Visual experience is necessary for maintenance but not development of receptive fields in superior colliculus.

Sensory deprivation is thought to have an adverse effect on visual development and to prolong the critical period for plasticity. Once the animal reaches adulthood, however, synaptic connectivity is understood to be largely stable. We reported previously that N-methyl-D-aspartate (NMDA) receptor blockade in the superior colliculus of the Syrian hamster prevents refinement of receptive fields (RFs) in normal or compressed retinotopic projections, resulting in target neurons with enlarged RFs but normal stimulus tuning. Here we asked whether visually driven activity is necessary for refinement or maintenance of retinotopic maps or if spontaneous activity is sufficient. Animals were deprived of light either in adulthood only or from birth until the time of recording. We found that dark rearing from birth to 2 mo of age had no effect on the timing and extent of RF refinement as assessed with single unit extracellular recordings. Visual deprivation in adulthood also had no effect. Continuous dark rearing from birth into adulthood, however, resulted in a progressive loss of refinement, resulting in enlarged, asymmetric receptive fields and altered surround suppression in adulthood. Thus unlike in visual cortex, early visually driven activity is not necessary for refinement of receptive fields during development, but is required to maintain refined visual projections in adulthood. Because the map can refine normally in the dark, these results argue against a deprivation-induced delay in critical period closure, and suggest instead that early visual deprivation leaves target neurons more vulnerable to deprivation that continues after refinement.

NMDA antagonists in the superior colliculus prevent developmental plasticity but not visual transmission or map compression.

Partial ablation of the superior colliculus (SC) at birth in hamsters compresses the retinocollicular map, increasing the amount of visual field represented at each SC location. Receptive field sizes of single SC neurons are maintained, however, preserving receptive field properties in the prelesion condition. The mechanism that allows single SC neurons to restrict the number of convergent retinal inputs and thus compensate for induced brain damage is unknown. In this study, we examined the role of N-methyl-D-aspartate (NMDA) receptors in controlling retinocollicular convergence. We found that chronic 2-amino-5-phosphonovaleric acid (APV) blockade of NMDA receptors from birth in normal hamsters resulted in enlarged single-unit receptive fields in SC neurons from normal maps and further enlargement in lesioned animals with compressed maps. The effect was linearly related to lesion size. These results suggest that NMDA receptors are necessary to control afferent/target convergence in the normal SC and to compensate for excess retinal afferents in lesioned animals. Despite the alteration in receptive field size in the APV-treated animals, a complete visual map was present in both normal and lesioned hamsters. Visual responsiveness in the treated SC was normal; thus the loss of compensatory plasticity was not due to reduced visual responsiveness. Our results argue that NMDA receptors are essential for map refinement, construction of receptive fields, and compensation for damage but not overall map compression. The results are consistent with a role for the NMDA receptor as a coincidence detector with a threshold, providing visual neurons with the ability to calculate the amount of visual space represented by competing retinal inputs through the absolute amount of coincidence in their firing patterns. This mechanism of population matching is likely to be of general importance during nervous system development.

Intrinsic and extrinsic factors that shape neocortical specification.

Increasing evidence points to the importance of intrinsic molecular cues in specifying the regional identity of mammalian neocortex. Few such cues, however, have been found to be restricted to individual functionally defined cortical areas before the arrival of afferent information. In contrast, thalamocortical axons are specifically targeted to individual cortical areas, raising the possibility that they can instruct some aspects of cortical areal identity. Cortical structure and function can be altered by modifying the source or pattern of activity in thalamocortical afferents. In particular, studies of cross-modal plasticity have shown that in many respects, one sensory cortical area can substitute for another after a switch of input modality during development. Afferent inputs might therefore direct the formation of their own processing circuitry, a possibility that has important implications for brain development, plasticity and evolution.

Development of inhibitory circuitry in visual and auditory cortex of postnatal ferrets: immunocytochemical localization of calbindin- and parvalbumin-containing neurons.

