Response properties of striate cortex neurons in cats raised with divergent or convergent strabismus

Recordings were made from striate cortex in five groups of cats that had been raised with strabismus produced by sectioning the extraocular muscles. These groups included animals reared with exotropia, unilateral or bilateral esotropia, and esotropia combined with lid suture of the unoperated eye. In addition, a group of esotropes was studied in which the unoperated eye was removed a few hours prior to recording. For comparison, five normal adult cats were also studied. In each of the above groups, cells were sampled in the representations of the central and peripheral visual fields in area 17 ipsilateral and contralateral to the deviated eye. We mapped the receptive field of each responsive cell, determined its ocularity, and tested it for selectivity. Confirming previous work, we found a marked loss of cortical binocularity in cats raised with strabismus. On average only 7% of the neurons that we recorded could be driven by both eyes. This percentage was relatively constant at all cortical locations that were studied and was not influenced by whether cats had been reared with exotropia, unilateral esotropia, or bilateral esotropia. The percentage of selective cells driven by the deviated eye in exotropes or esotropes did not appear to be different from normal at most cortical locations (but see 5, below). In addition, we did not observe any bias in the axial preference of selective cells in strabismic cats when compared with normal adult cats. In both exotropes and esotropes the deviated eye drove fewer cells when compared with the proportion that are driven by one eye in normal cats. In exotropes this deficit did not vary at different cortical representations of the visual field. In esotropes, however, this deficit was graded, being least in the representation of the peripheral visual field in area 17 contralateral to the deviated eye, intermediate in the representations of the central visual field in the contralateral and ipsilateral hemispheres, and greatest in the representation of the peripheral visual field in ipsilateral area 17. Furthermore, only when recording from the peripheral field representation in the ipsilateral hemisphere did we encounter significant numbers of cells driven by the deviated eye that lacked normal selectivity. Since it is possible that deprivation of the converged eye during development might account for the deficits noted above, we attempted to evaluate this factor using several independent lines of evidence. First, we could find no correlation between the angle of esotropia and the ability of the deviated eye to drive ipsilateral cortical cells representing the peripheral visual field.(ABSTRACT TRUNCATED AT 400 WORDS)     

Corticotectal circuit in the cat: a functional analysis of the lateral geniculate nucleus layers of origin

1. The dorsal lateral geniculate nucleus (LGN) of the cat is a major thalamic relay between the retina and several visual cortical areas. These cortical areas in turn project to the superior colliculus (SC). The aim of the present experiment was to determine which LGN layers provide a necessary input to the corticotectal circuit. 2. Individual layers of the LGN were reversibly inactivated by microinjection of cobalt chloride during recording of visual responses in the retinotopically corresponding part of the superior colliculus. 3. For cells driven through the contralateral eye, inactivation of layer A or the medial interlaminar nucleus (MIN) had little effect on visual responsiveness in the superior colliculus. In contrast, inactivation of layer C abolished visual responses at one-quarter of the SC recording sites, reduced responses at another quarter, and left half of the recording sites unaffected. 4. For cells driven through the ipsilateral eye, inactivation of layer C1 or the MIN had no effect. Inactivation of layer A1 uniformly reduced visual responses in the superior colliculus and usually abolished them entirely. 5. These results are compatible with previous work showing that cortical input to the SC originates from Y-cells. They indicate that two of the five Y-cell containing layers (A1 and C) provide major inputs to the corticotectal circuit. The results suggest that layer A1 is functionally allied to layer C as well as to layer A.     

Effects of altered visual input upon the development of the visual and somatosensory representations in the hamster's superior colliculus

