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We have identified the photoreceptors of Trachemys scripta elegans, an intensely studied species that is a model for color vision work. To recognize and count the different photoreceptor types, we labeled them with a combination of morphological and immunohistochemistry markers. The counts for the determination of the density of each photoreceptor type were made in wholemount retinas. The percentages found for each cone type were 29, 23, 21, 12, and 6%, respectively, for L (both types), double, M, S, and ultraviolet cones. The cones were found to be organized horizontally in a visual streak, a linear region with a higher density of photoreceptors that ends temporally in the periphery and more centrally in the nasal side. This region of high density of photoreceptors was not symmetrical along its extension; there was a region with conspicuous central density peaks in the temporal area, suggestive of an area centralis. We also observed a dorsoventral asymmetry in photoreceptor density, with greater density in the ventral region. This asymmetry was observed in cones and rods, but it was more pronounced in the rods. Our results corroborate and extend the findings of previous work in the literature describing the retinal photoreceptors of T. s. elegans and their spatial organization. The higher cone density within the visual streak reflects increased spatial resolution and its existence suggests the possibility of binocular vision. It is remarkable that within this region the entire potential for color vision is also present.

Chromaticity type horizontal cells (C-type HCs) are the first retinal neurons exhibiting spectral information processing in cold-blooded vertebrates. The simple input of hyperpolarizing responses of cone photoreceptors is transformed in the C-type HCs into spectral opponent output. Nitric oxide (NO), a known background neuromodulator in the distal retina, was tested here for its effects upon spectral information processing by C-type HCs in the retina of turtle. Photoresponses were intracellularly recorded from C-type HCs, using light stimuli of different wavelength, applied over backgrounds of different wavelengths, and changing retinal NO level. Raising retinal level of NO in darkness by adding the precursor for its synthesis (l-Arginine) augmented the depolarizing photoresponses elicited by long-wavelength light stimuli, and reduced the hyperpolarizing photoresponses elicited by short-wavelength light stimuli. Lowering retinal level of NO by l-NAME, an inhibitor of NO synthesis, induced the opposite effects. However, the total voltage range of operation remained constant regardless of the level of NO. Qualitatively similar effects were observed under background illuminations regardless of background strength and wavelength. Altering retinal level of NO exerted a small effect upon the null wavelength. Our findings are consistent with the known effects of NO upon turtle distal retinal neurons, with the addition of NO strengthening the negative feedback pathway from L-type horizontal cells onto medium-wavelength cones.

1. The Stiles two-colour increment threshold technique was applied to turtle cone photoreceptors in order to derive their field sensitivity action spectra. 2. Photoresponses of cone photoreceptors were recorded intracellularly. Flash sensitivities were calculated from small amplitude (1 mV) responses. The desensitizing effects of backgrounds of different wavelengths were measured and the background irradiance needed to desensitize the cone by a factor of 10 (1 log unit) was defined as threshold. The reciprocals of these thresholds were used to construct the field sensitivity action spectrum. 3. The field sensitivity action spectra of long-wavelength-sensitive (L) and medium-wavelength-sensitive (M) cones depended upon the wavelength of the test flash used to measure them. This excludes the possibility that turtle cones can function as single-colour mechanisms in the Stiles sense. 4. In fourteen L-cones, the average wavelength of peak sensitivity of the field sensitivity action spectrum was 613.7 +/- 7.7 nm for the 500 nm test and 635.6 +/- 9.6 nm for the 700 nm test. For six M-cones, these values were 558.5 +/- 6.8 and 628.8 +/- 10.6 nm for the 500 and 700 nm tests, respectively. 5. Two physiological mechanisms are suggested as contributing to the dependency of the field sensitivity action spectrum upon test wavelength. One is based upon the transmissivity properties of the coloured oil droplets, while the other hypothesizes excitatory interactions between cones of different spectral type. 6. Computer simulations of the field sensitivity action spectra indicate that both mechanisms are needed in order to account for the dependency of the field sensitivity action spectrum upon the wavelength of the test flash.

