Dendritic integration of motion information in visual interneurons of the blowfly

Non-uniform weighting of local motion inputs underlies dendritic computation in the fly visual system

The fly visual system offers a unique opportunity to explore computations performed by single neurons. Two previous studies characterized, in vivo, the receptive field (RF) of the vertical system (VS) cells of the blowfly (calliphora vicina), both intracellularly in the axon, and, independently using Ca²⁺ imaging, in hundreds of distal dendritic branchlets. We integrated this information into detailed passive cable and compartmental models of 3D reconstructed VS cells. Within a given VS cell type, the transfer resistance (TR) from different branchlets to the axon differs substantially, suggesting that they contribute unequally to the shaping of the axonal RF. Weighting the local RFs of all dendritic branchlets by their respective TR yielded a faithful reproduction of the axonal RF. The model also predicted that the various dendritic branchlets are electrically decoupled from each other, thus acting as independent local functional subunits. The study suggests that single neurons in the fly visual system filter dendritic noise and compute the weighted average of their inputs.

Influence of environmental information in natural scenes and the effects of motion adaptation on a fly motion-sensitive neuron during simulated flight

Gaining information about the spatial layout of natural scenes is a challenging task that flies need to solve, especially when moving at high velocities. A group of motion sensitive cells in the lobula plate of flies is supposed to represent information about self-motion as well as the environment. Relevant environmental features might be the nearness of structures, influencing retinal velocity during translational self-motion, and the brightness contrast. We recorded the responses of the H1 cell, an individually identifiable lobula plate tangential cell, during stimulation with image sequences, simulating translational motion through natural sceneries with a variety of differing depth structures. A correlation was found between the average nearness of environmental structures within large parts of the cell's receptive field and its response across a variety of scenes, but no correlation was found between the brightness contrast of the stimuli and the cell response. As a consequence of motion adaptation resulting from repeated translation through the environment, the time-dependent response modulations induced by the spatial structure of the environment were increased relatively to the background activity of the cell. These results support the hypothesis that some lobula plate tangential cells do not only serve as sensors of self-motion, but also as a part of a neural system that processes information about the spatial layout of natural scenes.

Robust coding of ego-motion in descending neurons of the fly

In many species, motion-sensitive neurons responding to optic flow at higher processing stages are well characterized; however, less is known how this representation of ego-motion is further transformed into an appropriate motor response. Here, we analyzed in the blowfly Calliphora vicina the visuomotor transformation from motion-sensitive neurons in the lobula plate [V2 and vertical system (VS) cells] onto premotor descending neurons [descending neurons of the ocellar and vertical system (DNOVS) cells] feeding into the motor circuit of the fly thoracic ganglion. We found that each of these cells is tuned to rotation of the fly around a particular body axis. Comparing the responses of presynaptic and postsynaptic cells revealed that DNOVS cells have approximately the same tuning widths as V2 and VS cells. However, DNOVS signals cells are less corrupted by fluctuations arising from the spatial structure of the visual input than their presynaptic elements. This leads to a more robust representation of ego-motion at the level of descending neurons. Thus, when moving from lobula plate cells to descending neurons, the selectivity for a particular optic flow remains unaltered, but the robustness of the representation increases.

Local and global motion preferences in descending neurons of the fly

For a moving animal, optic flow is an important source of information about its ego-motion. In flies, the processing of optic flow is performed by motion sensitive tangential cells in the lobula plate. Amongst them, cells of the vertical system (VS cells) have receptive fields with similarities to optic flows generated during rotations around different body axes. Their output signals are further processed by pre-motor descending neurons. Here, we investigate the local motion preferences of two descending neurons called descending neurons of the ocellar and vertical system (DNOVS1 and DNOVS2). Using an LED arena subtending 240° × 95° of visual space, we mapped the receptive fields of DNOVS1 and DNOVS2 as well as those of their presynaptic elements, i.e. VS cells 1–10 and V2. The receptive field of DNOVS1 can be predicted in detail from the receptive fields of those VS cells that are most strongly coupled to the cell. The receptive field of DNOVS2 is a combination of V2 and VS cells receptive fields. Predicting the global motion preferences from the receptive field revealed a linear spatial integration in DNOVS1 and a superlinear spatial integration in DNOVS2. In addition, the superlinear integration of V2 output is necessary for DNOVS2 to differentiate between a roll rotation and a lift translation of the fly.

