ALBERT

Notes: Summary 1. Retinula cells ofPeriplaneta in the same state of morphological adaptation differ among themselves in absolute sensitivity by up to 1 log unit (tenfold). 2. Morphologically dark-adapted retinula cells are, on average, ten times as sensitive as light-adapted cells to light on the optical axis. 3. All retinula cells are about 5 times as sensitive to plane polarized light in the plane of its optimum effectiveness as in the orthogonal plane. 4. Retinula cells fall into two groups with optimum plane of polarization 90 ° apart, corresponding with the 2 planes of rhabdomeric microvilli.

Notes: Summary 1. Intracellularly recorded illumination potentials from retinula cells (probably the green sensitive cells) of the cockroachPeriplaneta show the typical tetraphasic responses to graded intensities of light. In the dark-adapted state, the plateau phase is relatively greater than in the light-adapted state, and, in general, the responses are larger. 2. Angles of acceptance in the light-adapted state are 2.4±0.9 ° SD for the horizontal plane and 2.3±0.6 ° SD for the vertical plane. Angles of acceptance were about three times larger for dark-adapted cells, being 6.7±1.8 ° SD in the horizontal plane and 6.9±1.3 ° SD in the vertical plane. 3. In each state, the visual fields are circularly symmetrical. 4. The pigment movements and the palisade which develops upon dark-adaptation (Butler, 1973b) are the only anatomical features which can account for this change in acuity. 5. The above changes are similar in all ommatidia. Combining these measurements with the map of interommatidial angles (Butler, 1973a) leads to the conclusion that movement perception is not constant in different parts of the eye, and that changes during adaptation have unequal effects on movement perception in different parts of the eye.

Notes: Summary The sensitivity of single retinula cells 1–6 of the flyEristalis towards monochromatic light was measured in two types of adaptation experiments. First, the sensitivity was measured at various wavelengths after adaptation of the visual pigment to the photochemical equilibrium at a fixed wavelength (Figs. 3, 4, 5 and 6). Then, on the same cell, the sensitivity was measured by a test light of a fixed wavelength after adaptation at each wavelength, from 333 nm to 594 nm (Figs. 8 and 9). For this fixed test wavelength the isosbestic wavelength was selected. The relative absorption coefficients of the visual pigment rhodopsin (R) and its photoproduct metarhodopsin (M) were calculated. The absorption spectra are comparable to those found by photometric measurements on cells 1–6 of the same population ofEristalis (Stavenga, 1976). The calculations also show that the percentage absorption at peak wavelength (near 450 nm) is 85–95% for unpolarized light, and when the isosbestic point is taken to be 485 nm, the relative quantum efficiency for the M toR conversion is about 0.8 of that of theR toM conversion. Moreover, an absorption spectrum for a single pigment, with a lowβ peak near 360 nm, is found, irrespective of whether the retinula cell has a greater sensitivity in the UV than at the standard peak of longer wavelength. The additional sensitivity in the UV, which is characteristic of some retinula cells 1–6, is therefore apparently unrelated to the basic properties of the visual pigment.

Notes: Summary The wavelength dependence of the afterpotentials following a bright illumination was studied in single photoreceptor cells of the droneflyEristalis. Cells with only a spectral sensitivity peak in the blue were selected. As previously demonstrated, these cells contain a rhodopsin absorbing maximally at about 450–460 nm, which upon photoconversion transforms into a metarhodopsin absorbing maximally at about 550 nm (Tsukahara and Horridge, 1977). With the visual pigment initially all in the rhodopsin form, a high rate of visual pigment conversion results in an afterhyperpolarization (AHP) when the fraction of metarhodopsin remains negligible after illumination as occurs at longer wavelengths if the intensity is high. Intensive illumination at short wavelengths is followed by a prolonged depolarizing afterpotential (PDA). The magnitude of the PDA peaks at low intensities at about 450–460 nm, corresponding to the peak of the cell's spectral sensitivity (i.e. the rhodopsin peak). With increasing intensity of illumination, however, the peak shifts progressively towards 430 nm, which corresponds to the photoequilibrium with maximum metarhodopsin that can be established by monochromatic light. From this result, it is inferred that the PDA is related to the induced fall in the rhodopsin fraction. The PDA can be abolished, or knocked down, by a long-wavelength flash which reconverts remaining metarhodopsin into rhodopsin. Therefore the decline of the PDA is restrained by the existing amount of metarhodopsin. Possible theories of afterpotentials are discussed.

