Inhibition in the eye of Limulus

In the compound lateral eye of Limulus each ommatidium functions as a single receptor unit in the discharge of impulses in the optic nerve. Impulses originate in the eccentric cell of each ommatidium and are conducted in its axon, which runs without interruption through an extensive plexus of nerve fibers to become a fiber of the optic nerve. The plexus makes interconnections among the ommatidia, but its exact organization is not understood. The ability of an ommatidium to discharge impulses in the axon of its eccentric cell is reduced by illumination of other ommatidia in its neighborhood: the threshold to light is raised, the number of impulses discharged in response to a suprathreshold flash of light is diminished, and the frequency with which impulses are discharged during steady illumination is decreased. Also, the activity that can be elicited under certain conditions when an ommatidium is in darkness can be inhibited similarly. There is no evidence for the spread of excitatory influences in the eye of Limulus. The inhibitory influence exerted upon an ommatidium that is discharging impulses at a steady rate begins, shortly after the onset of the illumination on neighboring ommatidia, with a sudden deep minimum in the frequency of discharge. After partial recovery, the frequency is maintained at a depressed level until the illumination on the neighboring receptors is turned off, following which there is prompt, though not instantaneous recovery to the original frequency. The inhibition is exerted directly upon the sensitive structure within the ommatidium: it has been observed when the impulses were recorded by a microelectrode thrust into an ommatidium, as well as when they were recorded more proximally in single fibers dissected from the optic nerve. Receptor units of the eye often inhibit one another mutually. This has been observed by recording the activity of two optic nerve fibers simultaneously. The mediation of the inhibitory influence appears to depend upon the integrity of nervous interconnections in the plexus: cutting the lateral connections to an ommatidium abolishes the inhibition exerted upon it. The nature of the influence that is mediated by the plexus and the mechanism whereby it exerts its inhibitory action on the receptor units are not known. The depression of the frequency of the discharge of nerve impulses from an ommatidium increases approximately linearly with the logarithm of the intensity of illumination on receptors in its vicinity. Inhibition of the discharge from an ommatidium is greater the larger the area of the eye illuminated in its vicinity. However, equal increments of area become less effective as the total area is increased. The response of an ommatidium is most effectively inhibited by the illumination of ommatidia that are close to it; the effectiveness diminishes with increasing distance, but may extend for several millimeters. Illumination of a fixed region of the eye at constant intensity produces a depression of the frequency of discharge of impulses from a nearby ommatidium that is approximately constant, irrespective of the level of excitation of the ommatidium. The inhibitory interaction in the eye of Limulus is an integrative process that is important in determining the patterns of nervous activity in the visual system. It is analogous to the inhibitory component of the interaction that takes place in the vertebrate retina. Inhibitory interaction results in the exaggeration of differences in sensory activity from different regions of the eye illuminated at different intensities, thus enhancing visual contrast.     

Inhibitory interaction of receptor units in the eye of Limulus

The inhibition that is exerted mutually among the receptor units (ommatidia) in the lateral eye of Limulus has been analyzed by recording oscillographically the discharge of nerve impulses in single optic nerve fibers. The discharges from two ommatidia were recorded simultaneously by connecting the bundles containing their optic nerve fibers to separate amplifiers and recording systems. Ommatidia were chosen that were separated by no more than a few millimeters in the eye; they were illuminated independently by separate optical systems. The frequency of the maintained discharge of impulses from each of two ommatidia illuminated steadily is lower when both are illuminated together than when each is illuminated by itself. When only two ommatidia are illuminated, the magnitude of the inhibition of each one depends only on the degree of activity of the other; the activity of each, in turn, is the resultant of the excitation from its respective light stimulus and the inhibition exerted on it by the other. When additional receptors are illuminated in the vicinity of an interacting pair too far from one ommatidium to affect it directly, but near enough to the second to inhibit it, the frequency of discharge of the first increases as it is partially released from the inhibition exerted on it by the second (disinhibition). Disinhibition simulates facilitation; it is an example of indirect effects of interaction taking place over greater distances in the eye than are covered by direct inhibitory interconnections. When only two interacting ommatidia are illuminated, the inhibition exerted on each (decrease of its frequency of discharge) is a linear function of the degree of activity (frequency of discharge) of the other. Below a certain frequency (often different for different receptors) no inhibition is exerted by a receptor. Above this threshold, the rate of increase of inhibition of one receptor with increasing frequency of discharge of the other is constant, and may be at least as high as 0.2 impulse inhibited in one receptor per impulse discharged by the other. For a given pair of interacting receptors, the inhibitory coefficients are not always the same in the two directions of action. The responses to steady illumination of two receptor units that inhibit each other mutually are described quantitatively by two simultaneous linear equations that express concisely all the features discussed above. These equations may be extended and their number supplemented to describe the responses of more than two interacting elements.     