The right superior colliculus and right eye were ablated in hamsters within 12 h of birth and the visual and somatosensory representations in the remaining (left) superior colliculus were evaluated using standard single unit recording and receptive field mapping techniques when the animals reached adulthood (at least 3 months of age). In a number of the hamsters used for recording, injections of [3H]leucine were made into the left eye 6-10 days prior to the terminal experiment. This was done to insure that the neonatal lesions did, in fact, produce the extensive recrossing of retinal fibers demonstrated by others who have employed this preparation. All of the hamsters which received [3H]leucine injections prior to the recording experiment exhibited a markedly expanded ipsilateral retinocollicular projection and retinal axons which recrossed the midline at the level of the tectum. The recording experiments showed further that this projection resulted in a visual map which was generally mirror symmetric to that in normal hamsters. There were, however, numerous irregularities and discontinuities in this representation and, in a few hamsters, it appeared almost completely disorganized. There were also a number of abnormalities in the somatosensory representation in the deep tectal laminae of the neonatally brain damaged hamsters. There was a substantial increase in the number of cells with receptive fields that extended onto the ipsilateral side of the body, neurons with split receptive fields were recorded and there were changes in the magnification of different portions of the body surface. These alterations did not, however, change the organization of the somatosensory map in a manner which brought it into alignment with the visual representation in the superficial laminae. Nevertheless, additional recording experiments in animals subjected to enucleation of both eyes and ablation of the superficial laminae of one superior colliculus did indicate that the existence of the aberrant retinal projection was a necessary condition for the somatosensory abnormalities which we observed. Additional anterograde and retrograde tracing experiments demonstrated only one abnormality in the organization of the somatosensory afferent input to the remaining colliculus. In 75% of the brain damaged hamsters, there was a weak crossed projection from the sensorimotor cortex that was never observed in normal animals. Ablation of this cortex at the time of the recording experiment did not, however, reduce the incidence of abnormal somatosensory receptive fields in these hamsters.     

Synaptic connections of cortical and retinal terminals in the superior colliculus of the rabbit: an electron microscopic double labelling study

Summary Retinal and cortical afferents to the superior colliculus of the rabbit were labelled simultaneously by injecting ³H-leucine into the right eye and HRP into the left visual cortex. It could be shown that there is some convergence of retinal and cortical input onto common postsynaptic elements in the superficial grey, but these cases were found to be rather rare indicating that most afferents from the retina and the visual cortex terminate either on different postsynaptic cells or on different parts of common postsynaptic cells.     

The initial stages of development of the retinocollicular projection in the wallaby (Macropus eugenii): distribution of ganglion cells in the retina and their axons in the superior colliculus

The time course of ingrowth of retinal projections to the superior colliculus in the marsupial mammal, the wallaby (Macropus eugenii), was determined by anterograde labelling of axons from the eye with horseradish peroxidase, from birth to 46 days, when axons cover the colliculus contralaterally and ipsilaterally. The position of retinal ganglion cells giving rise to these projections over this period was determined in fixed tissue by retrograde labelling from the colliculus with a carbocyanine dye. Axons first reach the rostrolateral contralateral colliculus 4 days after birth and extend caudally and medially, reaching the caudal pole at 18 days and the far caudomedial pole at 46 days. The first contralaterally projecting cells are in the central dorsal and temporal retina, followed by cells in the nasal and finally the ventral retina. They are distributed closer to the periphery with increasing age. The first sign of a visual streak appears by 18 days. Axons reach the ipsilateral colliculus a day later than contralateral axons and come from a similar region of the retina. The sparser ipsilateral projection reaches the caudal and medial collicular margins by 46 days but by 16–18 days, ganglion cells giving rise to this transient projection are already concentrated in the temporoventral retina. The orderly recruitment of ganglion cells from retinotopically appropriate regions of the retina as axons advance across the contralateral colliculus suggests that the projection is topographically ordered from the beginning. The ipsilateral projection is less ordered as cells are located in the temporoventral crescent at a time when their axons are still transiently covering the colliculus prior to becoming restricted to the rostral colliculus. Features of mature retinal topography such as the visual streak and the location of ipsilaterally projecting cells begin to be established very early in development, before the period of ganglion cell loss and long before eye opening at 140 days.     