Precise subcellular input retinotopy and its computational consequences in an identified visual interneuron

The Lobula Giant Movement Detector (LGMD) is a higher-order visual interneuron of Orthopteran insects that responds preferentially to objects approaching on a collision course. It receives excitatory input from an entire visual hemifield that anatomical evidence suggests is retinotopic. We show that this excitatory projection activates calcium-permeable nicotinic acetylcholine receptors. In vivo calcium imaging reveals that the excitatory projection preserves retinotopy down to the level of a single ommatidium. Examining the impact of retinotopy on the LGMD's computational properties, we show that sublinear synaptic summation can explain orientation preference in this cell. Exploring retinotopy's impact on directional selectivity leads us to infer that the excitatory input to the LGMD is intrinsically directionally selective. Our results show that precise retinotopy has implications for the dendritic integration of visual information in a single neuron.

Sensory integration: neuronal adaptations for robust visual self-motion estimation

Recent studies on the fly visual system have revealed how the morphology of visual interneurons and their lateral electrical connectivity helps overcome a notorious problem in visuomotor control - the ambiguity of local sensor signals.

The free-flight response of Drosophila to motion of the visual environment

In the present study we investigated the behavioural strategies with which freely flying fruit flies (Drosophila) control their flight trajectories during active optomotor stimulation in a free-flight arena. We measured forward, turning and climbing velocities of single flies using high-speed video analysis and estimated the output of a 'Hassenstein-Reichardt' elementary motion detector (EMD) array and the fly's gaze to evaluate flight behaviour in response to a rotating visual panorama. In a stationary visual environment, flight is characterized by flight saccades during which the animals turn on average 120 degrees within 130 ms. In a rotating environment, the fly's behaviour typically changes towards distinct, concentric circular flight paths where the radius of the paths increases with increasing arena velocity. The EMD simulation suggests that this behaviour is driven by a rotation-sensitive EMD detector system that minimizes retinal slip on each compound eye, whereas an expansion-sensitive EMD system with a laterally centred visual focus potentially helps to achieve centring response on the circular flight path. We developed a numerical model based on force balance between horizontal, vertical and lateral forces that allows predictions of flight path curvature at a given locomotor capacity of the fly. The model suggests that turning flight in Drosophila is constrained by the production of centripetal forces needed to avoid side-slip movements. At maximum horizontal velocity this force may account for up to 70% of the fly's body weight during yaw turning. Altogether, our analyses are widely consistent with previous studies on Drosophila free flight and those on the optomotor response under tethered flight conditions.

Nonlinear integration of binocular optic flow by DNOVS2, a descending neuron of the fly

For visual orientation and course stabilization, flies rely heavily on the optic flow perceived by the animal during flight. The processing of optic flow is performed in motion-sensitive tangential cells of the lobula plate, which are well described with respect to their visual response properties and the connectivity among them. However, little is known about the postsynaptic descending neurons, which convey motion information to the motor circuits in the thoracic ganglion. Here we investigate the physiology and connectivity of an identified premotor descending neuron, called DNOVS2 (for descending neuron of the ocellar and vertical system). We find that DNOVS2 is tuned in a supralinear way to rotation around the longitudinal body axis. Experiments involving stimulation of the ipsilateral and the contralateral eye indicate that ipsilateral computation of motion information is modified nonlinearly by motion information from the contralateral eye. Performing double recordings of DNOVS2 and lobula plate tangential cells, we find that DNOVS2 is connected ipsilaterally to a subset of vertical-sensitive cells. From the contralateral eye, DNOVS2 receives input most likely from V2, a heterolateral spiking neuron. This specific neural circuit is sufficient for the tuning of DNOVS2, making it probably an important element in optomotor roll movements of the head and body around the fly's longitudinal axis.

