Neural circuit tuning fly visual neurons to motion of small objects. II. Input organization of inhibitory circuit elements revealed by electrophysiological and optical recording techniques

A connectome of the Drosophila central complex reveals network motifs suitable for flexible navigation and context-dependent action selection

Flexible behaviors over long timescales are thought to engage recurrent neural networks in deep brain regions, which are experimentally challenging to study. In insects, recurrent circuit dynamics in a brain region called the central complex (CX) enable directed locomotion, sleep, and context- and experience-dependent spatial navigation. We describe the first complete electron-microscopy-based connectome of the Drosophila CX, including all its neurons and circuits at synaptic resolution. We identified new CX neuron types, novel sensory and motor pathways, and network motifs that likely enable the CX to extract the fly's head-direction, maintain it with attractor dynamics, and combine it with other sensorimotor information to perform vector-based navigational computations. We also identified numerous pathways that may facilitate the selection of CX-driven behavioral patterns by context and internal state. The CX connectome provides a comprehensive blueprint necessary for a detailed understanding of network dynamics underlying sleep, flexible navigation, and state-dependent action selection.

Towards Computational Models and Applications of Insect Visual Systems for Motion Perception: A Review

Motion perception is a critical capability determining a variety of aspects of insects' life, including avoiding predators, foraging, and so forth. A good number of motion detectors have been identified in the insects' visual pathways. Computational modeling of these motion detectors has not only been providing effective solutions to artificial intelligence, but also benefiting the understanding of complicated biological visual systems. These biological mechanisms through millions of years of evolutionary development will have formed solid modules for constructing dynamic vision systems for future intelligent machines. This article reviews the computational motion perception models originating from biological research on insects' visual systems in the literature. These motion perception models or neural networks consist of the looming-sensitive neuronal models of lobula giant movement detectors (LGMDs) in locusts, the translation-sensitive neural systems of direction-selective neurons (DSNs) in fruit flies, bees, and locusts, and the small-target motion detectors (STMDs) in dragonflies and hoverflies. We also review the applications of these models to robots and vehicles. Through these modeling studies, we summarize the methodologies that generate different direction and size selectivity in motion perception. Finally, we discuss multiple systems integration and hardware realization of these bio-inspired motion perception models.

Asymmetric processing of visual motion for simultaneous object and background responses

Visual object fixation and figure-ground discrimination in Drosophila are robust behaviors requiring sophisticated computation by the visual system, yet the neural substrates remain unknown. Recent experiments in walking flies revealed object fixation behavior mediated by circuitry independent from the motion-sensitive T4-T5 cells required for wide-field motion responses. In tethered flight experiments under closed-loop conditions, we found similar results for one feedback gain, whereas intact T4-T5 cells were necessary for robust object fixation at a higher feedback gain and in figure-ground discrimination tasks. We implemented dynamical models (available at http://strawlab.org/asymmetric-motion/) based on neurons downstream of T4-T5 cells—one a simple phenomenological model and another, physiologically more realistic model—and found that both predict key features of stripe fixation and figure-ground discrimination and are consistent with a classical formulation. Fundamental to both models is motion asymmetry in the responses of model neurons, whereby front-to-back motion elicits stronger responses than back-to-front motion. When a bilateral pair of such model neurons, based on well-understood horizontal system cells, downstream of T4-T5, is coupled to turning behavior, asymmetry leads to object fixation and figure-ground discrimination in the presence of noise. Furthermore, the models also predict fixation in front of a moving background, a behavior previously suggested to require an additional pathway. Thus, the models predict several aspects of object responses on the basis of neurons that are also thought to serve a key role in background stabilization.

Method and software for using m-sequences to characterize parallel components of higher-order visual tracking behavior in Drosophila

A moving visual figure may contain first-order signals defined by variation in mean luminance, as well as second-order signals defined by constant mean luminance and variation in luminance envelope, or higher-order signals that cannot be estimated by taking higher moments of the luminance distribution. Separating these properties of a moving figure to experimentally probe the visual subsystems that encode them is technically challenging and has resulted in debated mechanisms of visual object detection by flies. Our prior work took a white noise systems identification approach using a commercially available electronic display system to characterize the spatial variation in the temporal dynamics of two distinct subsystems for first- and higher-order components of visual figure tracking. The method relied on the use of single pixel displacements of two visual stimuli according to two binary maximum length shift register sequences (m-sequences) and cross-correlation of each m-sequence with time-varying flight steering measurements. The resultant spatio-temporal action fields represent temporal impulse responses parameterized by the azimuthal location of the visual figure, one STAF for first-order and another for higher-order components of compound stimuli. Here we review m-sequence and reverse correlation procedures, then describe our application in detail, provide Matlab code, validate the STAFs, and demonstrate the utility and robustness of STAFs by predicting the results of other published experimental procedures. This method has demonstrated how two relatively modest innovations on classical white noise analysis—the inclusion of space as a way to organize response kernels and the use of linear decoupling to measure the response to two channels of visual information simultaneously—could substantially improve our basic understanding of visual processing in the fly.

Impact of stride-coupled gaze shifts of walking blowflies on the neuronal representation of visual targets

During locomotion animals rely heavily on visual cues gained from the environment to guide their behavior. Examples are basic behaviors like collision avoidance or the approach to a goal. The saccadic gaze strategy of flying flies, which separates translational from rotational phases of locomotion, has been suggested to facilitate the extraction of environmental information, because only image flow evoked by translational self-motion contains relevant distance information about the surrounding world. In contrast to the translational phases of flight during which gaze direction is kept largely constant, walking flies experience continuous rotational image flow that is coupled to their stride-cycle. The consequences of these self-produced image shifts for the extraction of environmental information are still unclear. To assess the impact of stride-coupled image shifts on visual information processing, we performed electrophysiological recordings from the HSE cell, a motion sensitive wide-field neuron in the blowfly visual system. This cell has been concluded to play a key role in mediating optomotor behavior, self-motion estimation and spatial information processing. We used visual stimuli that were based on the visual input experienced by walking blowflies while approaching a black vertical bar. The response of HSE to these stimuli was dominated by periodic membrane potential fluctuations evoked by stride-coupled image shifts. Nevertheless, during the approach the cell's response contained information about the bar and its background. The response components evoked by the bar were larger than the responses to its background, especially during the last phase of the approach. However, as revealed by targeted modifications of the visual input during walking, the extraction of distance information on the basis of HSE responses is much impaired by stride-coupled retinal image shifts. Possible mechanisms that may cope with these stride-coupled responses are discussed.

