Correlated Firing in Rabbit Retinal Ganglion Cells

Steven H. DeVries

Department of Neurobiology, Stanford University, Stanford, California 94305


    ABSTRACT
Top
Abstract
Introduction
Methods
Results
Discussion
Conclusion
References

DeVries, Steven H. Correlated firing in rabbit retinal ganglion cells. A ganglion cell's receptive field is defined as that region on the retinal surface in which a light stimulus will produce a response. While neighboring ganglion cells may respond to the same stimulus in a region where their receptive fields overlap, it generally has been assumed that each cell makes an independent decision about whether to fire. Recent recordings from cat and salamander retina using multiple electrodes have challenged this view of independent firing by showing that neighboring ganglion cells have an increased tendency to fire together within ±5 ms. However, there is still uncertainty about which types of ganglion cells fire together, the mechanisms that produce coordinated spikes, and the overall function of coordinated firing. To address these issues, the responses of up to 80 rabbit retinal ganglion cells were recorded simultaneously using a multielectrode array. Of the 11 classes of rabbit ganglion cells previously identified, coordinated firing was observed in five. Plots of the spike train cross-correlation function suggested that coordinated firing occurred through two mechanisms. In the first mechanism, a spike in an interneuron diverged to produce simultaneous spikes in two ganglion cells. This mechanism predominated in four of the five classes including the ON brisk transient cells. In the second mechanism, ganglion cells appeared to activate each other reciprocally. This was the predominant pattern of correlated firing in OFF brisk transient cells. By comparing the receptive field profiles of ON and OFF brisk transient cells, a peripheral extension of the OFF brisk transient cell receptive field was identified that might be produced by lateral spike spread. Thus an individual OFF brisk transient cell can respond both to a light stimulus directed at the center of its receptive field and to stimuli that activate neighboring OFF brisk transient cells through their receptive field centers.


    INTRODUCTION
Top
Abstract
Introduction
Methods
Results
Discussion
Conclusion
References

Multielectrode recordings from cat (Mastronarde 1983a-c) and salamander (Brivanlou et al. 1998; Meister et al. 1995) retina show that adjacent ganglion cells have a pronounced tendency to fire together within ±5 ms. These tightly correlated spikes are not directly produced by a light stimulus: while a dim stimulus applied to the receptive fields of adjacent cells will increase activity in both cells, the increase will have a time course of ~50-100 ms, the duration of the single photon response at dim intensities (Brivanlou et al. 1998; Mastronarde 1983b). Rather, tight correlations apparently arise through circuitry within the retina. The existence of specific retinal circuits that coordinate firing raises the possibility that groups of ganglion cells could act together to encode a visual stimulus.

To learn about the possible functions of correlated activity, Mastronarde (1983a,b) examined the relative timing of the spikes in neighboring cat X- and Y-ganglion cells in the dark and in dim, diffuse light. Two types of tightly correlated activity were observed. The first type occurred indiscriminately among pairs of X and Y cells with the same center sign (i.e., ON or OFF). The timing of the correlated events suggested that they were initiated by a transient in a single excitatory interneuron that made synapses onto both ganglion cells. Consistent with this "divergent" mechanism, the spike train cross-correlation function had a single central peak. The second type of rapid correlation was seen only between Y cells of the same center sign. The cross-correlation function was bimodal with a trough at 0 ms and peaks at ±1 ms. A bimodal distribution could result if Y-ganglion cells activated each other reciprocally with a 1-ms delay. Indeed, Mastronarde (1983c) was able to cause one Y cell to fire at a short latency by retrogradely activating a neighboring Y cell of the same center sign. It is likely that reciprocal correlations occur through electrical synapses (Brivanlou et al. 1998; Mastronarde 1983c). For both divergent and reciprocal activity, correlations were strongest for adjacent cells and much weaker for distant pairs (Mastronarde 1983a). Although Mastronarde (1983a) found correlations among cells in the four classes studied, mammalian retinas have many additional ganglion cell types, and it is not known whether correlated firing is a general feature of ganglion cell encoding or whether it is limited to a few select classes.

Meister et al. (1995) have recently studied the stimulus properties encoded by tightly correlated spikes produced through a divergent mechanism. The receptive field profiles of two neighboring ganglion cells were measured and then compared with the "profile" associated with spikes in the two cell's trains that occurred jointly. Joint-spike receptive fields were located midway between those of either ganglion cell and were frequently smaller than either of the original receptive fields. Consequently, it has been suggested that divergently correlated spikes encode information about small-diameter stimuli. The stimulus properties encoded by reciprocally correlated spikes have not been investigated.

The rabbit retina is well suited to in vitro recordings with a multielectrode array because it is thin and avascular. In addition, the responses of rabbit ganglion cells have been characterized extensively both in single electrode (Amthor et al. 1989a,b; Caldwell and Daw 1978; Levick 1967) and paired or multielectrode recordings (Arnett and Spraker 1981; DeVries and Baylor 1995, 1997). The purpose of this work is to identify the classes of rabbit retinal ganglion cells that fire tightly correlated spikes. Once these classes are identified, the mechanisms that produce the correlated activity will be characterized. Finally, the receptive field properties of ON and OFF brisk transient cells will be compared with each other to show how reciprocal correlations extend the receptive field span of an individual ganglion cell.


    METHODS
Top
Abstract
Introduction
Methods
Results
Discussion
Conclusion
References

The methods for recording ganglion cell activity with a multielectrode array and the use of a flickering stimulus to measure receptive field profiles have been described previously (DeVries and Baylor 1995, 1997; Meister et al. 1994, 1995). In brief, pigmented New Zealand white rabbits were maintained in darkness overnight. All subsequent manipulations were done under dim red light or using an infrared to visible light image converter to maintain the retina in a dark-adapted state. An eye was enucleated, and the retina was separated from the pigment epithelium and mounted ganglion cell side down on a glass substrate bearing 61 closely spaced extracellular electrodes. Most recordings were obtained from the central retina, slightly below the visual streak. While mounted on the multielectrode array, voltage spikes occurring at each electrode were recorded continuously, and their amplitudes, widths, and times of occurrence stored in a computer. At the end of a recording session, the spikes were sorted and assigned to individual neurons (Meister et al. 1994). The responses of ganglion cells and spike-generating displaced amacrine cells could not be distinguished. It was assumed that a majority of the recorded cells were ganglion cells because displaced amacrine cells comprise <20% of the cells in the ganglion cell layer (Vaney 1980). During recordings, the retina was superfused continuously with bicarbonate-buffered Ames medium at 35°C. Pharmacological agents were added to the superfusate before solution entered the recording chamber. All reagents were obtained from Sigma Chemical (St. Louis) unless otherwise mentioned.

