Common noise in the firing of neighbouring ganglion cells in goldfish retina

PRANAS: A New Platform for Retinal Analysis and Simulation

The retina encodes visual scenes by trains of action potentials that are sent to the brain via the optic nerve. In this paper, we describe a new free access user-end software allowing to better understand this coding. It is called PRANAS (https://pranas.inria.fr), standing for Platform for Retinal ANalysis And Simulation. PRANAS targets neuroscientists and modelers by providing a unique set of retina-related tools. PRANAS integrates a retina simulator allowing large scale simulations while keeping a strong biological plausibility and a toolbox for the analysis of spike train population statistics. The statistical method (entropy maximization under constraints) takes into account both spatial and temporal correlations as constraints, allowing to analyze the effects of memory on statistics. PRANAS also integrates a tool computing and representing in 3D (time-space) receptive fields. All these tools are accessible through a friendly graphical user interface. The most CPU-costly of them have been implemented to run in parallel.

Signals and noise in an inhibitory interneuron diverge to control activity in nearby retinal ganglion cells

Information about sensory stimuli is represented by spatiotemporal patterns of neural activity. The complexity of the central nervous system, however, frequently obscures the origin and properties of signals and noise that underlie these activity patterns. We minimized this constraint by examining mechanisms governing correlated activity in mouse retinal ganglion cells (RGCs) under conditions in which light-evoked responses traverse a specific circuit, the rod bipolar pathway. Signals and noise in this circuit produced correlated synaptic input to neighboring On and Off RGCs. Temporal modulation of light intensity did not alter the degree to which noise in the input to nearby RGCs was correlated, and action potential generation in individual RGCs was largely insensitive to differences in network noise generated by dynamic and static light stimuli. Together, these features enable noise in shared circuitry to diminish simultaneous action potential generation in neighboring On and Off RGCs under a variety of conditions.

Remodelling of early axonal projections through the selective elimination of neurons and long axon collaterals

Studies using neuroanatomical techniques have shown that the connections characteristic of the mature vertebrate brain are brought about by a considerable refinement of the projections initially established during development. The selective loss of neurons and long axon collaterals plays a major role in this remodeling process as illustrated in the development of the retina and cortex of the rat. In the retina, two-thirds of the initial population of ganglion cells (RGCs) die early. This loss serves to remove selectively RGCs that make erroneous axonal projections, including those which project to an incorrect target, to an inappropriate part of a correct target, or to the wrong side of the brain. Studies using the sodium channel blocker, tetrodotoxin, suggest that in rats the selective elimination of erroneously projecting RGCs is based, in part, on patterns of impulse activity. In the cortex a different mechanism is illustrated. All neocortical areas initially give rise to callosal and pyramidal tract axons but through a process of selective collateral elimination not involving cell death these projections assume the limited distributions seen in adult rats. Manipulations resulting in the maintenance of such long collaterals suggest that their removal is functionally and locally determined. In contrast to error elimination, this phenomenon of collateral elimination may be a developmental strategy for generating connectional diversity while limiting the amount of information required for the regional specification of the cortex.

Variability in the firing of retinal ganglion cells of goldfish: a review

The isolated retina of the goldfish has proven a valuable resource for studying the variability of firing of retinal ganglion cells. Three major areas of study are considered here: the variability of maintained discharges, the correlated firing of neighboring ganglion cells, and the variability of responses to light. The sources of variability, its relationship to retinal processing, and its possible functional role in perception are examined through these three aspects of variability. The results are related to similar studies in mammals (mainly cats). This retrospective is biased toward my studies over 30 years.

The potential coding utility of intercell cross-correlations in the retina

The action potentials (impulses) produced by pairs of neighboring retinal ganglion cells often show a tendency either to fire in close temporal synchrony or to avoid temporal synchrony. This cross-correlation (a rate of "coincidences" that differs from that expected by chance) has been exploited as a window into retinal processing, but its possible functional significance has proven elusive. Previous work has failed to show that the coincidences serve as a direct code for visual stimuli. In this analysis it is shown that the coincidences serve neither as a key for reducing variability nor as a key for improving the coding by the individual cells. The residual impulse trains (trains with coincidences deleted) are more variable than the raw impulse trains and provide an inferior coding to that of the raw impulse trains. There is negative correlation between the firing rate of the residual impulse trains and that of the coincidence impulse trains, which is consistent with the lower variance of the raw impulse trains. There is no consistent cross-correlation between the rates of residual impulse trains of cells in pairs showing cross-correlation; however, it is found that this observation does not discriminate among models for generating coincidences.

Firing coincidences between neighboring retinal ganglion cells: inside information or epiphenomenon?