The responses of Limulus optic nerve fibers to patterns of illumination on the receptor mosaic

The inhibition that is exerted mutually among receptor units (ommatidia) of the compound eye of Limulus is less for units widely separated than for those close together. This diminution of inhibition with distance is the resultant of two factors: (1) the threshold of inhibitory action increases with increasing distance between the units involved; and (2) the coefficient of inhibitory action decreases with increasing distance. The discharge of nerve impulses from ommatidia at various distances from one another may be described quantitatively by a set of simultaneous linear equations which express the excitatory effects of the illumination on each ommatidium and the inhibitory interactions between each ommatidium and its neighbors. The values of the thresholds and coefficients of inhibitory action, which appear as parameters in these equations, must be determined empirically: their dependence on distance is somewhat irregular and cannot yet be expressed in an exact general law. Nevertheless the diminution of inhibitory influences with distance is sufficiently uniform that patterns of neural response generated by various patterns of illumination on the receptor mosaic can be predicted qualitatively. Such predictions have been verified experimentally for two simple patterns of illumination: an abrupt step in intensity, and a simple gradient between two levels of intensity (the so-called Mach pattern). In each case, transitions in the pattern of illumination are accentuated in the corresponding pattern of neural response.     

Properties of visual cells in the lateral eye of Limulus in situ.

Excitatory properties of visual cells in the lateral eye of Limulus, investigated by optic nerve recordings in situ, differ significantly from the properties of cells in the classical, excised eye preparation. The differences suggest the possibility that two receptor mechanisms function in the eye in situ: one mechanism encodes low light intensities and the other responds to high intensities. The two mechanisms enable each ommatidium to respond over an intensity range of approximately 10 log units. This hypothesis was tested by measuring the increment threshold and the spectral sensitivity, by studying light and dark adaptation, and by analyzing the variability of the impulse discharge. Although the results do not conclusively identify two receptor mechanisms, they indicate that a process or a part of a process that functions in the eye in situ is abolished by excising the eye or cutting off its blood supply.     

Preferential adhesion maintains separation of ommatidia in the Drosophila eye

In the Drosophila eye, neighboring ommatidia are separated by inter-ommatidial cells (IOCs). How this ommatidial spacing emerges during eye development is not clear. Here we demonstrate that four adhesion molecules of the Irre cell recognition module (IRM) family play a redundant role in maintaining separation of ommatidia. The four IRM proteins are divided into two groups: Kirre and Rst are expressed in IOCs, and Hbs and Sns in primary pigment cells (1°s). Kirre binds Hbs and Sns in vivo and in vitro. Reducing activity of either Rst or Kirre alone had minimal effects on ommatidial spacing, but reducing both together led to direct ommatidium:ommatidium contact. A similar phenotype was also observed when reducing both Hbs and Sns. Consistent with the role of these factors in sorting ommatidia, mis-expression of Hbs plus Sns within a single IOC led to complete separation of the cell from neighboring ommatidia. Our results indicate mutual preferential adhesion between ommatidia and IOCs mediated by four IRM proteins is both necessary and sufficient to maintain separation of ommatidia.     

