Comparative development of cell properties in cortical area 18 of normal and dark-reared kittens

Early postnatal development of the primary visual areas 17 and 18 of the cat cerebral cortex: An SMI-32 study

Using anti-neurofilament H non-phosphorylated antibodies (SMI-32) as markers for the neuronal maturation level and Y channel responsible for motion processing, we investigated early postnatal development of the primary visual areas 17 and 18 in cats aged 0, 10, 14, and 34 days and in adults. Two analyzed parameters of SMI-32-immunolabeling were used: the total proportion of SMI-32-labeling and the density of labeled neurons. (i) The developmental time course of the total proportion of SMI-32-labeling shows the general increase in the accumulation of heavy-chain neurofilaments. This parameter showed a different time course for cortical layer development; the maximal increment in the total labeling in layer V occurred between the second and fifth postnatal weeks and in layers II-III and VI after the fifth postnatal week. In addition, the delay in accumulation of SMI-32-labeling was shown in layer V of the area 17 periphery representation during the first two postnatal weeks. (ii) The density of SMI-32-labeled neurons decreased in all layers of area 18, but was increased, decreased, or had a transient peak in layers II-III, V, and VI of area 17, respectively. The transient peak is in good correspondence with some transient neurochemical features previously revealed for different classes of cortical and thalamic neurons and reflects the time course of the early development of the thalamocortical circuitry. Some similarities between the time courses for the development of SMI-32-labeling in areas 17/18 and in A- and C-laminae of the LGNd allow us to propose heterochronous postnatal development of two Y sub-channels.

Beyond Rehabilitation of Acuity, Ocular Alignment, and Binocularity in Infantile Strabismus

Infantile strabismus impairs the perception of all attributes of the visual scene. High spatial frequency components are no longer visible, leading to amblyopia. Binocularity is altered, leading to the loss of stereopsis. Spatial perception is impaired as well as detection of vertical orientation, the fastest movements, directions of movement, the highest contrasts and colors. Infantile strabismus also affects other vision-dependent processes such as control of postural stability. But presently, rehabilitative therapies for infantile strabismus by ophthalmologists, orthoptists and optometrists are restricted to preventing or curing amblyopia of the deviated eye, aligning the eyes and, whenever possible, preserving or restoring binocular vision during the critical period of development, i.e., before ~10 years of age. All the other impairments are thus ignored; whether they may recover after strabismus treatment even remains unknown. We argue here that medical and paramedical professionals may extend their present treatments of the perceptual losses associated with infantile strabismus. This hypothesis is based on findings from fundamental research on visual system organization of higher mammals in particular at the cortical level. In strabismic subjects (as in normal-seeing ones), information about all of the visual attributes converge, interact and are thus inter-dependent at multiple levels of encoding ranging from the single neuron to neuronal assemblies in visual cortex. Thus if the perception of one attribute is restored this may help to rehabilitate the perception of other attributes. Concomitantly, vision-dependent processes may also improve. This could occur spontaneously, but still should be assessed and validated. If not, medical and paramedical staff, in collaboration with neuroscientists, will have to break new ground in the field of therapies to help reorganize brain circuitry and promote more comprehensive functional recovery. Findings from fundamental research studies in both young and adult patients already support our hypothesis and are reviewed here. For example, presenting different contrasts to each eye of a strabismic patient during training sessions facilitates recovery of acuity in the amblyopic eye as well as of 3D perception. Recent data also demonstrate that visual recoveries in strabismic subjects improve postural stability. These findings form the basis for a roadmap for future research and clinical development to extend presently applied rehabilitative therapies for infantile strabismus.