Retinal ganglion cells often fire impulses in synchrony; is this synchronization an irrelevant by-product of processing shared inputs, or does it encode information? We examined the rate of occurrence of coincident impulses from pairs of ganglion cells responding to stimuli that varied along several dimensions. We find that coincidences convey little if any additional information about simple static stimuli beyond what could be determined from the firing rates of the two cells considered separately. In fact, at least one of the separate cells generally provided a better information channel than the coincidence rate, implying that under these conditions ganglion cells do not employ a strategy of encoding by coincidences.

Spontaneous retinal activity is tonic and does not drive tectal activity during activity-dependent refinement in regeneration

During development, waves of activity periodically spread across retina to produce correlated activity that is thought to drive activity-dependent ordering in optic fibers. We asked whether similar waves of activity are produced in the retina of adult goldfish during activity-dependent refinement by regenerating optic fibers. Dual-electrode recordings of spontaneous activity were made at different distances across retina but revealed no evidence of retinal waves in normal retina or during regeneration. Retinal activity was tonic and lacked the episodic bursting associated with waves. Cross-correlation analysis showed that the correlated activity that was normally restricted to near neighbors (typically seen across 100-200 microm and absent at >500 microm) was not altered during regeneration. The only change associated with regeneration was a twofold reduction in ganglion cell firing rates. Because spontaneous retinal activity is known to be sufficient to generate refinement during regeneration in goldfish, we examined its effect on tectal activity. In normal fish, acutely eliminating retinal activity with TTX rapidly reduced tectal unit activity by >90%. Surprisingly, during refinement at 4-6 weeks, eliminating retinal activity had no detectable effect on tectal activity. Similar results were obtained in recordings from torus longitudinalis. After refinement at 3 months, tectal activity was again highly dependent on ongoing retinal activity. We conclude that spontaneous retinal activity drives tectal cells in normal fish and after regeneration but not during activity-dependent refinement. The implications of these results for the role of presynaptic activity in refinement are considered.

Plasticity in the tectum of Xenopus laevis: binocular maps

Xenopus frogs exhibit dramatic changes in the binocular projections to the tectum during a critical period of development. Their eyes change position in the head, moving from lateral to dorsal and creating an increasing region of binocular overlap. There is a corresponding shift of binocular projections to the tectum that keeps the two eyes' maps in register with each other throughout this period. The ipsilateral input is relayed via the nucleus isthmi. Two factors bring the ipsilateral projection into register with the contralateral projection. First, chemoaffinity cues establish a crude topographic map beginning when the shift of eye position begins. Approximately 1 month later, visual cues bring the ipsilateral map into register with the contralateral map. The role of visual input is demonstrated by the ability of the axons that bring the ipsilateral eye's map to the tectum to reorganize in response to a surgical rotation of one eye and to come into register with the contralateral eye's map. This plasticity can be blocked by NMDA receptor antagonists during the critical period. In normal adults, reorganization is minimal. Eye rotation fails to induce reorganization of the ipsilateral map. However, plasticity persists indefinitely in animals that are reared in the dark, and plasticity can be restored in normally-reared animals by treatment with NMDA. The working model to explain this plasticity posits that correlated input from the two eyes triggers opening of NMDA receptor channels and initiates events that stabilize appropriately-located isthmotectal connections. Specific tests of this model are discussed.

Cross-correlation between neurons: a source of information about the nervous system

Impulse trains of spiking neurons are a stochastic process; however, the impulse trains of related neurons are generally not statistically independent. Cross-correlation may be due to either a common input that affects the firing of both cells, or to direct synaptic influence of the one neuron upon the other. By analyzing the distributions of intervals preceding and those following the coincident firings relative to that of all the intervals between impulses in each train, it is possible to infer restrictions upon the ways in which the correlating influence interacts with other sources of variability for each cell. This is applied to pairs of cells in two systems, one in which the coincident firing is due to an input common to the two cells, and one in which one cell is presynaptic to the other.

An analysis of the cross-correlation between ganglion cells in the retina of goldfish

Neighboring ganglion cells in the retinae of vertebrates show cross-correlation between their impulse trains. Cross-correlation is found both in maintained discharges and during responses to visual stimulation. There has been speculation about the function of this statistical dependence, but little is known about its genesis. This study examines the statistics of the interimpulse intervals preceding and those following impulses that coincide with an impulse in the other train. Short intervals are rarer than expected preceding a coincidence, regardless of the form of the cross-correlation. Short intervals are more common than expected following a coincidence when the cross-correlation is positive, but rarer than expected following coincidences during negative cross-correlation. These results contradict the extant models for cross-correlation, but may be explained by the multiplicative combination of a variable common input and the variability within each cell. In addition, the lag (relative timing of coincident impulses in the two cells) is found to be related to the maintained discharge rates of the cells, implying that the lags may be explained without invoking specific delay circuits.

