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Notes: Summary This paper considers the functional significance of fused rhabdoms. Since all rhabdomeres are joined tightly together, the possibility of optical and electrical coupling between retinula cells is greatly enhanced. We study the extent and consequences of this coupling in order to understand the functional significance of fused rhabdoms. Our methods include both theory and intracellular recordings. The results are as follows: Optical Coupling. Because rhabdomeres of different spectral types are fused into a common light guide, the absorption properties of each influence the manner in which light is transmitted along the composite rhabdom structure. 1. Each rhabdomere acts as if it were an absorption filter in front of all others, i.e. rhabdomeres function as lateral absorption filters (Fig. 4). 2. As a consequence of this filtering, the shape of the spectral sensitivity curve for each retinula cell is approximately independent of the amount of light it absorbs, i.e. independent of the rhabdomere's length and concentration of photopigment (Fig. 7). This is in direct contrast to the retinula cells of fly that have spectral sensitivity curves which become progressively flatter as more light is absorbed (Snyder and Pask, 1973). In other words, the flattening of curves by self absorption is prevented by optical coupling. 3. Thus, one functional advantage of the fused rhabdom (due to optical coupling) is that each retinula cell can have a high absolute sensitivity while preserving its spectral identity (narrow spectral sensitivity curves). (Compare Fig. 5 to Fig. 6.) Thus the same receptors can operate in a high sensitivity and in a colour vision system (cf. vertebrate rods and cones).Since all spectral cell types are together in one rhabdom, the animal can have hue discrimination in a small field of view (fine grain colour vision). Thus an individual ommatidium has the potential for providing excellent spectral discrimination. 4. If two cells have photopigments with absorption maxima close together, the maxima of their spectral sensitivity curves are moved further apart (Fig. 8). 5. In the absence of electrical coupling polarization sensitivity (PS) can depend dramatically on wavelength. The spectral composition of the rhabdom, in addition to the direction of the microvilli, profoundly influences the polarization sensitivity vs. wavelength PS (λ) curves of individual retinula cells. This is shown theoretically for the worker bee rhabdom (Fig. 10) where (a) there is a pronounced difference in PS (λ) between cells with orthogonal microvilli and (b) green retinula cells show a large PS in the green while the UV cells show a much smaller PS in the UV (Fig. 13).

Notes: Summary 1. The responses of retinula cells and large monopolar cells (LMC's) to axial light flashes were recorded intracellularly in dark-adapted dragonflies (Figs. 1 and 4). 2. LMC's respond to retinal illumination with a triphasic graded hyperpolarisation whose amplitude and waveform is intensity dependent. An initial hyperpolarising “on” transient is followed by a smaller amplitude sustained plateau. A rapid positive going “off” transient follows the cessation of the stimulus. Intensity is encoded as hyperpolarisation amplitude for action potentials are not recorded in these cells. 3. Measurements of the difference between LMC and retinula response latency (2 msec, Fig. 6) and the LMC angular sensitivity (Fig. 7) confirm the previous anatomical studies suggesting that the LMC's are post-synaptic to retinula axons and receive their major input from axons with the same fields of view. 4. Comparison of retinula and LMC response/intensity functions (Fig. 2) suggests that the visual signal is amplified when it is transferred from the retinula cell soma to a LMC. 5. The derivation of average normalised response/intensity functions (Fig. 3) leads to an estimation of gain during the transfer of the LMC “on” transient and plateau amplitudes (Fig. 8). Their maximum values are times 14 and times 12, respectively. 6. The possible mechanisms for producing amplification at this level in the visual system are discussed together with the significance of amplification in terms of the performance of the visual system. 7. The synaptic noise level in the LMC's is high, from 4.2% to 15.6% of the maximum response amplitude with an average value of 8.6%. It is shown that this is equivalent to a receptor signal of 400 μV at threshold. It is proposed that the high noise level is the result of multiple synapses. It is shown that multiple synapses increase the visual signal: synaptic noise ratio in proportion to the square root of the number of synapses, in a manner analagous to a signal averager. 8. It is concluded that the retinula-LMC pathway acts, in thedark-adapted state as a high sensitivity detection system, and shows several adaptations to maximise the signal:noise ratio.