Spatial summation of inhibitory influences in the eye of Limulus,and the mutual interaction of receptor units

The inhibitory influences exerted mutually among the receptor units (ommatidia) of the lateral eye of Limulus are additive. If two groups of receptors are illuminated together the total inhibition they exert on a "test receptor" near them (decrease in the frequency of its nerve impulse discharge in response to light) depends on the combined inhibitory influences exerted by the two groups. If the two groups are widely separated in the eye, their total inhibitory effect on the test receptor equals the sum of the inhibitory effects they each produce separately. If they are close enough together to interact, their effect when acting together is usually less than the sum of their separate effects, since each group inhibits the activity of the other and hence reduces its inhibitory influence. However, the test receptor, or a small group illuminated with it, may interact with the two groups and affect the net inhibitory action. A variety of quantitative effects have been observed for different configurations of three such groups of receptors. The activity of a population of n interacting elements is described by a set of n simultaneous equations, linear in the frequencies of the receptor elements involved. Applied to three interacting receptors or receptor groups equations are derived that account quantitatively for the variety of effects observed in the various experimental configurations of retinal illumination used.     

Die Verarbeitung stationärer optischer Nachrichten im Komplexauge von Limulus

Summary The functional properties of the processing of visual information by the complex eye of Limulus was studied. The spatial distribution of activity that results in the optic nerve when the Limulus eye is exposed to a stationary optical pattern depends upon the transfer characteristics of two subsystems: the dioptric apparatus and the nervous interactions comprising the lateral inhibition system. — The transfer characteristic of the dioptric apparatus is determined by the sensitivity distribution function of single ommatidia. This distribution was measured and found to be approximately of Gauss-function type. The sensitivity falls off to 1/e at a distance of one ommatidium; thus the visual fields of adjacent ommatidia strongly overlap. As a consequence of the overlap, amplitudes of the spatial Fourier components, of which the brightness distribution of the optical surround is made up, are more and more reduced with increasing frequency in the intensity distribution on the receptor mosaic. The amplitude of the spatial frequency 1/=0,25 ( in units of interommatidial distance) is reduced to half of the maximum value, which is attained at zero frequency. It is shown that the amplitude frequency characteristic of the sensitivity distribution function has no zeros, which means that no loss of optical information results from overlap of visual fields. Thus the resolving power of the dioptric apparatus is limited only by the number of receptors per unit area. — The transfer characteristic of the lateral inhibition system in the Limulus eye depends on the distribution of the inhibitory coefficients around the individual receptors. This distribution function was determined from excitatory responses in the optic nerve elicited by a spatial light intensity step function on the receptor mosaic. It is found that this distribution is also Gaussian in form, but decays to 1/e at a distance of eight to nine ommatidia along the major axis of the eye. The average value of the inhibitory coefficients between adjacent ommatidia was found to be 0,025. The amplitude frequency response of the inhibitory system is constant for high spatial frequencies down to 1/=0,1 while amplitudes of lower frequency sinusoids are reduced down to nearly half of the maximum value at frequency zero. The amplitude frequency characteristic of the inhibitory system ensures a one to one correspondence between the intensity distribution on the receptor mosaic and the excitation distribution in the optic nerve. The overall transfer characteristic of the eye is derived from the transfer characteristics of the dioptric apparatus and the inhibitory system. This characteristic is of bandpass type with a maximum amplitude response at a frequency of 1/=0,07. The overall transfer characteristic was independently confirmed in a separate experiment. The nature of the overall transfer characteristic shows that the inhibitory system does not exactly correct for the overlap of the visual fields of single ommatidia, which in principal the system could do if the distributions of inhibitory coefficients and ommatidia sensitivity were equal. The overall transfer characteristic of the Limulus eye garantees a one to one correspondence between patterns in the optical surround and excitation distributions in the optic nerve. — The average values of the inhibitory coefficients derived from these experiments are at least a factor ten smaller than those determined directly by other investigators. Possible explanations of this discrepency are discussed. — In a separate chapter the overall transfer characteristic for eyes submerged in water is described. It was found that this characteristic does not differ from that determined in air for the eye region which was investigated in the experiments. This result is explained by two properties of the eye which are dependent on the refractive index of the surround medium and whose influences cancel each other: the visual fields of ommatidia are reduced under water, while the divergence angles between the optical axes of adjacent ommatidia also diminish.

