Horizontal Interactions in Cat Striate Cortex: III. Ectopic Receptive Fields and Transient Exuberance of Tangential Interactions

Effects of divergent strabismus on the horizontal connections of neurons in the cat visual cortex

Neuron connections of eye dominance columns in the visual cortex were studied in kittens at age 5-6 months with experimental strabismus induced on days 13-16 of life (by tenotomy of the medial ocular muscle). A photographic method was used to assess the angle between the visual axes of the eyes. The operated oculomotor muscle was found to undergo partial reattachment to the eyeball. In some kittens, the operated eye returned to the normal position, while others showed persisting deviation of the eyes by 5-15 degrees. Loss of the binocular properties of cortical neurons have been demonstrated in such animals (Sireteanu et al., 1993). Differences were seen in the extents of connections of cortical eye dominance columns between these groups of cats. Elimination of diplopia and non-correspondence of the two retinal images in kittens of these groups is suggested to be mediated by different mechanisms: alternation of the eyes on fixation or suppression of the activity of the deviant eye.

Neuronal connections of eye-dominance columns in the cat cerebral cortex after monocular deprivation

Plastic changes in intrahemisphere neuronal connections of the eye-dominance columns of cortical fields 17 and 18 were studied in monocularly deprived cats. The methodology consisted of microintophoretic administration of horseradish peroxidase into cortical columns and three-dimensional reconstruction of the areas of retrograde labeled cells. The eye dominance of columns was established, as were their coordinates in the projection of the visual field. In field 17, the horizontal connections of columns receiving inputs from the non-deprived eye via the crossed-over visual tracts were longer than the connections of the "non-crossed" columns of this eye and were longer than in normal conditions; the connections of the columns of the deprived eye were significantly reduced. Changes in the spatial organization of horizontal connections in field 17 were seen for the columns of the non-deprived eye (areas of labeled cells were rounder and the density of labeled cells in these areas were non-uniform). The longest horizontal connections in deprived cats were no longer than the lengths of these connections in cats with strabismus. It is suggested that the axon length of cells giving rise to the horizontal connections of cortical columns has a limit which is independent of visual stimulation during the critical period of development of the visual system.

Exuberance in the development of cortical networks

The cerebral cortex is the largest and most intricately connected part of the mammalian brain. Its size and complexity has increased during the course of evolution, allowing improvements in old functions and causing the emergence of new ones, such as language. This has expanded the behavioural and cognitive repertoire of different species and has determined their competitive success. To allow the relatively rapid emergence of large evolutionary changes in a structure of such importance and complexity, the mechanisms by which cortical circuitry develops must be flexible and yet robust against changes that could disrupt the normal functions of the networks.

Horizontal Interactions in Cat Striate Cortex: II. A Current Source-Density Analysis

The current source-density (CSD) analysis was used to investigate the organization of tangential synaptic connections in primary visual cortex of normally reared (NR) kittens and of NR, binocularly deprived (BD) and dark-reared (DR) adult cats. Laminar profiles of field potentials, elicited by intracortical microstimulation were measured at various distances from the stimulating electrodes. To exclude contamination by axon collaterals of antidromically stimulated thalamo-cortical fibres, these were destroyed by injecting the cytotoxin N-methyl-D-aspartate (NMDA) into the lateral geniculate nucleus 13 - 27 days before recording. The CSD profiles revealed distinct layer-specific patterns of lateral spread of activity. Invariably, the most prominent, long-lasting and far reaching responses were recorded in supragranular layers. Responses in layer IV were brief and confined to the vicinity of the stimulation site. Responses in infragranular layers spread as far as those in supragranular layers, but were of smaller amplitude. Latency considerations, the results of double shock stimulation, and the effects of translaminar cuts, suggest that these responses were monosynaptic and mediated by intracortical pathways with a conduction velocity of 0.3-5 m/s. The spatial spread of these responses changed substantially with age but was not influenced by visual deprivation. In NR adults, supra- and infragranular responses were recordable up to 2.5 mm from the stimulation site and layer IV responses up to 1 mm from the stimulation site. In kittens, the former responses spread up to 5 mm and the latter up to 2 mm from the site of stimulation. The amplitude of the responses decreased with distance from the stimulation site. This decrease was not always monotonic suggesting inhomogeneities in the tangential projections. The laminar distribution of current sinks and sources indicates that the pathways mediating tangential interactions form excitatory synapses on apical dendrites of pyramidal cells. It is concluded that the spatial spread of tangential excitatory interactions decreases with age, but that neither the laminar pattern nor the age-dependent reduction in the strength of tangential interactions are influenced by visual deprivation.