Origins of strabismus and loss of binocular vision

Strabismus is a frequent ocular disorder that develops early in life in humans. As a general rule, it is characterized by a misalignment of the visual axes which most often appears during the critical period of visual development. However other characteristics of strabismus may vary greatly among subjects, for example, being convergent or divergent, horizontal or vertical, with variable angles of deviation. Binocular vision may also vary greatly. Our main goal here is to develop the idea that such “polymorphy” reflects a wide variety in the possible origins of strabismus. We propose that strabismus must be considered as possibly resulting from abnormal genetic and/or acquired factors, anatomical and/or functional abnormalities, in the sensory and/or the motor systems, both peripherally and/or in the brain itself. We shall particularly develop the possible “central” origins of strabismus. Indeed, we are convinced that it is time now to open this “black box” in order to move forward. All of this will be developed on the basis of both presently available data in literature (including most recent data) and our own experience. Both data in biology and medicine will be referred to. Our conclusions will hopefully help ophthalmologists to better understand strabismus and to develop new therapeutic strategies in the future. Presently, physicians eliminate or limit the negative effects of such pathology both on the development of the visual system and visual perception through the use of optical correction and, in some cases, extraocular muscle surgery. To better circumscribe the problem of the origins of strabismus, including at a cerebral level, may improve its management, in particular with respect to binocular vision, through innovating tools by treating the pathology at the source.

Cortical plasticity and preserved function in early blindness

The "neural Darwinism" theory predicts that when one sensory modality is lacking, as in congenital blindness, the target structures are taken over by the afferent inputs from other senses that will promote and control their functional maturation (Edelman, 1993). This view receives support from both cross-modal plasticity experiments in animal models and functional imaging studies in man, which are presented here.

Parallel development of orientation maps and spatial frequency selectivity in cat visual cortex

In an early stage of the postnatal development of cats, orientation maps mature and spatial frequency selectivity is consolidated. To investigate the time course of orientation map maturation associated with the consolidation of spatial frequency selectivity, we performed optical imaging of intrinsic signals in areas 17 and 18 of cats under the stimulation of drifting square-wave gratings with different orientations and spatial frequencies. First, orientation maps for lower spatial frequencies emerged in the entire part of the lateral gyrus, which includes areas 17 and 18, and then these orientation maps in the posterior part of the lateral gyrus disappeared as orientation maps for higher spatial frequencies matured. Independent of age, an anteroposterior gradient of response strengths from lower to higher spatial frequencies was observed. This indicates that the regional distribution of spatial frequencies is innately determined. The size of iso-orientation domains tended to decrease as the stimulus spatial frequency increased at every age examined. In contrast, orientation representation bias changed with age. In cats younger than 3 months, the cardinal (vertical and horizontal) orientations were represented predominantly over the oblique orientations. However, in young adult cats from 3 to 9 months old, the representation bias switched to predominantly oblique orientations. These age-dependent changes in the orientation representation bias imply that orientation maps continue to elaborate within postnatal 1 year with the consolidation of spatial frequency selectivity. We conclude that both intrinsic and mutual factors lead to the development of orientation maps and spatial frequency selectivity.

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.

Postnatal development of GFAP, connexin43 and connexin30 in cat visual cortex

In cat visual cortex, neurons acquire progressively mature functional properties during the first postnatal months. The aim of this study was to analyze the development of astrocytes during this period. The patterns of expression of the glial fibrillary acidic protein (GFAP) as well as of two gap junction proteins expressed in astrocytes, connexin43 (Cx43) and connexin30 (Cx30), were investigated by immunohistochemistry and optical density measurements, in visual cortical areas 17 and 18 at four different ages: 2 weeks (postnatal days 12 to 15, P12-15), 1 month (P27-31), 2 months (P60-62) and beyond 1 year. Since visual experience is a key factor for neural development, the patterns of expression of these three proteins were studied both in normally-reared and monocularly deprived animals. Interestingly, the distribution of GFAP, Cx43 and Cx30 was found to change dramatically but independently of visual experience, during postnatal development, even beyond P60. During the first postnatal month, GFAP and Cx43 were mainly localized in the white matter underlying the visual cortical areas 17 and 18. Then, their distributions evolved similarly with a progressive decrease of their density in the white matter associated with an increase in the cortex. Connexin30 expression appeared only from the second postnatal month, strictly in the cortex and with a laminar distribution which was similar to that of Cx43 at the same age. In adults, a specific laminar distribution was observed, that was identical for GFAP, Cx43 and Cx30: their density was higher in layers II/III and V than in the other cortical layers.