Akopian, Abram, Robert Gabriel, and Paul Witkovsky. Calcium released from intracellular stores inhibits GABAA-mediated currents in ganglion cells of the turtle retina. J. Neurophysiol. 80: 1105–1115, 1998. We studied spiking neurons isolated from turtle retina by the whole cell version of the patch clamp. The studied cells had perikaryal diameters >15 μm and fired multiple spikes in response to depolarizing current steps, indicating they were ganglion cells. In symmetrical [Cl−], currents elicited by puffs of 100 μM γ-aminobutyric acid (GABA) were inward at a holding potential of –80 mV. All of the GABA-evoked current was blocked by SR95331 (20 μM), indicating that it was mediated by a GABAAreceptor. The GABA-evoked currents were unaltered by eliciting a transmembrane calcium current either just before or during the response to GABA. On the other hand caffeine (10 mM), which induces Ca2+release from intracellular stores, inhibited the GABA-evoked current on average by 30%. The caffeine effect was blocked by introducing the calcium buffer bis-( o-aminophenoxy)- N, N, N′, N′-tetraacetic acid (BAPTA) into the cell but was unaffected by replacing [Ca2+]owith equimolar cobalt. Thapsigargin (10 μM), an inhibitor of intracellular calcium pumps, and ryanodine (20 μM), which depletes intracellular calcium stores, both markedly reduced a caffeine-induced inhibition of the GABA-evoked current. Another activator of intracellular calcium release, inositol trisphosphate (IP3; 50 μM), also progressively reduced the GABA-induced current when introduced into the cell. Dibutyryl adenosine 3′,5′-cyclic monophosphate (cAMP; 0.5 mM), a membrane-permeable analogue of cAMP, did not reduce GABA-evoked currents, suggesting that cAMP-dependent kinases are not involved in suppressing GABAAcurrents, whereas calmidazolium (30 μM) and cyclosporin A(20 μM), which inhibit Ca/calmodulin-dependent phosphatases, did reduce the caffeine-induced inhibition of the GABA-evoked current. Alkaline phosphatase (150 μg/ml) and calcineurin (300 μg/ml) had a similar action to caffeine or IP3. Antibodies directed against the ryanodine receptor or the IP3receptor reacted with the great majority of neurons in the ganglion cell layer. We found that these two antibodies colocalized in large ganglion cells. In summary, intracellular calcium plays a role in reducing the currents elicited by GABA, acting through GABAAreceptors. The modulatory action of calcium on GABA responses appears to work through one or more Ca-dependent phosphatases.

Model of a turtle's visual cortex has been developed and successfully used to discriminate di fferent input stimuli to different locations of its lateral geniculate nucleus. Turtle's retina model also shown the capability of discriminating motion targets verifying the biological experimental results. However, the two models are studied separately and driven by different input stimuli in the past. Here, in this thesis we will be combining the two models and let the output of the retinal model to drive the visual cortex model. We will be paying attention to the model parameters such as the connection strength between the two previous models and the white noise levels during this experiment.

Networks of physiologically identified H2 horizontal cells in the turtle retina were labeled by intracellular injection of Neurobiotin. We obtained a quantitative description of the neighbourhood relations in the dye-coupled cell mosaics by using the somata as centers for the Voronoi-Delaunay construction. Computational models simulating the experimental data are presented.

Intracellular recordings from luminosity-type horizontal cells of the turtle retina were used to analyze the effects of steady and flickering background illumination on the size of their receptive fields. Both types of background illumination reduce the size of the receptive field to about the same extent. The reduction seems largely due an increase in the coupling resistance between horizontal cells. The effects of both types of background illumination are sensitive to the dopamine antagonist fluphenazine. This suggests that steady and flickering illuminations stimulate the release of endogenous dopamine.