Notes: Summary 1. Positive potentials with the waveform of a smoothed retinula receptor potential are recorded from dragonfly lamina. All potentials of this type are calledlamina positive potentials, unless their origin is certain. To establish their origin and to see how information is processed in the lamina their sensitivity characteristics are examined in detail and compared with retinula cell somata. 2. By comparing response noise at low intensities (discrete potentials), polarised light sensitivity, angular sensitivity, spectral sensitivity (Fig. 1) and intensity/ response functions it becomes clear that not all lamina positive potentials originate from retinula cell axons. The potentials are divided into two groups,axon responses andlamina depolarisationa. 3. Axon responses have sensitivities and characteristics that resemble closely retinula cell somata (Fig. 2, 3, 5). In some cases recordings are correlated with a definite resting potential. It is concluded that axon responses probably originate intracellularly from retinula axons in the lamina. Some axon responses show small light induced action potentials (Fig. 6). 4. Lamina depolarisations show the properties of a summed response from several retinula cell somata, i.e. high signal:noise ratio at low intensities, no polarised light sensitivity (Fig. 1), broad or distorted spectral sensitivities (Fig. 4). On the basis of this evidence, together with their broader angular sensitivity functions and unique intensity/response functions (Figs. 2, 3) it is proposed that lamina depolarisations are extracellular in origin. 5. Previous studies also suggest an extracellular origin for the lamina depolarisation. It is concluded that this extracellular signal may act as a negative feed-back within the lamina. 6. No pre-synaptic summation of retinula axon signals can be found in dragonfly lamina. Voltage amplification and improved signal: noise ratio result from a summation of inputs upon second order neurons. The unique individual properties of retinula cells are maintained in the lamina and they may function as inputs to other systems.

Notes: Summary 1. The transfer ofangular sensitivity from photoreceptors (retinula cells) to second order neurons (large monopolar cells — LMC's) is investigated by means of intracellular recordings from the retina and lamina ofHemicordulia tau. Angular sensitivity is measured by using a single point light source in two different ways. In the first, the constant intensity test flash method, responses to test flashes delivered at different angles of incidence are compared with the axial intensity/response function of the unit (Pig. 1). In the second, the off-axis intensity/ response function method, complete LMC intensity/response functions are derived at a number of angular inclinations toaxis (defined as the point of maximum sensitivity within the unit's visual field) (Fig. 4, 5). 2. The constant intensity test flash method shows that dragonfly retinula cells have a high angular sensitivity when compared to other insects. The horizontal and vertical acceptance angles are 1.46°±0.44 and 1.31°±0.23 respectively. Application of this same method to LMC's demonstrates that they retain retinal acuity for their angular sensitivity functionsappear to be the same as those of retinula cells (Pig. 2, Table 1). 3. The off-axis intensity/response functions show that the shape of the triphasic LMC response waveform depends upon the angular inclination of the stimulus to axis. The relative amplitudes of “on” transient and plateau (Pig. 3) vary independantly with angle (Pig. 4). The slope of the plateau response/log intensity curve decreases as the stimulus moves off axis but the “on” transient curve's slope remains relatively constant (Pig. 5). 4. Constant intensity test flash methods cannot measure LMC angular sensitivity because the slope of the response/log intensity curves depend upon stimulus inclination. The off-axis intensity/response function method shows that lateral inhibition narrows the LMC visual fields (Fig. 6) and angular sensitivity is increased during the transfer of visual information (Pig. 7). 5. Examination of the LMC response waveform (Fig. 8) and the intensity/response characteristics (Fig. 5) shows that two types of inhibition shape the response to square wave stimuli. Intracartridge inhibition acts at the level of the first synapses to attenuate the response to maintained stimuli. Intercartridge inhibition acts with a time delay to depolarise the LMC membrane and increase angular sensitivity. 6. It is concluded that LMC's integrate retinal input by acting as high sensitivity detectors of contrast differences within the spatial domain. Their role as an input to the visual system is discussed in relationship to visual behaviour and its experimental analysis.

Notes: Summary 1. Intracellular recordings are made from second order interneurons in the visual system of the dragonfly,Hemicordulia tau. The dark-adapted spectral sensitivity functions of these large monopolar cells (LMC's) are measured by obtaining in tensity/response functions at 12 wavelengths between 317 and 614 nm. 2. The LMC response waveform depends only upon intensity and not upon wavelength (Fig. 1) and at all wavelengths except 614 nm the slopes of the V/logI curves are not significantly different (Fig. 2 and Table 1). It is concluded that dark-adapted LMC's are not spectrally opponent and the dark-adapted pathway shows no wavelength dependent inhibition. 3. The average spectral sensitivity function of LMC's is extremely broad (Fig. 5) and sensitivity is greater than 40% between 317 and 614 nm. Maximum sensitivity is at 475 nm and at this wavelength 1.2×109 quanta/cm2/s are required to give a response of 50% maximum amplitude. 4. LMC's have no polarisation sensitivity in the green and low PS in the UV. This, together with the shape of the spectral sensitivity curve and the lack of response noise in the UV, suggests that the “linked pigment” retinula cells with broad spectral sensitivity functions form the major receptor input to LMC's. 5. The LMC's lack of colour sensitivity is considered along with their absolute sensitivity, dynamic range of response, angular sensitivity and lightadaptation properties. It is concluded that the LMC's function to provide the visual system with a high sensitivity, low noise contrast input that is well suited for high acuity movement perception.