Horizontal Interactions in Cat Striate Cortex: I. Anatomical Substrate and Postnatal Development

The system of tangential connections was studied in area 17 of normally reared (NR), binocularly deprived (BD) and dark-reared (DR) kittens and adult cats. Connections were labelled antero- and retrogradely by intracortical micro-injections of several fluorescent markers and horseradish peroxidase conjugated with wheat-germ agglutinin (WGA-HRP). In 5-day-old kittens tangential connections consist of homogeneously distributed fibres extending maximally over 2.7 mm. Around postnatal day (pnd) ten these connections start to express the patchy pattern characteristic of the adult. Retrogradely stained somata and anterogradely labelled terminals become organized in individual 300 to 350 microm wide clusters with a centre-to-centre spacing of about 500 microm. During the first three postnatal weeks the horizontal connections increase their span to up to 10.5 mm and the spacing between individual patches increases to about 700 microm. Over the following 4 weeks these projections become reduced in length and number. In adult NR cats, tangential connections span a distance of up to 3 mm and form a lattice of 200 - 500 microm wide clusters, which have an average centre-to-centre spacing of 1050 microm. Tangential connections originate and terminate in all cortical laminae except layer I and they are organized in register. The distances spanned are largest in supragranular, intermediate in infragranular and shortest in granular layers. In BD and DR cats older than 10 weeks, the length of intracortical tangential fibres becomes reduced to the same extent as in NR animals, but individual clusters are less numerous. The authors conclude that the lattice-like structure of lateral connections evolves independently of visual experience, and that the selectivity of interactions results from pruning of initially exuberant connections. It is suggested that this pruning process is dependent on activity and influenced by visual experience.

Monocular focal retinal lesions induce short-term topographic plasticity in adult cat visual cortex

Electrophysiological recording in primary visual cortex (VI) was performed both prior to and in the hours immediately following the creation of a discrete retinal lesion in one eye with an argon laser. Lesion projection zones (LPZs; 21-64 mm2) were defined in the visual cortex by mapping the extent of the lesion onto the topographic representation in cortex. There was no effect on neuronal responses to the unlesioned eye or on its topographic representation. However, within hours of producing the retinal lesion, receptive fields obtained from stimulation of the lesioned eye were displaced onto areas surrounding the scotoma and were enlarged compared with the corresponding field obtained through the normal eye. The proportion of such responsive recording sites increased during the experiment such that 8-11 hours post-lesion, 56% of recording sites displayed neurons responsive to the lesioned eye. This is an equivalent proportion to that previously reported with long-term recovery (three weeks to three months). Responsive neurons were evident as far as 2.5 mm inside the border of the LPZ. The reorganization of the lesioned eye representation produced binocular disparities as great as 15 degrees, suggesting interactions between sites in VI up to 5.5 mm apart.

