Indirect, across-the-midline retinotectal projections and representation of ipsilateral visual field in superior colliculus of the cat

1. In agreement with previous work, we have found that the ipsilateral visual field is represented in an extensive rostral portion--from one-third to one-half--of the superior colliculus (SC) of the cat. This representation is binocular. The SC representation of the ipsilateral visual field can be mediated both directly, by crossed retinotectal connections originating from temporal hemiretina, and indirectly, by across-the-midline connections relaying visual information from one-half of the brain to contralateral SC. 2. In order to study the indirect, across-the-midline visual input to the SC, we have recorded responses of SC neurons to visual stimuli presented to either the ipsilateral or the contralateral eye of cats with a midsagittal splitting of the optic chiasm. Units driven by the ipsilateral eye, presumably through the direct retinotectal input and/or corticotectal connections from ipsilateral visual cortex, were found throughout the SC, except at its caudal pole, which normally receives fibers from the extreme periphery of the contralateral nasal hemiretina. Units driven by the contralateral eye, undoubtedly through an indirect across-the-midline connection, were found only in the anterior portion of the SC, in which is normally represented the ipsilateral visual field. Receptive fields in both ipsilateral and contralateral eye had properties typical of SC receptive fields in cats with intact optic pathways. 3. All units having a receptive field in the contralateral eye had also a receptive field in the ipsilateral eye; for each of these units, the receptive fields in both eyes invariably abutted the vertical meridian of the visual field. The receptive field in one eye had about the same elevation relative to the horizontal meridian and the same vertical extension as the receptive field in the other eye; the two receptive fields of each binocular unit matched each other at the vertical meridian and formed a combined receptive field straddling the vertical midline of the horopter...     

Spatial relation of the acetylcholinesterase-rich domain to the visual topography in the feline superior colliculus

Summary The superior colliculus (SC) of the cat shows a prominent compartmentalized organization at the level of its intermediate layers. The mosaic of these compartments is apparent in the pattern of acetylcholinesterase (AChE) staining. Patches of high AChE-activity are sharply set off from surrounding areas in the caudal SC while they are less distinct anteriorly. The rostral part lacks such obvious compartments. Thus, a structural reorganization apparently cuts across the topographical representations spread out in the SC. In order to test if this compartmental gradient relates to the topographic maps of the colliculus, retinotopic landmarks were visualized in the superficial layers by labeling the retinotectal pathway. In the SC ipsilateral to the eye injected with horseradish peroxidase (HRP) a paucity of labeling indicated the zone representing the ipsilateral visual half-field. Serial reconstructions of collicular sections, cut longitudinally or tangentially, revealed that the non-compartmentalized part of the intermediate layers corresponds to the representation of the ipsilateral visual half-field in the layers above, while an intricate mosaic array of compartments prevail in tectal zones related to the representation of the contralateral visual half-field.     