Reciprocal inhibitory connections within a neural network for rotational optic-flow processing

Neurons in the visual system of the blowfly have large receptive fields that are selective for specific optic flow fields. Here, we studied the neural mechanisms underlying flow–field selectivity in proximal Vertical System (VS)-cells, a particular subset of tangential cells in the fly. These cells have local preferred directions that are distributed such as to match the flow field occurring during a rotation of the fly. However, the neural circuitry leading to this selectivity is not fully understood. Through dual intracellular recordings from proximal VS cells and other tangential cells, we characterized the specific wiring between VS cells themselves and between proximal VS cells and horizontal sensitive tangential cells. We discovered a spiking neuron (Vi) involved in this circuitry that has not been described before. This neuron turned out to be connected to proximal VS cells via gap junctions and, in addition, it was found to be inhibitory onto VS1.

Synaptic organization of lobula plate tangential cells inDrosophila: γ-Aminobutyric acid receptors and chemical release sites

In flies, the large tangential cells of the lobula plate represent an important processing center for visual navigation based on optic flow. Although the visual response properties of these cells have been well studied in blowflies, information on their synaptic organization is mostly lacking. Here we study the distribution of presynaptic release and postsynaptic inhibitory sites in the same set of cells in Drosophila melanogaster. By making use of transgenic tools and immunohistochemistry, our results suggest that HS and VS cells of Drosophila express gamma-aminobutyric acid (GABA) receptors in their dendritic region within the lobula plate, thus being postsynaptic to inhibitory input there. At their axon terminals in the protocerebrum, both cell types express synaptobrevin, suggesting the presence of presynaptic specializations there. HS- and VS-cell terminals additionally show evidence for postsynaptic GABAergic input, superimposed on this synaptic polarity. Our findings are in line with the general circuit for visual motion detection and receptive field properties as postulated from electrophysiological and optical recordings in blowflies, suggesting a similar functional organization of lobula plate tangential cells in the two species.

Integration of lobula plate output signals by DNOVS1, an identified premotor descending neuron

Many motion-sensitive tangential cells of the lobula plate in blowflies are well described with respect to their visual response properties and the connectivity among them. They have large and complex receptive fields with different preferred directions in different parts of their receptive fields matching the optic flow that occurs during various flight maneuvers. However, much less is known about how tangential cells connect to postsynaptic neurons descending to the motor circuits in the thoracic ganglion and how optic flow is represented in these downstream neurons. Here we describe the physiology and the connectivity of a prominent descending neuron called DNOVS1 (for descending neurons of the ocellar and vertical system). We find that DNOVS1 is electrically coupled to a subset of vertical system cells. The specific wiring leads to a preference of DNOVS1 for rotational flow fields around a particular body axis. In addition, DNOVS1 receives input from interneurons connected to the ocelli.

Sharing receptive fields with your neighbors: tuning the vertical system cells to wide field motion

In the blowfly, the direction-selective response of the 60 lobula-plate tangential cells has been ascribed to the integration of local motion information across their extensive dendritic trees. Because the lobula plate is organized retinotopically, the receptive fields of the tangential cells ought to be determined by their dendritic architecture. However, this appears not always to be the case. One compelling example is the exceptionally wide receptive fields of the vertical system (VS) tangential cells. Using dual-intracellular recordings, Haag and Borst (2004) found VS cells to be mutually coupled in such a way that each VS cell is connected exclusively to its immediate neighbors. This coupling may form the basis of the broad receptive fields of VS cells. Here, we tested this hypothesis directly by photoablating individual VS cells. The receptive field width of VS cells indeed narrowed after the ablation of single VS cells, specifically depending on whether the receptive field of the ablated cell was more frontal or more posterior to the recorded cell. In particular, the responses changed as if the neuron lost access to visual information from the ablated neuron and those VS cells more distal than it from the recorded neuron. These experiments provide strong evidence that the lateral connections among VS cells are a crucial component in the mechanism underlying their complex receptive fields, augmenting the direct columnar input to their dendrites.