The inhibitory neurotransmitter gamma-aminobutyric acid (GABA) is thought to play an important role in activity-dependent stages of brain development. Previous studies have shown that different functional subclasses of cortical GABA-containing neurons can be distinguished by antibodies to the calcium-binding proteins parvalbumin and calbindin. Thus insight into the development of distinct subsets of inhibitory cortical circuits can be gained by studying the development of these calcium-binding protein-containing neurons. Previous studies in several mammalian species have suggested that calcium-binding proteins are upregulated in sensory cortex when thalamocortical afferents arrive. In ferrets, the ingrowth of thalamic axons into cortex occurs well into postnatal development, allowing access to early stages of cortical development and calcium-binding protein expression. We find in ferrets that both parvalbumin- and calbindin-immunoreactivity are present in primary visual and primary auditory cortex long before thalamocortical synapse formation, but that there is a sharp decline in immunoreactivity by postnatal day 20. Day 20 in ferrets corresponds to postnatal day 1 in cats, and thus previous studies in postnatal cats would have missed this early pattern of calcium-binding protein distribution. Another surprising finding is that the proportion of parvalbumin- and calbindin-immunoreactive neurons peaks secondarily late in development, between P60 and adulthood. This result suggests that the parvalbumin- and calbindin-containing subclasses of nonpyramidal neurons remain immature until late in the critical period for cortical plasticity, and that they are positioned to play an important role in experience-dependent modification of cortical circuits.

Visual behaviour mediated by retinal projections directed to the auditory pathway.

An unresolved issue in cortical development concerns the relative contributions of intrinsic and extrinsic factors to the functional specification of different cortical areas. Ferrets in which retinal projections are redirected neonatally to the auditory thalamus have visually responsive cells in auditory thalamus and cortex, form a retinotopic map in auditory cortex and have visual receptive field properties in auditory cortex that are typical of cells in visual cortex. Here we report that this cross-modal projection and its representation in auditory cortex can mediate visual behaviour. When light stimuli are presented in the portion of the visual field that is 'seen' only by this projection, 'rewired' ferrets respond as though they perceive the stimuli to be visual rather than auditory. Thus the perceptual modality of a neocortical region is instructed to a significant extent by its extrinsic inputs. In addition, gratings of different spatial frequencies can be discriminated by the rewired pathway, although the grating acuity is lower than that of the normal visual pathway.

Cross-modal reorganization of horizontal connectivity in auditory cortex without altering thalamocortical projections.

The development of the different, highly specialized regions of the mammalian cerebral cortex depends in part on neural activity, either intrinsic spontaneous activity or externally driven sensory activity. To determine whether patterned sensory activity instructs the development of intrinsic cortical circuitry, we have experimentally altered the modality of sensory inputs to cerebral cortex. Neonatal diversion of retinal axons to the auditory thalamus (cross-modal rewiring) results in a primary auditory cortex (AI) that resembles visual cortex in its response properties and topography (Roe et al., 1990, 1992). To test the hypothesis that the visual response properties are created by a visually driven reorganization of auditory cortical circuitry, we investigated the effect of early visual experience on the development of intrinsic, horizontal connections within AI. Horizontal connections are likely to play an important role in the construction of visual response properties in AI as they do in visual cortex. Here we show that early visual inputs to auditory thalamus can reorganize horizontal connections in AI, causing both an increase in their extent and a change in pattern, so that projections are not restricted to the isofrequency axis, but extend in a more isotropic pattern around the injection site. Thus, changing afferent modality, without altering the source of the thalamocortical axons, can profoundly alter cortical circuitry. Similar changes may underlie cortical compensatory processes in deaf or blind humans and may also have played a role in the parcellation of neocortex during mammalian evolution.

Cross-modal reorganization of callosal connectivity without altering thalamocortical projections.

Mammalian cerebral cortex is composed of a multitude of different areas that are each specialized for a unique purpose. It is unclear whether the activity pattern and modality of sensory inputs to cortex play an important role in the development of cortical regionalization. The modality of sensory inputs to cerebral cortex can be altered experimentally. Neonatal diversion of retinal axons to the auditory thalamus (cross-modal rewiring) results in a primary auditory cortex (AI) that resembles the primary visual cortex in its visual response properties and topography. Functional reorganization could occur because the visual inputs use existing circuitry in AI, or because the early visual inputs promote changes in AI's circuitry that make it capable of constructing visual receptive field properties. The present study begins to distinguish between these possibilities by exploring whether the callosal connectivity of AI is altered by early visual experience. Here we show that early visual inputs to auditory thalamus can reorganize callosal connections in auditory cortex, causing both a reduction in their extent and a reorganization of the pattern. This result is distinctly different from that in deafened animals, which have widespread callosal connections, as in early postnatal development. Thus, profound changes in cortical circuitry can result simply from a change in the modality of afferent input. Similar changes may underlie cortical compensatory processes in deaf and blind humans.

