The effect of vertical extent of stimuli on cockroach optomotor response.

Using tethered American cockroaches walking on a trackball in a spherical virtual reality environment, we tested optomotor responses to horizontally moving black-and-white gratings of different vertical extent under six different light intensities. We found that shortening the vertical extent of the wide-field stimulus grating within a light level weakened response strength, reduced average velocity, and decreased angular walking distance. Optomotor responses with the vertically shortened stimuli persisted down to light intensity levels of 0.05 lx. Response latency seems to be independent of both the height of the stimulus and light intensity. The optomotor response started saturating at the light intensity of 5 lx, where the shortest behaviourally significant stimulus was 1°. This indicates that the number of vertical ommatidial rows needed to elicit an optomotor response at 5 lx and above is in the single digits, maybe even just one. Our behavioural results encourage further inquiry into the interplay of light intensity and stimulus size in insect dim-light vision.

Effect of stimulus height on cockroach optomotor response.

Using tethered American cockroaches walking on a trackball in a spherical virtual reality environment, we tested optomotor responses to horizontally moving black-and-white gratings of different vertical extent under six different light intensities. We found that shortening the vertical extent of the wide-field stimulus grating within a light level weakened response strength, reduced average velocity and decreased angular walking distance. Optomotor responses with the vertically shortened stimuli persisted down to light intensity levels of 0.05 lx. Response latency seems to be independent of both the height of the stimulus and light intensity. The optomotor response started saturating at a light intensity of 5 lx, where the shortest behaviourally significant stimulus was 1 deg. This indicates that the number of vertical ommatidial rows needed to elicit an optomotor response at 5 lx and above is in the single digits, maybe even just one. Our behavioural results encourage further inquiry into the interplay of light intensity and stimulus size in insect dim-light vision.

The role of ocelli in cockroach optomotor performance.

Insect ocelli are relatively simple eyes that have been assigned various functions not related to pictorial vision. In some species they function as sensors of ambient light intensity, from which information is relayed to various parts of the nervous system, e.g., for the control of circadian rhythms. In this work we have investigated the possibility that the ocellar light stimulation changes the properties of the optomotor performance of the cockroach Periplaneta americana. We used a virtual reality environment where a panoramic moving image is presented to the cockroach while its movements are recorded with a trackball. Previously we have shown that the optomotor reaction of the cockroach persists down to the intensity of moonless night sky, equivalent to less than 0.1 photons/s being absorbed by each compound eye photoreceptor. By occluding the compound eyes, the ocelli, or both, we show that the ocellar stimulation can change the intensity dependence of the optomotor reaction, indicating involvement of the ocellar visual system in the information processing of movement. We also measured the cuticular transmission, which, although relatively large, is unlikely to contribute profoundly to ocellar function, but may be significant in determining the mean activity level of completely blinded cockroaches.

Non-linear amplification of graded voltage signals in the first-order visual interneurons of the butterfly Papilio xuthus.

Lamina monopolar cells (LMCs) are the first-order visual interneurons of insects and crustacea, primarily involved in achromatic vision. Here, we investigated morphological and electrophysiological properties of LMCs in the butterfly Papilio xuthus Using intracellular recording coupled with dye injection, we found two types of LMCs. Cells with roundish terminals near the distal surface of the medulla demonstrating no or small depolarizing spikes were classified as L1/2. Cells with elongated terminals deep in the medulla that showed prominent spiking were classified as L3/4. The majority of LMCs of both types had broad spectral sensitivities, peaking between 480 and 570 nm. Depending on the experimental conditions, spikes varied from small to action potential-like events, with their amplitudes and rates decreasing as stimulus brightness increased. When the eye was stimulated with naturalistic contrast-modulated time series, spikes were reliably triggered by high-contrast components of the stimulus. Spike-triggered average functions showed that spikes emphasize rapid membrane depolarizations. Our results suggest that spikes are mediated by voltage-activated Na+ channels, which are mainly inactivated at rest. Strong local minima in the coherence functions of spiking LMCs indicate that the depolarizing conductance contributes to the amplification of graded responses even when detectable spikes are not evoked. We propose that the information transfer strategies of spiking LMCs change with light intensity. In dim light, both graded voltage signals and large spikes are used together without mutual interference, as a result of separate transmission bandwidths. In bright light, signals are non-linearly amplified by the depolarizing conductance in the absence of detectable spikes.