Figure-ground discrimination behavior in Drosophila. I. Spatial organization of wing-steering responses

The behavioral algorithms and neural subsystems for visual figure–ground discrimination are not sufficiently described in any model system. The fly visual system shares structural and functional similarity with that of vertebrates and, like vertebrates, flies robustly track visual figures in the face of ground motion. This computation is crucial for animals that pursue salient objects under the high performance requirements imposed by flight behavior. Flies smoothly track small objects and use wide-field optic flow to maintain flight-stabilizing optomotor reflexes. The spatial and temporal properties of visual figure tracking and wide-field stabilization have been characterized in flies, but how the two systems interact spatially to allow flies to actively track figures against a moving ground has not. We took a systems identification approach in flying Drosophila and measured wing-steering responses to velocity impulses of figure and ground motion independently. We constructed a spatiotemporal action field (STAF) – the behavioral analog of a spatiotemporal receptive field – revealing how the behavioral impulse responses to figure tracking and concurrent ground stabilization vary for figure motion centered at each location across the visual azimuth. The figure tracking and ground stabilization STAFs show distinct spatial tuning and temporal dynamics, confirming the independence of the two systems. When the figure tracking system is activated by a narrow vertical bar moving within the frontal field of view, ground motion is essentially ignored despite comprising over 90% of the total visual input.

Spatial vision in insects is facilitated by shaping the dynamics of visual input through behavioral action

Insects such as flies or bees, with their miniature brains, are able to control highly aerobatic flight maneuvres and to solve spatial vision tasks, such as avoiding collisions with obstacles, landing on objects, or even localizing a previously learnt inconspicuous goal on the basis of environmental cues. With regard to solving such spatial tasks, these insects still outperform man-made autonomous flying systems. To accomplish their extraordinary performance, flies and bees have been shown by their characteristic behavioral actions to actively shape the dynamics of the image flow on their eyes ("optic flow"). The neural processing of information about the spatial layout of the environment is greatly facilitated by segregating the rotational from the translational optic flow component through a saccadic flight and gaze strategy. This active vision strategy thus enables the nervous system to solve apparently complex spatial vision tasks in a particularly efficient and parsimonious way. The key idea of this review is that biological agents, such as flies or bees, acquire at least part of their strength as autonomous systems through active interactions with their environment and not by simply processing passively gained information about the world. These agent-environment interactions lead to adaptive behavior in surroundings of a wide range of complexity. Animals with even tiny brains, such as insects, are capable of performing extraordinarily well in their behavioral contexts by making optimal use of the closed action-perception loop. Model simulations and robotic implementations show that the smart biological mechanisms of motion computation and visually-guided flight control might be helpful to find technical solutions, for example, when designing micro air vehicles carrying a miniaturized, low-weight on-board processor.

Neuronal encoding of object and distance information: a model simulation study on naturalistic optic flow processing

We developed a model of the input circuitry of the FD1 cell, an identified motion-sensitive interneuron in the blowfly's visual system. The model circuit successfully reproduces the FD1 cell's most conspicuous property: its larger responses to objects than to spatially extended patterns. The model circuit also mimics the time-dependent responses of FD1 to dynamically complex naturalistic stimuli, shaped by the blowfly's saccadic flight and gaze strategy: the FD1 responses are enhanced when, as a consequence of self-motion, a nearby object crosses the receptive field during intersaccadic intervals. Moreover, the model predicts that these object-induced responses are superimposed by pronounced pattern-dependent fluctuations during movements on virtual test flights in a three-dimensional environment with systematic modifications of the environmental patterns. Hence, the FD1 cell is predicted to detect not unambiguously objects defined by the spatial layout of the environment, but to be also sensitive to objects distinguished by textural features. These ambiguous detection abilities suggest an encoding of information about objects-irrespective of the features by which the objects are defined-by a population of cells, with the FD1 cell presumably playing a prominent role in such an ensemble.

Species-Specific Flight Styles of Flies are Reflected in the Response Dynamics of a Homolog Motion-Sensitive Neuron

Hoverflies and blowflies have distinctly different flight styles. Yet, both species have been shown to structure their flight behavior in a way that facilitates extraction of 3D information from the image flow on the retina (optic flow). Neuronal candidates to analyze the optic flow are the tangential cells in the third optical ganglion - the lobula complex. These neurons are directionally selective and integrate the optic flow over large parts of the visual field. Homolog tangential cells in hoverflies and blowflies have a similar morphology. Because blowflies and hoverflies have similar neuronal layout but distinctly different flight behaviors, they are an ideal substrate to pinpoint potential neuronal adaptations to the different flight styles. In this article we describe the relationship between locomotion behavior and motion vision on three different levels: (1) We compare the different flight styles based on the categorization of flight behavior into prototypical movements. (2) We measure the species-specific dynamics of the optic flow under naturalistic flight conditions. We found the translational optic flow of both species to be very different. (3) We describe possible adaptations of a homolog motion-sensitive neuron. We stimulate this cell in blowflies (Calliphora) and hoverflies (Eristalis) with naturalistic optic flow generated by both species during free flight. The characterized hoverfly tangential cell responds faster to transient changes in the optic flow than its blowfly homolog. It is discussed whether and how the different dynamical response properties aid optic flow analysis.

Object representation and distance encoding in three-dimensional environments by a neural circuit in the visual system of the blowfly

Three motion-sensitive key elements of a neural circuit, presumably involved in processing object and distance information, were analyzed with optic flow sequences as experienced by blowflies in a three-dimensional environment. This optic flow is largely shaped by the blowfly's saccadic flight and gaze strategy, which separates translational flight segments from fast saccadic rotations. By modifying this naturalistic optic flow, all three analyzed neurons could be shown to respond during the intersaccadic intervals not only to nearby objects but also to changes in the distance to background structures. In the presence of strong background motion, the three types of neuron differ in their sensitivity for object motion. Object-induced response increments are largest in FD1, a neuron long known to respond better to moving objects than to spatially extended motion patterns, but weakest in VCH, a neuron that integrates wide-field motion from both eyes and, by inhibiting the FD1 cell, is responsible for its object preference. Small but significant object-induced response increments are present in HS cells, which serve both as a major input neuron of VCH and as output neurons of the visual system. In both HS and FD1, intersaccadic background responses decrease with increasing distance to the animal, although much more prominently in FD1. This strong dependence of FD1 on background distance is concluded to be the consequence of the activity of VCH that dramatically increases its activity and, thus, its inhibitory strength with increasing distance.

Binocular interactions underlying the classic optomotor responses of flying flies

In response to imposed course deviations, the optomotor reactions of animals reduce motion blur and facilitate the maintenance of stable body posture. In flies, many anatomical and electrophysiological studies suggest that disparate motion cues stimulating the left and right eyes are not processed in isolation but rather are integrated in the brain to produce a cohesive panoramic percept. To investigate the strength of such inter-ocular interactions and their role in compensatory sensory–motor transformations, we utilize a virtual reality flight simulator to record wing and head optomotor reactions by tethered flying flies in response to imposed binocular rotation and monocular front-to-back and back-to-front motion. Within a narrow range of stimulus parameters that generates large contrast insensitive optomotor responses to binocular rotation, we find that responses to monocular front-to-back motion are larger than those to panoramic rotation, but are contrast sensitive. Conversely, responses to monocular back-to-front motion are slower than those to rotation and peak at the lowest tested contrast. Together our results suggest that optomotor responses to binocular rotation result from the influence of non-additive contralateral inhibitory as well as excitatory circuit interactions that serve to confer contrast insensitivity to flight behaviors influenced by rotatory optic flow.