Aberrant visual projections in the Siamese cat

1. Guillery has recently shown that the Siamese cat has a grossly abnormal lateral geniculate body. His anatomical study suggested that certain fibres originating in the temporal retina of each eye cross in the chiasm instead of remaining uncrossed. They thus reach the wrong hemispheres, but in the geniculate they terminate in the regions that the missing fibres from the ipsilateral eye would normally have occupied. The result is that each hemisphere receives an input from parts of the ipsilateral field of vision, this input being entirely from the opposite eye. The purpose of the present work was to study the physiological consequences of this aberrant projection, in the lateral geniculate body and visual cortex.2. Single-cell recordings from the lateral geniculate body confirmed the presence of projections from the ipsilateral visual field of the contralateral eye. The part of layer A(1) receiving these projections was arranged so that the receptive fields of the cells were situated at about the same horizontal level and at the same distance from the vertical meridian as the fields of cells in the layers above and below (layers A and B), but were in the ipsilateral visual field instead of the contralateral. They thus occupied a region directly across the mid line from their normal position.3. In the cortex of all animals studied, we found a systematic representation of part of the ipsilateral visual field, inserted between the usual contralateral representations in areas 17 and 18. When the visual cortex was crossed from medial to lateral the corresponding region of visual field moved from the contralateral periphery to the mid line, and then into the ipsilateral field for 20 degrees . The movement then reversed, with a return to the mid line and a steady progression out into the contralateral field. The entire double representation was, with some possible exceptions, a continuous one. The point of reversal occurred at or near the 17-18 boundary, as judged histologically, and this boundary was in about the same position as in ordinary cats.4. Cells in the part of the cortex representing the ipsilateral fields had normal receptive fields, simple, complex, or hypercomplex. These fields tended to be larger than those in corresponding parts of the contralateral visual fields. Receptive-field size varied with distance from the area centralis, just as it does in the normal cat, so that cells with the smallest fields, in the area centralis projection, were situated some distance from the 17-18 border.5. Projections originating from the first 20 degrees from the midvertical in both visual half-fields had their origin entirely in the contralateral eye, as would be expected from the abnormal crossing at the chiasm. Beyond this visual-field region, and out as far as the temporal crescents, there were projections from both eyes, but we found no individual cells with input from the two eyes. The cells were aggregated, with some groups of cells driven by one eye and some by the other.6. From previous work it is known that ordinary cats raised with squint show a decline in the proportion of cells that can be driven binocularly, whereas animals raised with both eyes closed show little or no decline. A Siamese cat raised with both eyes closed had binocular cells in the regions of 17 and 18 subserving the peripheral visual fields, suggesting that the absence of binocular cells seen in the other Siamese cats was indeed secondary to the squint.7. In two Siamese cats there were suggestions of an entirely different projection pattern, superimposed upon that described above. In the parts of 17 and 18 otherwise entirely devoted to the contralateral visual field, we observed groups of cells with receptive fields in the ipsilateral field of vision. The electrode would pass from a region where cells were driven from some part of the contralateral visual field, to regions in which they were driven from a part of the ipsilateral field directly opposite, across the vertical mid line. The borders of these groups were not necessarily sharp, for in places there was mixing of the two groups of cells, and a few cells had input from two discrete regions located opposite one another on either side of the vertical mid line. The two receptive-field components of such cells were identical, in terms of orientation, optimum direction of movement, and complexity. Stimulation of the two regions gave a better response than was produced from either one alone, and the relative effectiveness of the two varied from cell to cell. These cells thus behaved in a way strikingly reminiscent of binocular cells in common cats.8. The apparent existence of two competing mechanisms for determining the projection of visual afferents to the cortex suggests that a number of factors may cooperate in guiding development. There seems, furthermore, not to be a detailed cell-to-cell specificity of geniculocortical connexions, but rather a tendency to topographic order and continuity, with one part of a given area such as 17 able to substitute for another. Whether or not these tentative interpretations are ultimately proved correct, it seems clear that this type of genetic anomaly has potential usefulness for understanding mechanisms of development of the nervous system.     

Binocular interactions and spatial disparity sensitivity in the superior colliculus of the siamese cat

 In Siamese cats, a genetically determined massive misrouting of retinal ganglion cells toward the contralateral hemisphere, as well as an accompanying strabismus, is believed to underlie the extreme paucity of binocular cells in the primary visual cortex. However, binocular cells have been shown to be present in more important numbers at the collicular level. The present study aims at investigating binocular interactions and sensitivity to spatial disparity in the superior colliculus of the Siamese cat. The activity of single units was recorded in the superficial layers of paralyzed and anesthetized Siamese cats. Although most collicular cells were monocularly driven, a significant proportion could be driven through both eyes (34/216 or 16%). Upon isolation of a binocular cell, the receptive fields were separated, then simultaneously stimulated with two light bars. A temporal delay was introduced between the arrival of the bars in the receptive fields to generate spatial disparities (–3° to +3°, in 0.5° or 1° steps). Results showed that some binocular cells presented disparity tuning profiles similar to the tuned excitatory (12/34), tuned inhibitory (2/34), near (2/34) and far (3/34) cells found at various cortical levels in the normal cat. These interactions might allow for coarse binocular fusion as well as play a role in the initiation of vergence and the fixation of the eyes upon the appropriate plane of vision. Received: 24 March 1998 / Accepted: 24 August 1998     

Connections and visual-field mapping in cat's tectoparabigeminal circuit.