All of the amino acids which have been implicated as potential neurotransmitter candidates in the vertebrate retina have been focalized, or have been shown to have effects on other neurotransmitter pathways in the turtle retina. These amino acids include glutamate, aspartate, glycine, and GABA

on-off transient ganglion cells of the turtle retina show distinct spike-event patterns in response to abrupt intensity changes, such as during saccadic eye movements. These patterns consist of two main spike events, with the latency of each event showing a systematic dependency on stimulus contrast. Whereas the latency of the first event decreases monotonically with increasing contrast, as expected, the second event shows the shortest latency for intermediate contrasts and a longer latency for high and low contrasts. These spike-event patterns improve the discrimination of different light-intensity transitions based on ensemble responses of the on-off transient ganglion cell subpopulation. Although the discrimination results are far better than chance using either spike counts or latencies of the first spikes, they are further improved by using properties of the second spike event. The best classification results are obtained when spike rates and latencies of both events are considered in combination. Thus spike counts and temporal structure of retinal ganglion cells carry complementary information about the stimulus condition, and thus spike-event patterns could be an important aspect of retinal coding. To investigate the origin of the spike-event patterns in retinal ganglion cells, two computational models of retinal processing are compared. A linear–nonlinear model consisting of separate filters for on and off response components fails to reproduce the spike-event patterns. A more complex cascade filter model, however, accurately predicts the timing of the spike events by using a combination of gain control loop and spike rate adaptation.

Chromaticity-type (C-type) horizontal cells of the turtle retina receive antagonistic inputs from cones of different spectral types, and therefore their response to background illumination is expected to reflect light adaptation of the cones and the interactions between their antagonistic inputs. Our goal was to study the behavior of C-type horizontal cells during background illumination and to evaluate the role of wavelength in background adaptation. The photoresponses of C-type horizontal cells were recorded intracellularly in the everted eyecup preparation of the turtleMauremys caspicaduring chromatic background illuminations. The voltage range of operation was either reduced or augmented, depending upon the wavelengths of the background and of the light stimuli, while the sensitivity to light was decreased by any background. The response–intensity curves were shifted to brighter intensities and became steeper as the background lights were made brighter regardless of wavelength. Comparing the effects of cone iso-luminant backgrounds on the Red/Green C-type horizontal cells indicated that background desensitization in these cells could not solely reflect background adaptation of cones but also depend upon response compression/expansion and changes in synaptic transmission. This leads to wavelength dependency of background adaptation in C-type horizontal cells, that is expressed as increased light sensitivity (smaller threshold elevation) and improved suprathreshold contrast detection when the wavelengths of the background and light stimuli were chosen to exert opponent effects on membrane potential.

Pseudo-3-dimensional models and surface renderings of a small group of neighboring cone pedicles in the turtle retina, in which the 5 different spectral types are represented, have been made from scanned serial electron micrographs using a Macintosh personal computer, Autocad and Studio Pro software. The resultant computer generated images clarify the morphological differences between the spectral types and show how they are related to each other via telodendrial connections and different levels of ending in the neuropil of the outer plexiform layer (OPL). The double cone pedicles end highest in the neuropil and the single red and green pedicles lie slightly lower into the OPL with the single blue and ultraviolent (UV)-sensitive cone pedicles coming in from obliquely angled axons to assume positions between the others much more vitread in the OPL. Telodendria interconnect the double and single red and green cones but not the blue and UV-sensitive cones.

Ganglion cell activity in response to drifting or alternating stimulus patterns was recorded in an eyecup preparation of the turtle. The stimuli were sinusoidal gratings with various spatial frequencies and spatial phases. Spike histograms of the activity were Fourier-analyzed. The results indicated that the retinal circuitry always functioned in a nonlinear manner. The average, first, and second harmonics of the activity were all a function of stimulus conditions, including spatial phase. Similar outcomes were obtained with full-field (retina-wide) and a small-field (1 mm diameter) stimulation. In the middle of the spatial-frequency range, the ratio of second to first harmonic activity was higher in response to static, contrast-reversing gratings than for drifting, constant-contrast ones. The findings suggest that, as distinct from the usual X, Y, and W segregation of ganglion cell properties in other retinas, there is a single class of cells whose responses seem to have attributes of one or another of the usual categories depending on stimulus characteristics. This may be the result of a degree of stimulus-dependent functional modification in the retinal input circuitry to the turtle ganglion cells.