Binocular integration of visual information: a model study on naturalistic optic flow processing

The computation of visual information from both visual hemispheres is often of functional relevance when solving orientation and navigation tasks. The vCH-cell is a motion-sensitive wide-field neuron in the visual system of the blowfly Calliphora, a model system in the field of optic flow processing. The vCH-cell receives input from various other identified wide-field cells, the receptive fields of which are located in both the ipsilateral and the contralateral visual field. The relevance of this connectivity to the processing of naturalistic image sequences, with their peculiar dynamical characteristics, is still unresolved. To disentangle the contributions of the different input components to the cell's overall response, we used electrophysiologically determined responses of the vCH-cell and its various input elements to tune a model of the vCH-circuit. Their impact on the vCH-cell response could be distinguished by stimulating not only extended parts of the visual field of the fly, but also selected regions in the ipsi- and contralateral visual field with behaviorally generated optic flow. We show that a computational model of the vCH-circuit is able to account for the neuronal activities of the counterparts in the blowfly's visual system. Furthermore, we offer an insight into the dendritic integration of binocular visual input.

Neural action fields for optic flow based navigation: a simulation study of the fly lobula plate network

Optic flow based navigation is a fundamental way of visual course control described in many different species including man. In the fly, an essential part of optic flow analysis is performed in the lobula plate, a retinotopic map of motion in the environment. There, the so-called lobula plate tangential cells possess large receptive fields with different preferred directions in different parts of the visual field. Previous studies demonstrated an extensive connectivity between different tangential cells, providing, in principle, the structural basis for their large and complex receptive fields. We present a network simulation of the tangential cells, comprising most of the neurons studied so far (22 on each hemisphere) with all the known connectivity between them. On their dendrite, model neurons receive input from a retinotopic array of Reichardt-type motion detectors. Model neurons exhibit receptive fields much like their natural counterparts, demonstrating that the connectivity between the lobula plate tangential cells indeed can account for their complex receptive field structure. We describe the tuning of a model neuron to particular types of ego-motion (rotation as well as translation around/along a given body axis) by its ‘action field’. As we show for model neurons of the vertical system (VS-cells), each of them displays a different type of action field, i.e., responds maximally when the fly is rotating around a particular body axis. However, the tuning width of the rotational action fields is relatively broad, comparable to the one with dendritic input only. The additional intra-lobula-plate connectivity mainly reduces their translational action field amplitude, i.e., their sensitivity to translational movements along any body axis of the fly.

Mutation of the Drosophila vesicular GABA transporter disrupts visual figure detection

The role of gamma amino butyric acid (GABA) release and inhibitory neurotransmission in regulating most behaviors remains unclear. The vesicular GABA transporter (VGAT) is required for the storage of GABA in synaptic vesicles and provides a potentially useful probe for inhibitory circuits. However, specific pharmacologic agents for VGAT are not available, and VGAT knockout mice are embryonically lethal, thus precluding behavioral studies. We have identified the Drosophila ortholog of the vesicular GABA transporter gene (which we refer to as dVGAT), immunocytologically mapped dVGAT protein expression in the larva and adult and characterized a dVGAT(minos) mutant allele. dVGAT is embryonically lethal and we do not detect residual dVGAT expression, suggesting that it is either a strong hypomorph or a null. To investigate the function of VGAT and GABA signaling in adult visual flight behavior, we have selectively rescued the dVGAT mutant during development. We show that reduced GABA release does not compromise the active optomotor control of wide-field pattern motion. Conversely, reduced dVGAT expression disrupts normal object tracking and figure-ground discrimination. These results demonstrate that visual behaviors are segregated by the level of GABA signaling in flies, and more generally establish dVGAT as a model to study the contribution of GABA release to other complex behaviors.

Behavioural state affects motion-sensitive neurones in the fly visual system

The strength of stimulus-induced responses at the neuronal and the behavioural level often depends on the internal state of an animal. Within pathways processing sensory information and eventually controlling behavioural responses, such gain changes can originate at several sites. Using motion-sensitive lobula plate tangential cells (LPTCs) of blowflies, we address whether and in which way information processing changes for two different states of motor activity. We distinguish between the two states on the basis of haltere movements. Halteres are the evolutionarily transformed hindwings of flies. They oscillate when the animals walk or fly. LPTCs mediate, amongst other behaviours, head optomotor responses. These are either of large or small amplitude depending on the state of motor activity. Here we find that LPTC responses also depend on the motor activity of flies. In particular, LPTC responses are enhanced when halteres oscillate. Nevertheless, the response changes of LPTCs do not account for the corresponding large gain changes of head movements. Moreover, haltere activity itself does not change the activity of LPTCs. Instead, we propose that a central signal associated with motor activity changes the gain of head optomotor responses and the response properties of LPTCs.

Distributed dendritic processing facilitates object detection: a computational analysis on the visual system of the fly

Background

Detecting objects is an important task when moving through a natural environment. Flies, for example, may land on salient objects or may avoid collisions with them. The neuronal ensemble of Figure Detection cells (FD-cells) in the visual system of the fly is likely to be involved in controlling these behaviours, as these cells are more sensitive to objects than to extended background structures. Until now the computations in the presynaptic neuronal network of FD-cells and, in particular, the functional significance of the experimentally established distributed dendritic processing of excitatory and inhibitory inputs is not understood.

Methodology/Principal Findings

We use model simulations to analyse the neuronal computations responsible for the preference of FD-cells for small objects. We employed a new modelling approach which allowed us to account for the spatial spread of electrical signals in the dendrites while avoiding detailed compartmental modelling. The models are based on available physiological and anatomical data. Three models were tested each implementing an inhibitory neural circuit, but differing by the spatial arrangement of the inhibitory interaction. Parameter optimisation with an evolutionary algorithm revealed that only distributed dendritic processing satisfies the constraints arising from electrophysiological experiments. In contrast to a direct dendro-dendritic inhibition of the FD-cell (Direct Distributed Inhibition model), an inhibition of its presynaptic retinotopic elements (Indirect Distributed Inhibition model) requires smaller changes in input resistance in the inhibited neurons during visual stimulation.

Conclusions/Significance

Distributed dendritic inhibition of retinotopic elements as implemented in our Indirect Distributed Inhibition model is the most plausible wiring scheme for the neuronal circuit of FD-cells. This microcircuit is computationally similar to lateral inhibition between the retinotopic elements. Hence, distributed inhibition might be an alternative explanation of perceptual phenomena currently explained by lateral inhibition networks.

Exploratory behaviour in NO-dependent cyclase mutants of Drosophila shows defects in coincident neuronal signalling

Background

Drosophila flies explore the environment very efficiently in order to colonize it. They explore collectively, not individually, so that when a few land on a food spot, they attract the others by signs. This behaviour leads to aggregation of individuals and optimizes the screening of mates and egg-laying on the most favourable food spots.