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This research was supported in part by the United States Air Force under Grant No. AF-EOAR-62-41 and monitored by the European Office, Office of Aerospace Research.     

Thermal Sensitivity of Lateral Inhibition in Limulus Eye

The effectiveness of lateral inhibition, measured as spike response decrement in a test ommatidium, produced by activity in a group of neighboring ommatidia, decreases as temperature decreases (Q₁₀ of 2.6). The corresponding sensory transducer-spike encoding processes have a weaker temperature dependence (Q₁₀ of 1.6). Relative synaptic delay, the time difference between the latency of inhibition onset and the latency of test receptor excitation, has a strong temperature dependence (Q₁₀ of 5), while receptor potential onset latency (Q₁₀ of 1.4) and optic nerve spike conduction velocity (Q₁₀ of 1.7), two factors inherent in relative synaptic delay, are less temperature sensitive. Oscillations of optic nerve spike response ("bursting") may be produced by thermal adjustment of temperature-sensitive parameters of the lateral inhibitory network in the retina. Burst interval has a strong temperature dependence (Q₁₀ of 2.4) and broad interspike interval distribution compared to the thermal sensitivity (Q₁₀ of 1.4) and narrow spike interval spectrum of the response of a single unit within the bursting group.     

The color-vision circuit in the medulla of Drosophila

BACKGROUND: Color vision requires comparison between photoreceptors that are sensitive to different wavelengths of light. In Drosophila, this is achieved by the inner photoreceptors (R7 and R8) that contain different rhodopsins. Two types of comparisons can occur in fly color vision: between the R7 (UV sensitive) and R8 (blue- or green sensitive) photoreceptor cells within one ommatidium (unit eye) or between different ommatidia that contain spectrally distinct inner photoreceptors. Photoreceptors project to the optic lobes: R1-R6, which are involved in motion detection, project to the lamina, whereas R7 and R8 reach deeper in the medulla. This paper analyzes the neural network underlying color vision into the medulla. RESULTS: We reconstruct the neural network in the medulla, focusing on neurons likely to be involved in processing color vision. We identify the full complement of neurons in the medulla, including second-order neurons that contact both R7 and R8 from a single ommatidium, or contact R7 and/or R8 from different ommatidia. We also examine third-order neurons and local neurons that likely modulate information from second-order neurons. Finally, we present highly specific tools that will allow us to functionally manipulate the network and test both activity and behavior. CONCLUSIONS: This precise characterization of the medulla circuitry will allow us to understand how color vision is processed in the optic lobe of Drosophila, providing a paradigm for more complex systems in vertebrates.     

Electrical potentials from the eye and optic nerve of Strombus: effects of electrical stimulation of the optic nerve.

1. Photic stimulation of the mature eye of Strombus can evoke in the optic nerve 'on' activity in numerous small afferent fibres and repetitive 'off' bursts of afferent impulses in a smaller number of larger fibres. 2. Synchronous invasion of the eye by electrically evoked impulses in small optic nerve fibres (apparently the 'on' afferents, antidromically activated) can evoke a burst of impulses in the larger 'off' fibres which propagate away from the eye. Invasion of the eye via one branch of optic nerve can evoke an answering burst in another branch. 3. Such electrically evoked bursts are similar to light-evoked 'off' bursts with respect to their impulse composition, their ability to be inhibited by illumination of the eye, and their susceptibility to MgCl2 anaesthesia. 4. Invasion of the eye by a train of repetitive electrically evoked impulses in the absence of photic stimulation can give rise to repetitive 'off' bursts as well as concomitant oscillatory potentials in the eye which are similar to those normally evoked by cessation of a photic stimulus. 5. The electrically evoked 'off' bursts appear to be caused by an excitatory rebound following the cessation of inhibitory synaptic input from photoreceptors which can be antidromically activated by electrical stimulation of the optic nerve. 6. The experimental results suggest that the rhythmic discharge of the 'off' fibres evoked by the cessation of a photic stimulus is mediated by the abrupt decrease of inhibitory synaptic input from the receptors.     