The initial stages of development of the retinocollicular projection in the wallaby (Macropus eugenii): distribution of ganglion cells in the retina and their axons in the superior colliculus

The time course of ingrowth of retinal projections to the superior colliculus in the marsupial mammal, the wallaby (Macropus eugenii), was determined by anterograde labelling of axons from the eye with horseradish peroxidase, from birth to 46 days, when axons cover the colliculus contralaterally and ipsilaterally. The position of retinal ganglion cells giving rise to these projections over this period was determined in fixed tissue by retrograde labelling from the colliculus with a carbocyanine dye. Axons first reach the rostrolateral contralateral colliculus 4 days after birth and extend caudally and medially, reaching the caudal pole at 18 days and the far caudomedial pole at 46 days. The first contralaterally projecting cells are in the central dorsal and temporal retina, followed by cells in the nasal and finally the ventral retina. They are distributed closer to the periphery with increasing age. The first sign of a visual streak appears by 18 days. Axons reach the ipsilateral colliculus a day later than contralateral axons and come from a similar region of the retina. The sparser ipsilateral projection reaches the caudal and medial collicular margins by 46 days but by 16–18 days, ganglion cells giving rise to this transient projection are already concentrated in the temporoventral retina. The orderly recruitment of ganglion cells from retinotopically appropriate regions of the retina as axons advance across the contralateral colliculus suggests that the projection is topographically ordered from the beginning. The ipsilateral projection is less ordered as cells are located in the temporoventral crescent at a time when their axons are still transiently covering the colliculus prior to becoming restricted to the rostral colliculus. Features of mature retinal topography such as the visual streak and the location of ipsilaterally projecting cells begin to be established very early in development, before the period of ganglion cell loss and long before eye opening at 140 days.     

Cortical suppression of the ritino-collicular pathway in the monocularly deprived cat.

1. In nine cats monocularly deprived from birth, the responses of single neurones in the superior colliculus contralateral to the deprived eye were studied. 2. In six animals most units could be driven by visual stimuli only through the ipsilateral (experienced) eye despite the fact that this colliculus receives a major input from the contralateral (deprived) retina. 3. Immediately following removal of visual cortex, including areas 17, 18 and 19, the collicular units could be driven by the deprived eye. 4. We conclude that the cortex must exert a powerful suppression of the retino-collicular input, and we argue that this suppression occurs in normal as well as in monocularly deprived animals. 5. In three animals the retinal input from the deprived eye was not suppressed but instead dominated many collicular celld, apparently to the exclusion of the cortical input from the experienced eye.     

The effects of prenatal and neonatal monocular enucleation on visual topography in the uncrossed retinal pathway to the rat superior colliculus

Summary The visual representation in the uncrossed retinal projection to the superior colliculus (SC) was examined electrophysiologically by recording multiunit responses in paralysed, anaesthetised adult rats (both pigmented and albino), which had been monocularly enucleated either prenatally or soon after birth. This manipulation partially stabilises an exuberant neonatal projection from the remaining eye to the ipsilateral SC. Neuronal responses were also stronger and the multi-unit receptive fields larger than in intact animals. Many of the visual fields recorded on penetrations in caudal SC were located in the peripheral ipsilateral visual hemifield, corresponding to nasal retina. Such receptive fields are not seen in normal animals and were not found in animals enucleated on day 3 or later. The topographic representation of the dorso-ventral retinal axis, lateral to medial in the SC, was normal in all experimental animals. The representation of the naso-temporal retinal axis was abnormal and more variable. In all operated animals as the recording electrode was moved caudally away from the rostral pole of the SC, the corresponding receptive fields moved gradually from up to 40° in the ipsilateral visual hemifield to about 40° into the contralateral hemifield (a location corresponding to the peripheral edge of the temporal retina). This is the mapping polarity found in the normal uncrossed retinal projection. In the enucleated animals, the map was expanded and frequently displayed a clustering of fields arising from far temporal retina. In animals enucleated prenatally or on the day of birth, visual responses could be recorded in more caudal SC. The corresponding receptive fields now moved nasally on the retina, generating reversals in the map. The most caudal penetrations in these early enucleates frequently gave receptive fields located in retina nasal to the optic disc, up to 90 degrees into the ipsilateral visual hemifield. These results demonstrate that a temporal relationship exists between the order and mapping polarity of the visual field in SC and the time of enucleation. Prenatal enucleation produces reversals of the mapping polarity in caudal SC while neonatal enucleation produces an expanded map but one with a mapping polarity appropriate for an uncrossed projection     

Aberrant retinotectal projection induced by larval unilateral enucleation in Xenopus

The left eye was removed at late larval stages in Xenopus and the optic fibre projections of the remaining right eye assessed from 2 weeks to 13 months past metamorphosis. [3H]Proline autoradiography and electrophysiological recording of the visual field projection showed an aberrant optic fibre projection from the peripheral ventral retina to the right ipsilateral tectum. It is suggested that optic fibres, arising from the retina formed after the time of the operation, reach the ipsilateral tectum by following the axon debris of the removed eye.     