Results

Flies perform cycles of exploration/aggregation depending on the resources of the environment. This behavioural ecology constitutes an excellent model for analyzing simultaneous processing of neurosensory information. We reasoned that the decision of flies to land somewhere in order to achieve aggregation is based on simultaneous integration of signals (visual, olfactory, acoustic) during their flight. On the basis of what flies do in nature, we designed laboratory tests to analyze the phenomenon of neuronal coincidence. We screened many mutants of genes involved in neuronal metabolism and the synaptic machinery.

Conclusion

Mutants of NO-dependent cyclase show a specifically-marked behaviour phenotype, but on the other hand they are associated with moderate biochemical defects. We show that these mutants present errors in integrative and/or coincident processing of signals, which are not reducible to the functions of the peripheral sensory cells.

Nonlinear, binocular interactions underlying flow field selectivity of a motion-sensitive neuron

Neurons in many species have large receptive fields that are selective for specific optic flow fields. Here, we studied the neural mechanisms underlying flow field selectivity in lobula plate tangential cells (LPTCs) of the blowfly. Among these cells, the H2 cell responds preferentially to visual stimuli approximating rotational optic flow. Through double recordings from H2 and many other LPTCs, we characterized a bidirectional commissural pathway that allows visual information to be shared between the hemispheres. This pathway is mediated by axo-axonal electrical coupling of H2 and the horizontal system equatorial (HSE) cell located in the opposite hemisphere. Using single-cell ablations, we found that this pathway is sufficient to allow H2 to amplify and attenuate dendritic input during binocular visual stimuli. This is accomplished through a modulation of H2's membrane potential by input from the contralateral HSE cell, which scales the firing rate of H2 during visual stimulation but is not sufficient to induce action potentials.

Application of multiline two-photon microscopy to functional in vivo imaging

High spatial resolution and low risks of photodamage make two-photon laser-scanning microscopy (TPLSM) the method of choice for biological imaging. However, the study of functional dynamics such as neuronal calcium regulation often also requires a high temporal resolution. Hitherto, acquisition speed is usually increased by line scanning, which restricts spatial resolution to structures along a single axis. To overcome this gap between high spatial and high temporal resolution we performed TPLSM with a beam multiplexer to generate multiple laser foci inside the sample. By detecting the fluorescence emitted from these laser foci with an electron-multiplying camera, it was possible to perform multiple simultaneous linescans. In addition to multiline scanning, the array of up to 64 laser beams could also be used in x-y scan mode to collect entire images at high frame rates. To evaluate the applicability of multiline TPLSM to functional in vivo imaging, calcium signals were monitored in visual motion-sensitive neurons in the brain of flies. The capacity of our method to simultaneously acquire signals at different cellular locations is exemplified by measurements at branched neurites and 'spine'-like structures. Calcium dynamics depended on branch size, but 'spines' did not systematically differ from their 'parent neurites'. The spatial resolution of our setup was critically evaluated by comparing it to confocal microscopy and the negative effect of scattering of emission light during image detection was assessed directly by running the setup in both imaging and point-scanning mode.

Spatial integration of optic flow signals in fly motion-sensitive neurons

Neurons in the fly lobula plate integrate motion signals over large regions of visual space in a directionally selective manner. This study is concerned with the details of this integration process. We used a stimulus consisting of a 4 x 4 lattice of locally moving Gabor patches, in which each patch could take any direction independently. We also presented only one patch at a time or two patches at a time. Across all possible directions of motion, the firing rate response r1+2 to two simultaneously presented patches was well described by r1+2(d1, d2) = G x [r1(d1) + r2(d2)] + S, where r1 and r2 are responses to individual patches moving in directions d1 and d2, and G approximately 0.81, S approximately -23. However, this quasi-linear scaling expression failed to account for three main empirical observations: 1) the directional-tuning curve for one patch is broader in the presence of another patch moving in the neuron's preferred direction (PD); 2) the vertical compression of this curve is greater when the second patch moves in the antipreferred direction (AD) as opposed to PD; 3) the ability of the neuronal response to discriminate the direction of a patch is greater when the other patch is moving in the PD as opposed to AD, where this ability is assessed using both information theory and a standard discriminability index. To account for these departures from the simple scaling model, we used a normalization model very similar to one used for macaque area MT/V5. This model can qualitatively explain all three departures from the scaling equation described above, suggesting that a gain-control normalization network may be at work within the fly lobula plate.

Function and coding in the blowfly H1 neuron during naturalistic optic flow

Naturalistic stimuli, reconstructed from measured eye movements of flying blowflies, were replayed on a panoramic stimulus device. The directional movement-sensitive H1 neuron was recorded from blowflies watching these stimuli. The response of the H1 neuron is dominated by the response to fast saccadic turns into one direction. The response between saccades is mostly inhibited by the front-to-back optic flow caused by the forward translation during flight. To unravel the functional significance of the H1 neuron, we replayed, in addition to the original behaviorally generated stimulus, two targeted stimulus modifications: (1) a stimulus in which flow resulting from translation was removed (this stimulus produced strong intersaccadic responses); and (2) a stimulus in which the saccades were removed by assuming that the head follows the smooth flight trajectory (this stimulus produced alternating zero or nearly saturating spike rates). The responses to the two modified stimuli are strongly different from the response to the original stimulus, showing the importance of translation and saccades for the H1 response to natural optic flow. The response to the original stimulus thus suggests a double function for the H1 neuron, assisting two major classes of movement-sensitive output neurons targeted by H1. First, its strong response to saccades may function as a saccadic suppressor (via one of its target neurons) for cells involved in figure-ground discrimination. Second, its intersaccadic response may increase the signal-to-noise ratio (SNR) of wide-field neurons involved in detecting translational optic flow between saccades, in particular when flying speeds are low or when object distances are large.

An elaborated model of fly small-target tracking

Flies have the capability to visually track small moving targets, even across cluttered backgrounds. Previous computational models, based on figure detection (FD) cells identified in the fly, have suggested how this may be accomplished at a neuronal level based on information about relative motion between the target and the background. We experimented with the use of this "small-field system model" for the tracking of small moving targets by a simulated fly in a cluttered environment and discovered some functional limitations. As a result of these experiments, we propose elaborations of the original small-field system model to support stronger effects of background motion on small-field responses, proper accounting for more complex optical flow fields, and more direct guidance toward the target. We show that the elaborated model achieves much better tracking performance than the original model in complex visual environments and discuss the biological implications of our elaborations. The elaborated model may help to explain recent electrophysiological data on FD cells that seem to contradict the original model.