Characterization of the first-order visual interneurons in the visual system of the bumblebee (Bombus terrestris).

The bumblebee (Bombus terrestris) has become a common model animal in the study of various aspects of vision and visually guided behavior. Although the bumblebee visual system has been studied to some extent, little is known about the functional role of the first visual neuropil, the lamina. In this work, we provide an anatomical and electrophysiological description of the first-order visual interneurons, lamina monopolar cells (LMCs), of the bumblebee. Using intracellular recording coupled with dye injection, we found that bumblebee LMCs morphologically resemble those found in the honeybee, although only the LMC type L1 cells could be morphologically matched directly between the species. LMCs could also be classified on the basis of their light response properties as spiking or non-spiking. We also show that some bumblebee LMCs can produce spikes during responses to stimulation with naturalistic light contrasts, a property unusual for these neurons.

Frequency-selective transmission of graded signals in large monopolar neurons of blowfly Calliphora vicina compound eye.

The functional roles of voltage-gated K(+)(Kv) channels in visual system interneurons remain poorly studied. We have addressed this problem in the large monopolar cells (LMCs) of the blowfly Calliphora vicina, using intracellular recordings and mathematical modeling methods. Intracellular recordings were performed in two cellular compartments: the synaptic zone, which receives input from photoreceptors, and the axon, which provides graded potential output to the third-order visual neurons. Biophysical properties of Kv conductances in the physiological voltage range were examined in the dark with injections of current in the discontinuous current-clamp mode. Putative LMC types 1/2 and 3 (L1/2 and L3, respectively) had dissimilar Kv channelomes: L1/2 displayed a prominent inactivating Kv conductance in the axon, while L3 cells were characterized by a sustained delayed-rectifier Kv conductance. To study the propagation of voltage signals, the data were incorporated into the previously developed mathematical model. We demonstrate that the complex interaction between the passive membrane properties, Kv conductances, and the neuronal geometry leads to a resonance-like filtering of signals with peak frequencies of transmission near 15 and 40 Hz for L3 and L1/2, respectively. These results point to distinct physiological roles of different types of LMCs.

Visual ecology and potassium conductances of insect photoreceptors.

Voltage-activated potassium channels (Kv channels) in the microvillar photoreceptors of arthropods are responsible for repolarization and regulation of photoreceptor signaling bandwidth. On the basis of analyzing Kv channels in dipteran flies, it was suggested that diurnal, rapidly flying insects predominantly express sustained K(+) conductances, whereas crepuscular and nocturnally active animals exhibit strongly inactivating Kv conductances. The latter was suggested to function for minimizing cellular energy consumption. In this study we further explore the evolutionary adaptations of the photoreceptor channelome to visual ecology and behavior by comparing K(+) conductances in 15 phylogenetically diverse insects, using patch-clamp recordings from dissociated ommatidia. We show that rapid diurnal flyers such as the blowfly (Calliphora vicina) and the honeybee (Apis mellifera) express relatively large noninactivating Kv conductances, conforming to the earlier hypothesis in Diptera. Nocturnal and/or slow-moving species do not in general exhibit stronger Kv conductance inactivation in the physiological membrane voltage range, but the photoreceptors in species that are known to rely more on vision behaviorally had higher densities of sustained Kv conductances than photoreceptors of less visually guided species. No statistically significant trends related to visual performance could be identified for the rapidly inactivating Kv conductances. Counterintuitively, strong negative correlations were observed between photoreceptor capacitance and specific membrane conductance for both sustained and inactivating fractions of Kv conductance, suggesting insignificant evolutionary pressure to offset negative effects of high capacitance on membrane filtering with increased conductance.

Difference in dynamic properties of photoreceptors in a butterfly, Papilio xuthus: possible segregation of motion and color processing.