Concerted signaling by retinal ganglion cells

To analyze the rules that govern communication between eye and brain, visual responses were recorded from an intact salamander retina. Parallel observation of many retinal ganglion cells with a microelectrode array showed that nearby neurons often fired synchronously, with spike delays of less than 10 milliseconds. The frequency of such synchronous spikes exceeded the correlation expected from a shared visual stimulus up to 20-fold. Synchronous firing persisted under a variety of visual stimuli and accounted for the majority of action potentials recorded. Analysis of receptive fields showed that concerted spikes encoded information not carried by individual cells; they may represent symbols in a multineuronal code for vision.

Spontaneous bursting and long-lived local correlation in normal and denervated tectum of goldfish

The formation of fine retinotopic order by growing optic fibers in the goldfish is thought to be mediated by the correlated firing of optic fibers from neighboring retinal ganglion cells. Although the activity of the tectal cells must also be important for this activity-dependent refinement, few studies have analyzed the pattern and local correlation of the intrinsic activity of tectal neurons and the effect of denervation on this activity. To address this issue, spontaneous (nonoptic driven) activity was analyzed and cross-correlograms were computed between individual tectal neurons using single and double electrode extracellular recordings. Recordings were made in normally innervated tectum in which the contribution of optic activity was eliminated by short-term intraocular blockade with tetrodotoxin and in denervated tecta in which the optic nerve had been severed several weeks prior. Several observations were relevant to activity-dependent refinement: First, coupling between neighboring tectal cells is weak. Second, the time duration for local correlation is relatively long, as long as 200 ms. Third, tectal neurons exhibit spontaneous bursting. Fourth, denervation increased the level of spontaneous activity in the tectum. The increased spontaneous activity and bursting following denervation implies that tectal neurons are more excitable when optic fibers are beginning to reinnervate the tectum. This could make it possible for optic fibers to drive tectal neurons at a time when their input to individual neurons is severely weakened by a lack of spatial convergence. The weak coupling between tectal cells and the consequent long-time constant for correlated activity implies a constraint on the duration of correlated retinal activity that is used for activity-dependent refinement. Since optic fibers likely need to detect the postsynaptic activity of a local group of tectal neurons, rather than that of a single neuron, the long tectal time constant means that retinal activity need not be correlated with precision much better than 200 ms because the postsynaptic circuitry cannot generate shorter correlations.

Rapid remodeling of retinal arbors in the tectum with and without blockade of synaptic transmission

Dynamic rearrangements of axon terminal arbors may be critical for establishing appropriate connections in the developing nervous system. Here, the changes in complex retinal axon arbors in the tecta of live Xenopus larvae were followed during the formation of the topographic retinotectal projection. Three-dimensional reconstructions of terminal arbors made with a confocal microscope at hourly intervals revealed rapid remodeling of arbor extensions. Shorter branches were extended and retracted very rapidly, suggesting that they probe the environment for the optimal sites to form stable branches. About 27% of longer branches were present throughout the entire observation period and may be sites of stabilized synaptic contacts. Treatment of the animals to block postsynaptic activity resulted in increased rates of arbor rearrangements, which may coincide with decreased synapse stability. These studies reveal the dynamic behavior of nerve arbors and provide estimates for the lifetimes of retinotectal branches.

Topography and ocular dominance: a model exploring positive correlations

The map from eye to brain in vertebrates is topographic, i.e., neighbouring points in the eye map to neighbouring points in the brain. In addition, when two eyes innervate the same target structure, the two sets of fibres segregate to form ocular dominance stripes. Experimental evidence from the frog and goldfish suggests that these two phenomena may be subserved by the same mechanisms. We present a computational model that addresses the formation of both topography and ocular dominance. The model is based on a form of competitive learning with subtractive enforcement of a weight normalization rule. Inputs to the model are distributed patterns of activity presented simultaneously in both eyes. An important aspect of this model is that ocular dominance segregation can occur when the two eyes are positively correlated, whereas previous models have tended to assume zero or negative correlations between the eyes. This allows investigation of the dependence of the pattern of stripes on the degree of correlation between the eyes: we find that increasing correlation leads to narrower stripes. Experiments are suggested to test this prediction.