Ellipse mutations in the Drosophila homologue of the EGF receptor affect pattern formation, cell division, and cell death in eye imaginal discs.

Ellipse alleles are mutations of the EGF-receptor homologue that reduce the number of ommatidia in the eye imaginal disc. Cobalt sulfide staining, expression of hairy and scabrous proteins, and mosaic analysis indicated that Elp mutations affect ommatidial precluster formation in the morphogenetic furrow. BrdU incorporation studies suggest that cells diverted from precluster formation instead enter S-phase after the morphogenetic furrow. Genetic studies suggest that the DER has multiple functions during eye development and that several recessive hypomorphic alleles affect another aspect of DER function that is required after precluster formation. Elp mutations show genetic interactions with the neurogenic mutations Notch and Delta. The small number of ommatidia that differentiate in Elp/Elp are separated more than in wildtype and have been studied to investigate what aspects of ommatidium development are intrinsic to the ommatidium itself. It appears that each developing ommatidium cues the determination of photoreceptors, cone cells, and primary pigment cells, but that the secondary and tertiary pigment cells, and the mechanosensory bristles, can form independently. The normal rotation of ommatidia in the dorsal-ventral axis does not require the presence of the ommatidial array. A short-range signal from a nearby ommatidium is important for mitosis. Cells not close to an ommatidium do not go through mitosis and many die.     

Physiology of Photoreceptor Neurons in the Abdominal Nerve Cord of the Crayfish

Nerve fibers which respond to illumination of the sixth abdominal ganglion were isolated by fine dissection from connectives at different levels in the abdominal nerve cord of the crayfish. Only a single photosensitive neuron is found in each connective; its morphological position and pattern of peripheral connections are quite constant from preparation to preparation. These cells are "primary" photoreceptor elements by the following criteria: (1) production of a graded depolarization upon illumination and (2) resetting of the sensory rhythm by interpolated antidromic impulses. They are also secondary interneurons integrating mechanical stimuli which originate from appendages of the tail. Volleys in ipsilateral afferent nerves produce short-latency graded excitatory postsynaptic potentials which initiate discharge of one or two impulses; there is also a higher threshold inhibitory pathway of longer latency and duration. Contralateral afferents mediate only inhibition. Both inhibitory pathways are effective against both spontaneous and evoked discharges. In the dark, spontaneous impulses arise at frequencies between 5 and 15 per second with fairly constant intervals if afferent roots are cut. Since this discharge rhythm is reset by antidromic or orthodromic impulses, it is concluded that an endogenous pacemaker potential is involved. It is postulated that the increase in discharge frequency caused by illumination increases the probability that an inhibitory signal of peripheral origin will be detected.     

Development of the compound eyes of dragonflies (Odonata). II. Development of the larval compound eyes

The development of the compound eye was analyzed by marking individual ommatidia and by studying naturally occurring pigment band patterns. New ommatidia are added to the eye along its anterior margin. This changes the directions of view of the older ommatidia with the greatest change occurring in the fovea. New ommatidia are added to the fovea medially, and old ones are removed laterally as their interommatidial angles and directions of view in the visual field change. Over one-third of the aeshnid ommatidia are foveal during at least one of the early larval instars, and are then used for peripheral vision later in development. The design of each ommatidium is a compromise so that it is adapted for all stages of development, but sometimes better adapted for one instar than for others. Factors which are balanced for best vision are lens diameter, facet admission function, interommatidial angle, and inclination of the optic axis to the eye surface. Ommatidia are described in terms of these factors throughout their life history, from initial differentiation anteriorly, through passage through the fovea, to their final relatively posterior location.     