1. The aim of these experiments was to analyze the organization of the reciprocal connections between the cat's superior colliculus and parabigeminal nucleus. Both physiological and anatomical techniques were employed. 2. A population of cells in the superficial gray and upper optic layers of the colliculus was labeled retrogradely by horseradish peroxidase injections into the parabigeminal nucleus. No other sources of input to the nucleus were found in the brain stem or diencephalon. 3. A map of the visual field within the parabigeminal nucleus was reconstructed by plotting visual receptive fields at 350 parabigeminal sites with microelectrodes. The map resembled that found in the colliculus, although it was considerably less orderly. The entire contralateral visual field was represented and, in addition, roughly the central 40 degrees of the ipsilateral hemifield was included; futhermore, the expansion of the central visual field was similar to that of the tectal map. 4. The return parabigeminal projections to the caudal parts of the two colliculi, representing the contralateral hemifields, were in register with the tectal visual-field maps. In contrast, the parabigeminal pathways to the anterior segments of the two colliculi, representing part of the ipsilateral visual fields, were not clearly topographic. The projection to this part of the contralateral colliculus showed little order, while that to the ipsilateral colliculus was extremely sparse. 5. A single site in the colliculus can be the target of axons from nonhomologous locations in the two parabigeminal nuclei; so that both parabigeminal inputs are in register with the tectal map.     

Response properties of X and Y LGN neurons in Siamese cats

Summary We investigated quantitatively the receptive-field properties of neurons in the lateral geniculate nucleus of Siamese cats. The experimental animals, Mid-western Siamese cats, exhibited varied degrees of ocular misalignment. The percentage of Y-cells, which receive projections from the area centralis, was significantly greater in Siamese cats than in normally pigmented cats. The spatial resolution of many, but not all, X-cells within the central 5 degrees of Siamese cats was reduced. A substantial number of Y-cells had significantly longer latencies to stimulation of the optic chiasm. In addition, some Siamese cat units had abnormal contrast threshold and lineweighting functions along with large receptive field center size and weak inhibitory effects from the RF surround. Finally, large variability was found with respect to the abnormalities among cells within a given cat and more importantly among the individual Siamese cats which we studied.     

Interhemispheric influences on area 19 of the cat

Summary Anatomical studies have shown an extensive network of homotopic and heterotopic interhemispheric connections in area 19 of the cat visual cortex (Segraves and Rosenquist 1982a; 1982b). We have investigated their functional organization by recording visual responses in area 19 of cats following a midsagittal section of the optic chiasm. This operation interrupts all crossed optic fibers coming both from the nasal and the temporal retinae; as a result, each hemisphere receives optic fibers only from the lateral hemiretina of the ipsilateral eye which conveys information from the contralateral visual field. Visual information transmitted to the same hemisphere from the contralateral retina and the ipsilateral visual field must be attributed to an indirect, interhemispheric pathway. We found that a rather high proportion of neurons (31.8%) in area 19 of seven split-chiasm cats responded to visual stimuli presented to the contralateral eye. 1 — All neurons receiving this interhemispheric activation were also driven by the ipsilateral eye via an intrahemispheric pathway. 2 — The property of binocularity was significantly related to the visuotopic map in that both receptive fields of each binocular neuron adjoined or were in the immediate vicinity of the vertical meridian. 3 — Due to the small size of receptive fields in area 19, the contribution of the interhemispheric pathway to the representation of the visual field is rather limited and it is certainly less extensive than that predicted by anatomical studies. The representation of the ipsilateral visual field in area 19 of intact cats, as assessed electrophy-siologically, was comparable to that found in split-chiasm cats. Recordings in areas 17–18 of split-chiasm cats showed that the visual field represented through the corpus callosum in these visual areas is certainly not less and probably more, extensive than that found in area 19. The results support the conclusion that the relation to the vertical meridian and the receptive field size can explain the organization of the interhemispheric connections in the visual areas studied so far.     