Development of inhibitory circuitry in visual and auditory cortex of postnatal ferrets: immunocytochemical localization of GABAergic neurons.

The goal of this study was to describe the development of gamma-aminobutyric acid (GABA)-containing neurons in visual and auditory cortex of ferrets. The laminar and tangential distribution of neurons containing excitatory, inhibitory, and neuromodulatory substances constrain the potential circuits which can form during development. Ferrets are born at an early stage of brain development, allowing examination of inhibitory circuit formation in cerebral cortex prior to thalamocortical ingrowth and cortical plate differentiation. Immunocytochemically labelled nonpyramidal GABA neurons were present from postnatal day 1 throughout development, in all cortical layers, and generally followed the inside-out pattern of neuronal migration into the cortical plate. Prior to postnatal day 14, pyramidal neurons with transient GABA immunoreactivity were also observed. The density of Nissl-stained and GABA-immunoreactive neurons was high early in development, declined markedly by postnatal day 20, then remained relatively constant until adulthood. However, examination of the proportion of GABA neurons revealed an unexpected late peak at postnatal day 60, then a decrease in adulthood. Visual and auditory cortex were similar in most respects, but the peak at postnatal day 60 and the final proportion of GABA neurons was higher in auditory cortex. The late peak suggests that inhibitory circuitry is stabilized relatively late in sensory cortical development, and thus that GABA neurons could provide an important substrate for experience-dependent plasticity at late stages of development.

Morphology of retinal axons induced to arborize in a novel target, the medial geniculate nucleus. I. Comparison with arbors in normal targets.

Ferret retinal axons can be induced to innervate the medial geniculate nucleus (MGN) by a combination of brain lesions early in development. Our previous work suggests that the retinal ganglion cells responsible for this plasticity are W cells. The present study continues this work with a morphological investigation of normal retinal ganglion-cell axons and retinal ganglion-cell axons induced to arborize in the MGN. Retinal axons were bulk filled with horseradish peroxidase placed in the optic tract, and individual axons were serially reconstructed from sagittal sections. The control population consisted of fine-caliber axons arborizing in the superior colliculus (SC) and in the ventral C laminae of the lateral geniculate nucleus (LGN) of normal ferrets. We also compared the axons in the MGN of lesioned ferrets to intracellularly filled X and Y axons from normal ferrets as reported by Roe et al. ([1989] J. Comp. Neurol. 288:208). We have found that the retino-MGN axons in the lesioned ferrets do not resemble X or Y axons in normal ferrets in axon diameter, arbor volume, bouton number, or bouton density. However, they do resemble the fine-caliber, presumed W axons arborizing in the C laminae of the LGN and in the SC of normal ferrets. Thus, this study, in combination with previous studies, suggests strongly that W retinal ganglion cells are responsible for the retinal input to the MGN in lesioned animals. In addition, we find that the retino-MGN axons are of two types, branched and unbranched, which may correspond to different subtypes of retinal W cells.

Morphology of retinal axon arbors induced to arborize in a novel target, the medial geniculate nucleus. II. Comparison with axons from the inferior colliculus.

Specific neonatal lesions in ferrets can induce retinal axons to project into the medial geniculate nucleus (MGN). In the accompanying paper (Pallas et al., this issue), we described the morphology of these retinal ganglion cell axons. Those results and others (Roe et al. [1993] J. Comp. Neurol. 334:263) suggest that these axons belong to the W class of retinal axons. In this paper, the retino-MGN axons are compared with the normal inputs to the MGN from the brachium of the inferior colliculus (BIC). We first sought to determine further the extent to which a novel target might influence retinal axon arbor morphology. The second issue concerns retinal topography. Ferrets with retinal projections to the MGN have a two-dimensional retinotopic map in the MGN and the primary auditory cortex rather than the one-dimensional tonotopic map normally present (Roe et al. [1990] Science 250:818). To investigate whether there might be an anatomical substrate for a two-dimensional retinotopic map in the MGN, we compared the space-filling characteristics of the retino-MGN axons with the IC-MGN axons. Our results show that the branched retino-MGN axons resemble normal retinal W axons much more closely than they resemble the normal inputs to MGN. In addition, most of the axon arbors from the BIC are elongated along the rostrocaudal (isofrequency) axis, whereas the branched retino-MGN axons are more spatially restricted, suggesting an anatomical substrate for a retinotopic map in the MGN of the rewired ferrets.