Ganglion cell receptive field profiles were determined by analyzing responses to random flicker stimulation (DeVries and Baylor 1997). The retina was stimulated with a Macintosh high-resolution RGB color monitor the screen of which was imaged onto the photoreceptor layer. The stimulus was a randomly varying checkerboard with squares (100-160 µm on a side) that were made green or black according to a pseudorandom sequence. The mean stimulus intensity, calculated as described in DeVries and Baylor (1995), was 0.5-5.0 Rh* · rod-1 · s-1. At this intensity, rods were the predominantly active photoreceptor (DeVries and Baylor 1995). The checkerboard pattern was updated continuously at 13.3 or 16.7 Hz because a stimulus refresh interval of 60-75 ms is well matched to the response properties of the transduction cascade in dark-adapted rabbit rods (Nakatani et al. 1991). The sequence of patterns imaged on the retina then was cross-correlated with a ganglion cell's spike train to obtain the cell's mean effective stimulus. This is the stimulus, a function of space and time, that on average preceded the occurrence of a spike. It has units of light intensity. The spatial profile of a ganglion cell's receptive field was obtained from the mean effective stimulus at the temporal maximum. Plots of the spatial dependence of the mean effective stimulus usually were normalized such that +1 and -1 corresponded to the high and low intensities in the checkerboard, the average normalized intensity being zero.

Receptive field centers were fitted with equations that specified either a single generalized Gaussian surface or the sum of two generalized Gaussians. The single Gaussian fit provided estimates of Gaussian amplitude and width along the major and minor axes (sigma x or sigma y), the position of the receptive field center, and the angle relative to the checkerboard stimulus (DeVries and Baylor 1997). The two Gaussian fit provided estimates of the amplitudes of both Gaussians, the widths along the major and minor axes of each Gaussian, and the position of the receptive field center and angle relative to the checkerboard stimulus, which were constrained to the be same for both Gaussians. The root mean square (RMS) error was used to compare how well the same profile was fitted by the two surfaces.

In some experiments, the receptive field spatial profile was measured by using contrast reversal spatial sinewave gratings as described in DeVries and Baylor (1997; see also Hochstein and Shapley 1976). In brief, the gratings had a contrast, Imax - Imin/Imax Imin, of 1 where Imin and Imax are the peak and trough intensities. The mean intensity was 1.5 Rh* · rod-1 · s-1. The spatial intensity profile of the grating was periodically reversed by a temporal sinusoid at 0.5 Hz. Monitor intensities were corrected for the nonlinear relation between command voltage and phosphor emission. The retina was stimulated for 120-s intervals at each spatial frequency. Time histograms of each ganglion cell's spike train then were analyzed by integrating the peak in their Fourier transforms at the fundamental frequency. The peak area then was plotted as a function of the spatial frequency of the stimulus. The effective diameters of the center and surround of the receptive field were obtained by fitting plots of peak area versus spatial frequency with a sum of two Gaussian curves.

When the same receptive field profiles were measured with sinusoidal gratings and checkerboard patterns, the surrounds obtained from gratings appeared to be more prominent. However, for the larger receptive fields measured here, a checkerboard stimulus (containing 240 squares) that provided useful information about the receptive field center profile typically did not extend far into the receptive field surround. For ganglion cell types with small receptive fields encountered during the same experiments, the checkerboard stimulus usually encompassed most of the surround. In these cases, the center and surround profiles calculated from both checkerboard and sinusoidal stimuli were found to be similar after the making the appropriate conversion (see Eq. 5, DeVries and Baylor 1995; unpublished results).

Neurons were classified into functional groups by two criteria (DeVries and Baylor 1997). The first criterion was the shape of the autocorrelation function of the spike train. In the rabbit retina, the autocorrelation functions of ganglion cells in the same class have nearly identical shapes, which are distinct from those of other classes. The second criterion was the time course of the mean effective stimulus, which was obtained by spatially averaging the stimulus intensity over the center of a cell's receptive field. Ganglion cells in the same class had similarly shaped mean effective stimulus time courses.

The tendency for two neighboring ganglion cells to fire together was studied by plotting their cross-correlation function. A spike train cross-correlation function plots the mean firing rate of one cell as a function of time relative to the occurrence of a spike in its neighbor. Figure 2 shows typical cross-correlation functions for pairs of ON and OFF center ganglion cells. The function has up to three regions. First, at times greater than about ±100 ms, the function is flat, indicating that spikes are not correlated over long intervals. Within ±100 ms, there is a broad increase in the tendency of the two cells to fire together. Finally, a sharp peak in the center of the cross-correlation shows that the cells have a pronounced tendency to fire within ±5 ms of each other. The fraction of a cell's spikes that are "rapidly" correlated was calculated as follows: the crude number of rapidly correlated spikes was determined by counting the number of spikes in cell 1's train that occurred within ±5 ms of a spike in cell 2's train. The crude number was corrected for the pairings that would occur due to both chance and to the broad correlations in the same interval. This correction was based on the observation that the peak height of the broad correlation is similar to the height within the intervals of -15 to -10 ms and +10 to +15 ms. The number of spikes in cell 1 occurring within these intervals of a spike in cell 2 then was determined and subtracted from the crude value, and the result divided by the total number of spikes in the train to give the fraction of rapidly correlated spikes. The average fraction of correlated spikes for both cells usually is cited (Mastronarde 1983a).

The strength of correlated firing decreases with distance between receptive fields (Mastronarde 1983a). The spacing ratio, s, used to express the distance between receptive fields is defined as
<IT>s</IT><IT>=</IT><FR><NU><IT>d</IT></NU><DE><IT>r</IT><SUB><IT>1</IT></SUB><IT>+</IT><IT>r</IT><SUB><IT>2</IT></SUB></DE></FR>
where d is the distance between receptive field centers and r1 and r2 are the center to 1 sigma  border distances for each receptive field, as measured along the line connecting the field centers (DeVries and Baylor 1997). All values were obtained from the Gaussian fits. For profiles fitted by a sum of two Gaussians, a single value of sigma x or sigma y was obtained from the narrower Gaussian.


    RESULTS
Top
Abstract
Introduction
Methods
Results
Discussion
Conclusion
References

Rapid correlations occur in a subset of ganglion cell classes

Eleven functional classes of rabbit ganglion cells were identified previously during recordings with a multielectrode array (DeVries and Baylor 1997). The occurrence of tightly correlated firing among the cells in these classes is now characterized. In Fig. 1, the percentage of rapidly correlated spikes in pairs of ganglion cells is plotted as a function of the distance between receptive field centers. To compare results from different retinas, the distance between receptive fields was normalized so that Gaussian receptive field profiles which touched at their 1 sigma  borders had a spacing of 1. Rapid correlations were categorized arbitrarily as either strong (>8% of the spikes in adjacent cells were correlated rapidly), moderate (2-8%), or weak (<2%) according to the strength of the correlations at 0.5 spacing units. Figure 1A plots the results for pair-wise combinations of ON-center ganglion cells including the ON eta  cells, a type of ganglion cell with distinctive physiological properties that are described in the next section. Pairs of ON brisk transient cells had the strongest correlations with a shared activity of 13% at a spacing of 0.5. The spikes in ON brisk transient cells also were correlated strongly with those in neighboring ON eta  and ON brisk sustained cells. Moderate correlations were observed in ON brisk sustained cell pairs, ON eta  cell pairs, and in pairs consisting of an ON brisk sustained and an ON eta  cell (Table 1). Weak correlations were observed when pairs contained ON sluggish, ON-OFF direction selective (DS), or ON DS cells. For both moderately and strongly correlated pairs, the strength of coupling declined with distance so that an exponential curve fit to the data had a "length constant," lambda , of 0.42-0.67 spacing units (Table 1). A lambda  <1 indicates that rapid correlations were prominent between adjacent cells where the receptive fields overlapped significantly, but much weaker for pairs where there was little receptive field overlap.