Topographic variations in W-cell input to cat superior colliculus

Summary Most of the retinal input to the cat's superior colliculus (SC) arises from W-cells of the contralateral eye and terminates just below the tectal surface. The goal of this study was to determine whether the strength of this input is uniform over the collicular map or, instead, exhibits topographic variations as has been reported for the retinotectal Y-cell projection (McIlwain and Lufkin 1976). Monosynaptic inputs from the principal W-cell projection mediate the late negative potential (LNP), a collicular field potential that can be evoked by shocks to the optic pathway. We assumed that the amplitude of the potential provided a measure of the strength of the W-cell input to the upper superficial gray layer. Using a fixed stimulus, we measured the maximal amplitude of the LNP at 90 topographically identified tectal sites in 5 cats. The amplitude of the LNP varied as much as 5-fold over the SC and was systematically related to the azimuthal position of the recording site. LNP amplitudes were consistently smallest in the representation of the area centralis and vertical meridian and largest in the representations of the contralateral hemifield periphery and the ipsilateral hemifield. There was little systematic variation in LNP amplitude as a function of elevation in the map. The observed variations did not result from non-uniform activation of retinal afferents or drift in properties of the recording electrodes, stimuli, or preparation. The results suggest that the principal W-cell input to the SC is weaker in the representation of the area centralis than elsewhere in the map. These topographic variations are similar to those reported for the retinotectal Y-cell projection (McIlwain and Lufkin 1976) and are consistent with anatomical evidence for thinning of retinal input in the area-centralis representation (Graybiel 1975; Harting and Guillery 1976; Mize 1983). An important implication of these results is that the scaling of the collicular retinotopic map may not be proportional to the spatial density of tectally projecting W-cells.     

Roles of periventricular neurons in retinotectal transmission in the optic tectum

The midbrain roof is a retinorecipient region referred to as the optic tectum in lower vertebrates, and the superior colliculus in mammals. The retinal fibers projecting to the tectum transmit visual information to tectal retinorecipient neurons. Periventricular neurons are a subtype of these neurons that have their somata in the deepest layer of the teleostean tectum and apical dendrites ramifying at more superficial layers consisting of retinal fibers. The retinotectal synapses between the retinal fibers and periventricular neurons are glutamatergic, and ionotropic glutamate receptors mediate the transmission in these synapses. This transmission involves long-term potentiation, and is modulated by hormone action. Visual information processed in the periventricular neurons is transmitted to adjacent tectal cells and target nuclei of periventricular neuron axonal branches, some of which relay the visual information to other brain areas controlling behavior. We demonstrated that periventricular neurons play a principal role in visual information processing in the teleostean optic tectum; the effects of tectal output on behavior is discussed also in the present review.     

Visual experience and the maturation of the ipsilateral visuotectal projection in Xenopus laevis

