autoradiography and (ii) the retrograde transport of horseradish peroxidase (HRP). Injections of 3H-leucine largely restricted to the MTN and minimally involving adjacent ventral midbrain structures, produced heavy terminal axonal labeling within the ipsilateral nucleus of the optic tract (NOT) and the dorsal terminal nucleus (DTN) of the AOS. Terminal labeling was observed in all superficial portions of the NOT, except for a small ventromedial segment in the rostral two thirds and a larger medial segment in the caudal one third of this nucleus. Thus the MTN-NOT projections we describe entirely overlap the retinal-NOT projection and partially overlap the visual cortical-NOT, as reported by others. Within the DTN, the dense terminal fields covered the entire nucleus. After postinjection survival times of 3-7 days, the pattern of axonal labeling showed that the MTN-NOT projection consisted of three bundles: (i) a superficial mesencephalic bundle coursing within the superior fasciculus, posterior fibers of the AOS which enters the caudal portions of the NOT and the DTN; (ii) a deep mesencephalic bundle that traversed the midbrain tegmentum dorsolaterally, also reaching the caudal one-half of the NOT and all of the DTN; and (iii) a mesodiencephalic bundle that passed first laterally through midbrain tegmentum and then dorsally through lateral thalamus to enter the rostral one-half of the NOT. Pretectal injections of HRP that invade the NOT and DTN produced retrograde labeling of most (ca. 75%) of the neurons of the ipsilateral MTN, without labeling the adjacent substantia nigra or ventral tegmental area. This finding confirms our autoradiographic data by showing that the MTN provides the major, ventral tegmental projection to the NOT and DTN. The present finding of a MTN-NOT projection, combined with available anatomical and physiological data, suggests that the MTN may play a more significant role in visual-vestibular aspects of oculomotor control than formerly thought.

Velocity-tuning of motion-sensitive and direction-selective cells in the rabbit striate cortex

The receptive field characteristics of 268 striate cortical cells were classified in 10 Dutch-belted rabbits. From the 66 motion-sensitive and direction-selective units encountered, quantitative data describing the responses of 41 motion-sensitive, and 10 direction-selective cells to moving gratings were obtained. The results showed that all cells responded with increased spike discharges to increased velocities of the moving gratings up to a maximum, then they became less responsive to further increase of the velocities. The patterns of the response curves varied from cell to cell. When the 51 cells were taken as a whole, there were more cells responding to the velocities of 10–30 deg/sec than to higher or lower velocities. The total range over which these cells responded was 0.1–300 deg/sec. The preferred velocities for individual cells ranged from 1.5 to 150 deg/sec, showing a moderate concentration at the 6–40 deg/sec region. The extent of the effective velocities for individual cells formed a continuous distribution. No segregation into groups was seen in either of the two cell types studied or in all cells taken as a group. These data were compared with those reported for the rabbit retinal ganglion cells and the cells in the nucleus of the optic tract, as well as the cells in the cat visual system.

Connections between the frontal eye field and pretectum in the monkey: an anterograde/retrograde study using HRP gel and TMB neurohistochemistry.

Horseradish peroxidase (HRP) gel implants in the frontal eye field (FEF) of macaque monkeys, processed with tetramethylbenzidine (TMB) neurohistochemistry and studied with darkfield microscopy, demonstrated bidirectional HRP labeling of the afferents and efferents of this cortical area. It was evident that among the entire scope of its inputs, the FEF received a prominent afferent projection from the nucleus of the optic tract (NOT, nucleus limitans) and the suprageniculate nucleus, and projected to a medial subdivision of NOT, sublentiform nucleus, nucleus of the pretectal area, nucleus of the posterior commissure, and the rostral periaqueductal gray. The direct afferent projections to FEF from NOT could provide a route for visual input to reach FEF via the pretectum without first going to the visual cortex. The efferents probably represent the pathway through which FEF influences pupillary dynamics known to accompany, or occur independently of, eye movements.

Projection of brain stem neurons to the perigeniculate nucleus and the lateral geniculate nucleus in the cat

Small horseradish peroxidase injections in the perigeniculate nucleus (PGN) or the lateral geniculate nucleus (LGN) gave retrograde labeling of many cells in the pontomesencephalic reticular formation (RF), the nuclei raphe dorsalis and centralis linearis, locus coeruleus, nucleus of the optic tract and nucleus parabigeminalis. Antidromic stimulation was used to identify neurons in the RF projecting to the PGN-LGN complex. Threshold mapping through the PGN and the LGN shows separate projection from the reticular formation to the PGN and the LGN.