Binocular visual integration in the crustacean nervous system

Although the behavioral repertoire of crustaceans is largely guided by visual information their visual nervous system has been little explored. In search for central mechanisms of visual integration, this study was aimed at identifying and characterizing brain neurons in the crab involved in binocular visual processing. The study was performed in the intact animal, by recording intracellularly the response to visual stimuli of neurons from one of the two optic lobes. Identified neurons recorded from the medulla (second optic neuropil), which include sustaining neurons, dimming neurons, depolarizing and hyperpolarizing tonic neurons and on-off neurons, all presented exclusively monocular (ipsilateral) responses. In contrast, all wide field movement detector neurons recorded from the lobula (third optic neuropil) responded to moving stimuli presented to the ipsilateral and to the contralateral eye. In these cells, the responses evoked by ipsilateral or contralateral stimulation were almost identical, as revealed by analysing the number and amplitude of the elicited postsynaptic potentials and spikes, and the ability to habituate upon repeated visual stimulation. The results demonstrate that in crustaceans important binocular processing takes place at the level of the lobula.

Ca2+ Clearance in Visual Motion-Sensitive Neurons of the Fly Studied In Vivo by Sensory Stimulation and UV Photolysis of Caged Ca2+

In motion-sensitive visual neurons of the fly, excitatory visual stimulation elicits Ca2+ accumulation in dendrites and presynaptic arborizations. Following the cessation of motion stimuli, decay time courses of the cytosolic Ca2+ concentration signals measured with fluorescent dyes were faster in fine arborizations compared with the main branches. When indicators with low Ca2+ affinity were used, the decay of the Ca2+ signals appeared slightly faster than with high affinity dyes, but the dependence of decay kinetics on branch size was preserved. The most parsimonious explanation for faster Ca2+ concentration decline in thin branches compared with thick ones is that the velocity of Ca2+ clearance is limited by transport mechanisms located in the outer membrane and is thus dependent on the neurite's surface-to-volume ratio. This interpretation was corroborated by UV flash photolysis of caged Ca2+ to systematically elicit spatially homogeneous step-like Ca2+ concentration increases of varying amplitude. Clearance of Ca2+ liberated by this method depended on branch size in the same way as Ca2+ accumulated during visual stimulation. Furthermore, the decay time courses of Ca2+ signals were only little affected by the amount of Ca2+ released by photolysis. Thus Ca2+ efflux via the outer membrane is likely to be the main reason for the spatial differences in Ca2+ clearance in visual motion-sensitive neurons of the fly.

The computational basis of an identified neuronal circuit for elementary motion detection in dipterous insects

Based on comparative anatomical studies and electrophysiological experiments, we have identified a conserved subset of neurons in the lamina, medulla, and lobula of dipterous insects that are involved in retinotopic visual motion direction selectivity. Working from the photoreceptors inward, this neuronal subset includes lamina amacrine (α) cells, lamina monopolar (L2) cells, the basket T-cell (T1 or β), the transmedullary cell Tm1, and the T5 bushy T-cell. Two GABA-immunoreactive neurons, the transmedullary cell Tm9 and a local interneuron at the level of T5 dendrites, are also implicated in the motion computation. We suggest that these neurons comprise the small-field elementary motion detector circuits the outputs of which are integrated by wide-field lobula plate tangential cells. We show that a computational model based on the available data about these neurons is consistent with existing models of biological elementary motion detection, and present a comparable version of the Hassenstein-Reichardt (HR) correlation model. Further, by using the model to synthesize a generic tangential cell, we show that it can account for the responses of lobula plate tangential cells to a wide range of transient stimuli, including responses which cannot be predicted using the HR model. This computational model of elementary motion detection is the first which derives specifically from the functional organization of a subset of retinotopic neurons supplying the lobula plate. A key prediction of this model is that elementary motion detector circuits respond quite differently to small-field transient stimulation than do spatially integrated motion processing neurons as observed in the lobula plate. In addition, this model suggests that the retinotopic motion information provided to wide-field motion-sensitive cells in the lobula is derived from a less refined stage of processing than motion inputs to the lobula plate.

Input organization of multifunctional motion-sensitive neurons in the blowfly

Flies rely heavily on visual motion cues for course control. This is mediated by a small set of motion-sensitive neurons called lobula plate tangential cells. A single class of these, the centrifugal horizontal (CH) neurons, play an important role in two pathways: figure-ground discrimination and flow-field selectivity. As was recently found, the dendrites of CH cells are electrically coupled with the dendritic tree of another class of neurons sensitive to horizontal image motion, the horizontal system (HS) cells. However, whether motion information arrives independently at both of these cells or is passed from one to the other is not known. Here, we examine the ipsilateral input circuitry to HS and CH neurons by selective laser ablation of individual interneurons. We find that the response of CH neurons to motion presented in front of the ipsilateral eye is entirely abolished after ablation of HS cells. In contrast, the motion response of HS cells persists after the ablation of CH cells. We conclude that HS cells receive direct motion input from local motion elements, whereas CH cells do not; their motion response is driven by HS cells. This connection scheme is discussed with reference to how the dendritic networks involved in figure-ground detection and flow-field selectivity might operate.

Dendro-dendritic interactions between motion-sensitive large-field neurons in the fly

For visual course control, flies rely on a set of motion-sensitive neurons called lobula plate tangential cells (LPTCs). Among these cells, the so-called CH (centrifugal horizontal) cells shape by their inhibitory action the receptive field properties of other LPTCs called FD (figure detection) cells specialized for figure-ground discrimination based on relative motion. Studying the ipsilateral input circuitry of CH cells by means of dual-electrode and combined electrical-optical recordings, we find that CH cells receive graded input from HS (large-field horizontal system) cells via dendro-dendritic electrical synapses. This particular wiring scheme leads to a spatial blur of the motion image on the CH cell dendrite, and, after inhibiting FD cells, to an enhancement of motion contrast. This could be crucial for enabling FD cells to discriminate object from self motion.

Spike-triggered dendritic calcium transients depend on synaptic activity in the cricket giant interneurons

The relationship between electrical activity and spike-induced Ca2+ increases in dendrites was investigated in the identified wind-sensitive giant interneurons in the cricket. We applied a high-speed Ca2+ imaging technique to the giant interneurons, and succeeded in recording the transient Ca2+ increases (Ca2+ transients) induced by a single action potential, which was evoked by presynaptic stimulus to the sensory neurons. The dendritic Ca2+ transients evoked by a pair of action potentials accumulated when spike intervals were shorter than 100 ms. The amplitude of the Ca2+ transients induced by a train of spikes depended on the number of action potentials. When stimulation pulses evoking the same numbers of action potentials were separately applied to the ipsi- or contra-lateral cercal sensory nerves, the dendritic Ca2+ transients induced by these presynaptic stimuli were different in their amplitude. Furthermore, the side of presynaptic stimulation that evoked larger Ca2+ transients depended on the location of the recorded dendritic regions. This result means that the spike-triggered Ca2+ transients in dendrites depend on postsynaptic activity. It is proposed that Ca2+ entry through voltage-dependent Ca2+ channels activated by the action potentials will be enhanced by excitatory synaptic inputs at the dendrites in the cricket giant interneurons.