Changing patterns of binocular visual connections in the intertectal system during development of the frog, Xenopus laevis. III. Modifications following early eye rotation

In the frog, Xenopus laevis, a system of intertectal connections underlies the visual projection from an eye to its ipsilateral tectal lobe and is involved in the topographic representation of binocular visual space. Rotation of one eye in early life may be followed by a radical rearrangement of the connections in this system. The modified pattern which later emerges is that which keeps the visual projection through the ipsilateral eye in topographic registration with the direct visual projection from the contralateral eye to the same tectal lobe. This plasticity requires visual experience. In this paper we describe the time-course and sequence of events by which this plasticity is effected. Following rotation of one eye in larval animals or in animals undergoing metamorphic climax, the earliest evidence of intertectal modification was found 3-4 weeks after metamorphosis. With increasing intervals after metamorphosis an increasing proportion of animals displayed modified intertectal systems. At intermediate intervals many animals showed partial modifications, which were interpreted as transitional stages in the modification process. Analysis of these transitional stages indicated that the sequence of events involved in the elaboration of a modified intertectal system following the experimental alteration of eye alignment exhibits features in common with rearrangements of the system that occur during normal development in response to growth-related alterations in eye alignment.

Complicated substructure from simple circularly symmetric Gaussian processes within the centers of goldfish ganglion cell receptive fields

The center of the receptive field of some retinal ganglion cells exhibits an interesting fine structure: the relative amplitudes of responses to onset and responses to offset of a small spot of light varies systematically as the spot is positioned at various places within the center. Although this pattern may appear complicated, a simple model can account for it in detail. The model postulates that the ganglion cell receives input from separate ON and OFF processes within the center of its receptive field. These processes have the form of Gaussian functions and are laterally displaced from each other. These central ON and OFF input processes are not associated with the additional antagonistic surround of the receptive field. The model is examined for various parameters of the input processes. The observed systematic variation in the ratio of offset to onset responses is predicted when the two processes are of nearly equal width (standard deviation of the Gaussians). Receptive fields made of more than two Gaussians produce various patterns, depending on the relative standard deviations of the Gaussians. Oblong fields, reminiscent of those found in visual cortex, may be generated from a relatively small number of circularly symmetric Gaussian processes.

The effect of TTX-activity blockade and total darkness on the formation of retinotopy in the goldfish retinotectal projection

In the normal goldfish, neighboring retinal ganglion cells terminate in one small tectal locus to produce the precise retinotopy characteristic of this projection. This can be directly demonstrated by labeling neighboring ganglion cells with small "spot" injections of WGA-HRP, which yield a single small patch of product at the retinotopically appropriate part of the tectum. When the optic nerve is crushed, label from these spot injections was previously found to be widely dispersed during the early phase of regeneration. With time, label subsequently condensed, typically into several discrete patches reminiscent of ocular dominance columns. In this study, we tested whether the formation of these patches required impulse activity by injecting tetrodotoxin (TTX) into the eye during regeneration. We found that impulse blockade completely inhibited the formation of discrete patches while permitting considerable condensation of the label. This implies that these patches are generated by activity but that some map "refinement" utilized cellular processes that are activity independent. This activity-independent condensation progressed at a noticeably slower rate than the equivalent condensation seen with activity, thus suggesting that activity normally participates as a "helper factor," even though it is not strictly required. Since the formation of discrete patches during regeneration provides a sensitive measure of activity-dependent refinement, this was used to further address two controversial questions concerning the role of impulse activity. One is whether there is a chronologically defined critical period for activity-dependent refinement. This was tested by blocking impulse activity for 2 to 4 months, much longer than the activity-dependent refinement is thought to last, and then permitting activity to resume. We found that multiple patches were formed following this period of late activity, thus indicating that synaptic plasticity extends for several months beyond the supposed critical period. The other question was whether spontaneous retinal activity was sufficient for activity-dependent ordering. To test this, fish were kept in constant darkness during optic nerve crush and labelled with retinal spot injections at various times during regeneration. Condensation of label with the final formation of multiple patches formed at about the same time as fish with normal visual experience. This implies that the amount and extent of correlation of spontaneous activity in retina is adequate for driving activity-dependent refinement.

Activity-driven sharpening of the regenerating retinotectal projection: effects of blocking or synchronizing activity on the morphology of individual regenerating arbors