Optokinetic visual detection in the rat visual centres. A [14C]-2-deoxy-D-glucose study.

Following forty-five min of binocular optokinetic stimulation (OKS) the autoradiographic maps of [14C]-2-deoxy-D-glucose (2DG) assumption of Long-Evans brain reveal clearly different patterns of optical density within visual centres. The most superficial layers of superior colliculus (SC) and a pretectal area including the nucleus of the optic tract (NOT) appear symmetrically, strongly darker than other visual structures such as lateral geniculate nucleus (LGN) and visual cortex (VC). Whereas the lack of metabolic increase at LGN and VC levels entirely confirms the non-involvement of the geniculo-cortical path in mediating the optomotor response following OKS in Rodents, it is postulated that the symmetrical increase of 2DG uptake even upon unidirectional OKS found even at pretectal level may represent a commissural transfer of visual information between homologous pretectal areas like the nuclei of the optic tract.

Retinal projection to the nucleus of the optic tract in the cat as revealed by retrograde transport of horseradish peroxidase

Retrograde transport of horseradish peroxidase injected iontophoretically into the nucleus of the optic tract of cats revealed that the direction-selective cells in this pretectal nucleus receive direct retinal projections from small retinal ganglion cells, the so-called γ-cells. These cells from a horizontal band on the contralateral retina. Few labeled cells are found in the ipsilateral temporal retina. The input from the contralateral retina is 10 times more numerous than from the ipsilateral one. In both retinae the highest concentration of labeled cells is near the area centralis.

Projections from areas 17 and 18 of the visual cortex to the nucleus of the optic tract

The corticofugal pathway to the nucleus of the optic tract (NOT) in the cat was studied with visual and electrical stimulation in two experimental series. In 10 experiments cells in the NOT were identified, and orthodromic responses evoked by single electric shocks applied through microelectrodes situated at different loci in areas 17 and 18. All but 2 units gave clear responses to shocks applied to either cortical electrode. The mean of response latencies was calculated to be 3.29 ms for area 17 stimulation and 3.04 ms for area 18 stimulation. In 8 further experiments the stimulation and recording sites were reversed; i.e. single shocks through microelectrodes in the NOT were used to elicit antidromic discharges in areas 17 and 18. A third microelectrode was placed in the superior colliculus (SC) at a position in retinotopic register with the units recorded at the NOT stimulation site. Out of 231 cortical units tested, 42 (17%) gave an antidromic response to NOT and/or SC shock. Seventeen units responded both to NOT and SC stimulation, 18 to NOT alone, and 7 to SC alone. The response latencies after NOT shock (mean 2.8 ± 1.5S.D.) did not differ significantly from those to SC shock ( 2.9ms± 1.5S.D.). All cortical cells activated antidromically from NOT and/or SC were located in layer V of areas 17 and 18. These units showed the following response characteristics: they responded well to conventional light bars as well as to large area random dot patterns; they were binocular and showed direction tuning; as compared to NOT cells they were more sharply tuned for stimulus velocity preferring faster movements. The present findings suggest a convergent projection to the NOT from the same type of cortical cells that project to the superior colliculus.

Effects of ablation of flocculus and paraflocculus of eye movements in primate.