Spatial control of rhabdom shedding in the lateral eye of the American horseshoe crab, <Emphasis Type="Italic">Limulus polyphemus</Emphasis>

Membrane leaves the rhabdom of Limulusphotoreceptors either by transient shedding at dawn or throughout the day by light-driven shedding. We examined whether the light trigger for transient shedding and the light drive for light-driven shedding are localized properties of the illuminated photoreceptors or whether they are an array property of the retina. Four experiments were conducted during which the lateral eye was exposed to one of a variety of different illumination patterns for a day, fixed, dissected and cut into serial frozen sections. Immunocytochemistry with different antibodies to Limulus opsin and arrestin revealed the results of the two processes in a distinguishable way. Eyes stimulated with whole-eye illumination had both types of shedding or just light-driven shedding when transient shedding was blocked by cutting the optic nerve. Eyes exposed to whole-eye darkness had neither type of shedding. However, when only half of an eye was exposed to light, the dark half had the same kinds of shedding as the lighted half. We conclude that the signals to trigger or drive shedding must be communicated laterally from illuminated ommatidia to unilluminated ommatidia. Rhabdom shedding is an array property.     

The fine structure of the limulus optic nerve

Two complete composite photographs of the optic nerve of Limulus, made by electron microscopy, reveal the presence of neurosecretory granules in the large axons of the rudimentary eye neurons. The number of intermediate sized, (3–7 μ), of eccentric cells corresponds with the number of ommatidia as expected, but only their sheath of Schwann cells show an intimate interfolding. Based on the number of fine axons within the nerve each ommatidium has an average of 12–13 retinular cells. The diameter of their fibers is between 0.2 and 3 μ although the majority are between 1 and 1.5 μ. They are aggregated into bundles of six to seven fibers by the sheath cells although some bundles contain only two, others as many as 181 fibers. There is no indication in these studies that retinular cell axons within a bundle are associated with the same, adjacent, or other pattern of ommatidia. The photographs suggest that physiological activity in retinular cell axons might be detected most easily in the smallest bundles because they contain the fewest, but the larger retinular cell axons.     

The organization of perpendicular fibre pathways in the insect optic lobe.

High resolution serial photomicrography has been used to plot the axonal projection patterns between retina, lamina and medulla in the optic lobes of various insects with differing ommatidial receptor arrangements. Observations are reported on the cabbage white and skipper butterflies, the bee, locust, fly, backswimmer and waterbug. The patterns of these fibre pathways have previously eluded non-rigorous analyses primarily because of their physical dimensions but are revealed in this study to have striking precision and uniformity between species when examined at the level of individually identifiable cells. Axon bundles of the tracts between retina and lamina or lamina and medulla project between a single ommatidium and its corresponding lamina cartridge or between corresponding lamina and medulla cartridges. Lateral interweaving of axons between adjacent bundles is absent. The bundles preserve the retinotopic order within their total array, so transferring the pattern of retinulae directly upon the lamina and thence after horizontal inversion in the chiasma upon the medulla. Within the lamina neuropile on the other hand the trajectories of the individual terminals from a bundle have patterns which are species-specific, sometimes involving lateral divergences. In species with open-rhabdomere ommatidia the terminals distribute to a group of lamina cartidges with a pattern which resembles the receptor pattern in the overlying ommatidium. In species with fused-rhabdome ommatidia the terminals of a single retinula behave less interestingly and all enter the same cartridge, within which, again, each occupies a position related to its cell body position within the retinula. Long visual fibres in both eye types penetrate the lamina and terminate in the particular medulla cartridge that connects with the lamina cartridge underlying their ommatidium. The perpendicular fibre pathways therefore project the visual field exactly upon the medulla in all species while the lack of interweaving between adjacent fibre bundles precludes their involvement in lateral interactions between pathways with differing visual axes. Uniformity of these projection patterns between cell layers and species differences in retinular terminal locations in the lamina can be correlated with different modes of axon growth between and within neuropile layers during optic lobe neurogenesis. Further discussion surrounds the question of which particular receptors give rise to which type of axon, for which no clear generalization has yet emerged.     