Notes: Summary In the anterior region ofEristalis eye there is a type of retinula cell with the following features. The spectral sensitivity is broad and the slope of the V/log10I curve increases with increasing wavelength when the response is measured to the initital peak of the receptor potential. With a change in stimulus wavelength from 452 nm to 594 nm, the optical axis moves about 1° and the plane of maximum sensitivity to polarized light changes by a definite angle. These effects can be attributed to an additional depolarizing effect on this retinula cell from a neighbouring cell of a different kind. The receptor potential waveform of the first cell type is also wavelength dependent. Measurements made to the plateau or notch following the peak reveal the same interaction but now it is an inhibition with latency 75–200 ms. A candidate cell which could cause the lateral interaction has a spectral sensitivity peak near 540 nm. If this is the correct source, the lateral interaction is in one direction because the slope of the V/log10I curve of the cell with 540 nm peak is independent of wavelength and it has negligible sensitivity in the range 350–450 nm.

Notes: Summary 1. The butterfly retina exhibits strong interactions between photoreceptor responses recorded intracellularly. In general, the receptor which is locally giving the largest response suppresses the responses of neighbouring receptors that are less strongly stimulated. The effect enhances the differences between the primary photo-receptors and reduces responses to stimuli that excite all receptors together. 2. The interaction is explained in terms of a high extracellular resistance, so that receptor currents pass through other receptors in the opposite direction to those of their own responses. The result is that receptors are actively turned off by colours away from their own peak wavelength. 3. This effect applies most strongly to colour and to polarization plane when the stimulus is a point source on axis, and is therefore strong between the receptors of the same ommatidium. 4. The result is that spectral sensitivity peaks and angular sensitivity peaks are narrower, and polarisation sensitivity is greater, than expected from single retinula cells in isolation. The sensitivities measured electrophysiologically cannot be easily related to the physical properties of the visual pigments. Polarisation sensitivity (PS) can reach 50. 5. There are four types of primary photoreceptor, with peak near 380 nm, 450 nm, 550 nm and 610 nm. Cell marking usually reveals these as single retinula cells. Near the peak spectral sensitivity the responses are up to 60 mV positive-going, but away from the peak they can be negative-going. 6. Anatomically the retina ofPapilio has four distal, four proximal retinula cells, and a ninth basal cell. Narrow pigment cells and tracheoles squeeze through the substantial basement membrane along with each bundle of nine axons. 7. Two of the distal retinula cells contain red pigment grains near the rhabdom. The distal retinula cells are UV or blue sensitive. Green sensitive cells are proximal and can be coupled in opposite pairs. Red sensitive cells are proximal. 8. The UV sensitive cell with peak near 380 nm is the most sensitive of the cell types when measured by the position of theV/logI curve on the intensity axis at the spectral peak of each type. The red-sensitive cells are also sensitive. By its inhibitory effect, interaction between receptors reduces the sensitivity measurement on this scale. 9. Angular sensitivities measured with positive-going responses near the spectral peaks are narrow (Δρ-2°); when measured with negative-going responses they are wider (Δρ=3° to 5°). 10. One type of unit has only negative-going responses to −60 mV, with Δp=2° to 5°, spectral peak near 550 nm and sometimes also 380 nm or 450 nm. This type has not been marked and is regarded as a restricted channel for return current. ItsV/logI curve extends over an intensity range of 106. 11. The variety of the units suggests that their responses are not due to a simple regular network with all units connected indiscriminately to all others at all times through their terminals. There are selective channels for current flow and some retinula cells appear to be little influenced by others. 12. Theory shows that when there is a direct electrical coupling between a pair of retinula cells (not passing through the extracellular space) it is possible to balance out the negative interactions caused by current flow through their terminals. Far from degenerating the signals, direct electrical coupling can cancel the negative interaction, and this may be its normal function.