Dual action of a ligand for Eph receptor tyrosine kinases on specific populations of axons during the development of cortical circuits

The structural basis of cortical columns are radially oriented axon collaterals that form precise connections between distinct cortical layers. During development, these connections are highly specified from the initial outgrowth of collateral branches. Our previous work provided evidence for positional cues confined to individual layers that induce and/or prevent the formation of axon collaterals in specific populations of cortical neurons. Here we demonstrated with in situ hybridization techniques that mRNA of the Eph receptor tyrosine kinase EphA5 and one of its ligands, ephrin-A5, are present in distinct cortical layers, at a time when intrinsic connections are being formed in the cortex. Axonal guidance assays indicate that ephrin-A5 is a repellent signal for a populations of axons that in vivo avoid the cortical layer expressing ephrin-A5. In contrast to its established role as a repulsive axonal guidance signal, ephrin-A5 specifically mediates sprouting of those cortical axons that target the ephrin-A5-expressing layer in vivo. These results identify a novel function of ephrin-A5 on axonal arbor formation. The laminar distribution and the dual action on specific populations of axons suggest that ephrin-A5 plays a role in the assembly of local cortical circuits.

Development and plasticity of local intracortical projections within the vibrissae representation of the rat primary somatosensory cortex

Labelling with 1,1'-dioctadecyl-3,3,3',3'-tetramethylindocarbocyanine perchlorate (Di-A) was used to assess the development of projections within the primary somatosensory cortex (SI) of rats aged between postnatal day 2 and 8 (P-2 and P-8). 1,1'-Dioctadecyl-3,3,3,"3'-tetramethylindocarbocyanine perchlorate (Di-I) was used in these same animals to label thalamocortical afferents. Particular attention was paid to the emergence of lamina IV intracortical projections that form a pattern complementary to vibrissae-related thalamocortical afferents. A vibrissae-related pattern of Di-A-labelled cells and fibers that was restricted largely to the septa regions was not apparent in rats killed on P-2, but it was visible in animals killed on P-4 and later ages. Tracing with biotinylated dextran amine (BDA) was used to assess intra-SI projections of adult rats that sustained transection of the infraorbital nerve (ION) on P-0 or P-7 or implantation of a tetrodotoxin (TTX)-impregnated polymer chip over the cortex on P-0. Rats that sustained ION transection on P-7 or that had TTX implants demonstrated normal patterns of projections within SI. The patterns of labelling in the supra- and infragranular layers of the cortices of the rats that sustained ION transection on P-0 were generally similar to those in the other groups evaluated. However, in lamina IV, there was no organization that could be related to the distribution of the vibrissae. These results indicate that the vibrissae-related pattern of intracortical projections within SI develops shortly after birth and that two manipulations that alter cortical activity, but not the patterning of thalamocortical afferents (application of TTX and transection of the ION after thalamocortical afferent patterns are established), have no significant effect on it. However, a manipulation that alters thalamocortical development (transection of the ION on P-0) profoundly affects the patterning of intracortical connections.

Specification of layer-specific connections in the developing cortex

One of the basic tasks of neurobiology is to understand how the precision and specificity of neuronal connections is achieved during development. In this paper we reviewed some recent in vitro studies on the developing mammalian cerebral cortex that have been made towards this end. The results of these experiments provided evidence that membrane-associated molecules are instrumental for the formation of specific afferent and efferent cortical projections. Substrate-bound molecules guide growing axons towards their target, regulate the timing of thalamocortical innervation and mediate target cell recognition. Moreover, a newly described glycoprotein, defined by a monoclonal antibody, revealed a molecular heterogeneity in the developing white matter. Since this molecule has opposite effects on thalamic and cortical axons, it might play a role in the segregation of axons running to and from the cortex. Substrate-bound cues are important during the formation of local cortical circuits. In vitro assays demonstrated that molecular components confined to individual cortical layers control the laminar specificity of cortical axon branching. This suggests that similar developmental strategies contribute to the laminar specification of extrinsic and intrinsic cortical circuits. Thus substrate-bound molecules might provide the framework for subsequent activity-dependent mechanisms that control the elaboration of precise connections between the cortical columns. A major challenge ahead is to identify the factors that mediate these processes and to determine their mode of action. Recently, two families of proteins, the netrins and the semaphorins/collapsins, have been identified as growth cone signals in the developing spinal cord (reviewed in Goodman, 1994; Colamarino and Tessier-Lavigne, 1995a; Dodd and Schuchardt, 1995; Kennedy and Tessier-Lavigne, 1995). Semaphorins/collapsins appear to regulate axonal guidance by repelling growth cones and by inhibiting axonal branching and synapse formation. Originally, netrins have been purified as diffusible chemoattractants for commissural axons of the dorsal spinal cord, but it is now well established that they can also function as chemorepellent factors for other classes of neurons. Since netrins are related to extracellular matrix components and since they can bind to the cell surface, they might also act as local guidance cues. A possible role of netrins and semaphorins/collapsins in the development of cortical connections is likely to be resolved in the near future. The identification of the factors that regulate specific branching patterns of cortical neurons might provide a better understanding of cortical development, but it might also be relevant to some aspects of plasticity and repair in the adult cortex.

