Visual recognition of social signals by a tectothalamic neural circuit.

Social affiliation emerges from individual-level behavioural rules that are driven by conspecific signals1-5. Long-distance attraction and short-distance repulsion, for example, are rules that jointly set a preferred interanimal distance in swarms6-8. However, little is known about their perceptual mechanisms and executive neural circuits3. Here we trace the neuronal response to self-like biological motion9,10, a visual trigger for affiliation in developing zebrafish2,11. Unbiased activity mapping and targeted volumetric two-photon calcium imaging revealed 21 activity hotspots distributed throughout the brain as well as clustered biological-motion-tuned neurons in a multimodal, socially activated nucleus of the dorsal thalamus. Individual dorsal thalamus neurons encode local acceleration of visual stimuli mimicking typical fish kinetics but are insensitive to global or continuous motion. Electron microscopic reconstruction of dorsal thalamus neurons revealed synaptic input from the optic tectum and projections into hypothalamic areas with conserved social function12-14. Ablation of the optic tectum or dorsal thalamus selectively disrupted social attraction without affecting short-distance repulsion. This tectothalamic pathway thus serves visual recognition of conspecifics, and dissociates neuronal control of attraction from repulsion during social affiliation, revealing a circuit underpinning collective behaviour.

Major Feedforward Thalamic Input Into Layer 4C of Primary Visual Cortex in Primate.

One of the underlying principles of how mammalian circuits are constructed is the relative influence of feedforward to recurrent synaptic drive. It has been dogma in sensory systems that the thalamic feedforward input is relatively weak and that there is a large amplification of the input signal by recurrent feedback. Here we show that in trichromatic primates there is a major feedforward input to layer 4C of primary visual cortex. Using a combination of 3D-electron-microscopy and 3D-confocal imaging of thalamic boutons we found that the average feedforward contribution was about 20% of the total excitatory input in the parvocellular (P) pathway, about 3 times the currently accepted values for primates. In the magnocellular (M) pathway it was around 15%, nearly twice the currently accepted values. New methods showed the total synaptic and cell densities were as much as 150% of currently accepted values. The new estimates of contributions of feedforward synaptic inputs into visual cortex call for a major revision of the design of the canonical cortical circuit.

Laminar differences in the orientation selectivity of geniculate afferents in mouse primary visual cortex.

It has been debated whether orientation selectivity in mouse primary visual cortex (V1) is derived from tuned lateral geniculate nucleus (LGN) inputs or computed from untuned LGN inputs. However, few studies have measured orientation tuning of LGN axons projecting to V1. We measured the response properties of mouse LGN axons terminating in V1 and found that LGN axons projecting to layer 4 were generally less tuned for orientation than axons projecting to more superficial layers of V1. We also found several differences in response properties between LGN axons and V1 neurons in layer 4. These results suggest that orientation selectivity of mouse V1 may not simply be inherited from LGN inputs, but could also depend on thalamocortical or V1 circuits.

A systematic random sampling scheme optimized to detect the proportion of rare synapses in the neuropil.

Synapses can only be morphologically identified by electron microscopy and this is often a very labor-intensive and time-consuming task. When quantitative estimates are required for pathways that contribute a small proportion of synapses to the neuropil, the problems of accurate sampling are particularly severe and the total time required may become prohibitive. Here we present a sampling method devised to count the percentage of rarely occurring synapses in the neuropil using a large sample (approximately 1000 sampling sites), with the strong constraint of doing it in reasonable time. The strategy, which uses the unbiased physical disector technique, resembles that used in particle physics to detect rare events. We validated our method in the primary visual cortex of the cat, where we used biotinylated dextran amine to label thalamic afferents and measured the density of their synapses using the physical disector method. Our results show that we could obtain accurate counts of the labeled synapses, even when they represented only 0.2% of all the synapses in the neuropil.

Postnatal development of alkaline phosphatase activity correlates with the maturation of neurotransmission in the cerebral cortex.