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.

Velocity constancy and models for wide-field visual motion detection in insects

The tangential neurons in the lobula plate region of the flies are known to respond to visual motion across broad receptive fields in visual space. When intracellular recordings are made from tangential neurons while the intact animal is stimulated visually with moving natural imagery,we find that neural response depends upon speed of motion but is nearly invariant with respect to variations in natural scenery. We refer to this invariance as velocity constancy. It is remarkable because natural scenes, in spite of similarities in spatial structure, vary considerably in contrast, and contrast dependence is a feature of neurons in the early visual pathway as well as of most models for the elementary operations of visual motion detection. Thus, we expect that operations must be present in the processing pathway that reduce contrast dependence in order to approximate velocity constancy. We consider models for such operations, including spatial filtering, motion adaptation, saturating nonlinearities, and nonlinear spatial integration by the tangential neurons themselves, and evaluate their effects in simulations of a tangential neuron and precursor processing in response to animated natural imagery. We conclude that all such features reduce interscene variance in response, but that the model system does not approach velocity constancy as closely as the biological tangential cell.

On the computations analyzing natural optic flow: quantitative model analysis of the blowfly motion vision pathway

For many animals, including humans, the optic flow generated on the eyes during locomotion is an important source of information about self-motion and the structure of the environment. The blowfly has been used frequently as a model system for experimental analysis of optic flow processing at the microcircuit level. Here, we describe a model of the computational mechanisms implemented by these circuits in the blowfly motion vision pathway. Although this model was originally proposed based on simple experimenter-designed stimuli, we show that it is also capable to quantitatively predict the responses to the complex dynamic stimuli a blowfly encounters in free flight. In particular, the model visual system exploits the active saccadic gaze and flight strategy of blowflies in a similar way, as does its neuronal counterpart. The model circuit extracts information about translation velocity in the intersaccadic intervals and thus, indirectly, about the three-dimensional layout of the environment. By stepwise dissection of the model circuit, we determine which of its components are essential for these remarkable features. When accounting for the responses to complex natural stimuli, the model is much more robust against parameter changes than when explaining the neuronal responses to simple experimenter-defined stimuli. In contrast to conclusions drawn from experiments with simple stimuli, optimization of the parameter set for different segments of natural optic flow stimuli do not indicate pronounced adaptational changes of these parameters during long-lasting stimulation.

Dye-coupling visualizes networks of large-field motion-sensitive neurons in the fly

In the fly, visually guided course control is accomplished by a set of 60 large-field motion-sensitive neurons in each brain hemisphere. These neurons have been shown to receive retinotopic motion information from local motion detectors on their dendrites. In addition, recent experiments revealed extensive coupling between the large-field neurons through electrical synapses. These two processes together give rise to their broad and elaborate receptive fields significantly surpassing the extent of their dendritic fields. Here, we demonstrate that the electrical connections between different large-field neurons can be visualized using Neurobiotin dye injection into a single one of them. When combined with a fluorescent dye which does not cross electrical synapses, the injected cell can be identified unambiguously. The Neurobiotin staining corroborates the electrical coupling postulated amongst the cells of the vertical system (VS-cells) and between cells of the horizontal system (HS-cells and CH-cells). In addition, connections between some cells are revealed that have so far not been considered as electrically coupled.

Spatial distribution of inputs and local receptive field properties of a wide-field, looming sensitive neuron