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Fig. 1. Percentage of correlated spikes is plotted against receptive field spacing, a normalized measure of receptive field center-to-center distance. Spacing of 1 unit means that the Gaussians fitted to the receptive field profiles of adjacent cells touched at their 1 sigma  borders (see METHODS). A: plots for several pairings of ON center cells (results from 5 retinas). B: plots for several pairings of OFF center cells (results from 7 retinas). Solid curves were obtained by fitting the data points with a single-exponential curve using the least-squares method. Fits were constrained to approach 0 at infinite spacing. BT, brisk transient; BS, brisk sustained; Slug, sluggish; D, delayed.


                              
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Table 1. Fraction of correlated spikes for the various cell classes

A plot of correlated activity versus spacing is shown in Fig. 1B for pairs of OFF-center cells. The largest correlations were observed in pairs of OFF brisk transient cells, where >20% of the spikes in nearest neighbors were correlated tightly. Spikes in OFF brisk transient cells were correlated moderately with those in OFF brisk sustained cells (2.4% at 0.5 spacing units), although the strength of the correlations varied significantly from retina to retina. Thus rapid correlations were not evident between OFF brisk transient and OFF brisk sustained cells in most retinas (48 of 50). In two retinas, however, the percentage of rapidly correlated spikes was found to exceed 4%. The plot in Fig. 2B includes results from these two retinas. The source of this variability is not known. OFF delayed, OFF sluggish, and local edge detector (LED) cells showed no evidence of rapid correlations. In addition, rapid correlations were not seen between OFF brisk transient and ON-OFF DS cells. When rapid correlations were observed, they were strong between neighboring OFF cells but much weaker for cells with receptive fields that did not overlap.



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Fig. 2. Rapid correlations arise from circuits that are located proximal to the photoreceptor synapse. A: spike train cross-correlation functions for 2 neighboring OFF brisk transient cells obtained in the dark and during stimulation with a dim checkerboard (mean intensity = 1.7 Rh* · rod-1 · s-1). Cross-correlation functions are normalized by their peak firing rate (top right). B: cross-correlation functions for a pair of ON brisk transient cells in the same retina. C: cross-correlation functions for a pair of OFF brisk transient cells obtained during stimulation with the checkerboard while the retina was bathed in solution containing 40 µM 2-amino-4-phosphonobutyric acid (APB) and under control conditions (mean stimulus intensity = 3.3 Rh* · rod-1 · s-1). Different retina from A and B. D: cross-correlation functions for a pair of OFF brisk transient cells obtained by repeating the same checkerboard stimulus. Results from the 1st run are shown (left; mean firing rates: cell 1, 8.4 Hz; cell 2, 8.7 Hz) as is the 2nd run (middle; cell 1, 7.3 Hz; cell 2, 7.6 Hz). Left: result of cross-correlating the spike train from cell 1 obtained during the 1st trial with the spike train from cell 2 obtained during the 2nd trial. Similar cross-correlation function was obtained using cell 2 from the 1st trial and cell 1 from the 2nd trial. Each run lasted 20 min. Same retina as A and B.

Three experimental approaches suggested that the rapid correlations were not produced by photoreceptor activity during stimulation with the dim checkerboard. The first approach compared correlated activity in the dark and during exposure to the checkerboard stimulus. A dim checkerboard stimulus was used to probe correlated activity because most rabbit ganglion cells were silent in the dark or in dim, diffuse light. For those cells that did maintain a modest firing rate in the dark, primarily brisk transient and ON brisk sustained cells, rapid correlations could be observed, and their strength and extent were similar to that measured during checkerboard stimulation (Table 1). Although the narrow central peak changed little in the dark and during stimulation (Fig. 2, A and B), the height of the broad hump was increased markedly in the presence of the stimulus. Results from cat (Mastronarde 1983b) and salamander (Brivanlou et al. 1998) suggest that the broad correlations are produced by light that activates photoreceptors the signals of which drive both recorded ganglion cells. Indeed, the temporal width of the broad correlation is similar to the duration of the dim flash response in dark-adapted rabbit rods (Nakatani et al. 1991).

The second approach used 2-amino-4-phosphonobutyric acid (APB) to block transmission at the rod synapse. As a consequence of its action at receptors on rod bipolar cells, APB eliminates activity in ON center ganglion cells while strongly suppressing light responses and increasing spontaneous activity in OFF center ganglion cells (DeVries and Baylor 1995; Muller et al. 1988). In APB, the rapid correlations between OFF brisk transient cells were strengthened and the correlations in OFF brisk transient-OFF brisk sustained pairs were more evident (Fig. 2C; Table 1). Cells that lacked rapidly correlated spikes in control solution also lacked these spikes in APB-containing solution. Thus blocking communication at the rod synapse did not stop the rapid correlations.

Further evidence that the rapid correlations were not directly caused by the light stimulus was obtained by recording from adjacent OFF brisk transient cells during two presentations of the same 20-min stimulus sequence (Fig. 2D). Within a trial, the cross-correlation function had the expected broad region and a narrow central peak. However, when the spike trains were cross-correlated across trials, the narrow peak disappeared while the broad hump remained. The simplest interpretation is that the broad correlations are produced by the light stimulus, while the rapid correlations originate in synaptic activity within the retina.

A narrow central peak was not observed when the spikes trains of ON center and OFF center ganglion cells were cross-correlated, as was also the case in cat retina (Mastronarde 1983a). Although tight negative or anticorrelations were seen commonly between cat ON and OFF center ganglion cells (Mastronarde 1983), tight negative correlations were not prominent in the rabbit. The lack of distinctive tight negative correlations in the rabbit retina probably results from the relatively low firing rate of rabbit ganglion cells. In summary, tightly correlated activity was significant in 5 of the 11 identified classes of rabbit ganglion cells. Correlations were strongest for cells the receptive fields of which overlapped but much weaker for second or third order neighbors.

ON eta cells

Correlated activity was observed in four classes of ganglion cells in cat retina: ON and OFF X and ON and OFF Y. As shown above, correlated activity is found in similar cell types, the brisk cells (Caldwell and Daw 1978), in the rabbit. However, in rabbit, there appears to be an additional type of ON-center cell, the ON eta  cell, the spikes of which were correlated strongly with those of neighboring ON brisk transient and brisk sustained cells. The criteria for distinguishing between ON eta  and other rabbit ON center ganglion cells are described in the next paragraph.