Regulation of retinal ganglion cell axon arbor size by target availability: mechanisms of compression and expansion of the retinotectal projection.

The ability of pre- and postsynaptic populations to achieve the proper convergence ratios during development is especially critical in topographically mapped systems such as the retinotectal system. The ratio of retinal ganglion cells to their target cells in the optic tectum can be altered experimentally either by early partial tectal ablation, which results in an orderly compression of near-normal numbers of retinal projections into a smaller tectal area, or by early monocular enucleation, which results in the expansion of a reduced number of axons in a near-normal tectal volume. Our previous studies showed that changes in cell death and synaptic density consequent to these manipulations can account for only a minor component of this compensation for the population mismatch. In this study, we examine other mechanisms of population matching in the hamster retinotectal system. We used an in vitro horseradish peroxidase labeling method to trace individual retinal ganglion cell axons in superior colliculi partially ablated on the day of birth, as well as in colliculi contralateral to a monocular enucleation. We found that individual axon arbors within the partially lesioned tectum occupy a smaller area, with fewer branches and fewer terminal boutons, but preserve a normal bouton density. In contrast, ipsilaterally projecting axon arbors in monocularly enucleated animals occupy a greater area than in the normal condition, with a much larger arbor length and greater number of boutons and branches compared with normal ipsilaterally projecting cells. Alteration of axonal arborization of retinal ganglion cells is the main factor responsible for matching the retinal and tectal cell populations within the tectum. This process conserves normal electrophysiological function over a wide range of convergence ratios and may occur through strict selectivity of tectal cells for their normal number of inputs.

Visual projections induced into the auditory pathway of ferrets: II. Corticocortical connections of primary auditory cortex.

Although the development of corticocortical projections has been well studied, less is known about the role of sensory inputs in the specification of these connections. As part of an ongoing series of studies in our laboratory, we have examined the role of thalamic input modality in the development of corticocortical connections. These studies involve making unilateral lesions and inducing retinal inputs into the auditory thalamus (MGN) during early development in ferrets, thereby conferring visual responsiveness on primary auditory cortex (AI). In this way we can examine the role of input identity in cortical specification in general, and connectivity patterns specifically. A previous paper (Pallas et al. [1990] J. Comp. Neurol. 298:50-68) described the pattern of thalamocortical and corticothalamic connections of auditory cortex in normal and lesioned animals. This study compares the pattern of auditory corticocortical connections in normal and lesioned animals. We injected neuroanatomical tracers into AI and mapped out the distribution of retrogradely labelled cells in the cortex. We report that the cortical inputs to ferret AI resembled those in cats, and that the pattern of ipsi- and contralateral corticocortical connections of ferret AI with visual input was similar to the normal pattern. Auditory cortex with visual input did not make ectopic connections with visual cortex, but maintained its connections with other auditory cortical areas. These results suggest that the overall corticocortical connections of an area are not influenced by the modality or activity pattern of its inputs. In particular, altering the input activity to a cortical area does not seem to promote the formation of entirely new connections, although small changes in the strength of existing connections are possible (Sur et al. [1990] Trends Neurosci. 13:227-233).

Visual projections routed to the auditory pathway in ferrets: receptive fields of visual neurons in primary auditory cortex.

How does cortex that normally processes inputs from one sensory modality respond when provided with input from a different modality? We have addressed such a question with an experimental preparation in which retinal input is routed to the auditory pathway in ferrets. Following neonatal surgical manipulations, a specific population of retinal ganglion cells is induced to innervate the auditory thalamus and provides visual input to cells in auditory cortex (Sur et al., 1988). We have now examined in detail the visual response properties of single cells in primary auditory cortex (A1) of these rewired animals and compared the responses to those in primary visual cortex (V1) of normal animals. Cells in A1 of rewired animals differed from cells in normal V1: they exhibited larger receptive field sizes and poorer visual responsivity, and responded with longer latencies to electrical stimulation of their inputs. However, striking similarities were also found. Like cells in normal V1, A1 cells in rewired animals exhibited orientation and direction selectivity and had simple and complex receptive field organizations. Furthermore, the degree of orientation and directional selectivity as well as the proportions of simple, complex, and nonoriented cells found in A1 and V1 were very similar. These results have significant implications for possible commonalities in intracortical processing circuits between sensory cortices, and for the role of inputs in specifying intracortical circuitry.