Notes: Summary 1. The spectral, polarisation and absolute sensitivities of darkadapted retinula cells of the ventral retina of the dragonflyHemicorduliatau are measured by making intracellular recordings of receptor potential. 2. On the basis of their spectral sensitivities the retinula cells fall into two distinct classes. The “single pigment” cells have narrow spectral sensitivity functions corresponding to the absorption spectrum of either a UV (360 nm) or a blue (440 nm) or a green (510 nm) rhodopsin photopigment (Fig. 1). The “linked pigment” cells have broadened spectral sensitivity functions which suggest that at least three rhodopsins contribute to their response (Table 1, Fig. 2). 3. The “single pigment” UV cell has a high PS of 7.1 whereas the “linked pigment” cells are insensitive to polarised light (Figs. 4, 6). The PS(λ) functions of “linked pigment” cells (plots of PS against stimulus wavelength) show that the UV cell acts as a dichroic filter placed in front of the “linked pigment” cells (Figs. 5, 6, 7) and that self-screening plays no role in downgrading “linked pigment” cell PS (Fig. 6). 4. The absolute sensitivity of all cell types is precisely calibrated using monochromatic parallel rays of light of the most effective (peak) wavelength directed along the optical axis of the ommatidium. The PAQ50 (PeakAxial cornealQuantal irradiance required to give a transient response of 50% maximum) is measured and its reciprocal defines the APS50 (AxialPeakSensitivity as determined at the 50% level). 5. When one knows the spectral and angular sensitivities of units APS50 measurements are comparable from cell to cell and organism to organism. In dragonfly ventral retina the “linked pigment” cells and “single pigment” green cells have almost identical absolute sensitivities (APS50 = 1.5× 10−12 S.D. = 1.2× 10−12 whereas the “single pigment”UV cell is 12 times more sensitive (APS50 = 1.8× 10−11, S.D. = 2.2× 10−11 (Fig. 8). 6. The UV cell has a peak-to-peak voltage noise level 7 times greater than that of the “linked pigment”cells, (Figs. 9, 10). The analysis of noise in terms of equivalent intensity (Appendix 1) shows that this voltage noise is generated by the random absorption of photons (photon shot noise) and/or intrinsic noise that is statistically identical (Figs. 11, 12). 7. The high voltage noise levels of the UV cell result from its transducer having a voltage gain greater than that of the other cells. Thus higher gain gives the UV cell a greater absolute sensitivity which compensates for the relative scarcity of UV photons and enables the UV cell to operate in sunlight with a voltage output similar to that of “linked pigment” cells. 8. It is concluded that the retinula cells of the ventral retina show a division of labour into colour, PS and contrast-coding types but absolute sensitivities are carefully matched so that all the cell types described can operate simultaneously with almost identical dynamic response ranges.

Notes: Summary The capacity of the compound eye to perceive its spatial environment is quantified by determining the number of different pictures that can be reconstructed by its array of retinula cells. We can then decide on the best compromise between an animal's capacity for fine detail and contrast sensitivity. The theory accounts for imperfect optics, photon noise, and angular motion limitations to acuity. 1. There is an optimum parameterp = D Δ φ, whereD is the facet diameter andΔ φ is the interommatidial angle, for each mean luminance, angular velocity and mean object contrast. We find that this value ofp is approximately that found by Snyder (1977) for threshold resolution of a sinusoidal grating at the ommatidial sampling frequency. 2. A diffraction limited eye (D Δ φ ≅λ/√¯3) is the optimum design only for those animals that are active in the brightest sunlight, and have a region of their eye that normally experiences low angular velocity, otherwise it is better to have a largerD Δ φ. λ is the wavelength of light in vacuum. 3. The design of the flyMusca is consistent with that of an animal with high angular velocity.