The effect of visual deprivation upon the maturation of the ipsilateral visuotectal projection has been studied in Xenopus laevis. This topographically ordered projection is polysynaptic. The first stage involves the retinal projection to the contralateral optic tectum. The tectum projects to the nucleus isthmi on the same side. The final stage is the crossed isthmotectal projection from the nucleus isthmi to the tectum ipsilateral to the eye. The topographic precision of connections at various points in this polysynaptic pathway has been investigated by quantifying single-unit and multi-unit receptive field sizes in the contralateral and ipsilateral visuotectal projections. Observations have been made on normal animals of different ages to plot the normal maturational course of events. The effects of visual deprivation on this maturational process has been studied. Between one week and one year after metamorphosis there is an increase in the precision of connections in both the contralateral and ipsilateral visuotectal projections. Visual deprivation had no effect upon the parameters of the contralateral visuotectal projection. Ipsilateral visuotectal single units in dark-reared animals had normal receptive field sizes. Ipsilateral multi-unit receptive fields in dark-reared animals were considerably larger than in normal animals. It was concluded that the effects of visual deprivation are limited to effects on the crossed isthmotectal component of the intertectal system. In this component, however, visual experience seems to play an important role in the normal development and modification of connections. It is suggested that visual experience is utilized to accommodate changes in the system required to respond to normal changes in interocular geometry that take place with development in Xenopus.     

Physiological effects of chronic and acute application of N-methyl-D-aspartate and 5-amino-phosphonovaleric acid to the optic tectum of Rana pipiens frogs.

Visually elicited activity contributes to the formation of orderly connections in the optic tectum of frogs. Glutamate receptors of the N-methyl-D-aspartate class participate in this process. Blocking those receptors interferes with activity-dependent refinement of maps in normal frogs and of ocular dominance bands in surgically produced animals with three eyes. Chronic application of N-methyl-D-aspartate sharpens the bands. The possibility that 5-amino-phosphonovaleric acid depresses tectal responsiveness was motivation for studying the effects of 5-amino-phosphonovaleric acid and N-methyl-D-aspartate applied both chronically and acutely. We evaluated tectal responsiveness to visual input by presenting flashes of light to one eye and recording responses in the ipsilateral tectal lobe. This method reveals the output of the tectal cells contralateral to the stimulated eye. These cells project via the nucleus isthmi to the opposite tectal lobe. We also mapped the receptive field dimensions of the crossed isthmotectal axons. Our results show that acute topical application of 500 microM or 1 mM N-methyl-D-aspartate dramatically increases spontaneous activity, while 100 microM N-methyl-D-aspartate causes little change. Chronic treatment with N-methyl-D-aspartate at a low dose (estimated to be in the micromolar range) shown to influence retinotectal mapping, reduces response latencies but produces no statistically significant changes in tectal cell firing rates or receptive field size. Acute application of 5-amino-phosphonovaleric acid produces complex results: 10 microM produces no changes in firing, 100 microM 5-amino-phosphonovaleric acid decreases firing, and doses of 500-100 microM increase the firing.(ABSTRACT TRUNCATED AT 250 WORDS)     

Abnormal responses of retinal ganglion cells in Siamese cats to moving stimuli

Summary The responses of X and Y-Type retinal ganglion cells in Siamese cats to moving slits were compared to those in common cats, in order to assess the center/surround interactions in Siamese cat receptive fields. Responses in the latter animal were quantitatively lower than those in common cats, the RF centers were larger, and the encounter rate for Y-type optic tract fibers was significantly lower than in common cats. Whereas response enhancement of common cat units was predictable following masking of the surround, such an effect could only be observed in Siamese cats when the contrast between the stimulus and background was increased considerably. These data suggest anomalous center/surround interactions in Siamese cats, probably due to the presence of weaker surround influences in that animal.This investigation was supported by NIH grants EY01444 to Y.M.C. and EY00701 to D.I.H.     

Gabor analysis of auditory midbrain receptive fields: spectro-temporal and binaural composition