The lobula giant movement detector (LGMD) in the locust visual system and its target neuron, the descending contralateral movement detector (DCMD), respond to approaching objects looming on a collision course with the animal. They thus provide a good model to study the cellular and network mechanisms underlying the sensitivity to this specific class of behaviorally relevant stimuli. We determined over an entire locust eye the density distribution of optical axes describing the spatial organization of local inputs to the visual system and compared it with the sensitivity distribution of the LGMD/DCMD to local motion stimuli. The density of optical axes peaks in the equatorial region of the frontal eye. Local motion sensitivity, however, peaks in the equatorial region of the caudolateral visual field and only correlates positively with the dorso-ventral density of optical axes. On local stimulation, both the velocity tuning and the response latency of the LGMD/DCMD depend on stimulus position within the visual field. Spatial and temporal integration experiments in which several local motion stimuli were activated either simultaneously or at fixed delays reveal that the LGMD processes local motion in a strongly sublinear way. Thus the neuron's integration properties seem to depend on several factors including its dendritic morphology, the local characteristics of afferent fiber inputs, and inhibition mediated by different pathways or by voltage-gated conductances. Our study shows that the selectivity of this looming sensitive neuron to approaching objects relies on more complex biophysical mechanisms than previously thought.

Passive spatial and temporal integration of excitatory synaptic inputs in cerebellar Purkinje cells of young rats

We have investigated the integration of excitatory (parallel fiber) synaptic inputs in cerebellar Purkinje cells of young rats in vitro and in a compartmental model of such a cell, based on 3D morphological reconstruction. Excitatory synaptic inputs at two independent dendritic sites were activated by electrical stimulation with various delays between the two stimuli. Population postsynaptic potentials summed linearly under current clamp condition when the two dendritic input sites were spatially separated (>200 microm) but sublinearly, in a delay dependent manner, when the input sites were close (<50 microm) to each other. Population postsynaptic currents measured under voltage clamp conditions summed linearly independent of the spatial and temporal separation of inputs. Summation of inputs in a passive compartmental model of a Purkinje cell was similar to that of Purkinje cells in vitro. We show that sublinear summation of neighboring inputs is independent of inhibitory mechanisms and suggest that sublinearity is mainly due to a locally reduced driving force.

Different mechanisms of calcium entry within different dendritic compartments

From our experiments combining in vivo calcium imaging and electrophysiology on fly vertical motion-sensitive cells (VS-cells) during visual stimulation, we infer different mechanisms of calcium entry within different dendritic compartments; while in the main dendritic branches calcium influx from extracellular space takes place only via voltage-activated calcium channels (VACCs), calcium enters the dendritic tips through VACCs as well as nicotinic acetylcholine receptors (nAChRs). Consequently, neuronal nACHRs of insects have to be assumed to be permeable to some extent for calcium under in vivo conditions.

Transfer of visual motion information via graded synapses operates linearly in the natural activity range

Synaptic transmission between a graded potential neuron and a spiking neuron was investigated in vivo using sensory stimulation instead of artificial excitation of the presynaptic neuron. During visual motion stimulation, individual presynaptic and postsynaptic neurons in the brain of the fly were electrophysiologically recorded together with concentration changes of presynaptic calcium (Delta[Ca(2+)](pre)). Preferred-direction motion leads to depolarization of the presynaptic neuron. It also produces pronounced increases in [Ca(2+)](pre) and the postsynaptic spike rate. Motion in the opposite direction was associated with hyperpolarization of the presynaptic cell but only a weak reduction in [Ca(2+)](pre) and the postsynaptic spike rate. Apart from this rectification, the relationships between presynaptic depolarizations, Delta[Ca(2+)](pre), and postsynaptic spike rates are, on average, linear over the entire range of activity levels that can be elicited by sensory stimulation. Thus, the inevitably limited range in which the gain of overall synaptic signal transfer is constant appears to be adjusted to sensory input strengths.

Mechanisms of dendritic calcium signaling in fly neurons

We examined the mechanisms underlying dendritic calcium accumulation in lobula plate tangential cells of the fly visual system using an in vitro preparation of the fly brain. Local visual stimulation evokes a localized calcium signal in the dendrites of these cells in vivo. Here we show that a similar localized calcium accumulation can be elicited in vitro by focal iontophoretic application of the cholinergic agonist carbachol. The calcium signal had at least two sources: first, voltage-dependent calcium channels contributed to the carbachol-induced signal and were concentrated on the dendrite, the soma, and the terminal ramification of the axon. However, the dendritic calcium signal induced by carbachol stimulation was only weakly dependent on membrane depolarization. The most likely explanation for the second, voltage-independent part of the dendritic calcium signal is calcium entry through nicotinic acetylcholine receptors. We found no indication of second-messenger or calcium-mediated calcium release from intracellular stores. In summary, the characteristic spatiotemporal calcium signals in the dendrites of lobula plate tangential cells can be reproduced in vitro, and result from a combination of voltage- and ligand-gated calcium influx.

