Asymmetrical interhemispheric connections develop in cat visual cortex after early unilateral convergent strabismus: anatomy, physiology, and mechanisms.

In the mammalian primary visual cortex, the corpus callosum contributes to the unification of the visual hemifields that project to the two hemispheres. Its development depends on visual experience. When this is abnormal, callosal connections must undergo dramatic anatomical and physiological changes. However, data concerning these changes are sparse and incomplete. Thus, little is known about the impact of abnormal postnatal visual experience on the development of callosal connections and their role in unifying representation of the two hemifields. Here, the effects of early unilateral convergent strabismus (a model of abnormal visual experience) were fully characterized with respect to the development of the callosal connections in cat visual cortex, an experimental model for humans. Electrophysiological responses and 3D reconstruction of single callosal axons show that abnormally asymmetrical callosal connections develop after unilateral convergent strabismus, resulting from an extension of axonal branches of specific orders in the hemisphere ipsilateral to the deviated eye and a decreased number of nodes and terminals in the other (ipsilateral to the non-deviated eye). Furthermore this asymmetrical organization prevents the establishment of a unifying representation of the two visual hemifields. As a general rule, we suggest that crossed and uncrossed retino-geniculo-cortical pathways contribute successively to the development of the callosal maps in visual cortex.

Direct projections of omnipause neurons to reticulospinal neurons: a double-labeling light microscopic study in the cat.

Omnipause neurons (OPNs) are inhibitory neurons located in the midline region of the caudal pons. Their role in gating the discharges of saccade-related burst neurons is well known, but there is no agreement concerning their influence on brainstem neurons that control other muscle groups participating in rapid gaze shifts. In the present study, we inquired whether OPNs project directly to pontobulbar reticulospinal neurons (RSNs) in the cat. Retrograde transport of horseradish peroxidase from the cervical spinal cord was used to label RSNs and an anterograde tracer (biocytin) was iontophoresed at sites of extracellular recording of the OPN activity. Somadendritic characteristics of biocytin-labeled OPNs were largely similar to those obtained previously with intracellular labeling. Three-dimensional reconstruction of axonal trajectories and collaterals revealed that projections of OPNs, regarded as a population, are bilateral. Their terminals were restricted to the reticular formation and midline structures throughout the rostral bulbar and pontine tegmentum. Appositions of synaptic boutons originating from five fully stained OPNs were detected on 38 retrogradely labeled RSNs, each of the OPNs contacting 3-13 cells. The numbers of boutons (1-46; mean 11.8) on the RSN somata and proximal dendrites indicate that the anatomical strength of paired OPN-RSN connections is comparable to that of other similarly studied inhibitory neurons in the cat. The existence of connections with RSNs supports the hypothesis of a generalized influence of OPNs on several effectors participating in orienting gaze shifts as opposed to the idea of their strict specialization for the control of eye saccades.

Control of orienting movements: role of multiple tectal projections to the lower brainstem.

In movement neuroscience this past decade, a conceptual approach that puts emphasis on population coding was clearly dominant. The purpose of numerous studies has been to define presumably homogeneous groups of neurons on the basis of the correlation of their discharges with sensory and motor events. The goal of this chapter is to stress the importance of taking into account individual properties of neurons, this being an essential prerequisite for a biologically meaningful definition of neuron populations. Taking as an example the executive limb of the neural network controlling gaze movements, we demonstrate the functional and anatomical diversity of tectal and reticular neurons, which are generally considered as homogeneous populations and used, accordingly, as lumped elements in models. We argue that the extraction of effector-specific signals from the global command of gaze displacement is based not on the interplay between discrete neural modules, but rather on a gradual process of signal specification at all levels of the executive network. An eventual accurate description of this network will require knowledge of the unique combinations of afferent inputs and efferent connections for as many subsets of its constituent neurons as is conceivably possible.

Density gradients of trans-synaptically labeled collicular neurons after injections of rabies virus in the lateral rectus muscle of the rhesus monkey.

We evaluated the two-dimensional distribution of superior colliculus (SC) neurons visualized after retrograde transneuronal transport of rabies virus injected into the lateral rectus muscle of rhesus monkeys to test whether the density of projection neurons might play a role in the spatiotemporal transformation and vector decomposition. If this were the case, the number of horizontal eye movement-related SC neurons should increase with their distance from the rostral pole of the SC and decrease with their distance from the representation of the horizontal meridian. Labeled neurons of the intermediate SC layers were counted inside a 1-mm-wide band that matched the horizontal meridian of the collicular motor map. Local areal densities were plotted against distance from the rostral SC pole. At 2.5 days after inoculation, there was no labeling in the SC. At 3 days, moderate labeling appeared on both sides, mostly in the intermediate layers. At 3.5 days, cell numbers substantially increased and the laminar distribution changed as cells appeared in the superficial SC layers. At 3 days, rostrocaudal density profiles were unimodal, with peaks at locations near 50 degrees (contralateral SC) and 25-30 degrees (ipsilateral SC) horizontal eccentricity. At 3.5 days, distributions were bimodal due to the appearance of a second high-density region near the rostral pole of the SC. The distribution of SC neurons influencing the abducens nucleus, thus, was nonuniform. Caudal sites contained more neurons, but the experimentally observed density gradients were shallower than the theoretically predicted ones that would be necessary to fully account for the spatiotemporal transformation. Similarly, we studied the distributions of cell densities in the intermediate SC layers along an isoamplitude line (representing saccades of equal amplitudes but different directions). Consistent with theoretical estimates of the density gradients required for vector decomposition, we found that the concentrations of labeled cells were highest in the vicinity of the horizontal meridian but their decrease toward the periphery of the motor map was steeper than predicted. We conclude that SC cell density gradients cannot fully account for the spatiotemporal transformation and vector decomposition in the absence of an additional mechanism such as the previously demonstrated (Grantyn et al., [1997] Soc. Neurosci. Abstr. 23:1295; Moschovakis et al., [1998] J. Neurosci. 18:10219-10229) locus-dependent weighting of the strength of efferent projections to the saccade generators.