Rabbit ganglion cells were divided into classes based on three functional properties: the mean effective stimulus time course when stimulated with a checkerboard pattern, the autocorrelation function shape, and the response to a light pulse (DeVries and Baylor 1997). The functional properties of ON eta and ON brisk transient cells are compared because some of their characteristics are similar. Figure 3A shows the superimposed mean effective stimulus time courses of ON eta (black) and ON brisk transient (gray) cells from the same retina. Cells in the two classes have similar time courses with peaks at -142 and -135 ms, respectively. Figure 3B plots the autocorrelation functions for the same cells. The minimum interspike interval was shorter for the ON brisk transient cells (2.0 ms) than for the ON eta cells (2.8 ms), and the interval between the first and second peaks was longer for ON eta cells. ON eta cells could be distinguished readily from ON brisk transient cells by their poor response to a diffuse light pulse (Fig. 3C) and by receptive field width. At the onset of the stimulus, ON brisk transient cells responded with a burst of spikes the rate of which typically exceeded 100 spikes/s. ON eta cells fired throughout the stimulus at a rate that rarely exceeded 10 spikes/s. The receptive field widths of ON eta  cells were one-half to one-third that of neighboring ON brisk transient cells (Fig. 3D). ON eta cells were distinguished from ON sluggish and ON brisk sustained cells by their mean effective stimulus time course and autocorrelation shape and from ON DS cells by their nonselective response to a sinewave grating drifting in one of four orthogonal directions (DeVries and Baylor 1997).



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Fig. 3. Classification of ON eta cells. A: superimposed, normalized mean effective stimulus time courses for 3 ON eta (black lines) and 2 ON brisk transient (gray lines) cells recorded simultaneously. Mean stimulus intensity was 22.5 Rh* · rod-1 · s-1 (dashed line). Peak amplitudes before normalization for the ON eta cells were 29.4, 28.3, and 29.1 Rh* · rod-1 · s-1 and for the ON brisk transient cells were 25.0 and 25.9 Rh* · rod-1 · s-1. Time to peak for the mean effective stimulus ranged between -141 and -143 ms for the ON eta cells and -132 and -137 ms for the ON brisk transient cells. B: normalized autocorrelation functions for the same cells. Peak rates before normalization for the ON eta cells were 179, 204, and 196 spikes/s and for the ON brisk transient cells were 350 and 304 spikes/s. C: pulse response of an ON eta and an ON brisk transient cell. Bar above both traces shows the timing of the light stimulus, which was spatially uniform and produced approximately 1.7 Rh* · rod-1 · s-1. Each response histogram is normalized by the peak spike rate (shown at left for the ON eta cell and at right for the ON brisk transient cell). Bin width, 50 ms; average of 20 repetitions. D: receptive field locations for 3 classes of ON-center cells and the ON eta cells. Receptive field locations are shown by a central dot and the 1 sigma  border of the generalized Gaussian fit to the receptive field profile (DeVries and Baylor 1997). Mean stimulus intensity was 1.6 Rh* · rod-1 · s-1. Width of the hexagon showing the borders of the array is 545 µm.

In Fig. 3D, the spatial profiles of ON eta cells and the ON center ganglion cells in three classes were fitted with a generalized Gaussian surface and the centers and 1 sigma  ovals are shown. All plots were from the same retina. ON eta cell profiles demonstrate a 2 sigma  center-center spacing, similar to that previously reported for most classes of ganglion cells in rabbit retina (DeVries and Baylor 1997). The profiles of cells in the three classes also were distributed with this characteristic spacing, which would be corrupted if ovals from any two classes were combined. In the mammalian retina, the receptive fields and dendritic trees of ganglion cells in the same class often are observed to evenly tile retina. The dendritic trees of ganglion cells in different classes appear to form independent mosaics (Wassle and Boycott 1991).

ON eta  cells may correspond to the "hybrid" ganglion cells recognized by Caldwell and Daw (1978). Hybrid cells were described as having properties intermediate between those of ON brisk transient, ON DS, and ON sluggish cells. Alternatively, the ON eta cell could correspond to a type of ON sluggish cell because two sluggish types were identified by Caldwell and Daw (1978) but only one type was recognized in a previous study (DeVries and Baylor 1997). In addition, there is a possibility that the ON eta  cell could be a type of displaced amacrine cell.

Mechanisms of correlated firing

The shape of a cell's cross-correlation function may provide clues about the retinal circuits that produce the correlated spikes (Brivanlou et al. 1998; Mastronarde 1983a,b; Meister et al. 1995). Figure 4A shows the cross-correlation functions for several pairwise combinations of ON brisk transient and sustained cells in one retina. The functions have a prominent central peak the temporal width of which at half-height was 5-9 ms. Single peaks in the cross-correlation function also were characteristic of ON brisk transient-ON eta pairs, ON brisk sustained pairs, ON eta pairs, ON brisk sustained-ON eta pairs, and OFF brisk transient-OFF brisk sustained cell pairs (Table 2 and Fig. 5). One way in which a single central peak might occur is if both ganglion cells are excited simultaneously by a spike or another type of transient depolarization occurring in a common presynaptic interneuron. The circuit for this "divergent" mechanism is illustrated by the diagram in Fig. 4A. An excitatory mechanism involving a presynaptic voltage transient followed by a excitatory postsynaptic current is consistent with the narrow temporal width of the central peak and assumes only a modest firing rate in a common presynaptic cell (Mastronarde 1983a). An alternative explanation for the shape of the cross-correlation function is that the ganglion cells receive tonic inhibition from a common interneuron. Periodic gaps in the tonic inhibition would allow both cells to fire at nearly the same time. gamma -Aminobutyric acid (GABA) and glycine are the two main inhibitory transmitters in the retina. However, an inhibitory mechanism involving a GABAergic interneuron appears unlikely because 20-100 µM picrotoxin increased the strength of rapid correlations in two retinas (4 ON brisk transient pairs, mean change = 47%; ON brisk transient-brisk sustained pairs, 11%; 1 OFF brisk transient pair, 55%). The role of glycinergic transmission could not be assessed because strychnine dramatically increased activity in nearly all ganglion cells.



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Fig. 4. Cross-correlation functions for pairs of ON brisk cells and OFF brisk transient cells. A: cross-correlation functions for 3 pairs of ON center cells. Inset: circuit that might produce a cross-correlation function with a single central peak. This circuit consists of an interneuron (open circle ) and 2 ganglion cells (). Arrows symbolize excitatory synaptic input. B: cross-correlation functions for 3 pairs of OFF brisk transient cells. Inset: circuit that might produce a cross-correlation function with 2 central peaks separated by a trough. In this circuit, 2 ganglion cells activate each other reciprocally. All responses were recorded simultaneously. Retina was stimulated with a dim, randomly varying checkerboard (mean intensity = 1.7 Rh* · rod-1 · s-1).


                              
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Table 2. Peak width at half-height for the cell pairs with cross-correlation functions that had a single central peak



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Fig. 5. Central peak splitting in the cross-correlation functions of different pairs of ganglion cells. A, left: typical profile of an ON brisk transient cell pair central peak (width at half-height = 7.82 ms). Right: center profile of an OFF brisk transient cell pair with the peaks located at ±2.25 ms. In between are shown the profiles that would result if 10-30% of the correlated events within the central peak region were produced by a "reciprocal" mechanism. Peak ratio is the average height of the curve at ±2.25 ms divided by the height at 0 ms. B: cross-correlation functions for representative cell pairs from 4 different retinas. Pairings are (from left to right): OFF brisk transient-OFF brisk sustained, ON brisk transient-ON eta , and 2 pairs of OFF brisk transient cells. C: peak ratios were determined for pairs of ON and OFF center cells. With the exception of the OFF brisk transient cell pairs, all pairings had similar peak ratios, which were typically <1. Horizontal bar denotes the mean value. Peak ratio for the simulated and experimentally determined cross-correlation functions were measured in the same way. Height of the cross-correlation function at the intersections with the dashed lines was obtained by averaging the height in the 5 adjacent time bins centered on the line. Before calculating the peak ratio from the experimentally obtained cross-correlation function, the broad correlation was subtracted by fitting it with a Gaussian curve. Bin width was 0.25 ms.