Distribution of circadian photoreceptors in the compound eye of the cricket Gryllus bimaculatus

Adult male crickets (Gryllus bimaculatus) show a nocturnal circadian locomotor rhythm, which is driven by the pacemaker in the optic lamina-medulla complex and synchronizes to the light-dark (LD) cycle received by the compound eye. To see whether there was any specially differentiated circadian photoreceptor area in the eye, we examined the effect of a partial reduction of various areas of the compound eye, in addition to a removal of the contralateral optic lamina-medulla-compound eye complex, on entrainability of the locomotor rhythm. All operated animals showed a response to the LD cycle in their locomotor rhythm, no matter which area of the eye was left intact: They either stably entrained to an LD cycle or showed a sign of weak entrainment. The capacity for stable entrainment was still retained when only 262 ommatidia were left. Transient cycles needed for re-entrainment, following a 6-hr phase advance of the LD cycle, were measured in 20 reduced-eye animals showing clear stable entrainment. They were in inverse proportion to the number of ommatidia in the reduced eye: The fewer ommatidia there were, the more transient cycles were observed (r = -0.76, p less than 0.001). These results suggest that almost the whole area of the compound eye may contain circadian photoreceptors, and that the photic information from each ommatidium may additively affect the circadian clock to entrain via neural integration mechanisms.     

Mach bands in the lateral eye of Limulus

Patterns of optic nerve activity were computed for stationary step patterns of illumination from theoretical models of lateral inhibiton based on revised Hartlin-Ratliff equations. The computed response patterns contain well-defined Mach bands which match closely in amplitude and shape those recorded from single optic nerve fibers of the Limulus lateral eye. Theory and experiment show that the amplitude of the Mach bands is reduced by in inhibitory nonlinearity, the width of the Mach bands is approximately equal to the lateral dimension of the inhibitory field, but the shapes of the Mach bands are poor indices of the precise configuration of the inhibitory field. Theorems are proved establishing the equivalence of Mach-band patterns for models of different dimensions and a uniqueness condition for solutions of the piecewise linear model.     

Localization of visual pigments within rhabdoms of the compound eye of Spodoptera exempta (Insecta,Noctuidae)

Summary In the noctuid moth Spodoptera exempta, the distribution of visual pigments within the fused rhabdoms of the compound eyes was investigated by electron microscopy. Each ommatidium regularly contains eight receptor cells belonging to three morphological types: one distal, six medial, and one basal cell (Meinecke 1981); four different visual pigments — absorption maxima at approximately 355, 465, 515, and 560 nm — are known to occur within the eye (Langer et al. 1979). The compound eyes were illuminated in situ by use of monochromatic light of different wavelengths. This illumination produced a wide scale of structural changes in the microvilli of the rhabdomeres of individual cells. Preparation of eyes by freeze-substitution revealed the structural changes in the rhabdomeres to be effects of light occurring in vivo.The degree of structural changes may be considerably different in rhabdomeres within the same ommatidium; it was found to depend on the wavelength and the duration of illumination, the intensity received by the ommatidia as well as the spectral sensitivity of the receptor cells. Therefore, it was possible to estimate the spectral sensitivities of the morphological types of receptor cells. Generally, all medial cells are green receptors and all basal cells red receptors; distal cells are blue receptors in about two-thirds of the ommatidia, while in the remaining third of them distal cells are sensitive to ultraviolet light.Supported by Deutsche Forschungsgemeinschaft, Sonderforschungsbereich 114 (Bionach)     

Localization of the pupil trigger in insect superposition eyes

Summary In the superposition eyes of the sphingid moth Deilephila and the neuropteran Ascalaphus, adjustment to different intensities is subserved by longitudinal migrations of screening pigment in specialized pigment cells. Using ophthalmoscopic techniques we have localized the light-sensitive trigger that controls pigment position.In both species, local illumination of a small spot anywhere within the eye glow of a dark-adapted eye evokes local light adaptation in the ommatidia whose facets receive the light. Details of the response pattern demonstrate that a distal light-sensitive trigger is located axially in the ommatidium, just beneath the crystalline cone, and extends with less sensitivity deep into the clear zone. The distal trigger in Deilephila was shown to be predominantly UV sensitive, and a UV-absorbing structure, presumably the distal trigger, was observed near the proximal tip of the crystalline cone.In Ascalaphus we also found another trigger located more proximally, which causes local pigment reaction in the ommatidia whose rhabdoms are illuminated (the centre of the eye glow). The light-sensitive trigger for this response appears to be the rhabdom itself.