We have shown previously that the tissue nonspecific alkaline phosphatase (TNAP) is selectively expressed in the synaptic cleft of sensory cortical areas in adult mammals and, by using sensory deprivation, that TNAP activity depends on thalamocortical activity. We further analyzed this structural functional relationship by comparing the developmental pattern of TNAP activity to the maturation of the thalamocortical afferents in the primate brain (Callithrix jacchus). Cortical expression of alkaline phosphatase (AP) activity reflects the sequential maturation of the modality-specific sensory areas. Within the visual cortex, the regional and laminar distribution of AP correlates with the differential maturation of the magno- and parvocellular streams. AP activity, which is transiently expressed in the white matter, exhibits a complementary distributional pattern with myelin staining. Ultrastructural analysis revealed that AP activity is localized exclusively to the myelin-free axonal segments, including the node of Ranvier. It was also found that AP activity is gradually expressed in parallel with the maturation of synaptic contacts in the neuropile. These data suggest the involvement of AP, in addition to neurotransmitter synthesis previously suggested in the adult, in synaptic stabilization and in myelin pattern formation and put forward a role of AP in cortical plasticity and brain disorders.

VGluT2 immunochemistry identifies thalamocortical terminals in layer 4 of adult and developing visual cortex.

A vesicular glutamate transporter, VGluT2, has been suggested to be the transporter utilized in the thalamocortical pathway. We examined the reliability of this marker in identifying and discriminating thalamic terminals in adult and developing ferret visual cortex. We studied brain sections stained for the transporter protein and/or anterogradely filled thalamocortical or intracortical axons, by using light, confocal, and electron microscopy. Under light microscopy, VGluT2 immunoreactivity (ir) in adult animals [past postnatal day (P)90] and in neonatal animals as early as P27 formed a dense band in layer 4 and appeared as scattered puncta in layers 6 and 1. Confocal dual-labeling analyses of P46 and adult striate cortices indicated that VGluT2 was present in thalamocortical axons, suggesting that thalamic projections utilize this transporter during postnatal development as well as adulthood. In contrast, extracellularly filled intracortical axons failed to colocalize with VGluT2-ir, suggesting that no significant terminal population originating in cortex contained VGluT2 in layer 4. Electron microscopic analysis revealed that, in adult layer 4, VGluT2-ir was present in large terminals, forming asymmetric synapses. Similar to anterogradely labeled thalamocortical terminals, VGluT2-ir synaptic terminals were different from their unlabeled counterparts in terms of terminal area (0.6 vs. 0.3 microm), synaptic length (486 vs. 353 nm), and preference for synapsing on spines (77% vs. 59%). Moreover, no significant differences were found between VGluT2-ir and anterogradely labeled thalamocortical terminals. Comparable similarities were also demonstrated at P46. These results indicate that thalamocortical terminals in layer 4 of visual cortex utilize VGluT2 and suggest that this marker can be used to identify thalamic axons specifically in adult and developing animals.

Histological findings at the boundary of craniopharyngiomas.

Although a craniopharyngioma is grossly well circumscribed, microscopically the borders are frequently irregular and may be associated with gliosis in the adjacent brain tissue. In the current study, we investigated the histology of the interface between craniopharyngiomas and surrounding normal structures such as the hypothalamus and pituitary gland. Histologically, we classified the findings at the boundary of craniopharyngiomas into three types. In type 1, a relatively thick capsule-like tissue was identified at the boundary between the craniopharyngioma and surrounding normal structure composed of tumor cells and inflammatory changes. In type 2, a craniopharyngioma had a relatively clear cleavage between the surrounding gliosis. In type 3, the boundary had some interdigitation of the tumor in the surrounding gliotic layer adjacent to the craniopharyngioma. In types 1 and 3, surgeons may fail to accomplish complete resection of the tumor. These histological features may result in recurrence of craniopharyngioma even after gross total resection.

Mesencephalic and diencephalic afferent connections to the thalamic nucleus rotundus in the lizard, Psammodromus algirus.

The present work is an analysis of the afferent projections to the thalamic nucleus rotundus in a lizard, both at the light- and electron-microscopic level, using biotinylated dextran amine (BDA) as a neuroanatomical tracer. This study has confirmed previously reported afferent projections to nucleus rotundus in reptiles and has also identified a number of new cellular aggregates projecting to this dorsal thalamic nucleus. After BDA injections into nucleus rotundus, retrogradely labelled neurons were observed consistently within the following neuronal groups in the midbrain and the diencephalon: (i) the stratum griseum centrale of the optic tectum; (ii) the nucleus subpretectalis in the pretectum; (iii) the nucleus ansa lenticularis posterior, the posterior nucleus of the ventral supraoptic commissure, and the posteroventral nucleus, in the dorsal thalamus and (iv) the lateral suprachiasmatic nucleus and part of the reticular complex in the ventral thalamus. Tectal axons entering nucleus rotundus were fine and varicose and formed exclusively asymmetric synaptic contacts, mainly on small dendritic profiles. Rotundal neurons had symmetric synapses made by large boutons probably of nontectal origin. After comparing our results with those in other reptiles, birds and mammals, we propose that the sauropsidian nucleus rotundus forms part of a visual tectofugal pathway that conveys mesencephalic visual information to the striatum and dorsal ventricular ridge, and is similar to the mammalian colliculo-posterior/intralaminar-striatoamygdaloid pathway, the function of which may be to participate in visually guided behaviour.