The eyes of the Japanese yellow swallowtail butterfly, Papilio xuthus, contain six spectral classes of photoreceptors, each sensitive either in the ultraviolet, violet, blue, green, red or broadband wavelength regions. The green-sensitive receptors can be divided into two subtypes, distal and proximal. Previous behavioral and anatomical studies have indicated that the distal subtype appears to be involved in motion vision, while the proximal subtype is important for color vision. Here, we studied the dynamic properties of Papilio photoreceptors using light stimulation with randomly modulated intensity and light pulses. Frequency response (gain) of all photoreceptor classes shared a general profile-a broad peak around 10 Hz with a declining slope towards higher frequency range. At 100 Hz, the mean relative gain of the distal green receptors was significantly larger than any other receptor classes, indicating that they are the fastest. Photoreceptor activities under dim light were higher in the ultraviolet and violet receptors, suggesting higher transduction sensitivities. Responses to pulse stimuli also distinguished the green receptors from others by their shorter response latencies. We thus concluded that the distal green receptors carry high frequency information in the visual system of Papilio xuthus.

Effect of light intensity on flight control and temporal properties of photoreceptors in bumblebees.

To control flight, insects rely on the pattern of visual motion generated on the retina as they move through the environment. When light levels fall, vision becomes less reliable and flight control thus becomes more challenging. Here, we investigated the effect of light intensity on flight control by filming the trajectories of free-flying bumblebees (Bombus terrestris, Linnaeus 1758) in an experimental tunnel at different light levels. As light levels fell, flight speed decreased and the flight trajectories became more tortuous but the bees were still remarkably good at centring their flight about the tunnel's midline. To investigate whether this robust flight performance can be explained by visual adaptations in the bumblebee retina, we also examined the response speed of the green-sensitive photoreceptors at the same light intensities. We found that the response speed of the photoreceptors significantly decreased as light levels fell. This indicates that bumblebees have both behavioural (reduction in flight speed) and retinal (reduction in response speed of the photoreceptors) adaptations to allow them to fly in dim light. However, the more tortuous flight paths recorded in dim light suggest that these adaptations do not support flight with the same precision during the twilight hours of the day.

Developmental changes in biophysical properties of photoreceptors in the common water strider (Gerris lacustris): better performance at higher cost.

Although the dependence of invertebrate photoreceptor biophysical properties on visual ecology has already been investigated in some cases, developmental aspects have largely been ignored due to the general research emphasis on holometabolous insects. Here, using the patch-clamp method, we examined changes in biophysical properties and performance of photoreceptors in the common water strider Gerris lacustris during postembryonic development. We identified two types of peripheral photoreceptors, green and blue sensitive. Whole cell capacitance (a measure of cell size) of blue photoreceptors was significantly higher than the capacitance of green photoreceptors (69 ± 20 vs. 43 ± 12 pF, respectively). Most of the measured morphological and biophysical parameters changed with development. Photoreceptor capacitance increased progressively and was positively correlated with sensitivity to light, magnitudes and densities of light-induced (LIC) and delayed rectifier K(+) (IDR) currents, membrane corner frequency, and maximal information rate [Spearman rank correlation coefficients: 0.70 (sensitivity), 0.79 (LIC magnitude), 0.79 (IDR magnitude), 0.48 (corner frequency), and 0.57 (information rate)]. Transient K(+) current increased to a smaller extent, while its density decreased. We found no significant changes in the properties of single photon responses or levels of light-induced depolarization, the latter indicating a balanced channelome expansion associated with IDR expression. However, the dramatic ∼7.6-fold increase in IDR from first instars to adults indicated a development-related rise in the metabolic cost of information. In conclusion, this study provides novel insights into functional photoreceptor adaptations with development and illustrates remarkable variability in patterns of postembryonic retinal development in hemimetabolous insects with dissimilar visual ecologies and behaviors.

Large variation among photoreceptors as the basis of visual flexibility in the common backswimmer.