Synaptic contacts of photoreceptors in the turtle retina were studied by intracellular injection of horseradish peroxidase (HRP) and electron microscopy. Both cone and rod photoreceptors radiated basal processes (telodendria) from their terminal endings. These telodendria ran laterally in the outer plexiform layer. The telodendria of cones gave rise to many fine branches that penetrated synaptic cavities of several neighboring cones. Tips of these branches terminated near the walls of synaptic cavities. Some of the telodendrial contact formed two types of basal junction: symmetrical and punctate. The distribution of cones that made telodendrial contacts with the HRP-filled cone were quantitatively investigated. Green-sensitive cones (n = 3) made telodendrial contacts with neighboring red- and blue-sensitive cones, blue-sensitive cones (n = 4) with red- and green-sensitive cones, and red-sensitive cones (n = 9) with red- and green-sensitive cones. In contrast to these cone connections, rod telodendria did not penetrate neighboring photoreceptors. Direct synaptic contacts were not found between rods and cones. Our results clarify the variety of cone couplings in turtle retina: the three chromatic classes of cones are selectively coupled by the basal junctions at the ends of telodendrial processes.

The internal organization of the plasma membrane at specialized contacts in the outer plexiform layer of the turtle, Pseudemys scripta elegans, was analyzed with the aid of the freeze-fracturing technique. In the invaginating synapse of cone pedicles the plasma membrane of the photoreceptor ending contains an aggregate of P-face particles, images of synaptic vesicle exocytosis, and rows of forming coated vesicles which are arranged in sequence from apex to base of the synaptic ridge. Thus, freeze-fracturing provides positive evidence that the synaptic ridge represents the active zone at the surface of the photoreceptor endings. Horizontal cell processes of dyads and triads have an aggregate of P-face particles opposite the apex of the ridge, but lack images of vesicle exocytosis. Deep-etching and rotary-shadowing demonstrate that an array of minute protrusions decorates the true outer surface of the horizontal cell membrane at the site of the intramembrane particle aggregate. The membrane of the invaginating bipolar dendrite is unspecialized. At basal junctions, the cone pedicle membrane has a sparse complement of P-face particles, but images of vesicle exocytosis are absent. The adjoining bipolar membrane is characterized by a prominent aggregate of E-face particles, often arranged in an orthogonal lattice. The freeze-fracture profile therefore suggests the existence of (1) a sign-conserving cone-to-horizontal cell synapse; (2) a sign-inverting synapse between cones and invaginating bipolar dendrites; and (3) a sign-conserving synapse between cones and bipolar dendrites at basal junctions. No freeze-fracture evidence was found for a horizontal-to-cone or horizontal-to-bipolar cell synapse within the synaptic invaginations.