Postnatal dendritic development of Y-like geniculocortical relay neurons

We describe the dendritic development of neurons in the dorsal lateral geniculate nucleus (LGNd) projecting to cortical area 18 in the postnatal cat. LGN neurons were identified by retrograde labeling from area 18 with fluorescent latex microspheres and injected in the fixed slice with Lucifer yellow (LY) and horseradish peroxidase (HRP) to visualize their dendritic arborizations. Both topological (measures of the patterns of dendritic branching and their territorial coverage) and metric parameters (measures of the quantitative parameters describing the size, length, extent and diameter of the dendritic arbors) were measured in three-dimensions for 25 LGN neurons in cats between 1 and 18 postnatal weeks. In addition, dendritic growth was compared to the changing dimensions of the LGNd. At all ages, neurons projecting to area 18 have large somata and radiate dendrites. From 1 to 18 weeks neurons increase in size--both soma area and the length of all dendritic segments double during this period. Intermediate and terminal dendritic segments show comparable growth until 5 weeks. However, only terminal segments continue to grow significantly from 5 until 18 weeks. Dendrites become straighter during development, the angle between daughter branches decreases and dendritic segment diameter increases, with terminal segments showing a greater increase relative to intermediate segments. The density of dendritic appendages increases transiently at 5 weeks and a differential redistribution occurs, so that by 18 weeks dendrites further from the soma have a greater density of appendages than those near the soma. Some dendritic relationships remain invariant during development--intermediate segments are always shorter, thicker and straighter than terminal segments. During these changes however, area 18 projecting neurons maintain a constant number of primary dendrites and have, on average, a constant branching pattern. The relative volume of the LGNd occupied by an area 18 projecting neuron increases 2.4-fold between 1 and 18 weeks as the dendrites grow with the result that the coverage of a given point of the LGNd by dendrites of area 18 projecting nearly doubles from 24 to 45 neurons per unit volume. This increased net dendritic overlap provides a substrate for enhanced numerical synaptic divergence of the Y-cell pathway from a point source in the retina to the visual cortex.

Pattern of development of the callosal transfer of visual information to cortical areas 17 and 18 in the cat

The aim of this study was to investigate the development of visual callosal transfer in the normally reared cat. Two- to nine-week-old kittens and adults (used as controls) underwent section of the optic chiasm. Three days later, the animals were placed under anesthesia and paralysed; unit activities were recorded from visual cortical areas 17 and 18 and from the white matter in one hemisphere. The units were tested for their responses to visual stimulation of each eye successively. Out of 1036 recorded neurons, 185 could be activated through the eye contralateral to the explored cortex via callosal transfer. Most of them could also be driven through the ipsilateral eye via the 'direct' geniculo-cortical pathway. For animals aged > or = 2 weeks, virtually all of these units were located at the 17/18 border zone, with a majority in the supragranular layers. When activated through the corpus callosum, they displayed receptive fields located either on the central vertical meridian of the visual field or in the hemifield ipsilateral to the explored cortex. Such extension into the ipsilateral hemifield as well as receptive field disparities of binocular units decreased with age, while spontaneous activity, strength of response, orientation selectivity and ability to respond to slits moving at middle-range velocity increased. The main conclusion is that the transient callosal projections described by anatomists, which are present until 3 months of age, do not achieve supraliminar synaptic contacts with parts of areas 17 and 18 other than the 17/18 border zone, at least from 12 days after birth. However the visual callosal transfer in young animals displays some characteristics which disappear with age.