1. Eye movements were recorded in four rhesus monkeys, before and after bilateral ablations of the flocculi and portions of the paraflocculi (“flocculectomy”). Animals were trained to fixate and follow targets so that pursuit, saccades, and vestibular and optokinetic nystagmus could be quantitated. 2. The vestibuloocular reflex (VOR) was mildly affected by flocculectomy. In darkness the VOR gain (eye velocity/head velocity) for steps of head velocity (normal range, 0.84-0.96) increased postoperatively in one monkey to 1.17, decreased in another to 0.62, and did not change significantly in the other two (0.96, 0.94). VOR phase lead at 0.05-Hz oscillation in darkness (normal range, O,O--LOO) increased in two monkeys by 12 and 9.5 O but did not change significantly in the others. 3. Visual suppression of inappropriate vestibular nystagmus was impaired. During oscillation at 0.25 Hz with a head-fixed visual scene, the average amount of attenuation of the VOR decreased from 5 1% preoperatively to 20% postoperatively. Visual suppression of caloric nystagmus was comparably affected. 4. Smooth tracking of small targets moving in space was impaired either with the head still (smooth pursuit) or moving (cancellation of the VOR by fixating a target rotating with the head). The average gain (gaze velocity/target velocity) in either case was about 0.65 (normal, 0.95-0.98). The pursuit deficit was less than that reported after total cerebellectomy, suggesting other cerebellar structures also participate in smooth tracking. 5. Optokinetic nystagmus (OKN) was present postoperatively but, in response to a constant-velocity stimulus, the initial slowphase velocity decreased by 50% and the rise time to a steady state nearly doubled. For stimuli belowr 6O”/s the average steady-state gain was only mildly diminished to 0.86 (normal, 0.98) and the time course of optokinetic afternystagmus (OKAN) in darkness was normal. The response to higher velocity optokinetic stimuli was impaired. These abnormalities can largely be attributed to the coexisting pursuit deficit although the protracted rise to a steady state suggests a change in the ability of the brain stem optokinetic system to handle large amounts of retinal slip. 6. Flocculectomized monkeys showed horizontally, gaze-paretic nystagmus with exponentially decaying centripetal drift (time constant, 1.56 s) and vertically, downbeat nystagmus with exponentially decaying (in three monkeys) or exponentially increasing (in one monkey) slow phases. All animals showed rebound nystagmus. These results implicate the flocculus and possibly the paraflocculus in the control of the time constant and stability of the brain stem oculomotor integrators. 7. Saccadic velocities and accuracy were normal but flocculectomized monkeys showed brief (40150 ms duration), approximately exponential, postsaccadic drift with amplitudes up to 15% of the size of the saccade. The eyes usually drifted in the same direction as the saccade but each monkey showed an idiosyncratic pattern depending on saccade direction and eye position. Postsaccadic drift may reflect a mismatch between the phasic (pulse) and tonic (step) innervational changes that create saccades.

Inversion of direction-selectivity to anterior fields in neurons of nucleus of the optic tract in rabbits with ocular albinism

Extracellular recordings were made of 46 well-isolated single units in the nucleus of the optic tract (NOT) from 27 rabbits which were paralyzed but unanesthetized. Rabbits were selected from 3 varieties with ocular albinism. These units showed short latencies (mean = 2.2 msec, S.D. = 0.4) to chiasmic stimulation suggestive of monosynaptic innervation from retinal ganglion cells. Receptive fields were large and receptive field centers were confined to the projection of the visual streak. All units showed direction selectivity in which preferred stimulus motions were always nearly horizontal in the anterior or posterior direction. The direction selectivity of the units was broad in that excitatory acceptance angles were 180 degrees. Single units recorded from NOT in albino rabbits could be classified into 4 types based on regions of direction selectivity. The most common (64%) type of unit had receptive fields with two regions of opposed direction selectivity. The preferred direction of motion was anterior in lateral and posterior visual fields and was posterior in anterior visual fields. Two smaller groups of units (13% and 11%) had receptive fields with only one preferred direction. When the receptive field was anterior, the preferred direction was posterior. When the receptive field was lateral or posterior, the preferred direction was anterior. The least commonly encountered type of unit (7%) had receptive fields with two regions of direction selectivity. Anterior visual fields gave rise to anterior direction selectivity and lateral and posterior visual fields gave rise to posterior direction selectivity. The characteristics of NOT units are similar to those described for NOT units in pigmented rabbits except for an inversion of direction selectivity to stimulation from anterior visual fields. NOT units in albino rabbits show posterior direction selectivity when stimulation arises from anterior visual fields. The difference in directional preference in the visual receptive fields of the units in the NOT of albino rabbit may be responsible for the inversion of optokinetic pursuit eye movements characteristic of albino rabbits.

Metabolic response to optic centers to visual stimuli in the albino rat: anatomical and physiological considerations.