Compensation for population size mismatches in the hamster retinotectal system: alterations in the organization of retinal projections.

Unilateral partial ablation of the superior colliculus in the hamster results in a compression of the retinotopic map onto the remaining tectal fragment. In a previous electrophysiological study (Pallas & Finlay, 1989a), we demonstrated that receptive-field properties of single tectal units (including receptive-field size) remain unchanged, despite the increased afferent/target convergence ratios in the compressed tecta. The present study was done to investigate the mechanism that produces increased convergence from retina to tectum at the population level while maintaining apparent stability of convergence at the single neuron level. We injected comparable quantities of horseradish peroxidase into the tecta of normal adult hamsters and adult hamsters that had received neonatal partial tectal ablations of varying magnitude. We then compared the area of retina backfilled from the injection and the number and density of labeled retinal ganglion cells within it to the size of the remaining tectal fragment. As expected from earlier anatomical (Jhaveri & Schneider, 1974) and physiological (Finlay et al., 1979a; Pallas & Finlay, 1989a) studies demonstrating compression of the retinotectal projection, we found that the area of retina labeled from a single tectal injection site increases linearly with decreasing tectal fragment size. However, for fragment sizes down to 30% of normal, total number of retinal ganglion cells projecting to the injection site remains in or above the normal range. For large lesions (less than 30% of tectum remaining), total number of labeled retinal ganglion cells declines from normal, despite the fact that a larger absolute area of retina is represented on each unit of tectum under these conditions. Comparison of retinal ganglion cell density with tectal fragment size shows an initial decline with decreasing fragment size, which becomes sharper with very large lesions (small tectal fragments). The maintenance of the normal number of retinal ganglion cells innervating each patch of tectum could be accomplished by an elimination of the tectal collaterals of some retinal ganglion cells. Our results suggest that, in addition to collateral elimination, reduction in the size of ganglion cell arbors is occurring, since the peak density of backfilled ganglion cells declines less rapidly than backfilled retinal area increases, especially for small lesions. However, arbor reduction and collateral elimination must occur in such a way that individual tectal cells represent the same amount of visual space as normal. Thus, collateral elimination and arbor reduction are two mechanisms that operate to maintain afferent/target convergence ratios (and thus receptive-field properties) over large variations in afferent availability.(ABSTRACT TRUNCATED AT 400 WORDS)

A map of visual space induced in primary auditory cortex.

Maps of sensory surfaces are a fundamental feature of sensory cortical areas of the brain. The relative roles of afferents and targets in forming neocortical maps in higher mammals can be examined in ferrets in which retinal inputs are directed into the auditory pathway. In these animals, the primary auditory cortex contains a systematic representation of the retina (and of visual space) rather than a representation of the cochlea (and of sound frequency). A representation of a two-dimensional sensory epithelium, the retina, in cortex that normally represents a one-dimensional epithelium, the cochlea, suggests that the same cortical area can support different types of maps. Topography in the visual map arises both from thalamocortical projections that are characteristic of the auditory pathway and from patterns of retinal activity that provide the input to the map.

Visual projections induced into the auditory pathway of ferrets. I. Novel inputs to primary auditory cortex (AI) from the LP/pulvinar complex and the topography of the MGN-AI projection.