Connection from cortical area V2 to MT in macaque monkey.

The extrastriate visual area of the macaque monkey called MT or V5, receives its input from multiple sources. We have previously examined the synaptic connections made by V1 cells that project to MT (Anderson et al., 1998). Here, we provide a similar analysis of the projection from V2 to MT. The major target of the V2 projection in MT is layer 4, where it forms clusters of asymmetric (excitatory) synapses. Unlike the V1 projection, it also forms synapses in layers 1 and 2 and does not form synapses in layer 6. The most frequently encountered targets of boutons labeled from V2 were spines (67% in layer 4; 82% in layer 2/3). Unusually, only 5/12 boutons examined in layer 1 actually formed synapses. Unlike the V1 projection, multisynaptic boutons were rare (mean, 1.1 synapses per bouton vs. 1.7 for the V1 projection). Like the V1 projection, the input to MT from any point in V2 is sparse (contributing approximately 4-6% of the asymmetric synapses in the densest clusters in layer 4). The synapses of the V2 projection were similar in size to those of the V1 projection (0.1 microm(2) vs. 0.09 microm(2)) and both formed more complex postsynaptic densities on spines than on dendritic shafts. The clear differences between the V1 and V2 projection to MT indicate that their functions are complementary rather than completely overlapping.

Growth-associated protein 43 is located in type I corticothalamic terminals in the cat visual thalamus.

Growth-associated protein 43 (GAP 43) is a presynaptic protein that has been proposed to be involved in synaptic plasticity. To determine the location of GAP 43 within the synaptic circuitry of the thalamus, immunocytochemical staining for GAP 43 was examined in a relay nucleus, the dorsal lateral geniculate nucleus (dLGN), and two association nuclei, the pulvinar nucleus and the lateral subdivision of the lateral posterior (LP) nucleus. In the dLGN, moderate neuropil staining was seen in the A laminae, and denser staining was found in the interlaminar zones and the C laminae. Uniform dense staining of the neuropil was found in both the pulvinar and LP nuclei. At the ultrastructural level, the GAP 43 staining was restricted to small-diameter myelinated axons, thin unmyelinated fibers, and small terminals that contained densely packed round vesicles (RS profiles) and made asymmetric synaptic contacts with small-caliber dendrites in the extraglomerular neuropil. The distribution of immunocytochemical label within the visual thalamus suggests that GAP 43 is confined to type I corticothalamic terminals and axons that originate from extrastriate cortical areas. These results also suggest that in both relay and association nuclei GAP 43 may be used to augment the cortical control of thalamic activity. In addition, these results underscore the distinction between the small type I corticothalamic terminals, which appear to contain GAP 43 throughout the visual thalamus, and the large type II corticothalamic terminals that, like the type II retinal terminals in the dLGN, do not contain GAP 43.

Location of muscarinic type 2 receptors within the synaptic circuitry of the cat visual thalamus.