Both blocking activity with intraocular tetrodotoxin (TTX) and synchronizing activity with a xenon strobe light (1 Hz) prevent retinotopic sharpening of regenerating optic projection in goldfish (Meyer, 1983; Schmidt, 1985; Cook and Rankin, 1986). In this study, we tested, in both normal and regenerating projections, the effects of these two treatments on individual optic arbors. Arbors were stained via anterograde transport of HRP, drawn in camera lucida from tectal whole mounts, and analyzed for spatial extent in the plane of the retinotopic map, order of branching, number of branch endings, depth of termination, and the caliber of the parent axon. In normal tectum, fine, medium, and coarse caliber axons gave rise to small, medium, and large arbors, which averaged 127 microns, 211 microns and 275 microns in horizontal extent, and terminated at characteristic depths. All three classes averaged roughly 21 branch endings. Optic arbors that regenerated with normal patterns of activity returned to a roughly normal appearance by 6-11 weeks postcrush: the same three calibers of axons gave rise to the same three sizes of arbors at the same depths, but they were much less stratified and well on average about 16% larger in horizontal extent. At this time point, arbors regenerated under TTX or strobe were on the average 71 and 119% larger, respectively, than the control-regenerated arbors (larger in all classes), although they had approximately the same number of branch endings and were equally poorly stratified. Synapses formed under strobe were also normal in appearance. Thus the only significant effect of both strobe and TTX treatment was to enlarge the spatial extent of arbor branches. Arbors that were not regenerating were very slightly (but significantly) enlarged by TTX block of activity or strobe illumination. As previous staining showed that regenerating axons initially make widespread branches and later retract many of those branches (Schmidt, Turcotte, Buzzard, and Tieman, 1988; Stuermer, 1988), the present findings support the idea that blocking activity or synchronizing activity prevents retinotopic sharpening by interfering with the elimination of some of the errant branches.

Spontaneous Activity as a Determinant of Axonal Connections

To investigate the role of spontaneous retinal activity in map refinement, we studied goldfish kept in darkness during regeneration of a cut optic nerve. In one experiment, such fish (with lenses ablated to blur vision) were maintained for 70 days in stroboscopic light, diurnal light, or total darkness interrupted daily by 15 minutes of stroboscopic light. The retinotectal projection was then assessed for retinotopy by standard methods, using retrograde transport of wheat germ agglutinin-horseradish peroxidase. As in previous work, significantly more refinement was found in diurnal than in stroboscopic light. In darkness, refinement was as complete as in diurnal light. In a second experiment, similar fish were kept in stroboscopic light for 63 days. Some were then assessed to confirm that refinement had been delayed, while others were transferred to darkness or diurnal light for assessment later. After 7 days in either environment, no further refinement was seen; but after 21 days, substantial and significant refinement has occurred in both. Thus the effects of darkness and diurnal light were indistinguishable, and very different from those of stroboscopic light and (in previous studies) tetrodotoxin. Map refinement is evidently activity-dependent but not experience-dependent, and can effectively use the correlated spontaneous firing of neighbouring ganglion cells as its basis. Locally correlated spontaneous activity, which appears also to drive eye- and class-specific axon segregation in mammals, occurs widely in the nervous system. It could potentially generate systematic interconnection patterns even between neuronal populations without an overtly topographic organization.

NMDA receptor antagonists disrupt the retinotectal topographic map

We tested the effect of two NMDA receptor antagonists, APV or MK801 (with NMDA), and the receptor agonist NMDA on the maintenance of retinal topography in frogs. Topography was assayed by measuring the dispersion of retrogradely labeled ganglion cells following a local HRP injection into the tectum. In untreated tadpoles, labeled cells covered about 5% of the retinal area. In APV- or MK801/NMDA-treated tadpoles, labeled ganglion cells covered 17% and 10% of the retinal area, respectively. Neither treatment with L-APV nor with NMDA disrupts the fidelity of the retinotectal projection. Neither APV- nor NMDA-treated ganglion cell terminals differed from untreated terminals with respect to tangential area, branch number, or branch density. These data support a role for the NDMA receptor in visual system development.

Correlated firing of retinal ganglion cells

Even in the absence of visual stimulation, retinal ganglion cells have a substantial maintained discharge. This maintained discharge is not generated independently within each ganglion cell, because the unstimulated activity of two neighboring ganglion cells can be remarkably correlated. These correlations show that two such cells respond together to strong, spontaneous signals from more distal retinal neurons and that, in some cases, ganglion cells even have effects on each other. Observations of correlated firing can give insights not only into the sources of maintained activity but also into retinal connections and signal processing. Correlating firing at the retinal level also has important implications for the use of correlation analysis to study connections between cells in higher visual centers. Much recent attention has focused on the role that correlated firing may play in forming appropriate, ordered connections to a target structure.