We have performed a serial-section electron microscopic study of the inner plexiform layer (IPL) of the retina of the turtle Pseudemys scripta elegans. A qualitative and quantitative assessment of the neuropil of the IPL has been done from photomontages taken from the linear visual streak area and peripheral retina. Counts of conventional, ribbon, serial, and reciprocal synapses, of ganglion cell dendrites, and of profiles containing large, dense-cored vesicles were made in five equal-thickness strata in each montage. Averages of these different features were plotted for each stratum in the linear visual streak and compared with peripheral retina. The trend was for stratum 2 to have the highest overall absolute number of amacrine and bipolar interactions, and also of serial synapses, both in the linear visual streak and in peripheral regions. Stratum 4 tended to have the second-highest number of synapses. The total number of synapses for the entire thickness of the IPL, regardless of stratification, is higher in the streak than in the periphery. The total amacrine-to-bipolar-synapse ratio in the IPL is the highest of any vertebrate studied to date (11.0 in the streak and 14.5 in the periphery) but the number of synapses/micron 2 was found to be similar to that reported for other vertebrates. Amacrine-to-amacrine synaptic contacts greatly outnumber other types of synapses; amacrines constitute the principal input to ganglion cells, whereas bipolar output is mainly onto amacrines. The trend for higher numbers of synaptic interactions in strata 2 and 4 of the streak region of the turtle IPL can be correlated with the branching of small-field amacrine and ganglion cells described in Golgi studies (Kolb: Philos. Trans. R. Soc. Lond. B 298:355-393, '82). In peripheral retina, branching of large-field amacrines and a lower number of bipolar pathways may account for the trend for larger numbers of amacrine synapses in strata 2 and 4. Profiles having large, dense-cored vesicles tend to occur most frequently in strata 1 and 5, which correlates well with the stratification in the IPL of the processes of immunoreactive amacrine cells described in other studies.

Light stimuli depolarize green cones of turtle retina through a circuit involving a feedback connection from luminosity horizontal cells (L-HC) to green cones. In turtle retina two types of L-HC have been distinguished: large-field L-HC and small-field L-HC. The spatial properties of the feedback depolarizations of green cones were compared with those of both large- and small-field L-HC. Green cones were found to be more effectively depolarized by relatively small spots of red light than by large red annuli. Moreover, red light stimulation of the periphery of the receptive field could reduce the depolarizing influence of central red stimuli. These spatial properties greatly differ from those of the large-field L-HC, whereas they strongly resemble those of the small-field L-HC. These results suggest that the small-field L-HC mediate the feedback action on green cones.

1. The spatial properties of the small-field luminosity horizontal cell (L2-HC) were investigated in the retina of the turtle by using circular or annular light stimuli of varying dimensions. 2. The amplitude of the hyperpolarizing response induced by dim light spots decreased when the diameter of the spot was increased beyond about 1,000 micrometer. 3. With bright-light stimuli the peak amplitude of the light responses increased monotonically when increasing the illuminated area, but a delayed antagonistic effect appeared with spots of more than 1,000-micrometer diameter. 4. Depolarizing responses were observed if the periphery of the receptive field was stimulated with annuli of light in the presence of central background illumination. 5. The interaction of the inputs converging on the L2-HC from the peripheral and central regions of the receptive field was found to be nonlinear. The same peripheral stimulation could result either in an enhancement or in a depression of the central response according to the intensity of the central illumination. 6. By comparing the receptive-field properties of the L2-HC and the red cones a model is proposed that explains the antagonistic surround mechanism of the L2-HC on the basis of the antagonistic peripheral mechanism in cones. '

Criteria were established whereby the chief and accessory members of double cones, red-, green-, and blue-sensitive single cones, and rods could be distringuished form one another in 1-μm sections through the retinas of Pseudemys scripta elegans and Chelydra serpentina. Criteria included the presence or absence of an oil droplet, the size, color, and radial position of the oil droplet, the size and shape of the inner and outer segments, and the shape and orientation of the connection between the perikaryon and synaptic terminal of each receptor cell. Using these criteria, the color-specific interconnections between horizontal cells and receptor cells were analyzed using 1-μm serial sections through Golgistained examples of horizontal cells of each type. The results of this analysis area: (a) H1 cell bodies contact both the red- and green-sensitive members of double cones, as well as red- green-sensitive single cones. (b) H1 aoxn terminals contact only the red-sensitive chief members of doulbe cones, red-sensitive single cones, and rods. (c) H2-type cells contact only green- and blue senstive single cones. (d) H2-type cells contact only blue-sensitive single cones. (e) H2-type cells contact only green-sensitive accessory members of double cones. With the exception of H1 cell bodies, all horizontal cells generally contacted every apporpriate receptor cell type within their receptive field. A model for the generation of complex horizontal cell light responses is proposed and discussed.