Recurrent network interactions underlying flow-field selectivity of visual interneurons

Motion-sensitive large-field neurons found at higher processing stages in many species often exhibit a remarkable selectivity for particular flow fields. However, the underlying neural mechanisms are not yet understood. We studied this problem in the so-called lobula plate tangential cells (LPTCs) of the fly. Investigating the connectivity between LPTCs by means of dual recordings, we find two types of connections: (1) heterolateral connections between LPTCs of both hemispheres and (2) ipsilateral connections between LPTCs within one lobula plate. The circuit is suitable to amplify incoming, dendritic signals in the case of rotatory flow fields and to reduce them in the case of other flow-field structures. In addition to feedforward connectivity, thus, the flow-field selectivity of LPTCs may be significantly attributable to recurrent excitation involving the network of large-field neurons in both brain hemispheres.

Identified nerve cells and insect behavior

Studies of insect identified neurons over the past 25 years have provided some of the very best data on sensorimotor integration; tracing information flow from sensory to motor networks. General principles have emerged that have increased the sophistication with which we now understand both sensory processing and motor control. Two overarching themes have emerged from studies of identified sensory interneurons. First, within a species, there are profound differences in neuronal organization associated with both the sex and the social experience of the individual. Second, single neurons exhibit some surprisingly rich examples of computational sophistication in terms of (a) temporal dynamics (coding superimposed upon circadian and shorter-term rhythms), and also (b) what Kenneth Roeder called "neural parsimony": that optimal information can be encoded, and complex acts of sensorimotor coordination can be mediated, by small ensembles of cells. Insect motor systems have proven to be relatively complex, and so studies of their organization typically have not yielded completely defined circuits as are known from some other invertebrates. However, several important findings have emerged. Analysis of neuronal oscillators for rhythmic behavior have delineated a profound influence of sensory feedback on interneuronal circuits: they are not only modulated by feedback, but may be substantially reconfigured. Additionally, insect motor circuits provide potent examples of neuronal restructuring during an organism's lifetime, as well as insights on how circuits have been modified across evolutionary time. Several areas where future advances seem likely to occur include: molecular genetic analyses, neuroecological syntheses, and neuroinformatics--the use of digital resources to organize databases with information on identified nerve cells and behavior.

Two classes of visual motion sensitive interneurons differ in direction and velocity dependency of in vivo calcium dynamics

Neurons exploit both membrane biophysics and biochemical pathways of the cytoplasm for dendritic integration of synaptic input. Here we quantify the tuning discrepancy of electrical and chemical response properties in two kinds of neurons using in vivo visual stimulation. Dendritic calcium concentration changes and membrane potential of visual interneurons of the fly were measured in response to visual motion stimuli. Two classes of tangential cells of the lobula plate were compared, HS-cells and CH-cells. Both neuronal classes are known to receive retinotopic input with similar properties, yet they differ in morphology, physiology, and computational context. Velocity tuning and directional selectivity of the electrical and calcium responses were investigated. In both cell classes, motion-induced calcium accumulation did not follow the early transient of the membrane potential. Rather, the amplitude of the calcium signal seemed to be related to the late component of the depolarization, where it was close to a steady state. Electrical and calcium responses differed with respect to their velocity tuning in CH-cells, but not in HS-cells. Furthermore, velocity tuning of the calcium response, but not of the electrical response differed between neuronal classes. While null-direction motion caused hyperpolarization in both classes, this led to a calcium decrement in CH-cells, but had no effect on the calcium signal in HS-cells, not even when calcium levels had been raised by a preceding excitatory motion stimulus. Finally, the voltage-[Ca2+]i-relationship for motion-induced, transient potential changes was steeper and less rectifying in CH-cells than in HS-cells. These results represent an example of dendritic information processing in vivo, where two neuronal classes respond to identical stimuli with a similar electrical response, but differing calcium response. This highlights the capacity of neurons to segregate two response components.

Binocular contributions to optic flow processing in the fly visual system

Integrating binocular motion information tunes wide-field direction-selective neurons in the fly optic lobe to respond preferentially to specific optic flow fields. This is shown by measuring the local preferred directions (LPDs) and local motion sensitivities (LMSs) at many positions within the receptive fields of three types of anatomically identifiable lobula plate tangential neurons: the three horizontal system (HS) neurons, the two centrifugal horizontal (CH) neurons, and three heterolateral connecting elements. The latter impart to two of the HS and to both CH neurons a sensitivity to motion from the contralateral visual field. Thus in two HS neurons and both CH neurons, the response field comprises part of the ipsi- and contralateral visual hemispheres. The distributions of LPDs within the binocular response fields of each neuron show marked similarities to the optic flow fields created by particular types of self-movements of the fly. Based on the characteristic distributions of local preferred directions and motion sensitivities within the response fields, the functional role of the respective neurons in the context of behaviorally relevant processing of visual wide-field motion is discussed.

Response latency of a motion-sensitive neuron in the fly visual system: dependence on stimulus parameters and physiological conditions

The response latency of an identified motion-sensitive neuron in the blowfly visual system strongly depends on stimulus parameters. The latency decreases with increasing contrast and temporal frequency of a moving pattern, but changes only little when the pattern size and thus the number of activated inputs is increased. The latency does not only depend on visual stimuli, but is also affected by temperature changes and the age of the fly. Since response latencies cover a range of one order of magnitude, the latency changes are expected to be of relevance in visually guided orientation behaviour.

Dendritic calcium accumulation associated with direction-selective adaptation in visual motion-sensitive neurons in vivo

Motion adaptation in directionally selective tangential cells (TC) of the fly visual system has previously been explained as a presynaptic mechanism. Based on the observation that adaptation is in part direction selective, which is not accounted for by the former models of motion adaptation, we investigated whether physiological changes located in the TC dendrite can contribute to motion adaptation. Visual motion in the neuron's preferred direction (PD) induced stronger adaptation than motion in the opposite direction and was followed by an afterhyperpolarization (AHP). The AHP subsides in the same time as adaptation recovers. By combining in vivo calcium fluorescence imaging with intracellular recording, we show that dendritic calcium accumulation following motion in the PD is correlated with the AHP. These results are consistent with a calcium-dependent physiological change in TCs underlying adaptation during continuous stimulation with PD motion, expressing itself as an AHP after the stimulus stops. However, direction selectivity of adaptation is probably not solely related to a calcium-dependent mechanism because direction-selective effects can also be observed for fast moving stimuli, which do not induce sizeable calcium accumulation. In addition, a comparison of two classes of TCs revealed differences in the relationship of calcium accumulation and AHP when the stimulus velocity was varied. Thus the potential role of calcium in motion adaptation depends on stimulation parameters and cell class.

Synaptic interactions increase optic flow specificity

Representations of optic flow are encoded in fly tangential neurons by pooling the signals of many retinotopically organized local motion-sensitive inputs as well as of other tangential cells originating in the ipsi- and contralateral half of the brain. In the so called HSE cell, a neuron involved in optomotor course control, two contralateral input elements, the H1 and H2 cells, mediate distinct EPSPs. These EPSPs frequently elicit spike-like depolarizations in the HSE cell. The synaptic transmission between the H2 and the HSE cell is analysed in detail and shown to be very reliable with respect to the amplitude and time-course of the postsynaptic potential. As a consequence of its synaptic input, the HSE cell responds best to wide-field motion, such as that generated on the eyes when the animal turns about its vertical body axis. It is shown that the specificity of the HSE cell for this type of optic flow is much enhanced if rapid membrane depolarizations, such as large-amplitude EPSPs or spike-like depolarizations, are taken into account rather than the average membrane potential.