Activity sharpens the regenerating retinotectal projection in goldfish: sensitive period for strobe illumination and lack of effect on synaptogenesis and on ganglion cell receptive field properties

The regenerating optic nerve of goldfish first reestablishes a rough retinotopic map on the contralateral tectum and then sharpens it. Disruption of visual activity, either by blocking activity with intraocular tetrodotoxin (TTX; Schmidt and Edwards, 1983) or by synchronizing activity with strobe illumination (Schmidt and Eisele, 1985), disrupts the sharpening process: the map is correctly oriented but the multiunit receptive fields at each point average 25-40 degrees in diameter. In order to test whether strobe and TTX interfere with the same mechanism, we have tested whether their sensitive periods are the same, and whether strobe, like TTX treatment, does not affect either ganglion cell receptive field properties or synaptogenesis. In parallel studies, we exposed fish to 2 weeks of either strobe illumination or intraocular TTX beginning at various times after crush and determined via electrophysiological recordings that the periods of sensitivity were nearly identical. There was no effect of either treatment during the first 2 weeks (before the fibers arrive at the tectum), maximal disruption of sharpening between 14 and 50 days (the period of rapid synaptogenesis), decreasing disruption between 50 and 125 days, and no effect beyond that point or in the normal projection. In addition, long strobe exposures of up to 142 days produced no greater disruptions than shorter 2-3-week exposures, indicating no cumulative effect. The reestablishment of synaptic transmission in tectum, assayed by recording field potentials elicited by optic nerve shock, was not affected by stroboscopic illumination. Finally, individual ganglion cells, recorded intraretinally following long-term strobe exposure, had receptive fields that were normal both in size and in their characteristic responses to light-on, to light-off, or to both on and off. These findings support the hypothesis that strobe-like TTX prevents retinotopic refinement by preventing the correction of errors initially made by the ingrowing optic axons (Schmidt et al., 1988).

Locally correlated activity in the goldfish tectum in the absence of optic innervation

Despite a substantial literature on the role of correlated presynaptic activity in the patterning of nerve connections during synaptogenesis in the central nervous system, few studies have focuses on postsynaptic activity during this process. To address the possibility that the target exhibits correlated activity independently of presynaptic activity, extracellular activity was recorded from the main optic innervation layer stratum fibrosum et griseum superficiale (SFGS) in goldfish in which the optic nerve was crushed or the eye removed. At about 2 weeks after denervation, multiunit recordings revealed phasic temporally correlated discharge between different tectal units. Auto-correlation analysis of these trains showed a broad peak 75-100 ms wide confirming temporal correlation. Using cross-correlation analysis of two simultaneous recordings at different distances across tectum, this correlation was shown to be local. Strong positive correlations were detected over about 200 micron and decrease with greater distances disappearing beyond about 400 micron. These correlograms showed a broad symmetrical peak about 75-100 ms wide. This pattern of activity persisted from the day following nerve crush into the period of activity dependent reinnervation at 1 month. When the eye was removed, the pattern could be demonstrated for up to 3 months of denervation indicating the circuitry responsible for the correlated activity was quite stable in the absence of optic innervation. We conclude that tectal elements are capable of locally correlated discharge independently of optic innervation. We propose that locally correlated discharge represents cooperative groups of tectal cells and that these groups, rather than single cells, are the target of the activity dependent synaptic rearrangement such as ocular dominance columns which occurs during synaptogenesis.

Cortical auditory neuron interactions during presentation of 3-tone sequences: effective connectivity

The role of cat's primary auditory cortex (AI) in both pattern discrimination and sound localization has been demonstrated by observing that ablations of it disrupt these functions. This research studied effective connectivity variations as a function of modifications in the temporal pattern of acoustic stimulation. Recordings of 10-15 neurons (simultaneously and separably) were made in AI of sedated cats. A bundle of 7 microelectrodes was stereotaxically placed along a tangential path. Stimuli were permutations of 3-tone bursts sequences. Each recorded neuron pair was analyzed off-line by cross-correlation. Cross-correlation of spike trains from neuron pairs showed signatures of direct and/or shared input. These appeared individually or in combination and for most pairs were present in spontaneous conditions. However, in stimulated conditions these spontaneous interactions were strongly modulated. The analysis detected differences in neuronal interaction during presentation of different tones. Similar differences occurred during presentation of any single particular stimulus if there was a history of different immediately previous tones. When individual neuron pair cross-correlations were put together to form an effective connectivity diagram among all recorded neurons, they turned out as different diagrams for different stimulus conditions.

Antibodies to ependymin block the sharpening of the regenerating retinotectal projection in goldfish