Comparison of receptive-field organization of the superior colliculus in Siamese and normal cats

1. The superior colliculus has been studied in Siamese and normal cats by recording the responses of single tectal units to visual stimuli.

2. The retinotopic organization of the superior colliculus has been compared in the two breeds. In the normal cat, the contralateral half-field is represented in the central and caudal part of the colliculus, and a vertical strip of the ipsilateral half-field, 15-20° wide, is represented at the anterior tip. The Siamese cat superior colliculus receives an abnormally large projection from the ipsilateral half-field so that units with visual receptive fields which extend as far as 40° into the ipsilateral half-field can be found. The area of the tectal surface devoted to the representation of the ipsilateral half-field is about twice as large in Siamese cats as in normal cats. The enhanced representation of the ipsilateral half-field in Siamese cats is reflected in a displacement of the vertical meridian and the area centralis on the tectal surface.

3. The area centralis in the Siamese cat is located at about the same point on the tectal surface as would be occupied by a point in the visual field about 6-7° contralateral to the area centralis in the normal cat. The smallest receptive fields in both breeds are located near the area centralis. The size of the receptive field for a tectal unit seems to be determined by the retinal location of the receptive field and not by the absolute position of the unit on the tectal surface.

4. The receptive-field characteristics of tectal units show many similarities in the two breeds. The receptive fields of individual units consist of activating regions flanked by suppressive surrounds. Units respond well to stimuli of different shapes and orientation provided they are moving. The optimum stimulus for a given unit can be much smaller than the size of the activating region. About two thirds of the units studied in both breeds show directional selectivity. Most of the units studied in normal cats can be activated by stimulation of either eye, while in Siamese cats, 80% of the units studied can be driven only by the contralateral eye. A few monocularly driven units with two separated receptive fields have been observed in Siamese cats.

5. In the left tectum of both breeds, units respond well to left-to-right stimulus movement. The reverse situation obtains in the right tectum. In Siamese cats, units located at the anterior tip of the tectum with their receptive fields located in the visual half-field ipsilateral to the tectum under study respond better to stimulus movement toward the area centralis than away from it. The preferred direction for a tectal unit seems to be determined by its tectal location rather than by the location of its receptive field in the retina.

6. Visual cortex lesions in both breeds increase the responsiveness of tectal units to flashing spots and almost entirely remove the directional selectivity exhibited by tectal units, although units with asymmetric surrounds are still found. In normal cats, the lesions change the ocular dominance distribution, skewing it more strongly toward the contralateral eye. In Siamese cats, the ocular dominance distribution remains unchanged after a visual cortex lesion.

7. The squint commonly exhibited by Siamese cats is regarded as a compensation for the anomalous retinotectal topography. It is suggested that, in the absence of an adaptive modification, the anomalous retinotectal projection would lead to mislocalization in Siamese cats just as it does in frogs and hamsters whose retinotectal projection has been experimentally altered. The convergent strabismus which Siamese cats commonly exhibit may be a cure for the abnormal retinal projections rather than a disease.

     

The ipsilateral field representation in the striate cortex of the opossum

Summary Reference axes for the visuotopic study of the opossum's striate cortex were estimated from corresponding binocular response fields using multi-unit recording. These central binocular axes (CBA) were derived from experimental data based on the concept that corresponding receptive fields for each eye should be mostly in register under natural conditions. Vertical reference meridians, orthogonal to these axes, define a contralateral and an ipsilateral field for each eye with respect to the recording site. An ipsilateral field representation was observed for both eyes in the striate cortex at the transition zone with peristriate. Maximal values for the center and border of ipsilateral receptive fields were, respectively, 8 and 20 degrees for the contralateral eye and 6 and 14 degrees for the ipsilateral eye. An equivalent ipsilateral field representation was found in animals that had the anterior commissure cut prior to the recording session. This suggests that the ipsilateral field of both eyes may be represented in the striate cortex via the ipsilateral optic tract. Additionally, it was observed that the region of higher ganglion cell density in the retina shows a flattened distribution and that the CBA intersects the retina at the temporal aspect of this region.     