Notes: Summary 1. Intracellular recordings from single retinula cells of the locust compound eye show that the visual field is about twice as wide in dark-adapted (horizontal Δ ρ=6.6°) as in light-adapted eyes (horizontal Δ ρ=3.4°). 2. The time course of this increase in acceptance angle is correlated with the replacement of a dense packing of mitochondria by a fluid-filled palisade around the rhabdom during dark adaptation. 3. This correlation is taken as evidence that the acceptance angle in light-adapted eyes is limited by the internal reflection at the boundary between rhabdomere and cytoplasm. 4. Direct tests of the acuity of single retinula cells to equally spaced black and white stripes of various repeat distances show reasonable agreement with calculation of the contrast caused by such stripes in single receptor cells, with the observed angles of acceptance, and with optomotor and other acuity studies (Palka). 5. No evidence from the retinula cells supports either optical or electrical interaction between adjacent ommatidia.

Notes: Summary Intracellular electrode recording techniques were used to measure the responses of single visual cells in the locust to various intensities of monochromatic lights of wavelength 350 to 600 mμ. Response-energy curves, found for ten cells to be parallel at all wavelengths, were used to deduce the relative number of quanta required to evoke a constant response. The average response-energy curve from these ten cells was used in obtaining spectral sensitivity data for another ten short-lived cells, where only one response at each wavelength could be obtained. Spectral sensitivity curves for all twenty cells showed a peak in the blue-violet (maximum about 430 mμ) region of the spectrum. In addition, all cells showed some green sensitivity (maximum about 515 mμ) but in different cells this varied from 15% to 100% of the blue-violet maximum for the cell. The magnitude of the errors is discussed. The results are consistent with the hypothesis that two visual pigments which fit a Dartnall nomogram for rhodopsin are contributing, in various ratios, to the spectral sensitivity of every cell.

Notes: Summary The spectral sensitivity of movement perception in Carcinus has a peak near 508 mμ, irrespective of the direction of the movement or of the plane of polarization of the stimulating light, although the primary sensory cells are 10 times as sensitive to light in one plane of polarization as in the plane at right angles. This balance in the input to the movement perception system breaks down with a stimulating light at 〈439 and at 〉650 mμ. Only one set of connexions between the primary receptors in the movement perception system agrees with all the results of manipulating the various attributes of the visual input, and this model has unique properties which allow the subsequent discrimination of each attribute of the stimulus independent of the other attributes.

Notes: Summary 1. The retinula cells ofVanessa are similar in physiological properties, including spectral sensitivityS(λ) and negative electrical coupling, to those already described inPapilio, except that red-sensitive cells have not been found in either retina or lamina. 2. In the dark-adapted eye, with a stimulus of one colour, lamina ganglion cells yield only hyperpolarizations. 3. Some lamina ganglion cells (LMC's) have a broad flatS(λ) with angular sensitivity showing that they receive summed input from only one ommatidium. Others have narrowS(λ) and narrow field suggesting that primary receptors from single ommatidia interact on or before reaching them. Narrow peaks are near 500 or 550 nm, but not spread through the spectrum, suggesting colour specific behaviour rather than colour vision. 4. Vanessa itea, V. kershawi, Precis villida andHeteronympha merope all have optomotorS(λ) similar to the curves for green-sensitive photoreceptors, with broad peak between 500 and 550 nm. 5. Optomotor responses of butterflies fall off rapidly around 0.1 cd·m−2, whereas insects with superposition eyes are 100 times more sensitive. Calibrations suggest that the butterfly optomotor threshold is well above the photon flux that yields abundant bumps in the retinula cells. 6. There is difficulty in reconciling theS(λ) of the optomotor response with theS(λ) of any of the individual LMC's. 7. The physiological properties of receptors, LMC's and deep optic lobe units are brought together in a discussion of insect colour vision.