The regenerating optic nerve of goldfish first reestablishes a rough retinotopic map on the tectum, then goes through an activity dependent refinement that appears to involve the elimination of inappropriate branches from early regenerated arbors. Retinotopically appropriate branches and synapses may be stabilized because the normally correlated firing of neighboring ganglion cells could cause summation of their postsynaptic responses, making them more effective. Thus, refinement of the map may be similar in several ways to associative learning. In this study, we therefore tested whether ependymin, a major protein component of the extracellular fluid that has been implicated in synaptic changes thought to be associated with learning a simple task in goldfish, may also be involved in refinement of the retinotopic map. Goldfish that had undergone unilateral optic nerve crush received intraventricular infusion of antiependymin IgG or of control IgG's beginning at 21 days postcrush. Tectal recordings from these fish at 39-56 days postcrush showed that the projection had failed to sharpen, much as in the fish with activity blocked or synchronized; the average size of the multiunit receptive fields was 31 degrees vs 11 degrees normally. The field potentials elicited from these tecta by optic nerve shock were not significantly smaller than in controls, suggesting normal levels of synaptogenesis. Control projections, identically treated but infused with either unrelated IgG or Ringer's alone regenerated normally, giving multiunit receptive fields of 12 degrees. Intact (non-regenerating) projections of the experimental fish were not rendered abnormal by the IgG treatment. Histology showed the retinas and tecta of the infused fish to be normal in appearance. The results show a specific block of sharpening by antiependymin IgG. The ependymal glia of the tectum stain positively for ependymin in normal fish, particularly the cell bodies in the ependymal layer. The tectum, particularly the ependymal layer, stains more intensely during regeneration, which appears to trigger increased synthesis of ependymins in the ependymal glia. This increase and the block of sharpening by specific antibodies to ependymin suggest a possible role for ependymin in activity dependent synaptic stabilization, possibly through its polymerization when calcium is focally depleted at active synapses.

Responsivity and absolute sensitivity of retinal ganglion cells in goldfish of different sizes, when measured under "psychophysical" conditions

Retinal neurogenesis occurs in adult goldfish, and more rods are added to the retina than any other class of cell as the fish grows. To determine whether the disproportionate addition of rods affects the responsivity and sensitivity of dark adapted retinal ganglion cells, we recorded activity from optic tract fibers in goldfish of different sizes. Experimental conditions were as similar as possible to those used in a separate study in which psychophysical absolute thresholds were measured: large, dim, monochromatic spots 1 sec in duration were projected close to the right eye of alert, self-respiring goldfish. A total of 214 fibers were recorded in small (5.0-5.7 cm), medium (9.5-11.0 cm) and large (13.0-20.0 cm) fish. Neither maintained activity (mean and variance of the discharge rate in darkness) nor responsivity (quantum-to-spike ratios) nor absolute threshold (quantal irradiance required to produce a difference of 1 spike/trial from spontaneous rates) varied reliably with size of fish. However, some Off cells were more active in the dark than On and On/Off cells; these had low QSR's and absolute thresholds, and were found in all sizes of fish. Fifty percent (50%) of Off cells (compared to 8% of On cells) had thresholds comparable to or lower than psychophysical threshold, and Off cell thresholds (but not On cell thresholds) tended to be lower in larger fish. Because psychophysical threshold is closely related to the planimetric density of rods in goldfish, the similarity between Off cell threshold and psychophysical threshold suggests that Off cells may be influenced relatively more than On cells by the addition of new rods to the retina.

A sharp retinal image increases the topographic precision of the goldfish retinotectal projection during optic nerve regeneration in stroboscopic light

Locally-correlated neural activity appears to play a key role in refining topographically mapped projections. The retinotectal projection of the goldfish normally regains a high degree of spatial precision after regeneration of a cut optic nerve, but it fails to do so if retinal ganglion cell activity is blocked by tetrodotoxin, or if local correlations in activity are masked by the synchronizing effect of stroboscopic light. A sharp retinal image is not normally needed for a sharp map because local correlation occurs even in darkness or diffuse light, but the possibility that a sharp image might restore local correlation and sharpen the map in stroboscopic light, though taken into account in earlier experiments, has not previously been tested. The precision of the retinotectal map was therefore studied, by retrograde transport of WGA-HRP from a standard tectal injection site and quantitative analysis of the labelled ganglion cell distribution, after regeneration of a cut optic nerve for 83-84 days in either continuous stroboscopic light or normal diurnal light. The lens of the eye was either ablated to blur the retinal image or sham-operated. Two different strobe flash patterns used in previous experiments were also compared. With the lens ablated, stroboscopic light impaired map refinement significantly, confirming previous results. A rapid, irregular flash pattern averaging about 5 Hz was rather more effective than a regular 1 Hz pattern. With the lens intact, however, neither pattern had any detectable effect. The significant gain in precision resulting from a sharp retinal image in these circumstances suggests that common mechanisms could underlie both the internal refinement of the retinotectal map and such directly experience-sensitive processes as the experimental realignment of binocular maps in the frog Xenopus, and of auditory and visual maps in the barn owl.