Figure 4B shows the cross-correlation functions for three pairs of OFF brisk transient cells recorded at the same time as the cells shown in Fig. 4A. The distinctively shaped functions have two peaks separated by a trough at 0 ms. The apices of the peaks are at 2.1 ± 0.2 ms. In a larger sample consisting of 10 OFF brisk transient pairs from different retinas, peaks were displaced from the center by 2.27 ± 0.49 ms. A cross-correlation function consisting of two peaks centered about a trough at 0 ms could result if the two ganglion cells activated each other reciprocally. Thus a spike in one ganglion cell might trigger a spike in its neighbor after a delay of ~2 ms and vice versa. The circuit is illustrated by the diagram in Fig. 4B. If the distance between somas is taken as the distance that a spike must travel before it can be detected by the array, then the 2 ms interval indicates a propagation velocity of ~100 mm/s.

The results in Fig. 4 suggest that reciprocal correlations predominate in OFF brisk transient cell pairs. Although the presence of reciprocal firing in ON-center cells cannot be excluded, it is possible to place an upper limit on the percentage of correlated spikes that might occur through such a mechanism. Figure 5A shows a range of peak shapes. On the left is a typical central peak shape for an ON-center cell pair. On the right is the shape of the cross-correlation function for an OFF brisk transient cell pair with a well-defined trough (obtained from Fig. 4B, middle). In between are the central peak shapes that would result if 10-30% of the correlated events were produced by a reciprocal mechanism, the remainder being generated by a divergent mechanism. The shape of the central portion of a peak was described by its peak ratio, which is the average height of the peak at ±2.25 ms divided by the height at 0 ms. The peak ratios obtained when different percentages of reciprocal and divergent spikes occur together are shown to the left of each peak in Fig. 5A. Figure 5B shows four cross-correlation functions obtained from cell pairs in different retinas demonstrating a range of peak shapes. The two functions on the left were from an OFF brisk transient-OFF brisk sustained pair and an ON brisk transient-ON eta pair, whereas the two on the right were from OFF brisk transient cells. Because the cross-correlation function peaks for pairs of OFF brisk transient cells were not always centered at ±2.25 ms, the peak ratio tends to underestimate the true extent of splitting. Figure 5C shows the peak ratios obtained from the cross-correlation functions of >100 pairs. With the exception of OFF brisk transient cell pairs, peak ratios were similar for all pairings and had a mean value slightly less than one, consistent with a minor contribution from a reciprocal mechanism. In contrast, about half of the OFF brisk transient cells pairs had distinct splitting (peak ratio >1.6), although, not all pairs showed troughs as distinctive as those in Fig. 4B. In the other half of the sample, the central trough was partially or fully obscured. Although a filled-in trough might suggest that divergent and reciprocal mechanism coexist in OFF brisk transient cell pairs, the central trough may be obscured by mechanisms that do not involve divergent spikes. These mechanisms will be addressed further in the DISCUSSION.

Receptive field profiles of OFF brisk transient ganglion cells

The ability of spikes to flow between OFF brisk transient cells suggests that a light stimulus that produces a spike in one cell, the spike donor, could cause a spike in a neighboring OFF brisk transient cell, the spike recipient. Because potential spike donors are arrayed circumferentially around a central recipient, the recipient cell might be expected to display an extended receptive field "skirt." To search for this peripheral skirt, the receptive field profiles of OFF brisk transient cells, which have reciprocal correlations, were compared with those of ON brisk transient cells, which lack reciprocal correlations. This comparison is useful because the anatomic substrates of the ON and OFF brisk transient cells, the ON and OFF alpha  cells, have similarly sized dendritic trees at the same eccentricity (Peichl et al. 1987), and thus their receptive fields would be expected to have similar profiles.

CHECKERBOARD STIMULUS. Previous results (DeVries and Baylor 1997) suggested that the receptive field profiles of OFF brisk transient cells differed from those of cells in other classes in that they were fitted poorly by a single generalized Gaussian surface. As shown in the next paragraph, the OFF brisk transient cell receptive field profiles were better fitted by a sum of two Gaussian surfaces.

The spatial mean effective stimulus for a typical OFF brisk transient cell is plotted in Fig. 6A. Inspection suggests that there are two components in the profile: a narrow central component and peripheral skirt or shoulder. Figure 6B shows a slice through the center of the profile (red circles) in Fig. 6A after it was fitted with a single generalized Gaussian surface (green line) and with the sum of two Gaussian surfaces (yellow line). The single Gaussian approximates the sides of the receptive field profile but poorly approximates the central peak. The two-Gaussian fit follows both the sides and the peak. The goodness of the fit was determined by calculating the root mean square error for the fitted surface (Fig. 6C). For 47 OFF brisk transient cells, the error of the two-Gaussian fit was smaller than that of the one-Gaussian fit by an average of 17.4 ± 10.1%. In a few cases (6 of 47), the one- and two-Gaussian fits had errors that differed by <7%. However, the profiles of the six receptive fields were an average of 1.74-fold wider (range = 1.14-2.92) than those of nearby ON brisk transient cells, suggesting that two components were present but not adequately resolved. For the ON brisk transient cells, a sum of two Gaussian surfaces provided only a slightly better fit than a single Gaussian surface (RMS error decrease = 1.9 ± 3.0%; n = 50). Similar results were obtained when the stimulus pattern square size was increased from 100 to 160 µm on a side (Fig. 6C, 5 retinas). Because the width of the OFF brisk transient cell narrow central component was comparable with that of the ON brisk transient cell profile (21% wider, 15 ON and 17 OFF brisk transient cells in 3 retinas, Gaussian widths ranged from 100 to 200 µm), the peripheral component adds to the overall width of the OFF brisk transient cell receptive field.



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Fig. 6. Mean effective stimulus profile of an OFF brisk transient cell. A: spatial mean effective stimulus at its peak (-105 ms relative to the spike) is plotted as a function of relative retinal position. Mean stimulus intensity was 5.8 Rh* · rod-1 · s-1. Normalized intensity is plotted on the z axis (DeVries and Baylor 1997). B: mean effective stimulus profile (red circles) was fitted either with a single generalized Gaussian surface (green line) or the sum of 2 Gaussian surfaces (yellow line). Cross-section through the peak shows both the data points and the Gaussian fits. Root mean square errors for the 1- and 2-Gaussian fits to the whole profile were 0.315 and 0.252, respectively. C: percent decrease in root mean square error when the same receptive field profile was fitted either with 1 or 2 Gaussians. Profiles were measured with a checkerboard containing squares that were either 100 µm (red) or 160 µm (green) on a side. Overall decreases in RMS error were 1.9 ± 3.0% (n = 50) for the ON brisk transient cells and 17.4 ± 10.1% (n = 47) for the OFF brisk transient cells. For the 160-µm checkerboard alone, decreases were 2.0 ± 2.4% (n = 9) for ON brisk transient cells and 13.4 ± 6.6% (n = 11) for OFF brisk transient cells. Results from 18 retinas.