Handedness in strabismics. Etiological implications

The incidence of non right handedness was found to be significantly higher in a group of 228 strabismics when compared with a control group of neurologically normal non strabismic subjects. The difference was still present after elimination of strabismics with known neurological or general health problems, or who were born prematurely. The strabismics showed no inter group differences in handedness in relation to the type of retinal correspondence present or the ability to achieve normal or abnormal binocular single vision. Evidence is reviewed for the presence of a developmental defect leading to strabismus, and results of this survey support a previously proposed idea that a developmental defect leads to strabismus, the type of which reflects the severity of the defect (although external factors may also be an influence).

Visual callosal connections and strabismus

Strabismus is a condition that exists when the visual axes of the two eyes fail to intersect at the fixation point under binocular viewing conditions. When it occurs in mammals during the critical period which corresponds to the period of maximal plasticity early in life, strabismus is known to induce both morphological anomalies and abnormal connections from the retina to the cortex; it further leads to binocular neural changes and to spatial vision deficits, especially at the cortical level. After a brief review of the already known data about the consequences of early strabismus in cats, monkeys and humans on the development of the visual system and of visual perception, new data are presented here concerning interhemispheric connections in the cat. In normally-reared kittens, visual callosal transfer is shown to be almost adult-like as soon as 12 days after birth: it is almost limited to the 17/18 border of the visual cortex when using visual stimulations in spite of the presence of still numerous juvenile exuberant callosal projections. In contrast, callosal transfer of visual information is extended to both areas 17 and 18 after strabismus, leading to the conclusion that at least some juvenile exuberant callosal projections are not only anatomically but also functionally stabilized after such an oculomotor disease. The possibility that similar abnormalities might be present in monkeys and humans is discussed.

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.

Dynamics of orientation tuning in the cat striate cortex neurons

Dynamics of orientation tuning was studied by a flashing light bar in 233 single units of cat primary visual cortex. Orientation tuning was plotted by the criterion of spike number in successive fragments of responses with a step of 10 or 20 ms. Preferred orientation and the width of orientation tuning were measured for each of such curves. It was found that successive dynamic change of preferred orientation during response was typical for 63% of units under study, while such change of orientation tuning width was typical for 93%. Neurons with or without shift in preferred orientation differed in their response characteristics: latency and duration of discharge, its frequency and response rise time, as well as quality of orientation tuning. Possible functional significance of two groups of cortical units, "scanners" (with systematic preferred orientation shift) and "timers" (without it), is discussed in accordance with the idea of spatiotemporal orientation coding in the primary visual cortex.

The contribution of intracortical connections to horizontal spread of activity in the neocortex as revealed by voltage sensitive dyes and a fast optical recording method