The spectro-temporal receptive field (STRF) is a model representation of the excitatory and inhibitory integration area of auditory neurons. Recently it has been used to study spectral and temporal aspects of monaural integration in auditory centers. Here we report the properties of monaural STRFs and the relationship between ipsi- and contralateral inputs to neurons of the central nucleus of cat inferior colliculus (ICC) of cats. First, we use an optimal singular-value decomposition method to approximate auditory STRFs as a sum of time-frequency separable Gabor functions. This procedure extracts nine physiologically meaningful parameters. The STRFs of approximately 60% of collicular neurons are well described by a time-frequency separable Gabor STRF model, whereas the remaining neurons exhibited obliquely oriented or multiple excitatory/inhibitory subfields that require a nonseparable Gabor fitting procedure. Parametric analysis reveals distinct spectro-temporal tradeoffs in receptive field size and modulation filtering resolution. Comparisons between an identical model used to study spatio-temporal integration areas of visual neurons further shows that auditory and visual STRFs share numerous structural properties. We then use the Gabor STRF model to compare quantitatively receptive field properties of contra- and ipsilateral inputs to the ICC. We show that most interaural STRF parameters are highly correlated bilaterally. However, the spectral and temporal phases of ipsi- and contralateral STRFs often differ significantly. This suggests that activity originating from each ear share various spectro-temporal response properties such as their temporal delay, bandwidth, and center frequency but have shifted or interleaved patterns of excitation and inhibition. These differences in converging monaural receptive fields expand binaural processing capacity beyond interaural time and intensity aspects and may enable colliculus neurons to detect disparities in the spectro-temporal composition of the binaural input.     

Binocular neuronal responsiveness in Clare-Bishop cortex of Siamese cats

Summary Retinotopy and binocular responsiveness were studied extracellularly in a total of 278, 61, 110 and 275 cells sampled in areas 17, 18, 19 and Clare-Bishop (CB) of Siamese cats. The misalignment of the visual axes of the two eyes was determined by the pupil reflex method in the behaving animal. The recording sessions were conducted under N₂O anesthesia, supplemented with continuous infusion of short-lasting anesthetics (Saffan, Glaxo) and muscle relaxants (Gallamine triethiodide) using two types of visual stimulators presenting two-dimensional (2D) motion stimuli and the visual cues for three-dimensional (3D) motion. All of the nine Siamese cats demonstrated Boston type retinotopic abnormalities in all of cortical areas 17–19 and CB. Very few binocular cells were present in areas 17–19 and the posterior (A1-P2) CB but they were numerous in most of CB (A9-4). A significant fraction (36/78) of binocular cells in the major CB of the Siamese cats demonstrated similar response selectivity to that reported in normal CB cortex for stimulation with the 3D motion cues under both null disparity and strabismic conditions (binocular receptive fields for two eyes were optically superposed or separated by the strabismic angles estimated in the individual animals). These findings indicate that the binocular signals converging to the CB cells through different pathways (signals coming from the contralateral eye via the ipsilateral hemisphere including the interlaminar nucleus and areas 17–19, and commissural signals from the ipsilateral eye via the contralateral areas 17–19 and CB) were integrated to yield useful information for the recognition of 3D motion, and that the major CB is an actual site of binocular integration at least in Siamese cats, rather than being merely a reflection of the information processing before the CB cortex.     

The visual areas in the splenial sulcus of the cat

1. The extreme periphery of the visual field is represented in the upper wall of the splenial sulcus where the sulcus is horizontal, and in its anterior wall more posteriorly where the sulcus runs downwards and laterally. About half the cells whose fields lie between 50 and 90° from the area centralis have a sharply horizontal preferred orientation.

2. Beyond the lateral edge of visual I there is a narrow band of visual cortex in which the receptive fields return towards the area centralis as one moves 1-1·5 mm laterally. Their receptive fields are usually about 20-30° degrees across, but all orientations are found. The more central fields may be binocular and those at the area centralis may be as small as 1° in diameter.

3. This band has been called the splenial visual area. It does not seem to have properties corresponding to those of visual II nor of visual III.