Development of visual inhibitory interactions in kittens

This study was designed to monitor the development of inhibitory interactions elicited in the cat visual system by oriented visual stimuli. Steady-state visual-evoked potentials (VEPs) were recorded from the scalp of 11 behaving and alert kittens while they viewed contrast-reversed sinusoidal gratings. In adult cats, the form of VEP contrast-response curves (the amplitude of second harmonic modulation as a function of stimulus contrast) was modified by superimposing a mask grating on the test. Parallel masks displaced the curves to a higher contrast region (probably via contrast gain-control mechanisms), increasing contrast threshold without affecting the slope of the curve. Orthogonal gratings, on the other hand, decrease the slope of the curve without affecting threshold (so called cross-orientation inhibition: Morrone et al., 1981). These effects are similar to those previously reported in human VEPs (Morrone & Burr, 1986; Burr & Morrone, 1987) and single cortical cat cells (Morrone et al., 1982). For young kittens of 20 days, the orthogonal mask had no effect whatsoever on the response curves, and the effect of the parallel mask was much less than for adult cats. At about 40 days, the orthogonal mask began to attenuate responses multiplicatively, and by 50 days the amount of multiplicative attenuation had reached adult levels. The effect of the parallel mask (as indicated by the increase in threshold elevation) increased gradually from 20-50 days. The results are consistent with the existence of at least two types of inhibition in cat visual neurones that develop at different rates.

Development of Y-axon innervation of cortical area 18 in the cat

1. Geniculocortical Y-axons (n = 38) in the optic radiations of 4-5-week-old kittens (n = 20) and adult cats (n = 18) were studied both physiologically and morphologically. Axons were recorded from intracellularly and subsequently filled ionophoretically with horseradish peroxidase (HRP). The HRP filled the axons' terminal arborizations in visual cortex (particularly well for those innervating area 18). Fourteen axons appeared to be completely filled with HRP (n = 8 in kitten, n = 6 in adult) and served as the basis for the quantitative analysis of the terminal arborizations reported in this study. 2. The distribution and correspondence of the axonal boutons to presynaptic elements in cortical layer 4A was analysed at both the light and electron microscope level using computerized three-dimensional analysis and serial section reconstruction, respectively. Compared to adult axons, the boutons of the kitten axons were smaller (means = 0.75 vs. 1.75 microns length, P less than 0.001) and more densely spaced both along individual axon branches (means = 6.60 vs. 11.20 microns interbouton interval, P less than 0.001) and between neighbouring branches of the same axon (means = 4.7 vs. 6.4 microns nearest-neighbour distance, P less than 0.01). 3. Most kitten boutons made a single Gray's type 1 synapse on a cortical neurone, unlike adult boutons which usually contacted two or more postsynaptic targets. Both kitten and adult axons had dendritic spines as their major target. Occasionally, HRP reaction-product was observed in cortical neurones postsynaptic to the labelled geniculocortical axon, which gave some estimate of the number of synaptic contacts between a single geniculocortical axon and target cell (about five). 4. The kitten Y-axons innervated the visual cortex in a pattern similar to that of the adult, with the richest terminal branching and bouton density in layer 4A with some additional boutons distributed in layers 3, 4B and 6. The extent of the terminal arborizations primarily in layer 4A (as measured in surface views) of kitten Y-axons in area 18 was significantly less than that of adult Y-axons in area 18 (means = 0.9 mm2 vs. means = 1.2 mm2, P = 0.04). 5. We conclude that between 4 and 5 postnatal weeks and 1 year, geniculocortical Y-axons projecting to cortical area 18 undergo four major changes. These include a reduction in synaptic bouton density (both in three-dimensional space and along individual branches), a concomitant moderate expansion in the surface area of cortex innervated, an increase in bouton size and an increase in the number of synaptic contacts made by each bouton. A general proportional growth of the individual axons' terminal arborization together with fusion and/or separation of neighbouring boutons is sufficient to explain this development.