The functional organization of the visual system was studied in the albino rat. Metabolic differences were measured using the 14-C-2-deoxyglucose (DG) autoradiographic technique during visual stimulation of one entire retina in unrestrained animals. All optic centers responded to changes in light intensity but to different degrees. The greatest change occurred in the superior colliculus, less in the lateral geniculate, and considerably less in second-order sites such as layer IV of visual cortex. These optic centers responded in particular to on/off stimuli, but showed no incremental change during pattern reversal or movement of orientation stimuli. Both the superior colliculus and lateral geniculate increased their metabolic rate as the frequency of stimulation increased, but the magnitude was twice as great in the colliculus. The histological pattern of metabolic change in the visual system was not homogenous. In the superior colliculus glucose utilization increased only in stratum griseum superficiale and was greatest in visuotopic regions representing the peripheral portions of the visual field. Similarly, in the lateral geniculate, only the dorsal nucleus showed an increased response to greater stimulus frequencies. Second-order regions of the visual system showed changes in metabolism in response to visual stimulation, but no incremental response specific for type or frequency of stimuli. To label proteins of axoplasmic transport to study the terminal fields of retinal projections 14C-amino acids were used. This was done to study how the differences in the magnitude of the metabolic response among optic centers were related to the relative quantity of retinofugal projections to these centers. Fast and slow axoplasmic transport were studied using three separate amino acids. In each case over 64% of the radioactivity projecting contralateral from the eye was found in superior colliculus. considerably less isotope was found in dorsal lateral geniculate (11-17%), ventral lateral geniculate (3, 7-6.2%), pretectal nuclei (5-12%), and the accessory optic system (3-7%). The greatest concentration of radioactivity within each optic center was found in the visuotopic aspect subserving the superior visual field; particularly the medial aspects of the superior colliculus, olivary pretectal nucleus, and posterior pretectal nucleus, and the anterior portion of the nucleus of the optic tract. The representation of central vision in the colliculus was relatively pale, as was a zone within the middle of the contralateral dorsal lateral geniculate. The anatomical and physiological results of this study suggest that differences in deoxyglucose metabolism among optic centers are primarily related to the number of retinofugal endings and the kind of visual stimulation. Changes within any one center primarily reflect the density of retinal endings subserving the visual field.

Sources of descending afferents to the inferior olive from the upper brain stem in the cat as revealed by the retrograde transport of horseradish peroxidase.

The retrograde transport of horseradish peroxidase (HRP) was employed in 35 cats to trace the sources of descending afferents to the inferior olivary nucleus. The majority of HRP-labeled cells were located in the ipsilateral mesencephalon, from the rostral pole of the red nucleus to the caudomedial border of the thalamus. Among the heavily labeled nuclear groups were the the parvocellular red nucleus, the interstitial nucleus of Cajal, the nucleus of Darkschewitsch, and the caudomedial extremity of the subparafascicular nucleus. Occasional cells were also labeled in the reticular formation lateral to the interstitial nucleus of Cajal in the caudomedial parafascicular nucleus, in the nucleus of the fields of Forel, and in the central gray. It was not possible to recognize a topographic distribution of the labeled cells corresponding to the localization of the HRP deposits. Elsewhere in the midbrain, there were HRP-labeled cells in the deep layers of the superior colliculus (IV and VI), predominantly on the side contralateral to the injection and in the nucleus of the optic tract, and in the anterior and posterior pretectal nuclei, ipsilaterally. These groups were best labeled after caudally located injections in the olive. Certain areas previously reported to project to the inferior olive were devoid of labeled cells; these included the basal ganglia, the rostral raphe nuclei, and the nucleus of Edinger-Westphal. It appears that the mesencephalic nuclei bordering the central gray represent the major source of descending afferents to the olive.

The immunocytochemical localization of enkephalin in the central nervous system of the rat.

The immunocytochemical localization of enkephalin-like immunoreactivity (ELI) throughout the rat central nervous system (CNS) was investigated. The detection of ELI-containing structures was facilitated through the use of (1) brains from colchicine-treated rats, (2) the proteolytic pretreatment of sections with pronase and (3) the "double-bridge" staining technique. Our findings confirm the presence of ELI in perikarya, neuronal processes and terminals in many areas of the CNS. In addition, the localization of ELI-containing perikarya is reported for the first time in the following areas: the olfactory bulb, the olfactory tubercle, the lateral preoptic nucleus, several nuclei within the amygdaloid nuclear complex, the hippocampus, the neocortex, the cingulate cortex, the posterior mammillary nucleus, the medial nucleus of the optic tract, the brachium of the inferior colliculus, the ventral tegmental nucleus, the locus ceruleus, the sub-ceruleal region, the lateral trapezoid nucleus, the nucleus reticularis lateralis, and lamina VII of the cervical spinal cord. Our results demonstrate ELI in neurons which are heterogeneous in size, some probably functioning as interneurons and others as projection neurons in different areas of the CNS. The location of these neurons within the brain suggests that these pentapeptides serve diverse functions which include, in addition to nociception, the regulation of neuroendocrine, respiratory, auditory, vestibular, and olfactory functions.