Notes: Summary 1. Intracellular recordings from photoreceptors and large monopolar cells (LMC's) of the flyCalliphora stygia, and the dragonflyHemicordulia tau, were used to examine the peripheral light adaptation processes of the insect compound eye. 2. Photoreceptor and lamina adaptation mechanisms were separated by comparing the response waveforms and intensity/response functions (plotted as V/log I curves) of receptors (Figs. 1 and 3) and LMC's (Figs. 2 and 4), subjected to identical regimes of adaptation. 3. Photoreceptor adaptation occurs in two phases, a rapid one lasting 100 ms, and a slow phase taking up to 60 s to complete (Fig. 1). This adaptation shifts theV/logI curves to higher intensities without changing their shape or slope (Fig. 3). Adaptation is negligible at low intensities but with stronger adaptation range sensitivity changes approach proportionality to background increments (Fig. 7). 4. Lamina adaptation mechanisms adjust the LMCV/logI curve in response to new background levels within 200 ms, producing a phasic response waveform within which background signals are annihilated (Figs. 1, 3, 8). The shape and amplitude of the saturated LMC ‘on’ and ‘off’ transient responses change with light adaptation (Figs. 2, 3). 5. At all background intensities examined the slopes of the LMC V/log I curves remain about 8–10 times that of the photoreceptors under the same conditions, implying that lamina adaptation does not change the voltage gain of the first synapse. We propose that light induced depolarisation of the lamina extracellular space subtracts away the standing background signal from the photoreceptor terminals. 6. During dark adaptation the faster lamina mechanism can be superimposed upon slower photoreceptor processes (Fig. 9). 7. A comparison of our findings with studies of higher order neurons of the compound eye suggests that peripheral adaptation mechanisms play an important role in determining the response of the entire visual system. 8. The peripheral light adaptation processes of fly and dragonfly are similar, and the intensity/response functions of retinula cells and LMC's resemble those of vertebrate cones and bipolar cells respectively (Fig. 11). We propose that this analogy has a functional basis. Both vertebrate and invertebrate systems use a ‘log transform-subtraction-multiplication” strategy to match the response bandwidth of peripheral neurons to the expected intensity fluctuation about any one mean, and in so doing maximise the image detail sent to higher centres.

Notes: Summary Photoreceptor membrane is dichroic, i.e. its absorption depends on the direction of the electric vectorE of linearly polarised light. Dichroism depends on the arrangement and packing of the membrane (form dichroism) in addition to the orientation and dichroic properties of the chromophores associated with the membrane (intrinsic dichroism). 1. The amount of light absorbed by a photoreceptor depends on the orientation of the dipoles within its membrane. There is an optimum orientation for the photoreceptor to have a maximum absolute sensitivity tounpolarised light andthis orientation depends on the photoreceptor structure. In both vertebrate discs and fly rhabdomeres 1 to 6, the optimum dipole orientation is a random alignment within the plane of the membrane; while in fused rhabdoms the dipoles must be highly aligned with the microvillus axis for maximum absolute sensitivity to unpolarised light. Measurements of dichroism indicate that this optimum dipole orientation is indeed present in vertebrate outer segments, fly rhabdomeres 1 to 6 and the crustacean rhabdom. This furnishes strong evidence for the view that dipoles are orientated in the plane of the membrane to provide the majority of photoreceptors with maximum absolute sensitivity to unpolarised light. Membrane dichroism is only a secondary consequence of this objective [See Section VII]. 2. The measured side-on dichroism of vertebrate outer segments is shown to represent aproduct of form and intrinsic dichroism, where the form factor is approximately 1.6. MSP measurements must be corrected for form dichroism to obtain the intrinsic dichroism. 3. An extremely simple derivation is given for the dichroism of microvilli (rhabdomeric photoreceptor) membranes in which each miorovillus is treated as if constructed from 4 vertebrate outer segment discs. The analysis accounts for both form and intrinsic dichroism. 4. The dichroic ratio of microvilli is estimated using contemporary knowledge of membrane and its associated chromophores. We find that the long (dipole) axis of the chromophore shows preferential axial alignment whenever the dichroic ratio of microvilli exceeds 1.67 to 1. This is to be contrasted to the value of 2 to 1 determined from the ideal (but unrealizable) model of Moody and Parriss (1962). An alignment of about 17° with the microvillus axis gives a dichroism of about 10 to 1 while perfect alignment gives 20 to 1. 5. We propose that the microvillus is fluid membrane rolled into a cylindrical shell. The bending causes the preferential dipole alignment when the rhodopsin molecule is asymmetrical in the membrane plane; a spherical molecule is associated with random dipole orientation in thecunved microvillus. Thus, the shape of the rhodopsin molecule is postulated to be asymmetric in crustacea and spherical in fly 1–6 microvilli.