A cholinergic projection from the parabrachial region (PBR) of the brainstem to the visual thalamus has been studied in great detail during the past 20 years. A number of physiological studies have demonstrated that this projection causes a dramatic change in thalamic activity during the transition from sleep to wakefulness. Additionally, the PBR may mediate more subtle changes in thalamic activity as attentional levels fluctuate during the waking state. The synaptic circuitry underlying these events has been identified in the cat thalamus. However, there is currently no anatomical information regarding the distribution of cholinergic receptors in relation to this circuitry. To begin to understand how the PBR projection modulates thalamic activity, we used immunocytochemical techniques to examine the distribution of muscarinic type 2 (M2) receptors in the visual thalamus of the cat. The distribution of M2 receptors correlates well with previous reports of the distribution of cholinergic terminals in the visual thalamus. At the light microscopic level, dense M2 staining was seen in the neuropil of the dorsal lateral geniculate nucleus (dLGN) and pulvinar nucleus and in somata and proximal dendrites of cells in the thalamic reticular nucleus (TRN). In the dLGN and pulvinar nucleus, we quantitatively analyzed the distribution of M2 receptors using electron microscopy. Postembedding immunocytochemistry for gamma aminobutyric acid (GABA) was used to determine whether M2 receptors are present on interneurons or thalamocortical cells. In particular, we examined the distribution of M2 receptors with respect to the known sites of PBR terminations. The dendrites of both thalamocortical cells and interneurons were stained for the M2 receptors in both the glomerular and extraglomerular neuropil. However, the densest staining was found in glomerular GABAergic profiles that displayed the morphology associated with interneuron dendritic terminals (F2 profiles). Our data suggest that M2 receptors play an important role both in blocking thalamic spindle oscillations and in increasing the efficacy of signal transmission during increased attentional states.

Retinal axon guidance by region-specific cues in diencephalon.

Retinal axons show region-specific patterning along the dorsal-ventral axis of diencephalon: retinal axons grow in a compact bundle over hypothalamus, dramatically splay out over thalamus, and circumvent epithalamus as they continue toward the dorsal midbrain. In vitro, retinal axons are repulsed by substrate-bound and soluble activities in hypothalamus and epithalamus, but invade thalamus. The repulsion is mimicked by a soluble floor plate activity. Tenascin and neurocan, extracellular matrix molecules that inhibit retinal axon growth in vitro, are enriched in hypothalamus and epithalamus. Within thalamus, a stimulatory activity is specifically upregulated in target nuclei at the time that retinal axons invade them. These findings suggest that region-specific, axon repulsive and stimulatory activities control retinal axon patterning in the embryonic diencephalon.

Quantitative analyses of synaptic contacts of interneurons in the dorsal lateral geniculate nucleus of the squirrel monkey.

Three interneurons were recorded from and then injected with horseradish peroxidase in the parvocellular laminae of the squirrel monkey's (Saimiri sciureus) dorsal lateral geniculate nucleus. They were then examined using the electron microscope for their synaptic contacts, both the afferent contacts onto their dendrites and their presynaptic dendritic contacts onto presumptive projection (relay) neuron dendrites. The somata of these interneurons were small (mean = 178 microns 2), but the dendritic trees were large compared with those of projection neurons. All three interneurons had similar synaptic patterns onto their dendrites with about equal numbers of retinal, cortical, and GABAergic contacts. The distribution of these contacts was more uniform compared with the same types of contacts made onto projection neurons. The presynaptic dendrites were observed to contact only the dendrites of presumptive projection neurons, and these contacts were nearly all in the form of geniculate triads. None of the three interneurons displayed an axon. The receptive fields of these interneurons were similar to those of projection cells, but were larger and had center-response signs that were the opposite of the projection neurons around them (e.g. OFF center for the dorsal part of the parvocellular mass where ON-center projection neurons reside). The squirrel monkey data provides additional evidence that one aspect of the laminar pattern observed in the parvocellular pathway of the primate's dLGN might be related to a segregation of projection neurons of one center-response sign with interneurons of the opposite center-response sign.

Bilateral projections from the superior colliculus to the suprageniculate nucleus in the cat: a WGA-HRP/double fluorescent tracing study.

Following WGA-HRP injection into the right suprageniculate nucleus of the cat brain, retrogradely-labeled neurons were found not only in the ipsilateral, but also in the contralateral superior colliculi. After WGA-HRP injection into the unilateral superior colliculus, anterogradely-labeled axon terminals were observed in the bilateral suprageniculate nuclei, and electron microscopic examination revealed that these were large terminals which made asymmetric synaptic contacts with dendrites. When a different kind of fluorescent tracer was injected into each suprageniculate nucleus (Fast blue and Nuclear yellow), double-labeled neurons were observed in the rostral and middle portions of both superior colliculi. These results suggest that there are direct bilateral projections from the superior colliculus to the suprageniculate nuclei, and that some of these projections originate from branching colliculo-suprageniculate axons.

Synaptic output of physiologically identified spiny stellate neurons in cat visual cortex.