The crossed projection from the temporal retina to the dorsal lateral geniculate nucleus in the rat

The crossed projection from the temporal crescent in the rat's retina was studied by producing a discrete retinal lesion in one eye and examining the dorsal lateral geniculate nucleus and superior colliculus contralateral to the lesion for anterograde degeneration products. The position of this crossed degeneration was described in relation to the uncrossed retinal termination in the same structures by injecting the opposite eye with [³H]proline and processing the tissue for autoradiography. The location of the retinal lesion in relation to the temporal cresent was identified by injecting the dorsal lateral geniculate nucleus ipsilateral to the lesioned eye with a fluorescent tracer, to retrogradely label the ipsilaterally projecting retinal ganglion cells in the lesioned eye.

Retinal lesions that were histologically verified to be restricted to the temporal crescent produced crossed degeneration in the superior colliculus at its rostral border, in accord with this projection's published visual topography. These same lesions consistently yielded a very circumscribed and sparse amount of degeneration in the contralateral dorsal lateral geniculate nucleus at its dorsomedial border, abutting the optic tract dorsally and the lateroposterior nucleus medially. The degeneration bore no consistent relationship to the position of the uncrossed retinal terminal field, which is situated further 9ventrally in the dorsal lateral geniculate nucleus; rather, this crossed temporal projection terminated in the outer shell of the nucleus along its medial border.

This crossed temporal retinogeniculate projection, together with the crossed projection from nasal retina, forms a continuous map of the complete contralateral retina in the outer shell of the dorsal lateral geniculate nucleus, likely to arise from a population of retinal ganglion cells possessing small soma sizes. This dorsomedial part of the rat's dorsal lateral geniculate nucleus, receiving a crossed projection from the temporal retina, may by similar to the cat's lamina 3 in the medial interlaminar nucleus of its retinogeniculate pathway. This result clarifies the homologous subdivisions of the dorsal lateral geniculate nucleus in the rodent and feline thalamus.     

Increase by visual stimuli in turnover of 5-hydroxytryptamine in the superior colliculi of the rabbit.

1. Irregular light falshes were played on to one eye of dark adapted rabbits for periods of 20-80 min. The concentration of 5-hydroxyindol-3-ylacetic acid (5-HIAA) and of 5-hydroxytryptamine (5-HT) were estimated in left and right superior colliculi, thalami and hippocampi. 2. In rabbits exposed to such visual stimuli for 30-60 min, there was an increase in the 5-HIAA content of the colliculus contralateral to the stimulated retina which aberaged 17% (P = 0-02), but no rise was seen if the exposure was shortened to 20 or prolonged to 80 min. At no time was there a difference in 5-HIAA content between right and left thalamus or right and left hippocampus. 3. Stationary or strictly repetitive visual stimuli produced no difference between the 5-HIAA content of left and right superior colliculus. 4. No difference in 5-HT concentration between the two colliculi was found after any form of visual stimulation, nor did any changes occur in the other parts of the brain which were examined. 5. Irregular, prolonged visual stimualtion thus appears to activate tryptaminergic neurones terminating in the colliculi. The possibility is discussed that the 5-HT released at this site might act as a brake to neuronal activity under conditions when habituation to the stimuli is not yet complete.     

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...     