SINEWAVE GRATING STIMULUS. Determining the mean effective stimulus provided a rapid method to find the size and shape of many receptive fields. The generality of the results obtained with this method were tested by stimulating with a series of contrast reversal spatial sinewave gratings (Enroth-Cugell and Robson 1966) the spatial period of which at the retinal surface varied incrementally between 57 µm and infinity. Previously, the profile centers of ON brisk transient, ON brisk sustained, and OFF brisk sustained cells were found to have similar widths when measured with either spatial sinusoids or the checkerboard pattern (DeVries and Baylor 1997). Figure 7 plots the area under the peak at the fundamental temporal frequency of the stimulus (0.5 Hz) as a function of the spatial frequency of the sinusoid (Hochstein and Shapley 1976) for two ON and two OFF brisk transient cells in the same retina. Center and surround widths were determined by fitting the results with an equation specifying the difference between two unidimensional Gaussians (Eq. 4 in DeVries and Baylor 1997; Enroth-Cugell and Robson 1966). The fits indicate that the centers of the OFF brisk transient cells (sigma  = 244 and 247 µm) were almost twice as wide as those of the On brisk transient cells (sigma  = 130 and 132 µm). In five retinas, OFF brisk transient cell centers were larger than ON brisk transient cell centers by a factor of 2.08 ± 0.33 (8 ON brisk transient and 11 OFF brisk transient cells). One of the retinas additionally was bathed in 100 µM picrotoxin; this diminished input from the inhibitory surround and permitted an adequate fit with a single Gaussian. The center widths of two OFF brisk transient cell profiles were each 289 µm, whereas that of a single ON brisk transient cell was 159 µm.



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Fig. 7. Receptive field widths of ON and OFF brisk transient cells measured with spatial sinusoids. Symbols plot the area under the peak of the fundamental response obtained during stimulation with alternating gratings the periods of which ranged from 57 µm (20 min of visual angle) to infinity (spatially uniform stimulus). Lines show the least-squares fit to the experimental points using Eq. 4 of DeVries and Baylor (1997). On brisk transient cell profiles: open circles, sigma center = 132 µm, sigma surround = 760 µm; open squares, sigma center = 130 µm, sigma surround = 560 µm. OFF brisk transient cell profiles: closed circles, sigma center = 244 µm, sigma surround = 2,130 µm; closed squares, sigma center = 247 µm, sigma surround = 1,470 µm. Receptive field centers were within 200 µm of each other on the retina. Inset: comparison between the fundamental response vs. stimulus frequency plot obtained directly from an OFF brisk transient cell (data points and thick solid line were replotted from the main figure; sigma center = 244 µm) and by calculation from the sum of 2 Gaussians fitted to that cell's mean effective stimulus profile. Curve obtained from the sum of 2 Gaussians fit has 2 "center" components (sigma 1 = 182 µm, sigma 2 = 410 µm) and is shown normalized (dashed line).

The sinusoidal gratings revealed a twofold difference in the center widths of ON and OFF brisk transient cell receptive fields. However, instead of the expected two component center, the profiles of OFF brisk transient cells were well fitted by a single Gaussian. The inset in Fig. 7 replots the data points and Gaussian fit (---) for one of the OFF brisk transient cells in the main part of the figure; the dashed line shows the receptive field center profile of the same cell obtained with the checkerboard stimulus. The spatial receptive field profile was fitted with the sum of two Gaussian surfaces, which then were used to calculate the profile in the frequency domain (for methods, see DeVries and Baylor 1997). Both lines closely followed the data points at high spatial frequencies, but the two Gaussian fit deviated from the results slightly at low spatial frequencies. The overlap between the two curves in the inset suggests that two central components might have been present during measurements with sinusoidal gratings but incompletely resolved.

Reciprocal firing can blur receptive field boundaries

Occasionally, pairs of OFF brisk transient cells had >30% correlated spikes. When reciprocal firing occurred to this extent, receptive fields could be extended markedly. Figure 8A shows the receptive field profile of an OFF brisk transient cell along with the receptive field center locations of five adjacent OFF brisk transient cells. The percentage of rapidly correlated spikes for the cell the receptive field profile of which is shown versus the three adjacent cells was 32.2, 18.4, and 16.9%. Figure 8B plots the receptive profile of an ON brisk transient cell in the same retina, and the center locations of five adjacent ON brisk transient cells. It is evident from Fig. 8 that the profile of the OFF brisk transient cell is considerably broader than that of the ON brisk transient cell. Because the distance between receptive field centers is similar for ON and OFF brisk transient cells, the receptive field coverage must be greater for OFF brisk transient cells. The equivalent receptive field center was calculated for the ON and OFF receptive fields shown in Fig. 8 as the cylinder that contained the same overall "volume" as the receptive field profile, with a height equal to the profile's peak. The radius of the equivalent center for the OFF brisk transient cell was 370 µm whereas that for the ON brisk transient cell was 190 µm. Profile peaks amplitudes were similar. If the density of ON and OFF brisk transient cells is assumed to be equal (DeVries and Baylor 1997; Peichl et al. 1987; unpublished observation), the OFF brisk transient cells would have a coverage factor that is 3.8-fold greater than that of the ON brisk transient cells. In the retina shown in Fig. 8, the distance between the receptive field centers of OFF brisk transient cells was ~1.2-fold greater than that between ON brisk transient cells. The slight increase in spacing reduces the coverage factor ratio to 3.1-fold. The increased overlap suggests that reciprocally correlated spikes can significantly extend the receptive field center of an OFF brisk transient cell.



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Fig. 8. Comparison between the receptive field profiles of an OFF brisk transient cell and an ON brisk transient cell at similar retinal locations. A: spikes in the OFF brisk transient cell were correlated reciprocally with those in adjacent OFF brisk transient cells. Percentages of correlated spikes were (counterclockwise around criterion cell): 18.4, 16.9, and 32.2. B: spikes in an ON brisk transient cell were correlated divergently with those of adjacent ON brisk transient cells. Percentages of correlated spikes were (counterclockwise around criterion cell): 6.5, 3.6, and 6.0. Mean stimulus intensity was 1.7 Rh* · rod-1 · s-1. Both profiles were normalized to peak value of 1, and the profile of the OFF brisk transient cell was inverted. Actual peak intensities were 1.09 Rh* · rod-1 · s-1 for the OFF brisk transient cell and 2.24 Rh* · rod-1 · s-1 for the ON brisk transient cell. Gray scale shows 15 even gradations between the values of 0 and 1. Hexagon gives the borders of the multielectrode array. Dimensions of the checkerboard square are shown (bottom right).


    DISCUSSION
Top
Abstract
Introduction
Methods
Results
Discussion
Conclusion
References

While reciprocal firing should expand a ganglion cell's receptive field center, an expansion due to this mechanism has not previously been recognized. In rabbit retina, reciprocal spikes were observed in OFF brisk transient ganglion cells but not in ON brisk transient cells or in the other classes of ON and OFF center cells. Because ON and OFF brisk transient cells (the rabbit alpha  cells) have dendritic trees of comparable widths at a given eccentricity (Peichl et al. 1987), their receptive field profiles can be usefully compared. In this comparison, the receptive field centers of ON brisk transient cells were well fitted by a single Gaussian surface, whereas the profiles of OFF brisk transient cells were wider overall and had two components, a central Gaussian, similar in width to that of an ON brisk transient cell's, and a peripheral skirt. The peripheral skirt is most likely produced by the centripetal flow of signals from neighboring OFF brisk transient cells.