     

Binaural mechanisms of spatial tuning in the cat's superior colliculus distinguished using monaural occlusion

This study explores the mechanisms of auditory spatial tuning in the superior colliculus of the anesthetized cat by correlating spatial tuning within specific regions of space with particular types of binaural interaction. The auditory spatial tuning of units was measured using a movable, broad-band stimulus presented in a free sound field. The contribution of each ear to the response of a unit was identified by acutely plugging one or the other ear. Every unit became largely or entirely unresponsive when a foam-rubber earplug was placed in the ear contralateral to the recording site. Thus, every unit exhibited an excitatory or facilitatory influence from the contralateral ear. A plug placed in the ipsilateral ear had different effects on different units. For half of the units (16/32), an ipsilateral earplug produced increases in the sizes of the units' receptive fields and increases in the magnitudes of their responses to stimuli presented from most locations. Thus, these units exhibited inhibition from the ipsilateral ear. Another class of units (9/32) exhibited ipsilateral facilitation, in that an ipsilateral earplug caused decreases in the sizes of the units' receptive fields and prominent decreases in their response magnitudes. For the remaining units (7/32), an ipsilateral earplug resulted in decreases in the sizes of the units' receptive fields, but produced both decreases in the responses of units to stimuli presented in their best areas and increases in the responses to stimuli presented away from the best areas. Thus these units exhibited mixed facilitatory and inhibitory ipsilateral influences. The influence of an ipsilateral earplug on a unit's response tended to correlate with its spatial tuning. The region of space within which a sound source was most effective in activating a unit was its "best area". The best areas of units exhibiting ipsilateral inhibition were located furthest peripherally, those of units showing ipsilateral facilitation were located furthest frontally, and the best areas of units showing mixed ipsilateral influences were located in an intermediate area. The frequency tuning of units measured using a free-field tone source also tended to correlate with the locations of their best areas. Half of the units tested (27/54) responded to tones of the sound pressure levels (SPLs) that were used (up to 50 dB SPL).(ABSTRACT TRUNCATED AT 400 WORDS)     

Visual input induces long-term potentiation of developing retinotectal synapses

Early visual experience is essential in the refinement of developing neural connections. In vivo whole-cell recording from the tectum of Xenopus tadpoles showed that repetitive dimming-light stimulation applied to the contralateral eye resulted in persistent enhancement of glutamatergic inputs, but not GABAergic or glycinergic inputs, on tectal neurons. This enhancement can be attributed to potentiation of retinotectal synapses. It required spiking of postsynaptic tectal cells as well as activation of NMDA receptors, and effectively occluded long-term potentiation (LTP) of retinotectal synapses induced by direct electrical stimulation of retinal ganglion cells. Thus, LTP-like synaptic modification can be induced by natural visual inputs and may be part of the underlying mechanism for the activity-dependent refinement of developing connections.     

Apparent movement of optic terminals out of a local postsynaptically blocked region in goldfish optic tectum

In goldfish tectum, alpha-bungarotoxin (alpha BTX) blocks postsynaptically generated field potentials elicited by either photic or electrical stimulation, but leaves presynaptic activity unaffected. To assess the chronic effects of blocking transmission on synaptic stability, small restricted areas of synaptic block were created by slow, low-pressure microinjection of toxin-Ringer's solution from a micropipette. Local transmission was blocked, but field potentials outside the injected region were unchanged. Radioautography of 125I-alpha BTX demonstrated that 6 days postinjection it was still highly localized and concentrated in discrete synaptic laminae of the tectal neuropil, in agreement with its known tight binding in goldfish brain (18). Electrophysiological maps made 6-8 days postinjection showed that few if any optic terminals could be recorded in the toxin-blocked zone, but there was no scotoma in the visual field. Terminals with receptive fields that would normally have corresponded to the region of injection were instead recorded from surrounding tectal areas. Since the normal units were also recorded there, this produced very large multiunit receptive fields. Apparently the optic terminals within the toxin-blocked area moved outward and innervated neighboring areas. In a minority of cases, no silent areas were noted and only the enlarged multiunit receptive fields were seen in the injection zone. All tecta injected with alpha BTX, but none injected with Ringer's, showed these disturbances in the map at the treated area at 6-8 days postinjection. By 2-3 wk, the maps in the alpha BTX-injected tecta had returned to normal. There were no silent areas and no enlarged receptive fields. The movement of the optic arbors was shown to be dependent on activity in the optic fibers. In fish receiving intraocular injections of tetrodotoxin (TTX) for the first 4-6 days, no changes were seen in the retinotectal maps recorded after the effects of the TTX wore off. When the entire surface of the tectum was blocked with multiple injections of alpha BTX, normal maps were recorded at 6-8 days postinjection. Thus in order to produce movements in optic terminals, the fibers require activity and an opportunity to make effective synapses in a nearby zone.     

