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.

1. Responses of the leg vibratory receptor neurons of the bugNezara viridula to substrate vibrations were recorded electrophysiologically and analysed. 2. The low frequency receptor neurons (LFR) respond in a phase-locked manner in the frequency range below 0.12 kHz. The low frequency receptor neurons of the first type (LFR 1) have been analysed in detail. Receptor neurons of this type respond to the upward movement of the leg in a vibration cycle. Their threshold curves follow the line of equal displacement values above 0.05 kHz of the stimulus carrier frequency (Fig. 1). Increasing displacement brings about an increase in the number of spikes per phase (Fig. 2). 3. The higher frequency vibratory receptor neurons are of two types. The middle frequency receptor neurons (MFR) show the highest sensitivity to the velocity component of vibration at frequencies around 0.2 kHz (Fig. 1); the high frequency receptor neurons (HFR) are most sensitive to velocity at frequencies between 0.75 and 1 kHz (Fig. 1). In the frequency range below the best velocity sensitivity, the threshold curves of both types of neurons follow the line of equal acceleration values; above the best frequency the curves follow the line of equal displacement values (Fig. 1). The shapes and positions of the response curves of both types depend on the stimulus carrier frequency (Fig. 3). The middle (MFR) and high frequency receptor neurons (HFR) respond with characteristically prolonged responses to applied vibrational stimuli of 0.2 kHz carrier frequency (Figs. 4–6). The phase-locked response pattern is observed in both neuron types in the frequency range up to 0.2 kHz (Figs. 4–6). 4. The frequency and time characteristics of the femaleNezara viridula calling song (FS 1) are well followed by the middle (MFR) and high frequency receptor neurons (HFR) (Figs. 7, 8), but the low frequency receptor neurons (LFR) follow the lower frequency components of the same female sound emission only at higher displacement values. 5. The origins of the responses of the low (LFR), middle (MFR) and high frequency receptor neurons (HFR) are discussed. The special response characteristics of the higher frequency receptor neurons, i.e. the middle (MFR) and high frequency (HFR) receptor neurons, at 0.2 kHz stimulus frequency may be due to the resonance of the special flaglike structure of cap cells of the subgenual organ.

1. Four spectral types of photoreceptors have been characterized by double electrode recordings, with one electrode inside the cell and the other immediately outside. The colour types are: UV — peak sensitivity at 390 nm, blue — 450 nm, green — 540 nm and red — 610 nm. Recordings from the red receptors presented here are the first complete electrophysiological confirmations of the microspectrophotometrically measured red receptors in the retina of butterflies (Bernard 1979). 2. Differential recording (extracellular subtracted from intracellular) shows that the hyperpolarizing afterpotential (HAP) is eliminated when a unit is stimulated at peak wavelength, and greatly reduced during off-peak stimulation. This implies that the HAP is at least partially due to passive lateral electrical inhibition, in addition to the ionic conductance changes previously postulated. 3. The change from a point source stimulus to a diffuse extended source increases both the extracellular and the intracellular hyperpolarizations (at off-peak wavelengths), and reduces the intracellular depolarization, but differential recordings show that the transmembrane potential does not change. 4. The existence of the secondary peak (UV) in green-peak cells and its changes in response to various stimuli are adequately explained by a rhodopsin with a β-peak and by interactions between the photoreceptors. 5. Differential recording brings most of the spectral sensitivities to the curves predicted by the Dartnall nomograms. The remaining discrepancies can be attributed to the inadequacy of the placement of the extracellular electrode. 6. A model of electrical lateral inhibition is presented that accounts for the observed effects. The model is an extended version of the one presented by Shaw (1975).

1. The organization of the pacemaker driving the circadian rhythm of N-acetyltransferase activity in the rat pineal gland was studied by observing changes of the rhythm caused by 1 min light pulses applied at night. These pulses proved to be effective phase-shifting signals. 2. After 1 min light pulses applied in the first half of the night. N-acetyltransferase activity began to increase anew following a lag period, as if the evening rise in N-acetyltransferase were phase-delayed. After 1 min light pulses applied past midnight, N-acetyltransferase activity declined rapidly and did not increase to the high night value through the rest of night as if the morning decline in N-acetyltransferase were phase-advanced. 3. The phase-response curve (PRC) showing phase-shifts of the evening N-acetyltransferase rise one day after 1 min light pulses had only phase-delays. The PRC showing phase-shifts of the morning N-acetyltransferase decline one day after the pulses had phase-advances as well as phase-delays; however the phase-advances were more pronounced. 4. The data are consistent with a hypothesis of two-oscillator pacemaking system for the N-acetyltransferase rhythm, proposed originally by Pittendrigh and Daan (1976) for the locomotor activity rhythm in nocturnal rodents.

1. Intracellular recordings from the photoreceptors of the locustsLocusta migratoria andValanga irregularis and the flyCalliphora stygia, were used to examine the differences between theV/logI curve (intensity/response function) determined in the dark adapted and the light adapted state. 2. For response amplitudes less than half the maximum the fully light adaptedV/logI curve follows the simple self-shunting model, where the number of channels activated is proportional to the number of photons absorbed. 3. We fitted ourV/logI curves with the commonly employed hyperbolic function Eq. (1), and found a consistent deviation of the experimental data from the predicted curves when responses exceeded half maximal amplitude. 4. The light adaptedV/logI curves of locusts exhibit a ‘kink’ deformation (Fig. 3) and 4 of the 80 cells recorded from locusts had a small spike superimposed on the rising phase of the photo-response (Fig. 1). No kink or spike was observed in the flyCalliphora. 5. Transmembrane recording with a double electrode eliminated the ERG as a candidate for changing the slope and producing the kink or the spike. Stimulation of single ommatidia also ruled out interommatidial interactions. 6. Change in pulse duration did not affect the slope or the shape of theV/logI curves.

In the thoracic part of the ventral cord ofNezara viridula four vibratory interneuron types were electrophysiologically identified and classified. They show different frequency-intensity characteristics, the LFVI, LFTVI and LFSVI being most sensitive in the low frequency region between 100 and 600 Hz and the HFVI being most sensitive in the higher frequency region between 600 and 1,500 Hz (Fig. 1).

1. This study considers a compound eye with a constant angular separation between the optical axes of individual ommatidia. When the modulation transfer function (MTF) of the system is a function which is constant for all spatial frequencies below the limit imposed by the sampling theorem, and zero otherwise, the eye will achieve the theoretically maximal information recording capacity. 2. Assuming a linear interaction between individual ommatidia, the presented computation (based on a Gaussian form of the angular sensitivity) leads to lateral interaction coefficients which provide the best approximation to the ideal MTF. Such lateral interaction greatly improves MTF's over those obtained when the ommatidia act independently. 3. Angular sensitivity functions which approach those obtained with the assumptions of optimal coupling between the ommatidia should be observable in second order cells of the lamina or medulla if the compound eye obeys this optimization. The improvement achieved by lateral interactions should be particularly significant for eyes which have a large overlap between the angular sensitivity functions of adjacent ommatidia.