The organization of cortical circuitry responsible for processing sensory information is a subject of intense examination. However, it is not known whether cortical cells in different sensory cortices process information in a way that is specific to the modality of their input, or whether there are commonalities in processing circuitry across different cortices. In our laboratory, this question has been investigated at the level of the geniculocortical pathway by routing information of one sensory modality into the processing circuitry of another modality. Appropriate early lesions cause growth of retinal axons into the auditory thalamus (MGN) (Sur et al., Science 242:1437, '88). Previously, we have established that the MGN carries the resulting visual information on to primary auditory cortex (AI), which thus contains visually responsive neurons and a topographic representation of the retina (Roe et al., Soc. Neurosci. Abstr. 14:460, '88; Sur et al., Science 242:1437, '88). In this paper, we describe anomalous projections from the dorsal part of the thalamus, specifically the lateral posterior/pulvinar complex, into AI. This result demonstrates that thalamic neurons belonging to one modality can be induced to project to cortex that is normally of a different modality. In addition, we have studied in detail the nature of the MGN to AI projection in these animals as compared to the normal projection. The MGN to AI projection appears to be unaltered by the lesions; the location and topography of labelled cells are similar to that in normal animals. Because the MGN to AI projection is still highly divergent along the "isofrequency" dimension when compared to the tonotopic dimension, our data suggest that visual topography in the cortical map is created within the auditory cortex, perhaps by activity-dependent sharpening of the retinal representation during development.

Cross-modal plasticity in cortical development: differentiation and specification of sensory neocortex.

Early developmental manipulations can induce sensory afferents of one modality to project to central targets of a different sensory modality. We and other investigators have used such cross-modal plasticity to examine the role of afferent inputs and their patterns of activity in the development of sensory neocortex. We suggest that the afferent rewiring can significantly influence the internal connectivity or microcircuitry of sensory cortex, aspects of which appear to be determined or specified relatively late in development, but that they cannot influence, or influence only to a minor extent, the laminar characteristics and external connectivity patterns of cortex, which appear to be specified earlier.

Conservation of receptive-field properties of superior colliculus cells after developmental rearrangements of retinal input.

The formation of topographic maps requires not only that afferents synapse with the appropriate targets, but that the spatial relationships between the afferents be maintained. During development, in addition to the formation of the topographic map, the connectivity patterns responsible for the receptive-field properties of the target cells are being formed. The extent of interaction between these two processes is unknown. The present study addresses this question by manipulating afferent/target ratios during development, thus altering the topography of the map, and studying the effects of this alteration on the receptive-field properties of single target cells in the adult. Partial unilateral lesions of the superior colliculus (SC) were made in neonatal hamsters. These lesions result in a compression of the retinotopic map onto the remaining collicular fragment. Single cells were recorded from the superficial gray layer of the SC in the adult in response to visual stimuli. Receptive-field properties observed in lesioned animals were compared to those in normal animals and in sham operates. Receptive-field properties were largely unaffected by the change in the topographic map. There was no difference in the receptive-field size of single tectal cells of lesioned and unlesioned animals. Stimulus velocity and stimulus size tuning functions remained the same. This raises the possibility that, rather than the expected increase in convergence of retinal ganglion cells (RGC) onto single collicular cells, single SC cells receive input from ganglion cells representing the same amount of retinal area as in unlesioned animals. The excess ganglion cells created by the partial target removal would then project elsewhere and/or reduce their arbor within the SC. Regardless of the mechanism, it is clear from our results that circuitry in the retinotectal system of the hamster can compensate for conditions of increased afferent availability and thus maintain receptive-field properties.

Control of cell number in the developing neocortex. I. Effects of early tectal ablation.

Target availability is an important factor in the early control of neuron number in many structures in the developing vertebrate nervous system. In early neocortical development, the role of target availability in the survival of subcortically projecting neurons is not yet understood, particularly because these cells' axons are widely distributed and highly branched. In this study, we have looked for alterations in the pattern of early cell death, adult cell density and adult morphology of pyramidal cells in layer V of visual cortex consequent to removal of one of their principal targets, the ipsilateral superior colliculus. After neonatal tectal ablation, there was no difference in the incidence of pyknotic cells in the cortex overall, or in layer V during the period of normal cell death in the cortex. Neither in adulthood, nor at any point in development did the density of layer V cells or cortical cell density overall differ from normal in Nissl material. Soma size of cells in layer V overall did not differ from normal in Nissl material. In addition, the soma size of the subpopulation of cells labelled with horseradish peroxidase (HRP) from midbrain injections was unaltered. In summary, this cell population appears unresponsive in both number and morphology to deletions of a major component of its target pool. This observation has some interesting implications for reasons of constancy of cell number in layer V across cytoarchitectonic areas.