Müller cells are the major type of glial cell in the vertebrate retina, and appear to participate in important structural and metabolic functions. Although the morphological features of Müller cells have been extensively studied, their topographic distribution across the retina has not been previously reported. We have used a Müller cell-specific monoclonal antibody, 19-33, to study the distribution of Müller cells in turtle retina. The antibody was obtained during a search for cell type-specific monoclonal antibodies in the rat retina. Immunoblotting studies show that 19-33 reacts with a 58 KDa protein that is present in Müller cells. Immunocytochemical studies with en face sections of turtle retina show that the density of Müller cells is fairly uniform across the retina although there are small regional differences. We estimate that the mean Müller cell density is about 1600 cells mm-2 of turtle retina and that each turtle retina contains about 54,000 Müller cells.

Interaction between horizontal cells of the turtle retina was studied by two microelectrodes (polarizing and recording), inserted into different cells at different distances apart. The presence of a direct electrical connection was demonstrated between the L cells of the same type (I, with large, and II, with small receptive fields). Its magnitude depends on the conditions of illumination and the level of the membrane potential, possibly because of the properties of the subsynaptic and nonsynaptic membranes of the horizontal cells. No direct electrical connection exists between L cells of different types. However, hyperpolarization of the type I cells through the microelectrode or by stimulation with a circle of light evoked depolarization in the type II cells. This indirect connection between the horizontal cells, also dependent on the conditions of illumination, can probably be explained by feedback to these cells from the photoreceptors. Polarization of L cells of both types had no effect on horizontal cells of color type.

Co-localization of retrograde transported Nuclear yellow and serotonin immunoreactivity revealed the existence of a central serotonergic projection to the retina in the turtle Pseudemys scripta elegans. This serotonergic retinopetal system consists of only one fiber originating in the contralateral caudal mesencephalon. In the retina, the fiber arborizes exclusively in the temporal hemisphere, covering about one third of the total retinal surface. Interestingly this projecting area lies in the part of the retina, to which binocular perception can be attributed.

Vertebrate retinal photoreceptors containing colorless oil droplets have often been described as blue-sensitive. This identification is reexamined by intracellular recording and Lucifer yellow dye injections in turtle retina. Contrary to previous reports, the majority of the cones with colorless oil droplets were found to be red-sensitive; only one-third of the cones sampled were blue-sensitive. In addition, colorless oil droplets contained in the red-sensitive cones displayed a bright green fluorescence upon near-ultraviolet irradiation, while those of the blue-sensitive cones did not fluoresce.

Action spectra were obtained by intracellular recordings from horizontal cells of the retina of turtle ( Pseudemys scripta ). Dark-adapted L 1 type horizontal cells received their major input from either red cones or from blue and green cones. With chromatic adaptation, red-cone dominated L 1 cells could be further subdivided on the basis of receptor input. They received input from only red cones (L 1 -R), red and green cones (L 1 -RG) or all cones, red, green and blue (L 1 -RGB). L 1 -RG and L 1 -RGB cells most likely receive their green-cone input from double cones. C-type horizontal cells were found to be of two types, biphasic and triphasic. Biphasic cells were depolarized by red light and hyperpolarized by green light (C-R/GB). Chromatic adaptation showed that blue and green cones acted together and opposed the activity of red cones. Triphasic C-type cells depolarized to green light and hyperpolarized to blue and red light (C-G /BR). Thus, blue cones acted with red cones and opposed the action of green cones. From these results it is concluded that: (1) red cones are the main determinant in the turtle luminosity system. (2) green cones and blue cones have their major role in the chromaticity system, (3) these results are consistent with the feedback hypothesis for the formation of C-type responses in turtle horizontal cells.