The comparison between the receptive field profiles of rabbit ON and OFF brisk transient cells rests on the assumption that their anatomic substrates are the ON and OFF alpha  cells described by Peichl et al. (1987). In a recent study, rabbit ganglion cells with the physiological properties of ON and OFF brisk transient cells were injected with tracer (Muller and Dacheux 1997). The injected cells had the morphological characteristics of the previously described rabbit alpha  cells (Peichl et al. 1987). In the cat retina, it has been shown conclusively that ganglion cells with the physiological properties of ON and OFF Y cells are the same as morphological alpha  cells (reviewed in Wassle and Boycott 1991). In common with cat Y cells, rabbit ON and OFF brisk transient cells produce a burst of spikes during a step change in light intensity (DeVries and Baylor 1997). In addition, rabbit ON and OFF brisk transient cells have a strong second harmonic response during stimulation with inverting sinusoidal gratings that have a high spatial frequency (DeVries and Baylor 1997; unpublished observation), a defining characteristic of cat Y cells (Hochstein and Shapley 1976). Finally, the occurrence of reciprocally correlated spikes in OFF brisk transient cells is characteristic of Y cells in the cat (Mastronarde 1983a,c). Based on the anatomic and physiological results in the rabbit and the similarities with cat Y cells, it is likely that the rabbit ON and OFF brisk transient cells correspond to the morphological ON and OFF alpha  cells.

Peichl et al. (1987) used Lucifer yellow injections to compare the dendritic tree widths of rabbit ON (inner) and OFF (outer) alpha  cells. When measured at the same retinal eccentricity, no systematic differences were found. A difference in width approaching twofold would have been noticed because it would have produced a fourfold difference in the anatomic coverage factor, given a similar percentage of inner and outer cells (44:56%). Rather, the coverage factor for ON cells was 1.45, whereas that for OFF cells was 1.85. A study of cat ON and OFF alpha  cells produced similar results (Wassle et al. 1981). A small systematic difference in the widths of rabbit ON and OFF alpha  cells cannot be excluded. Indeed, a comprehensive study of midget ganglion cells in human retina shows that ON cells have tree widths that are a factor of 1.3 greater than those of OFF cells (Dacey 1993). However, it is unlikely that subtle differences in rabbit alpha  cell dendritic tree width could account for the observed twofold difference in receptive field profile and the fourfold difference in coverage factor.

Previous qualitative studies did not find a difference in receptive field width between rabbit ON and OFF brisk transient cells (Caldwell and Daw 1978; Levick 1967). A more recent quantitative study (Amthor et al. 1989a) also failed to show a systematic difference, although the sample size was limited. Similarly in cat retina, detailed measurements have shown that ON and OFF Y cell receptive fields have the same width at a given eccentricity when measured under mesopic conditions (Peichl and Wassle 1979, 1983). However, receptive fields may enlarge when ambient light intensity is reduced (Barlow et al. 1957; see also Muller and Dacheux 1997), and this increase may occur disproportionately for OFF Y cells (Peichl and Wassle 1983).

Although previous studies have not recognized a receptive field skirt, the skirt is not likely to result from a hidden bias in the present measurement because two different methods were used. In the first method, the retina was stimulated with a randomly varying checkerboard pattern and a ganglion cell's mean effective stimulus was computed by cross-correlation with the spike train. In most cases (41/47), the receptive field center profile had central and flanking components. The two-component profile did not vary when the checkerboard square size was changed over a significant range (100-160 µm per side) when compared with the receptive field width (1 sigma  radius of 100-250 µm) (DeVries and Baylor 1997). In the second method, the retina was stimulated with spatial sinewave gratings, and the fundamental response was plotted as a function of stimulus frequency. The receptive field profiles of OFF brisk transient cells were found to be twice as wide as those of neighboring ON brisk transient cells. Although the increase in width is consistent with having two central components, the two central components were not clearly resolved. In a few cases (6/47), the checkerboard stimulus also failed to resolve two center components. In these experiments, the OFF brisk transient cell receptive field center width was twice that of the neighboring ON brisk transient cells.

There are several reasons why the peripheral skirt may not have been noted in previous studies. First, with the multielectrode array, it is possible to compare the receptive fields of adjacent ganglion cells that are measured simultaneously. Differences in receptive field shape might be overlooked when receptive fields are measured serially at slightly different eccentricities in one animal or in different animals during separate experiments. Second, qualitative measures of receptive field width were used in older studies (Caldwell and Daw 1978; Levick 1967). Thus the transition from a strong field center to a weaker, same-sign skirt might have been missed. Third, ganglion cells have complex receptive field structures. For example, detailed measurements of receptive field shape indicate that most cat Y cell receptive fields have an extensive same-sign "far periphery" (Derrington et al. 1979; McIlwain 1966), along with the classical center and opponent surround. The dynamic nature and complexity of the Y cell receptive field suggest that a same-sign skirt could have been present during previous studies but not recognized.

Although most OFF brisk transient cell receptive fields had a peripheral skirt, a distinctive split in the cross-correlation function was seen in less than half of OFF brisk transient cell pairs. In the remaining cases, the trough largely was obscured. Although spikes that are generated through a divergent mechanism could fill in the trough, other mechanisms also might contribute. For example, two coupled OFF brisk transient cells could activate each other reciprocally but also might be activated simultaneously by a spike in a third OFF brisk transient cell, the three cells being located at the apices of a triangle. In effect, a divergent mechanism might exist solely among OFF brisk transient cells. A second possibility is that the "filling in" could result from structure within the spike train of the individual OFF brisk transient cell. Both ON and OFF brisk transient spike trains consisted of doublets or triplets in which spikes occurred at intervals of 2.3-2.5 ms (DeVries and Baylor 1997). If the first spike in an OFF brisk transient cell's train activated a spike in a neighboring cell with a 2.2-ms delay, the spike in the neighbor is likely to be temporally coincident with the subsequent spike in the initiating cell. The coincidence would add to the events in the center of the cross-correlation function. Occasionally, an OFF brisk transient cell fired mostly "singlets" as judged by its autocorrelation function. The most distinctive splits in the cross-correlation function were observed in such pairs of OFF brisk transient cells (Fig. 4). Pairs of ON brisk transient cells that fired singlets showed a very minimal split in the central peak.

Mechanism of correlated activity

RECIPROCAL. The injection of the low-molecular-weight tracer neurobiotin into cat alpha  ganglion cells and their likely homologs in monkey (Dacey and Brace 1992), rabbit (Vaney 1991; Xin and Bloomfield 1997), and ferret (Penn et al. 1994) reveals that alpha  cells are dye coupled to amacrine cells and neighboring alpha  cells. It has been suggested that tracer coupling between alpha  cells occurs through amacrine cells (Dacey and Brace 1992; Jacoby et al. 1996; Vaney 1991) because tracer stains the local amacrine cells more intensely than neighboring ganglion cells. In addition, gap junctions have been observed in ultrastructural studies between monkey parasol cells and the dye-coupled long-range amacrine cells but are apparently rare between parasol cells (Jacoby et al. 1996). Either direct alpha  cell coupling by gap junctions or coupling through an intermediary amacrine cell are consistent with the slow rate of transmission (100 mm/s) and relatively weak synchrony observed in rabbit OFF brisk transient cells.