The common backswimmer, Notonecta glauca, uses vision by day and night for functions such as underwater prey animal capture and flight in search of new habitats. Although previous studies have identified some of the physiological mechanisms facilitating such flexibility in the animal's vision, neither the biophysics of Notonecta photoreceptors nor possible cellular adaptations are known. Here, we studied Notonecta photoreceptors using patch-clamp and intracellular recording methods. Photoreceptor size (approximated by capacitance) was positively correlated with absolute sensitivity and acceptance angles. Information rate measurements indicated that large and more sensitive photoreceptors performed better than small ones. Our results suggest that backswimmers are adapted for vision in both dim and well-illuminated environments by having open-rhabdom eyes with large intrinsic variation in absolute sensitivity among photoreceptors, exceeding those found in purely diurnal or nocturnal species. Both electrophysiology and microscopic analysis of retinal structure suggest two retinal subsystems: the largest peripheral photoreceptors provide vision in dim light and the smaller peripheral and central photoreceptors function primarily in sunlight, with light-dependent pigment screening further contributing to adaptation in this system by dynamically recruiting photoreceptors with varying sensitivity into the operational pool.

Performance of blue- and green-sensitive photoreceptors of the cricket Gryllus bimaculatus.

The compound eye of the cricket Gryllus bimaculatus contains a specialized dorsal rim area (DRA) populated by distinct blue-sensitive photoreceptors responsible for perception of polarized light. The rest of the eye is dominated by green-sensitive photoreceptors. Using patch clamp we studied dissociated ommatidia of nocturnal adults and diurnal eight-instar nymphs with the goals (1) of characterizing the biophysical properties of cricket photoreceptors in general and (2) describing the functionally dissimilar blue- and green-sensitive photoreceptors in terms of voltage-gated channel composition and signal coding. Despite different lifestyles, adult and nymph photoreceptors were indistinguishable. No significant circadian changes were observed in K⁺ currents. In contrast, prominent differences were seen between blue- and green-sensitive photoreceptors. The former were characterized by relatively low absolute sensitivity, high input resistance, slow quantum bumps with long latencies, small light-induced and K⁺ currents and low steady-state depolarization. Information rate, a measure of photoreceptor performance calculated from voltage responses to bandwidth-limited white noise-modulated light contrast, was 87 ± 8 bits s⁻¹ in green-sensitive photoreceptors vs. 59 ± 14 bits s⁻¹ in blue-sensitive photoreceptors, implying a limited role of DRA in the perception of visual contrasts. In addition, evidence of electrical coupling between photoreceptors is presented.

Cockroach optomotor responses below single photon level.

Reliable vision in dim light depends on the efficient capture of photons. Moreover, visually guided behaviour requires reliable signals from the photoreceptors to generate appropriate motor reactions. Here, we show that at behavioural low-light threshold, cockroach photoreceptors respond to moving gratings with single-photon absorption events known as 'quantum bumps' at or below the rate of 0.1 s(-1). By performing behavioural experiments and intracellular recordings from photoreceptors under identical stimulus conditions, we demonstrate that continuous modulation of the photoreceptor membrane potential is not necessary to elicit visually guided behaviour. The results indicate that in cockroach motion detection, massive temporal and spatial pooling takes place throughout the eye under dim conditions, involving currently unknown neural processing algorithms. The extremely high night-vision capability of the cockroach visual system provides a roadmap for bio-mimetic imaging design.

Equilibrating errors: reliable estimation of information transmission rates in biological systems with spectral analysis-based methods.

Shannon's seminal approach to estimating information capacity is widely used to quantify information processing by biological systems. However, the Shannon information theory, which is based on power spectrum estimation, necessarily contains two sources of error: time delay bias error and random error. These errors are particularly important for systems with relatively large time delay values and for responses of limited duration, as is often the case in experimental work. The window function type and size chosen, as well as the values of inherent delays cause changes in both the delay bias and random errors, with possibly strong effect on the estimates of system properties. Here, we investigated the properties of these errors using white-noise simulations and analysis of experimental photoreceptor responses to naturalistic and white-noise light contrasts. Photoreceptors were used from several insect species, each characterized by different visual performance, behavior, and ecology. We show that the effect of random error on the spectral estimates of photoreceptor performance (gain, coherence, signal-to-noise ratio, Shannon information rate) is opposite to that of the time delay bias error: the former overestimates information rate, while the latter underestimates it. We propose a new algorithm for reducing the impact of time delay bias error and random error, based on discovering, and then using that size of window, at which the absolute values of these errors are equal and opposite, thus cancelling each other, allowing minimally biased measurement of neural coding.