Spiny stellate neurons of area 17 of the cat's visual cortex were physiologically characterised and injected intracellularly with horseradish peroxidase. Six neurons from sublamina 4A were selected. Five had the S-type of simple receptive fields; one had a complex receptive field. Their axons formed boutons mainly in layers 3 and 4. An electron microscopic examination of 45 boutons showed that each bouton formed one asymmetric synapse on average. Spines were the most frequent synaptic target (74%); dendritic shafts formed the remainder (26%). On the basis of ultrastructural characteristics, 8% of the target dendrites were characterised as originating from smooth gamma-aminobutyrate-ergic (GABAergic) neurons. Thus the major output of spiny stellate neurons is to other spiny neurons, probably pyramidal neurons in layer 3 and spiny stellates in layer 4.

Polyneuronal innervation of spiny stellate neurons in cat visual cortex.

Our hypothesis was that spiny stellate neurons in layer 4 of cat visual cortex receive polyneuronal innervation. We characterised the synapses of four likely sources of innervation by three simple criteria: the type of synapse, the target (spine, dendritic shaft), and the area of the presynaptic bouton. The layer 6 pyramids had the smallest boutons and formed asymmetric synapses mainly with the dendritic shaft. The thalamic afferents had the largest boutons and formed asymmetric synapses mainly with spines. The spiny stellates had medium-sized boutons and formed asymmetric synapses mainly with spines. We used these to make a "template" to match against the boutons forming synapses with the spiny stellate dendrite. Of the asymmetric synapses, 45% could have come from layer 6 pyramidal neurons, 28% from spiny stellate neurons, and 6% from thalamic afferents. The remaining 21% of asymmetric synapses could not be accounted for without assuming some additional selectivity of the presynaptic axons. Additional asymmetric synapses may come from a variety of sources, including other cortical neurons and subcortical nuclei such as the claustrum. Of the symmetric synapses, 84% could have been provided by clutch cells, which form large boutons. The remainder, formed by small boutons, probably come from other smooth neurons in layer 4, e.g., neurogliaform and bitufted neurons. Our analysis supports the hypothesis that the spiny stellate receives polyneuronal innervation, perhaps from all the sources of boutons in layer 4. Although layer 4 is the major recipient of thalamic afferents, our results show that they form only a few percent of the synapses of layer 4 spiny stellate neurons.

Direct and indirect retinohypothalamic projections to the supraoptic nucleus in the female albino rat.

Earlier studies have shown that retinohypothalamic projections terminate extensively within the hypothalamus of the rat. Recently, we identified a light retinal projection to the supraoptic nucleus as well as a larger, well-focused projection resulting in a peri-supraoptic nucleus terminal field. In this study, we employed a double labeling method with cholera toxin conjugated to horseradish peroxidase (CT-HRP) and pseudorabies virus, a transsynaptic neural tracer, to evaluate retinorecipient neurons in both the supraoptic nucleus and peri-supraoptic nucleus terminal field. In addition, we looked for evidence that cells in the peri-supraoptic nucleus terminal field project into the supraoptic nucleus. Three strains of pseudorabies virus were compared. A direct retinosupraoptic nucleus circuit was confirmed with all three strains. Retinorecipient neurons in the peri-supraoptic nucleus terminal field were also confirmed. However, there was a strain-based difference in the identification of these neurons. The wild-type Becker strain labeled cells in the peri-supraoptic nucleus terminal field in a manner paralleling the early, intermediate and late stages of infection of the suprachiasmatic nucleus. This parallel time course suggests that retinal ganglion cells terminate directly on cells in the peri-supraoptic nucleus terminal field. Conversely, the Bartha and PRV-91 strains showed appreciable labeling of peri-supraoptic neurons only at long survival times. This longer time course suggests that these mutant strains label neurons in the peri-supraoptic nucleus terminal field indirectly, after passing through additional neurons. In addition, experiments with monocular injection of CT-HRP and posterior pituitary injection of pseudorabies virus showed retrogradely labeled second-order cells in the peri-supraoptic nucleus amidst the CT-HRP labeled terminal field of the retinohypothalamic tract. These results demonstrate a direct projection from the retina to the supraoptic nucleus and provide evidence for an indirect circuit from the retina to the supraoptic nucleus via neurons located in the peri-supraoptic nucleus terminal field. The strain-based differences imply that those retinal ganglion cells that project to the peri-supraoptic nucleus terminal field differ from those that project to the suprachiasmatic nucleus. In addition, these results suggest a neuroanatomic basis for photic effects on physiological mechanisms that are not mediated by the circadian timing system.

