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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 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).