Postembryonic developmental changes in photoreceptors of the stick insect Carausius morosus enhance the shift to an adult nocturnal life-style.

Optimization of sensory processing during development can be studied by using photoreceptors of hemimetabolous insects (with incomplete metamorphosis) as a research model. We have addressed this topic in the stick insect Carausius morosus, where retinal growth after hatching is accompanied by a diurnal-to-nocturnal shift in behavior, by recording from photoreceptors of first instar nymphs and adult animals using the patch-clamp method. In the nymphs, ommatidia were smaller and photoreceptors were on average 15-fold less sensitive to light than in adults. The magnitude of A-type K(+) current did not increase but the delayed rectifier doubled in adults compared with nymphs, the K(+) current densities being greater in the nymphs. By contrast, the density of light-induced current did not increase, although its magnitude increased 8.6-fold, probably due to the growth of microvilli. Nymph photoreceptors performed poorly, demonstrating a peak information rate (IR) of 2.9 ± 0.7 bits/s versus 34.1 ± 5.0 bits/s in adults in response to white-noise stimulation. Strong correlations were found between photoreceptor capacitance (a proxy for cell size) and IR, and between light sensitivity and IR, with larger and more sensitive photoreceptors performing better. In adults, IR peaked at light intensities matching irradiation from the evening sky. Our results indicate that biophysical properties of photoreceptors at each age stage and visual behavior are interdependent and that developmental improvement in photoreceptor performance may facilitate the switch from the diurnal to the safer nocturnal lifestyle. This also has implications for how photoreceptors achieve optimal performance.

Membrane filtering properties of the bumblebee (Bombus terrestris) photoreceptors across three spectral classes.

Filtering properties of the membrane form an integral part of the mechanisms producing the light-induced electrical signal in insect photoreceptors. Insect photoreceptors vary in response speed between different species, but recently it has also been shown that different spectral photoreceptor classes within a species possess diverse response characteristics. However, it has not been quantified what roles phototransduction and membrane properties play in such diversity. Here, we use electrophysiological methods in combination with system analysis to study whether the membrane properties could create the variation of the response speed found in the bumblebee (Bombus terrestris) photoreceptors. We recorded intracellular responses from each photoreceptor class to white noise-modulated current stimuli and defined their input resistance and linear filtering properties. We found that green sensitive cells exhibit smaller input resistance and membrane impedance than other cell classes. Since green sensitive cells are the fastest photoreceptor class in the bumblebee retina, our results suggest that the membrane filtering properties are correlated with the speed of light responses across the spectral classes. In general, our results provide a compelling example of filtering at the sensory cell level where the biophysical properties of the membrane are matched to the performance requirements set by visual ecology.

Cellular elements for seeing in the dark: voltage-dependent conductances in cockroach photoreceptors.

BACKGROUND: The importance of voltage-dependent conductances in sensory information processing is well-established in insect photoreceptors. Here we present the characterization of electrical properties in photoreceptors of the cockroach (Periplaneta americana), a nocturnal insect with a visual system adapted for dim light. RESULTS: Whole-cell patch-clamped photoreceptors had high capacitances and input resistances, indicating large photosensitive rhabdomeres suitable for efficient photon capture and amplification of small photocurrents at low light levels. Two voltage-dependent potassium conductances were found in the photoreceptors: a delayed rectifier type (KDR) and a fast transient inactivating type (KA). Activation of KDR occurred during physiological voltage responses induced by light stimulation, whereas KA was nearly fully inactivated already at the dark resting potential. In addition, hyperpolarization of photoreceptors activated a small-amplitude inward-rectifying (IR) current mediated at least partially by chloride. Computer simulations showed that KDR shapes light responses by opposing the light-induced depolarization and speeding up the membrane time constant, whereas KA and IR have a negligible role in the majority of cells. However, larger KA conductances were found in smaller and rapidly adapting photoreceptors, where KA could have a functional role. CONCLUSIONS: The relative expression of KA and KDR in cockroach photoreceptors was opposite to the previously hypothesized framework for dark-active insects, necessitating further comparative work on the conductances. In general, the varying deployment of stereotypical K+ conductances in insect photoreceptors highlights their functional flexibility in neural coding.