Both striate cortex and superior colliculus contribute to visual properties of neurons in superior temporal polysensory area of macaque monkey

Although the tectofugal system projects to the primate cerebral cortex by way of the pulvinar, previous studies have failed to find any physiological evidence that the superior colliculus influences visual activity in the cortex. We studied the relative contributions of the tectofugal and geniculostriate systems to the visual properties of neurons in the superior temporal polysensory area (STP) by comparing the effects of unilateral removal of striate cortex, the superior colliculus, or of both structures. In the intact monkey, STP neurons have large, bilateral receptive fields. Complete unilateral removal of striate cortex did not eliminate visual responses of STP neurons in the contralateral visual hemifield; rather, nearly half the cells still responded to visual stimuli in the hemifield contralateral to the lesion. Thus the visual properties of STP neurons are not completely dependent on the geniculostriate system. Unilateral striate lesions did affect the response properties of STP neurons in three ways. Whereas most STP neurons in the intact monkey respond similarly to stimuli in the two visual hemifields, responses to stimuli in the hemifield contralateral to the striate lesion were usually weaker than responses in the ipsilateral hemifield. Whereas the responses of many STP neurons in the intact monkey were selective for the direction of stimulus motion or for stimulus form, responses in the hemifield contralateral to the striate lesion were not selective for either motion or form. Whereas the median receptive field in the intact monkey extended 80 degrees into the contralateral visual field, the receptive fields of cells with responses in the contralateral field that survived the striate lesions had a median border that extended only 50 degrees into the contralateral visual field. Removal of both striate cortex and the superior colliculus in the same hemisphere abolished the responses of STP neurons to visual stimuli in the hemifield contralateral to the combined lesion. Nearly 80% of the cells still responded to visual stimuli in the hemifield ipsilateral to the lesion. Unilateral removal of the superior colliculus alone had only small effects on visual responses in STP. Receptive-field size and visual response strength were slightly reduced in the hemifield contralateral to the collicular lesion. As in the intact monkey, selectivity for stimulus motion or form were similar in the two visual hemifields. We conclude that both striate cortex and the superior colliculus contribute to the visual responses of STP neurons. Striate cortex is crucial for the movement and stimulus specificity of neurons in STP.(ABSTRACT TRUNCATED AT 400 WORDS)     

The retinal location and fate of ganglion cells which project to the ipsilateral superior colliculus in neonatal albino and hooded rats

We have studied the organization of the ipsilateral retinocollicular pathway in neonatal rats by injecting the enzyme horseradish peroxidase (HRP) into the superior colliculus within 24 h of birth and later examining the location of labelled cells in the contralateral and ipsilateral retinae. One day after HRP injection, regardless of the location of the injection site in the superior colliculus, the great majority (over 80%) of ipsilaterally projecting cells was located in the lower peripheral retina. Five days after injection into the posterior pole of the superior colliculus (which in adult animals does not receive input from the ipsilateral retina), there were very few labelled cells in the ipsilateral retina, but labelled cells were quite numerous in the appropriate part of the contralateral retina. These results suggest that in the neonatal rat the great majority of ipsilaterally projecting retinal ganglion cells lie in the same part of the retina as do ipsilaterally projecting cells in the adult, but that many of those cells which project to inappropriate parts of the superior colliculus die by the fifth postnatal day.     

Effects of early monocular deprivation on visual input to cat nucleus of the optic tract

Single cells were recorded extracellularly in the nucleus of the optic tract (NOT) in monocularly deprived cats. Monocular deprivation had no effect on the direction specificity of these neurons, i.e. all cells in the left nucleus preferred movements from right to left and all units in the right nucleus preferred movements from left to right in the visual field. Neurons driven from the deprived eye failed to respond to stimuli moving at velocities above 10 degrees/s whereas neurons driven from the non-deprived eye responded to velocities up to and above 100 degrees/s as do neurons in normal cats. In 8 out of the 10 cats tested all cells in the two nuclei could be influenced only from the contralateral eye irrespective whether this was the deprived or the non-deprived eye. In the other two cats the influence from the non-deprived eye on cells in the ipsilateral NOT was found to be normal. This influence is mediated probably via cortico-fugal projections. In the 8 abnormal cats a clear deprivation effect could be assigned for the first time to the non-deprived eye consisting in a loss of its connections to the ipsilateral NOT. Electrical stimulation of the visual cortex revealed, however, the existence of a connection between the visual cortex and the NOT. A possible explanation for the specific deficit with visual stimulation in the cortico-pretectal synapse ipsilateral to the non-deprived eye is discussed in relation to developmental mechanisms. The conduction velocity of retinal input to the NOT and the output of the NOT to the inferior olive remained uninfluenced by visual deprivation.     