Structural asymmetry in the thalamofugal visual projections in 2-day-old chick is correlated with a hemispheric difference in synaptic density in the hyperstriatum accessorium.

Differences in visual discrimination ability between the left and right eyes of chicks, which are most prominent in young males, may result from a structural asymmetry in the organization of the visual projections from the thalamus to the visual Wulst. This asymmetry in projections is no longer present by 21 days in males when the contralateral projections from the right thalamus to the left hyperstriatum have developed. Since the asymmetry of the thalamo-hyperstriatal system results in a differential input of fibres to regions of the hyperstriatum which in turn project to the hyperstriatum accessorium (HA), one of the major differences expected within this region would be an asymmetry in the numerical density of synapses (Nv.syn/microns3). When this was examined in the hyperstriatum accessorium of 2-day-old male chicks, the density of synapses in the right HA was found to be significantly higher (22%, P less than 0.05) than in the left HA. The consequences of this asymmetry in synaptic density in the HA could be widespread and influential within the chick visual system.

Divergence and collateral axon branching in subsystems of visual cortical projections from the cat lateral posterior nucleus.

The projections from the lateral (LPl), intermediate (LPi) and medial (LPm) subdivisions of the cat lateral posterior nucleus (n. LP) to visual areas 17, the posteomedial (PMLS) and posterolateral (PLLS) lateral suprasylvian and anterior ectosylvian (AEV) were studied using the retrograde labeling technique following concomitant injections of fluorescent dyes (Fast blue, Nuclear yellow, Evans blue and Rhodamine beta-isothiocyanate) into the different cortical loci. The results showed a medial-lateral topographical reversal of the visual n. LP-cortical connections: The ventral portion of LPl projects to area 17 whereas more dorsolateral regions of LPl and lateral LPi provide input to PMLS. Cells in medial LPi project mainly to the PLLS cortex and AEV receives afferents from the LPm. Areas of overlap were identified within the ventral LPl which projects to both area 17 and PMLS and within the LPi/LPm border region at the origin of connections to both PLLS and AEV. Furthermore, some single neurons within the areas of overlap were found to be double-labeled indicating divergent projections to their respective cortical targets via collateral axon branching. The data show that divergence and axonal branching are common features of the different n.LP-visual cortical subsystems and support the notion of the existence of families of thalamo-cortical systems which are distinct in their connection patterns and underlying functional properties.

Ultrastructural evidence of the formation of synapses by retinal ganglion cell axons in two nonstandard targets.

After unilateral ablation of the optic tectum in the frog (Rana pipiens), retinal ganglion cell axons enter the lateral thalamic neuropil in large numbers. This area is normally a target of the tectal efferent projection but is not innervated directly from the retina in normal frogs nor in frogs undergoing optic nerve regeneration in the presence of an intact tectum. The ability of retinal axons to form synaptic contacts in this nonstandard target, previously suspected only from light microscope studies, has been ultrastructurally verified in the present investigation. Retinal axon terminals were selectively labeled for light and electon microscope study by introducing horseradish peroxidase (HRP) into the optic nerve 73-413 days after unilateral ablation of the contralateral optic tectum. In some of the frogs, the optic nerve had also been crushed to test the ability of retinal axons regenerating over a long distance to form this connection. The HRP-labeled retinal axon terminals had the same untrastructural morphology whether located in the lateral thalamic neuropil or in the correct regions of projection, e.g., the lateral geniculate complex. They contained clear, spherical synaptic vesicles and made Gray type I synapses on the unlabeled postsynaptic dendrites. The magnitude of the projection was disproportionately greater in animals having complete or nearly complete tectal ablation than in a specimen in which the lesion was significantly incomplete. An aberrant projection was also observed in the nucleus isthmi in some of the specimens. These findings have significance for chemoaffinity theories of the specification of synaptic connections in that the ability of retinal axons to synapse in nonstandard targets in this experimental context may be considered evidence for the expression of appropriate cell-surface recognition-molecules by the abnormally targeted postsynaptic neurons. The likelihood that the expression of these postsynaptic labels is normally repressed transynaptically by molecular signals from the intact tectal input is discussed.