Ibotenic acid lesions in the pedunculopontine region result in recovery of visual orienting in the hemianopic cat.

Cats rendered hemianopic by a unilateral visual cortical ablation can recover the visual orienting response in the hemianopic visual field following disruption of the caudal non-tectotectal containing half of the commissure of the superior colliculus. Ibotenic acid lesions of a small 'critical zone' in the contralateral substantia nigra result in a similar recovery effect. A conceptual framework developed by Wallace et al. (1990) [J. Comp. Neurol. 296, 222-252] proposed that elimination of contralateral substantia nigra 'critical zone' inhibition on the superior colliculus ipsilateral to a visual cortical lesion is responsible for the recovery. This model is insufficient, however, to explain the observation that hemi-decorticate cats with contralateral substantia nigra 'critical zone' lesions which include but extend beyond the 'critical zone' do not demonstrate the recovery. In these cats, subsequent transection of the commissure of the superior colliculus does lead to the recovery. We hypothesize that another projection through the caudal commissure of the superior colliculus, from the pedunculopontine nucleus, is involved in the recovery effect.Visual orienting behavior was recorded before and after ibotenic acid lesions made in the pedunculopontine nucleus region contralateral to a visual cortical ablation in 16 cats. Four cats with lesions in a small rostral region of the contralateral pedunculopontine nucleus recovered the visual orienting response in the previously hemianopic visual field. Contralateral tectal projections from the pedunculopontine nucleus are thought to be cholinergic and terminate as distinct patches in the intermediate gray layers of the superior colliculus.Since this region of the pedunculopontine nucleus also receives GABA-ergic afferents from the substantia nigra, we propose that a subcortical neural circuit including the substantia nigra, pedunculopontine nucleus, and superior colliculus is involved in the recovery of visual orienting.     

Responses of cat mesencephalic reticulospinal neurons to stimulation of superior colliculus,pericruciate cortex,and neck muscle afferents

Summary Neurons were recorded extracellularly in the mesencephalic reticular formation outside the interstitial nucleus of Cajal in cerebellectomized cats anesthetized with chloralose. Reticulospinal neurons were identified by antidromic stimulation of the upper cervical segments. Stimulation in the deep layers of the ipsilateral superior colliculus evoked firing in 36% of reticulospinal neurons. For many neurons thresholds for activation were high in the intermediate tectal layers and declined as the electrodes entered the underlying tegmentum. However, low threshold points were found above the deep fiber layer within the superior colliculus for some cells. Stimulation of the contralateral superior colliculus excited 10% of neurons and thresholds for activation were high above the deep fiber layer for all neurons. Stimulation of the ipsilateral and contralateral pericruciate cortex excited 39 and 21% of neurons, respectively. The lowest threshold area was found in the frontal eye fields. Sixteen percent of neurons received excitation from neck muscle afferents (C₂ biventer-cervicis) bilaterally. Comparison of responses between mesencephalic reticulospinal neurons and interstitiospinal neurons (Fukushima et al. 1981) showed that responses of the two groups of neurons were similar when the pericruciate cortex and neck muscle afferents were stimulated. However, a difference was observed in tectal responses, since low threshold points were rarely observed above the deep fiber layer for interstitiospinal neurons.Supported in part by a Grant-in-Aid for Scientific Research (No. 477063) from The Ministry of Education, Science, and Culture of Japan