Coronal slices from guinea-pig visual neocortex were stained with voltage-sensitive fluorescence dyes RH414 or RH795. Activity was evoked by electrical stimulation of either the white matter or layer I. Emitted light intensity changes representing summated changes of membrane potential were recorded by a 10 x 10 photodiode array with a temporal resolution of 0.4 ms and a spatial resolution of 94 microns. The distribution and spread of activity in the horizontal direction was analysed. Following stimulation of the white matter or layer I, two regions of activity were differentiated in the medio-lateral direction: a central region (approximately 1 mm wide) of high-amplitude activity close to the stimulation electrode and, distant from the stimulation electrode, peripheral regions of low-amplitude activity. Central and peripheral regions differed in their rates of decline, their relative extent with stimulation of different sites and within different layers. The total extent of non-synaptic evoked activity did not exceed that of the central region of high-amplitude activity. Along the extent of non-synaptic activity, onset latencies of potentials were almost constant. Thus, activity of high amplitude in the central region was likely mediated by simultaneous activation of distributed afferent fibres. In contrast, no non-synaptic activity was found in peripheral regions. Therefore it is suggested that this low-amplitude activity was mediated without direct afferent activation but via long-distance intracortical horizontal pathways. These pathways are known to terminate in layer III, and accordingly latencies of responses in the periphery were shortest in upper cortical layers, whereas in the central region, latencies increased from lower to upper cortical layers.

Dynamics of responses of V1 neurons evoked by stimulation of different zones of receptive field

The dynamics of receptive fields of 73 neurons in area 17 of cat visual cortex were studied using the temporal slice method. Three-dimensional maps of the receptive fields were plotted using the criterion of spike number in successive fragments (step 10 or 20 ms) of responses to 100 local flashes presented at different parts of the receptive field in random order. The size and configuration of such dynamically recorded receptive fields were then estimated. This allowed us to reveal the dynamic reorganization of all receptive fields 20-400 ms after stimulation. A small zone of responses appeared in the receptive field after initial latency, then it widened, received definitive configuration, and after that decreased and disappeared. The effect was reproducible under repeated estimations. The relationships between receptive field and the previously described orientation tuning dynamics, as well as between dynamics of receptive fields and their summation zones, mechanisms and possible functional meaning of the revealed effects for signal processing in the primary visual cortex are discussed.

A role for glial cells in activity-dependent central nervous plasticity? Review and hypothesis

Activity-dependent plasticity relies on changes in neuronal transmission that are controlled by coincidence or noncoincidence of presynaptic and postsynaptic activity. These changes may rely on modulation of neural transmission or on structural changes in neuronal circuitry. The present overview summarizes experimental data that support the involvement of glial cells in central nervous activity-dependent plasticity. A role for glial cells in plastic changes of synaptic transmission may be based on modulation of transmitter uptake or on regulation of the extracellular ion composition. Both mechanisms can be initiated via neuronal-glial information transfer by potassium ions, transmitters, or other diffusible factor originating from active neurons. In addition, the importance of changes in neuronal circuitry in many model systems of activity-dependent plasticity is summarized. Structural changes in neuronal connectivity can be influenced or mediated by glial cells via release of growth or growth permissive factors on neuronal activation, and by active displacement and subsequent elimination of axonal boutons. A unifying hypothesis that integrates these possibilities into a model of activity-dependent plasticity is proposed. In this model glial cells interact with neurons to establish plastic changes; while glial cells have a global effect on plasticity, neuronal mechanisms underlie the induction and local specificity of the plastic change. The proposed hypothesis not only explains conventional findings on activity-dependent plastic changes, but offers an intriguing possibility to explain several paradoxical findings from studies on CNS plasticity that are not yet fully understood. Although the accumulated data seem to support the proposed role for glial cells in plasticity, it has to be emphasized that several steps in the proposed cascades of events require further detailed investigation, and several "missing links" have to be addressed by experimental work. Because of the increasing evidence for glial heterogeneity (for review see Wilkin et al., 1990) it seems to be of great importance to relate findings on glial populations to the developmental stage and topographical origin of the studied cells. The present overview is intended to serve as a guideline for future studies and to expand the view of "neuro" physiologists interested in activity-dependent plasticity. Key questions that have to be addressed relate to the mechanisms of release of growth and growth-permissive factors from glial cells and neuronal-glial information transfer. It is said that every complex problem has a simple, logical, wrong solution. Future studies will reveal the contribution of the proposed simple and logical solution to the understanding of central nervous plasticity.