Retinal decussation patterns in pigmented and albino ferrets

The decussation patterns of retinal ganglion cells in adult pigmented and albino ferrets were determined from the distribution of cells labelled after large unilateral injections of horseradish peroxidase into the visual pathway, involving the lateral geniculate nucleus and fibres of passage to the superior colliculus. About 6000 retinal ganglion cells project ipsilaterally in pigmented ferrets compared with only about 1500 in albino ferrets. In both strains, the vast majority of these cells (99 and 87% in pigmented and albino animals, respectively) are located in the temporal crescent, although we describe one albino ferret in which an aberrant uncrossed projection arises from nasal retina. In pigmented ferrets, there is a sharp nasotemporal division that runs through the area centralis; a small proportion of the ganglion cells in temporal crescent (less than 10%) does project contralaterally. In albinos, however, the majority of cells in temporal retina project contralaterally. There is no clear nasotemporal division in the albino retina; the density of uncrossed ganglion cells is reduced throughout temporal crescent and at no location exceeds the comparable density of the crossed projection. The peak density within the reduced uncrossed projection is also displaced away from the area centralis into temporal retina. Analysis of cell type on the basis of soma size indicates that whereas large horseradish peroxidase injections into the visual pathway of pigmented ferrets label all types of ganglion cell in the crossed projection, injections restricted to the superior colliculus label only those ganglion cells with large or small somata. The distribution of cell sizes in the crossed projection from temporal retina is biased towards small cells in the pigmented ferret but in albinos resembles that seen in the crossed projection from nasal retina. Thus the adult pigmented ferret has both a well developed nasotemporal division in which decussation lines are obvious in the crossed and uncrossed pathways and also, unlike rodents but like cats, a class of ganglion cell that does not project to the superior colliculus. The albino mutation both reduces the uncrossed projection throughout temporal retina, although the reduction is greatest close to the area centralis, and also commensurately increases the crossed projection from temporal retina.     

Stimulus frequency affects c-fos expression in the rat visual system

We have characterised the c-fos expression patterns in various centers of the visual pathway of adult rats monocularly stimulated either by continuous or flickering light at different frequencies. Results show different immunocytochemical patterns in all centers studied, the geniculate lateral complex (LGC), superior colliculus (SC) and primary visual cortex (Oc1), depending on the physical characteristics of the stimulus (blinking frequency and light wavelength). After stimulation of the left eye, the ipsilateral pathway presents a substantial density of immunoresponsive cells, which is greater than expected with respect to the number of fibers that project ipsilaterally from the retina to the LGC and the superficial layers of the SC. A surprisingly high positive immunoresponsiveness is obtained in all cases with coherent light stimulation in the red spectrum (634 nm).     

Effects of eye-enucleation on substance P-immunoreactive fibers of some retinorecipient nuclei of the rat in relation to their origin from the superior colliculus.

We have previously shown that retinal deafferentation causes a decrease in immunoreactive dendrites of substance P-positive neurons of the superficial superior colliculus of the rat. Since some retinorecipient thalamic and pretectal nuclei are putative targets for substance P-containing cells of the superior colliculus, the present study attempted to ascertain whether substance P-immunoreactive fibers in these nuclei are also affected by retinal denervation. We found that unilateral eye removal produced a progressive increase in fibrous substance P immunoreactivity in the nucleus of the optic tract, lateral posterior nucleus, and lateral geniculate nucleus of the side contralateral to the enucleation. On the other hand, unilateral lesions to the superficial layers of the superior colliculus produced a dramatic reduction in substance P immunoreactivity in the ipsilateral nucleus of the optic tract, lateral posterior nucleus, and dorsal and ventral lateral geniculate nuclei. In bilaterally enucleated animals, unilateral lesion to the superior colliculus produced, as expected, loss of immunoreactive fibers only in the lateral posterior nucleus and the retinorecipient nuclei ipsilateral to the lesion. These results suggest that transneuronal changes in the distribution of substance P in collicular neurons observed after enucleation could be reflected in their projections to the other primary visual centers and to the lateral posterior nucleus.