In vivo calcium accumulation in presynaptic and postsynaptic dendrites of visual interneurons

In this comparative in vivo study of dendritic calcium accumulation, we describe the time course and spatial integration properties of two classes of visual interneurons in the lobula plate of the blowfly. Calcium accumulation was measured during visual motion stimulation, ensuring synaptic activation of the neurons within their natural spatial and temporal operating range. The compared cell classes, centrifugal horizontal (CH) and horizontal system (HS) cells, are known to receive retinotopic input of similar direction selectivity, but to differ in morphology, biophysics, presence of dendrodendritic synapses, and computational task. 1) The time course of motion-induced calcium accumulation was highly invariant with respect to stimulus parameters such as pattern contrast and size. In HS cells, the rise of [Ca(2+)](i) can be described by a single exponential with a time constant of 5-6 s. The initial rise of [Ca(2+)](i) in CH cells was much faster (tau approximately 1 s). The decay time constant in both cell classes was estimated to be at least 3.5 times longer than the corresponding rise time constant. 2) The voltage-[Ca(2+)](i) relationship was best described by an expansive nonlinearity in HS cells and an approximately linear relationship in CH cells. 3) Both cell classes displayed a size-dependent saturation nonlinearity of the calcium accumulation. Although in CH cells calcium saturation was indistinguishable from saturation of the membrane potential, saturation of the two response parameters differed in HS cells. 4) There was spatial overlap of the calcium signal in response to nonoverlapping visual stimuli. Both the area and the amplitude of the overlap profile was larger in CH cells than in HS cells. Thus calcium accumulation in CH cells is spatially blurred to a greater extent than in HS cells. 5) The described differences between the two cell classes may reflect the following computational tasks of these neurons: CH cells relay retinotopic information within the lobula plate via dendritic synapses with pronounced spatial low-pass filtering. HS cells are output neurons of the lobula plate, in which the slow, local calcium accumulation may be suitable for local modulatory functions.

Temperature-dependence of neuronal performance in the motion pathway of the blowfly calliphora erythrocephala

Raising the head temperature within a behaviourally relevant range has strong effects on the performance of an identified neuron, the H1 neuron, in the visual motion pathway of blowflies. The effect is seen as an increase in the mean amplitude of the responses to motion under both transient and steady-state conditions, a considerable decrease in the response latency and an improvement in the reliability of the responses to motion. These temperature-dependent effects are independent of whether the animal is exposed to transient temperature changes or is maintained continuously at the same temperature for its entire life. The changes in the neuronal response properties with temperature may be of immediate functional significance for the animal under its normal operating conditions. In particular, the decrease in latency and the improvement in the reliability with increasing temperature may be relevant for the fly when executing its extremely virtuosic flight manoeuvres.

Dendritic structure and receptive-field organization of optic flow processing interneurons in the fly

The third visual neuropil (lobula plate) of the blowfly Calliphora erythrocephala is a center for processing motion information. It contains, among others, 10 individually identifiable "vertical system" (VS) neurons responding to visual wide-field motions of arbitrary patterns. We demonstrate that each VS neuron is tuned to sense a particular aspect of optic flow that is generated during self-motion. Thus the VS neurons in the fly supply visual information for the control of head orientation, body posture, and flight steering. To reveal the functional organization of the receptive fields of the 10 VS neurons, we determined with a new method the distributions of local motion sensitivities and local preferred directions at 52 positions in the fly's visual field. Each neuron was identified by intracellular staining with Lucifer yellow and three-dimensional reconstructions from 10-micron serial sections. Thereby the receptive-field organization of each recorded neuron could be correlated with the location and extent of its dendritic arborization in the retinotopically organized neuropil of the lobula plate. The response fields of the VS neurons, i.e., the distributions of local preferred directions and local motion sensitivities, are not uniform but resemble rotatory optic flow fields that would be induced by the fly during rotations around various horizontal axes. Theoretical considerations and quantitative analyses of the data, which will be presented in a subsequent paper, show that VS neurons are highly specialized neural filters for optic flow processing and thus for the visual sensation of self-motions in the fly.

Directionality and inhibition in crayfish tangential cells

The purpose of this study was to characterize the inhibitory mechanism(s) associated with directionally selective motion detection (DS) in nonspiking tangential cells of crayfish optic lobe. The experiments employed intracellular recording of synaptic potentials elicited with sinewave gratings and pharmacological techniques. Previous studies established that tangential cells are subject to bicuculline-sensitive GABA-mediated inhibition. In this study DS was reduced by 90% by bicuculline. The reduction in DS was accompanied by a substantial increase in the response to null-direction motion. Bicuculline also altered the response to pulses of illumination. The magnitude and time course of inhibition were derived from the time varying difference between the control light response and that elicited during bicuculline perfusion. Both the inhibitory delay (relative to excitation) and the inhibitory amplitude are close to the expectations of a linear model of DS. The inhibition is not prolonged with respect to excitation but its risetime is approximately 2.5 times longer. The result implies a longer time constant in the inhibitory pathway relative to that in the excitatory pathway and places limits on the frequency response of inhibition and DS. The velocity-dependence of DS is related to the time course of inhibition. The stimulus drift velocity eliciting maximum directionality is inversely proportional to the inhibitory delay. Bicuculline did not influence orientation selectivity. It is concluded that the quantitative features of bicuculline-sensitive, GABA-mediated inhibition are consistent with a linear model of DS.

The intrinsic electrophysiological characteristics of fly lobula plate tangential cells: II. Active membrane properties

The voltage-gated currents in the fly lobula plate tangential cells were examined using the switched electrode voltage clamp technique. In CH cells, two currents were identified (Figs. 1, 2): a slow calcium inward current and a delayed rectifying, noninactivating potassium outward current. HS and VS cells appear to possess similar currents to CH cells, but in addition, exhibit a fast-activating sodium inward current and a sodium-activated potassium outward current (Figs. 3, 4). While the delayed rectifying potassium current in all three cell classes is responsible for the observed outward rectification described previously (Borst and Haag, 1996), the sodium inward current produced the fast and irregular spikelike depolarizations found in HS and VS cells but not in CH cells: When the sodium current is blocked by either. TTX or intracellular QX314, no more action potentials can be elicited in HS cells under current-clamp conditions (Fig. 5). As is demonstrated in HS cells, space clamp conditions are sufficient to suppress synaptically induced action potentials (Fig. 6). The currents described above were incorporated with the appropriate characteristics into compartmental models of the cells (Fig. 7, 8). The anatomical and electrically passive membrane parameters of these cells were determined in a preceding paper (Borst and Haag, 1996). After fitting the current parameters to the voltage-clamp data (Fig. 9), the model cells qualitatively mimicked the fly tangential cells under current clamp conditions in response to current injection (Fig. 10). The simulations demonstrated that the electrical compactness seen in the HS and VS cells, either in passive models or in active models during continuous hyperpolarization, decreased significantly in the active models during continuous depolarization (Fig. 11). Active HS models reproduce the frequency-dependent amplification of current injected into their axon (Fig. 12).