Spectral sensitivities of photoreceptors in the turtle (Geoclemys) retina were studied by intracellular recording, and each cell was filled with Lucifer yellow (LY). Photoreceptors were classified into seven morphological types: rod, four types of single cones, and two members of a double cone. Single cones contained one of four different oil droplets: red, pale-green, orange, and clear. Double cones consisted of two apposed cones; principal members contained yellow oil droplets, while accessory members contained no oil droplet. Spectral sensitivities recorded from these seven types of photoreceptors were classified into one type of rod and three chromatic types of cones. Rods (n = 19) showed peak sensitivity at 520 nm. Single cones containing either a red (n = 51) or a pale-green (n = 9) oil droplet were red-sensitive (lambda max at 620 nm). Single cones containing an orange oil droplet (n = 14) were green-sensitive (lambda max at 540 nm). Single cones containing a clear oil droplet (n = 3) were blue-sensitive (lambda max at 460 nm). Both members of the double cone, principal (n = 22) and accessory (n = 15), were red-sensitive (lambda max at 620 nm). No diffusion of LY was detected between the apposed members of double cones. Red-sensitive cones, therefore, consisted of four different morphological types of cones, and they occupy about 70% of the photoreceptor mosaic in the turtle retina.

Recently, the interactions between cones and horizontal cells in the turtle retina were studied and the schematic diagram of the neuronal connexions responsible for the colour coding of horizontal cells was proposed. We studied the dynamic characteristics of the receptive field of horizontal cells in the carp with a nonlinear analysis technique. In this paper we proposed a model of the receptive field of L 2- and R/G-cells in the carp for monochromatic lights on the basis of the proposed interactions and our experimental results. The spectral responses of the model were nearly coincided with the experimental ones for two monochromatic lights. The responses of the model were controlled by the recurrent action from L 2-cells to cones. Therefore, it is suggested that the recurrent action may control the spectral and spatial properties of horizontal cell responses.

The horizontal cells of the rabbit retina have been studied by light microscopy of Golgi-impregnated whole-mount retinas. The two types of horizontal cell of the rabbit retina are similar to the horizontal cells of the cat retina in most respects. However, the majority of the A-type horizontal cells of the rabbit have asymmetrical dendritic fields compared to the circular, symmetrical dendritic fields of this cell type in the cat. The A-type horizontal cells of the superior edge of the linear visual streak in the rabbit retina are the most strikingly asymmetric and most of them are elongated and oriented in a direction approximately parallel to the linear visual streak. Like H1 axon terminals of the turtle retina the oriented, elongated A-type horizontal cells of the rabbit visual streak region may play a role in the neurocircuitry which underlies orientation sensitive ganglion cells.

Center-depolarizing and center-hyperpo-larizing photoresponses were recorded intracellularly from different bipolar cells in the turtle retina and subsequently injected with Procion Yellow for morphological analysis. The results indicate that the type of photoresponse is neither related to large or small soma sized cells nor to the number of axonal ramifications within the inner plexiform layer (IPL). It was found that axonal arborizations of center-depolarizing cells are restricted to the inner 60% of IPL, whereas arborizations of the center-hyperpolarizing cells are present at all levels of IPL.

We recorded by intracellular means responses of horizontal cells of the turtle retina to light increase and decrease of different values against the starting adapting level. In measuring these responses, curves reflecting the dependence of membrane potential deflection on light intensity (amplitude characteristics — ACh) were plotted. It is demonstrated that the ACh of transitional processes (on- and off-peaks) is considerably steeper than ACh of the plateau of the potential, but embraces a much smaller range of light intensities (slightly more than 1 log. un.). During a change in intensity of the adapting background (up to 3 log. un.), the ACh of transitional processes shifts along the scale of light intensities in such a way that its steep part remains in the zone of adapting light. We followed the dynamics in time of ACh shift after the transition from one adapting brightness to another. The ACh of total impulse response was plotted for ganglionic cells of the turtle at different intensities of adapting light. Comparison of these curves with the ACh of horizontal cells shows that its peripheral components are responsible for adaptive shifts of ACh of the visual system and that horizontal cells play an important role in the mechanism of adaptation. It is hypothesized that adaptive ACh shifts are the consequence of positive feedback between the horizontal cells and receptors.