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.

Electron microscopic identification of axon terminals of retinopretectal fibers in the cat by a combined horseradish peroxidase and tritiated amino acids tracing method

The terminal nucleus of the optic tract in the pretectum was observed electron microscopically in the cat after intravitreous injection of a mixture of HRP and tritriated amino acids. Autoradiographic silver grains, and often also HRP granules, were seen in large axon terminals containing round synaptic vesicles and pale mitochondria. These terminals occasionally made synaptic contacts with the presynaptic dendrites and were involved in the formation of the synaptic triad.

A quantitative analysis of the direction-specific response of Neurons in the cat's nucleus of the optic tract.

All cells in the nucleus of the optic tract (NOT) of the cat, that could be activated antidromically from the inferior olive, were shown to be direction-specific, as influenced by horizontal movements of an extensive visual stimulus. Cells in the left NOT were activated by leftward and inhibited by rightward movement, while those in the right NOT were activated by rightward and inhibited by leftward movement. Vertical movements did not modulate the spontaneous activity of the cells. The mean spontaneous discharge rate in 50 NOT cells was 30 spikes/s. This direction-specific response was maintained over a broad velocity range (Less Than 0.1 degrees - Greater Than 100 degrees/s). Velocities over 200 degrees/s could inhibit NOT cells regardless of stimulus direction. All cells in the NOT were driven by the contralateral eye, about half of them by the ipsilateral eye also. In addition, activation through the contralateral eye was stronger in most binocular units. Binocular cells preferred the same direction in the visual space through both eyes. An area approximately corresponding to the visual streak in the cat's retina projected most densely onto NOT cells. This included an extensive ipsilateral projection. No clear retinotopic order was seen. The most sensitive zone in the very large receptive fields (most diameters being Greater Than 20 degrees) was along the horizontal zero meridian of the visual field. The retinal input to NOT cells was mediated by W-fibers. The striking similarities between the input characteristics of NOT-cells and optokinetic nystagmus are discussed. The direction selectivity and ocular dominance of the NOT system as a whole can provide a possible explanation for the directional asymmetry in the cat's optokinetic nystagmus when only one eye is stimulated.

A quantitative comparison of optokinetic nystagmus (OKN) and neural responses in the nucleus of the optic tract (NOT) in monocularly deprived (MD) cats

During the first few months of life after birth human infants when tested monocularly move their unoccluded eye nasalward in darkness after viewing a large textured visual field moving either nasalward or temporalward. The eye movements in darkness are optokinetic after nystagmus (OKAN) which is an aftereffect of a reflex horizontal following eye movement, optokinetic nystagmus (OKN). Not until 4–5 months of age did temporalward field motion evoke OKAN with temporalward slow phase. The nasalward slow phase of OKAN that responded earlier to temporalward field motion appears to underlie the delayed development of reflex following eye movements in the temporalward direction.

The pretectal complex in the opossum: projections from the striate cortex and correlation with retinal terminal fields

Two terminal fields were revealed in the pretectal complex of the opossum by the Fink-Heimer method after striate cortical lesions. A rostral field is located within a rostrolateral strip of the compact part of the anterior pretectal nucleus, where a partial topographic arrangement of this projection is present. A caudal field is located within the sub-brachial nucleus of the optic tract, located between the brachium of the superior colliculus and the posterior pretectal nucleus. The corticotopic projection to this field is mirror-symmetric to that found in the superior colliculus and overlaps a bilateral projection from the retina. Based on neural pathway evidence, it is concluded that the nucleus of the optic tract in the opossum can be subdivided in (a) an intrabrachial nucleus receiving a direct projection from the contralateral retina and (b) a sub-brachial nucleus receiving projections from both retinae and from the striate cortex. The pretectal complex, as the superior colliculus, can be anatomically subdivided in a superficial region receiving visual input (the optic pretectum) and a deep region only remotely connected to the visual system. The optic pretectum, however, differs from the superior colliculus in displaying a multiple-map arrangement within its constituent nuclei, instead of a single continuous representation of the visual field.