Extracellular potentials modify the transfer of information at photoreceptor output synapses in the blowfly compound eye.

Signal processing in fly photoreceptors and visual interneurons takes place with graded potentials. Photoreceptors drive large monopolar cells (LMCs) with synapses that, like their counterparts in vertebrates, have a high gain and introduce strong spatiotemporal antagonism (Laughlin et al., 1987) that implements predictive coding (Srinivasan et al., 1982). The synapses are contained in compartments, lamina cartridges, whose extracellular potentials change with illumination (Shaw, 1984). We described these extracellular field potentials (FPs) using a novel permeabilization technique that converts neurons into extracellular recording probes. Having characterized extracellular FPs, we went on to study them using conventional microelectrodes. Extracellular space in a cartridge is electrically isolated from the body cavity and retina [input resistance (R(in)) = 6.0 MOmega in dark], and light adaptation increases this isolation (R(in) = 7.8 MOmega). In the dark, the extracellular space is 30 mV hyperpolarized compared with retina, and this promotes tonic synaptic activity by depolarizing the synaptic terminals. Illumination depolarizes the extracellular space, and voltage-clamp studies suggest that the postsynaptic chloride current in LMCs contributes to this light response. The presynaptic transmembrane potential in the photoreceptor axon was estimated by subtracting the FP from intracellular recordings. By backing off the presynaptic input, the FP can reset the synaptic operating range, produce response transients, and contribute to predictive coding by subtracting redundant low frequencies.

Ultrasmall and customizable multichannel electrodes for extracellular recordings.

Increasing demand exists for smaller multichannel electrodes that enable simultaneous recordings of many neurons in a noninvasive manner. We report a novel method for manufacturing ultrasmall carbon fiber electrodes with up to seven closely spaced recording sites. The electrodes were designed to minimize damage to neuronal circuitry and to be fully customizable in three dimensions so that their dimensions can be optimally matched to those of the targeted neuron population.

Regulation of excitation-contraction coupling in mouse cardiac myocytes: integrative analysis with mathematical modelling.

BACKGROUND: The cardiomyocyte is a prime example of inherently complex biological system with inter- and cross-connected feedback loops in signalling, forming the basic properties of intracellular homeostasis. Functional properties of cells and tissues have been studied e.g. with powerful tools of genetic engineering, combined with extensive experimentation. While this approach provides accurate information about the physiology at the endpoint, complementary methods, such as mathematical modelling, can provide more detailed information about the processes that have lead to the endpoint phenotype. RESULTS: In order to gain novel mechanistic information of the excitation-contraction coupling in normal myocytes and to analyze sophisticated genetically engineered heart models, we have built a mathematical model of a mouse ventricular myocyte. In addition to the fundamental components of membrane excitation, calcium signalling and contraction, our integrated model includes the calcium-calmodulin-dependent enzyme cascade and the regulation it imposes on the proteins involved in excitation-contraction coupling. With the model, we investigate the effects of three genetic modifications that interfere with calcium signalling: 1) ablation of phospholamban, 2) disruption of the regulation of L-type calcium channels by calcium-calmodulin-dependent kinase II (CaMK) and 3) overexpression of CaMK. We show that the key features of the experimental phenotypes involve physiological compensatory and autoregulatory mechanisms that bring the system to a state closer to the original wild-type phenotype in all transgenic models. A drastic phenotype was found when the genetic modification disrupts the regulatory signalling system itself, i.e. the CaMK overexpression model. CONCLUSION: The novel features of the presented cardiomyocyte model enable accurate description of excitation-contraction coupling. The model is thus an applicable tool for further studies of both normal and defective cellular physiology. We propose that integrative modelling as in the present work is a valuable complement to experiments in understanding the causality within complex biological systems such as cardiac myocytes.