The scotopic spectral sensitivity curve ofChrysemys picta picta was determined, using an instrumental technique. The results showed a maximum in the region of 460–500 nm, with a second maximum at about 640 nm in the red. There was also a significant interaction between the effects of wavelength and intensity on the rate of response. At low intensities, the rate of response was highest in the blue (460–500 nm), but at higher intensities it was greatest in the red (about 640 nm). These results show that the turtle retina is not a single-mechanism retina, but contains at least two receptors. One of these, most sensitive in the red, agrees with electrophysiological data on related species, and is probably a result of the oil droplets shifting the photopic sensitivity into the red. The second receptor, which has a lower threshold, is maximally sensitive in the blue.

The intercellular junctions of cone pedicles in the turtle retina were studied electronmicroscopically in tissue prepared by conventional techniques or impregnated by the method of Golgi. Dendritic branchlets of bipolar cells make specialized contacts with the basal surface of the pedicles. Processes ending laterally to wedge-shaped projections (synaptic ridges) of the pedicles probably belong always to horizontal cells, and make proximal and distal (to the synaptic ridges) junctions with the pedicles. Processes ending opposite the apex of the synaptic ridges are engaged also in two kinds of specialized contacts, termed apical and distal junctions. Basal processes of the cone pedicles make specialized contacts with adjacent pedicles, and with unidentified processes at the outer plexiform layer; their endings abut upon horizontal cell processes lodged within the pedicles of other cone cells.

1. Responses to monochromatic lights were recorded intracellularly from red cones, green cones, and luminosity horizontal cells (L-cells) in the retinae of turtles.2. Both types of cones responded to small fields of illumination with graded hyperpolarizations. Red cones were only moderately more sensitive to deep red (680 nm) than to green (550 nm) light while green cones were much more sensitive to the green light than to the red. L-cells produced small responses for flashes of either colour covering small fields.3. Stimulation of large fields with monochromatic lights of moderate or high intensity evoked large L-cell responses and composite responses in cones. These latter include the hyperpolarizing action of the light absorbed by the cone itself (direct response), its enhancement by illumination of the near surround, and the depolarizing effects of L-cell feed-back.4. L-cells respond primarily to the activity of red cones; with sufficient intensity of the light, however, their responses are influenced also by green cones. As a result, if a red and a green light stimulate red cones equally, the L-cell response is larger for the green stimulus.5. Green cones were depolarized by deep red lights of moderate intensity applied over large fields. These depolarizing responses include oscillations which follow closely oscillations in L-cells. Green light applied to the same large fields produced hyperpolarization of green cones.6. Red cones were hyperpolarized by red or green light covering large fields, but the time course of their responses differed for the two colours, reflecting a corresponding difference in L-cell activity.7. Red light in the form of an annulus produced large responses in central L-cells without eliciting direct responses in central green cones. In these conditions green cones developed depolarizing waves which included a large, sharp transient.8. It is concluded from these and other results that the direct response of each cone is modified by two interactions: enhancement only from nearby cones of the same colour and depression controlled (through L-cell feed-back) by cones of all colours. In this way the response of any cone will change as the proportion of responses in cones of different colours changes, this proportion being a function of the wave-length of the light.

NEURONES which respond preferentially to slits of light of a given orientation have been found at the ganglion cell level of the retina1, as well as at higher centres of the visual system2–4. Present knowledge about the morphology of such cells and the organisation of the inputs to these cells does not account for the origins of their specialised receptive field properties. We report here that the horizontal cells impaled in the visual streak of the turtle retina (a specialised linear area centralis5 analogous to the fovea of higher vertebrates) respond preferentially to slits of light oriented parallel to the streak and thus parallel to the horizontal meridian of the animal's visual field5. Anatomical studies demonstrate that these horizontal cells are all elongated and oriented in the direction of the streak. Thus, our findings show that a class of neurones exists in the retina whose morphologies may form the basis for their specialised receptive field properties.