Tracer spread is observed in both ON and OFF alpha  cell networks in monkey retina (Dacey and Brace 1992), and reciprocal firing is seen in both ON and OFF Y cells in the cat retina (Mastronarde 1983c). However, reciprocal correlations in rabbit retina predominated in only one type of alpha  ganglion cell corresponding to the OFF brisk transient cell. The reasons for this apparent discrepancy are not known. Fewer than 30% of alpha  ganglion cells in rabbit retina demonstrate tracer coupling, allowing for the possibility of differences in tracer coupling between ON and OFF type cells (Xin and Bloomfield 1997). In addition, even in monkey retina, there could be quantitative differences in the amount of coupling within the ON and OFF alpha syncytia that are not well revealed by tracer injection. Alternatively, ON brisk transient cells might be connected by gap junction channels, but these channels could be modulated closed under the dark adapted conditions used in the present experiments (Mills and Massey 1995) or an amacrine cell intermediary may be polarized so as to retard communication. It is interesting to note that when Mastronarde (1983c) retrogradely activated Y cells in cat retina, rapid transmission between neighboring ganglion cells was observed more readily in pairs of OFF Y cells than in pairs of ON Y cells.

The demonstration of tracer spread suggests that rabbit alpha  cells are coupled electrically, and electrical coupling provides a pathway for reciprocal spikes. However, tracer spread to a ring of neighboring cells also has been observed in ~30% of injected ON-OFF DS ganglion cells (Vaney 1994), although rapid correlations and a receptive field extension (DeVries and Baylor 1997; Yang and Masland 1992) were not evident. In the tracer experiments, neurobiotin was allowed to diffuse from an ON-OFF DS cell for several hours after injection. In addition, the assay for neurobiotin used two powerful intensification steps. It is possible that the detection of tracer in adjacent cells may not always correspond to physiologically significant electrical coupling.

DIVERGENT. The interneurons that produce the divergent correlations have not been identified. An excitatory interneuron might make either electrical or chemical synapses with ganglion cells. Electrical synapses have been shown to mediate divergent correlations in salamander retina by using blockers of chemical synaptic transmission (Brivanlou et al. 1998). However in the mammalian retina, the pattern of connectivity revealed by tracer studies does not match the pattern of "connectivity" predicted by correlated activity. For example, cat beta  cells and their likely homologs in monkey and ferret are not dye coupled to other beta  cells; nor are they coupled to nearby amacrine or alpha  ganglion cells (Dacey and Brace 1992; Penn et al. 1994; Vaney 1991). Yet the spikes in cat X/beta cells are correlated with those in neighboring X/beta and Y/alpha cells. In rabbit retina, alpha  ganglion cells are dye coupled homologously to neighboring alpha  cells but apparently not to other types of ganglion cells. Yet the spikes in rabbit ON brisk transient cells are correlated with those in neighboring ON brisk transient, brisk sustained, and eta  cells. Thus excitatory chemical synapses are the likely source of the shared input to mammalian ganglion cells.

The shared excitatory input may come from either amacrine or bipolar cells. The cholinergic starburst cell is well situated to activate nearby ganglion cells, many of which respond to acetylcholine at nicotinic receptors. However, in one preliminary experiment, the nicotinic cholinergic blockers mecamylamine (50 µM) and dihydro-beta -erythoidine (50 µM) together failed to reduce the rapid correlations between ON center cells (a 7 ± 18% decrease, 5 pairs). These agents were effective because the activation of ON-OFF DS cells by an appropriately oriented sinusoidal stimulus was reduced by 60% (He and Masland 1997; Kittila and Massey 1997). The glutamatergic bipolar cells are another possible source of shared excitatory input to ganglion cells. However, mammalian bipolar cells are thought to lack a tetrodotoxin-sensitive Na+ current (Kaneko et al. 1989) and thus cannot produce Na+-based action potentials, although transients produced by Ca2+ currents might occur. An indirect mechanism involving AII amacrine and bipolar cells is possible. AII amacrine cells have a voltage-dependent Na+ current and generate voltage transients (Boos et al. 1993). The voltage transients that originate in the AII could flow across gap junctions into depolarizing bipolar cells, which make excitatory synapses with ON center ganglion cell. A pathway involving spike initiation in an AII cell has an interesting asymmetry: a depolarization in the AII produces a spike that travels across electrical synapses to activate underlying ON center ganglion cells. However, during AII cell hyperpolarization, which leads to activation of OFF center ganglion cells, AII spikes would be suppressed. This asymmetry may help to explain why divergent correlations predominate in ON center cells. In addition, there is evidence that signals flow from AII cells to only a subset of retinal ganglion cells (Cohen and Sterling 1990; DeVries and Baylor 1995), providing a possible explanation for why correlated activity is limited to only some ON center cell classes.


    CONCLUSION
Top
Abstract
Introduction
Methods
Results
Discussion
Conclusion
References

The ability of OFF brisk transient cells to exchange spikes blurs the boundaries between their receptive fields. This blurring seems counterintuitive because it would degrade the spatial resolving power of the ganglion cell lattice. However, resolving spatial detail may be only one of the tasks for which OFF brisk transient cells are optimized. One possibility is that weak electrical coupling would ensure that neighboring ganglion cells fire together when a stimulus covers their receptive fields (Neuenschwander and Singer 1996). The benefit of correlated firing in this instance might be that the short-lived excitatory postsynaptic potentials (5-10 ms) (Jonas et al. 1993; Zhang and Trussell 1994) produced by neighboring ganglion cells could overlap and sum in a postsynaptic lateral geniculate neuron. Otherwise, a dim uniform stimulus might produce spikes in neighboring cells that were separated by as much as 50 ms (Mastronarde 1983b; Nakatani et al. 1991), and simple summation in a postsynaptic cell might be precluded. ON and OFF brisk transient cells both generate rapidly correlated spikes but the mechanisms differ. It is tempting to speculate that this difference is due to constraints on retinal wiring rather than differences in ultimate function.


    ACKNOWLEDGMENTS

The author thanks Dr. Denis A. Baylor for advice and support and R. Schneeveis for technical assistance.

This work was supported by National Eye Institute Grant EY-05750, the Ruth and Milton Steinbach Fund, National Eye Institute Postdoctoral Fellowship EY-06387, and a fellowship from The Bank of America-Giannini Foundation.

Present address and address for reprint requests: Dept. of Ophthalmology and Visual Science 7.024, University of Texas Houston Health Sciences Center, 6431 Fannin, Houston, TX 77030.


    FOOTNOTES

The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.

Received 19 March 1998; accepted in final form 14 October 1998.


    REFERENCES
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Abstract
Introduction
Methods
Results
Discussion
Conclusion
References

0022-3077/99 $5.00 Copyright © 1999 The American Physiological Society