A fast stimulus procedure to determine local receptive field properties of motion-sensitive visual interneurons

We present a method to determine, within a few seconds, the local preferred direction (LPD) and local motion sensitivity (LMS) in small patches of the receptive fields of wide-field motion-sensitive neurons. This allows us to map, even during intracellular recordings, the distribution of LPD and LMS over the huge receptive fields of neurons sensing self-motions of the animal. Comparisons of the response field of a given neuron with the optic flow fields caused by different movements in space, allows us to specify the particular motion of the animal sensed by that neuron.

The intrinsic electrophysiological characteristics of fly lobula plate tangential cells: I. Passive membrane properties

The passive membrane properties of the tangential cells in the fly lobula plate (CH, HS, and VS cells, Fig. 1) were determined by combining compartmental modeling and current injection experiments. As a prerequisite, we built a digital base of the cells by 3D-reconstructing individual tangential cells from cobalt-stained material including both CH cells (VCH and DCH cells), all three HS cells (HSN, HSE, and HSS cells) and most members of the VS cell family (Figs. 2, 3). In a first series of experiments, hyperpolarizing and depolarizing currents were injected to determine steady-state I-V curves (Fig. 4). At potentials more negative than resting, a linear relationship holds, whereas at potentials more positive than resting, an outward rectification is observed. Therefore, in all subsequent experiments, when a sinusoidal current of variable frequency was injected, a negative DC current was superimposed to keep the neurons in a hyperpolarized state. The resulting amplitude and phase spectra revealed an average steady-state input resistance of 4 to 5 M omega and a cut-off frequency between 40 and 80 Hz (Fig. 5). To determine the passive membrane parameters Rm (specific membrane resistance), Ri (specific internal resistivity), and Cm (specific membrane capacitance), the experiments were repeated in computer simulations on compartmental models of the cells (Fig. 6). Good fits between experimental and simulation data were obtained for the following values: Rm = 2.5 k omega cm2, Ri = 60 omega cm, and Cm = 1.5 microF/cm2 for CH cells; Rm = 2.0 k omega cm2, Ri = 40 omega cm, and Cm = 0.9 microF/cm2 for HS cells; Rm = 2.0 k omega cm2, Ri = 40 omega cm, and Cm = 0.8 microF/cm2 for VS cells. An error analysis of the fitting procedure revealed an area of confidence in the Rm-Ri plane within which the Rm-Ri value pairs are still compatible with the experimental data given the statistical fluctuations inherent in the experiments (Figs. 7, 8). We also investigated whether there exist characteristic differences between different members of the same cell class and how much the exact placement of the electrode (within +/-100 microns along the axon) influences the result of the simulation (Fig. 9). The membrane parameters were further examined by injection of a hyperpolarizing current pulse (Fig. 10). The resulting compartmental models (Fig. 11) based on the passive membrane parameters determined in this way form the basis of forthcoming studies on dendritic integration and signal propagation in the fly tangential cells (Haag et al., 1997; Haag and Borst, 1997).

A preparation of the blowfly (Calliphora erythrocephala) brain for in vitro electrophysiological and pharmacological studies

We describe a method for the preparation and maintenance of the blowfly (Calliphora erythrocephala) brain in a recording chamber under in vitro conditions in a semi-slice configuration. Large identification neurones in the posterior part of the 3rd optic lobe (lobula plate) can be penetrated easily with microelectrodes. The so-called vertical system (VS) cells which respond to vertical image motion in vivo could be encountered best because their axons are escorted individually by specific tracheae. Fluorescent stained cells show their natural shape as being in vivo. Electrophysiological properties of the cells investigated so far, i.e., resting potential (about -40 mV) and firing properties (single rebound spikes), are comparable to recordings in intact flies. Initial pharmacological experiments on VS cells in this preparation reveal that iontophoretical application of acetylcholine and carbamylcholine results in depolarization. VS cells also respond to bath-applied nicotine (1 microM) with a slow depolarization of their membrane potential in normal fly saline as well as in a Ca(2+)-free saline, suggesting direct cholinergic input via nicotinic receptors. The suitability of the preparation for a wide range of electrophysiological and pharmacological studies is discussed.

Anatomical and functional imaging of neurons using 2-photon laser scanning microscopy

Light scattering by brain tissue and phototoxicity are major obstacles to the use of high-resolution optical imaging and photo-activation ('uncaging') of bioactive compounds from inactive ('caged') precursors in intact and semi-intact nervous systems. Optical methods based on 2-photon excitation promise to reduce these obstacles (Denk, 1994; Denk et al., 1990, 1994). Here we show a range of imaging modes based on 2-photon laser scanning microscopy (TPLSM) as applicable to problems in neuroscience. Fluorescence images were taken of neurons labeled with ion-sensitive and voltage-sensitive dyes in invertebrate ganglia, mammalian brain slices, and from the intact mammalian brain. Scanning photochemical images with whole-cell current detection (Denk, 1994) show how the distribution of neurotransmitter receptors on the surface of specific cells can be mapped. All images show strong optical sectioning and usable images can be obtained at depths greater than 100 microns below the surface of the preparation.

Dendritic processing of synaptic information by sensory interneurons

One of the most distinguishing features of nerve cells is the vast morphological diversity of their input regions, that is, their dendrites. These range from bulbous structures, with only small protrusions, to large tree-like arborizations. The diversity of nerve cells is further augmented by a continuously increasing number of types of voltage-dependent conductances in dendrites that might alter the postsynaptic signals in a pronounced way. Moreover, intracellular factors such as Ca2+ link electrical activity with biochemical processes, and can induce short and long-term changes in responsiveness. This complexity of neurons in general, and the uniqueness of each cell type, sharply contrasts with the comparatively simple and uniform design principle of the integrate-and-fire units of so-called neuronal net models. This raises the question of which particular structural and physiological details of nerve cells really matter for the performance of neuronal circuits. An answer to this basic problem of computational neurobiology might be given only if the task of the neurons and circuits is known. This review illustrates how the problem can be approached particularly well in sensory interneurons. The functional significance of sensory interneurons can often be assessed more easily than that of central nerve cells because of their vicinity to the sensory surface.

Dendritic calcium dynamics

Further characterization of the biochemical components that contribute to calcium handling, together with advances in optical imaging of ion concentration, are providing quantitative information on the dynamics of calcium in the dendrites of neurons in tissue culture, brain slices and in vivo. It has recently been demonstrated that strong spatial gradients and transient calcium elevations result from the geometry and membrane properties of dendrites. These studies are adding to our understanding of calcium's role in synaptic plasticity and in shaping the electrophysiological properties of neurons.