Impaired refinement of the regenerated retinotectal projection of the goldfish in stroboscopic light: a quantitative WGA-HRP study

The retinotectal projection of the goldfish was studied after regeneration of a cut optic nerve in stroboscopic light, constant light or diurnal light, with the lens removed to blur the retinal image. Retrograde transport of wheatgerm agglutinin, conjugated to horseradish peroxidase, from a standard tectal injection site was used to measure the topographic precision of the projection. The dispersion of labelled retinal ganglion cells, which reflects this precision, was assessed by a method based on distance to nearest neighbour. In normal fish treated similarly, these cells are known to be clustered into about 1% of the retinal area. Early in regeneration, however, they are widely dispersed. The projection map then re-acquires its precision over two or three months. In diurnal light, lens ablation had no effect on refinement of the regenerated map. Constant light increased the number of labelled cells but also had no significant effect on the map. But in stroboscopic light with a continuous pseudorandom pattern of flash intervals (average rate 4.8 Hz), much less refinement was seen. Even after 70-98 days of regeneration, labelled cells remained scattered, on average, over 20% of the retinal area. These retinae were indistinguishable by several criteria from those obtained in diurnal light after only 32-39 days. Mislocated axon terminals, which are largely eliminated during the second and third months of regeneration in diurnal light, evidently persist much longer in stroboscopic light that synchronizes ganglion cell activity across the retina. These results, like previous ones obtained by blocking the transmission of activity to the tectum, support a model of map refinement based on correlation in the firing of neighbouring neurons, which may have wide application within the nervous system.

Formation of retinotopic connections: selective stabilization by an activity-dependent mechanism

During regeneration of the optic nerve in goldfish, the ingrowing retinal fibers successfully seek out their correct places in the overall retinotopic projection on the tectum. Chemospecific cell-surface interactions appear to be sufficient to organize only a crude retinotopic map on the tectum during regeneration. Precise retinotopic ordering appears to be achieved via an activity-dependent stabilization of appropriate synapses and is based upon the correlated activity of neighboring ganglion cells of the same receptive-field type in the retina. Four treatments have been found to block the sharpening process: (a) blocking the activity of the ganglion cells with intraocular tetrodotoxin (TTX), (b) rearing in total darkness, (c) correlating the activation of all ganglion cells via stroboscopic illumination and (d) blocking retinotectal synaptic transmission with alpha-bungarotoxin (alphaBTX). These experiments support a role for correlated visually driven activity in sharpening the diffuse projection and suggest that this correlated activity interacts within the postsynaptic cells, probably through the summation of excitatory postsynaptic potentials (EPSPs). Other experiments support the concept that effective synapses are stabilized: a local postsynaptic block of transmission causes a local disruption in the retinotectal map. The changes that occur during this disruption suggest that each arbor can move to maximize its synaptic efficacy. In development, initial retinotectal projections are often diffuse and may undergo a similar activity-dependent sharpening. Indirect retinotectal maps, as well as auditory maps, appear to be brought into register with the direct retinotopic projections by promoting the convergence of contacts with correlated activity. A similar mechanism may drive both the formation of ocular dominance patches in fish tectum and kitten visual cortex and the segregation of different receptive-field types in the lateral geniculate nucleus. Activity-dependent synaptic stabilization may therefore be a general mechanism whereby the diffuse projections of early development are brought to the precise, mature level of organization.

Eye-specific segregation of optic afferents in mammals, fish, and frogs: the role of activity

Eye-specific patches or stripes normally develop in the visual cortex and superior colliculus of many (but not all) mammals and are also formed, after surgically produced binocular innervation, in the optic tectum of fish and frogs. The segregation of ocular dominance patches or columns has been studied using a variety of anatomical pathway-tracing techniques, by electrophysiological recording of postsynaptic units or field potentials, and by the 2-deoxyglucose method following visual stimulation of only one eye. In the tectum of both fish and frogs and in the cortex and colliculus of mammals, eye-specific patches develop from initially diffuse, overlapping projections. Of the various mechanisms that might cause such segregation, the evidence favors an activity-dependent process that stabilizes synapses from the same eye because of their correlated activity. First, several environmental manipulations affect the segregation of afferents in visual cortex: strabismus and alternate monocular exposure apparently enhance segregation, whereas dark rearing slows the segregation process, and monocular deprivation causes the experienced eye to form larger patches at the expense of those of the deprived eye. Second, blocking activity in both eyes is effective in preventing the segregation both in the tectum of fish and frog and in the visual cortex of cat. With the eyes blocked, alternate stimulation of the optic nerves permits the segregation of ocular dominance, at least onto single cells in the cat visual cortex. These findings are discussed in terms of an activity-dependent stabilization of those synapses having correlated activity (those from neighboring ganglion cells within one eye) but not of those lacking correlated activity (those from left and right eyes). We suggest that the eye-specific patches represent a compromise between total segregation of the projections from the two eyes and the formation of a single continuous retinotopic map across the surface of the cortex or tectum.