Untangling dendrites with quantitative models

Our understanding of the function of dendrites has been greatly enriched by an inspiring dialogue between theory and experiments. Rather than functionally ignoring dendrites, representing neurons as single summing points, we have realized that dendrites are electrically and chemically distributed nonlinear units and that this has important consequences for interpreting experimental data and for the role of neurons in information processing. Here, we examine the route to unraveling some of the enigmas of dendrites and highlight the main insights that have been gained. Future directions are discussed that will enable theory and models to keep shedding light on dendrites, where the most fundamental input-output adaptive processes take place.

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.

Spatial distribution and characteristics of voltage-gated calcium signals within visual interneurons

Most of our knowledge about insect calcium currents is derived from studies on cultured or dissociated somata. So far, only little data on calcium currents are available for neurons including their dendritic and presynaptic structures. Here we combined the switched-electrode voltage-clamp technique with optical recording using calcium-sensitive dyes in identified fly visual interneurons in vivo to characterize the voltage dependence and dynamics of calcium currents quantitatively and in a spatially resolved way. For all three cell types considered, i.e., centrifugal horizontal (CH), horizontal system (HS), and vertical system (VS) cells, the activation curve is rather flat and covers a voltage range from -60 to -20 mV in dendritic as well as presynaptic areas of the cells. The calcium increase is fastest for CH cells with a time constant of approximately 70 ms. In HS and VS cells, the time constant amounts to 400-700 ms. The calcium dynamics as determined in different regions of the cells are similar except for a small segment between the axon and the dendrite in HS and VS cells, where the calcium increase is significantly faster. In summary, the results show the existence of a low-voltage-activated calcium current with little or no inactivation in dendritic as well as presynaptic regions of fly lobula plate tangential cells.

The intrinsic electrophysiological characteristics of fly lobula plate tangential cells: III. Visual response properties

In this last paper in a series (Borst and Haag, 1996; Haag et al., 1997) about the lobula plate tangential cells of the fly visual system (CH, HS, and VS cells), the visual response properties were examined using intracellular recordings and computer simulations. In response to visual motion stimuli, all cells responded mainly by a graded shift of their axonal membrane potential. While ipsilateral motion resulted in a graded membrane potential shift, contralateral motion led to distinct EPSPs. For HS cells, simultaneous extracellular recorded action potentials of a spiking interneuron, presumably the H2 cell, corresponded to the EPSPs in the HS cell in a one-to-one fashion. When HS cells were hyperpolarized during ipsilateral motion, they mainly produced action potentials, but when they were hyperpolarized during contralateral motion only a slight increase of EPSP amplitude, could be observed. Intracellular application of the sodium channel blocker QX 314 abolished action potentials of HS cells while having little effect on the graded membrane response to ipsilateral motion. HS and CH cells were also studied with respect to their spatial integration properties. For both cell types, their graded membrane response was found to increase less than linearly with the size of the ipsilateral motion pattern. However, while for HS cells various amounts of hyperpolarizing current injected during motion stimulation led to different saturation levels, this was not the case for CH cells. In response to a sinusoidal velocity modulation, CH cells followed pattern motion only up to 10 Hz modulation frequency, but HS cells still revealed significant membrane depolarizations up to about 40 Hz. In the computer simulations, the compartmental models of tangential cells, as derived in the previous papers, were linked to an array of local motion detectors. The model cells revealed the same basic response features as their natural counterparts. They showed a response saturation as a function of stimulus size. In CH-models, however, the saturation was less pronounced than in real CH-cells, indicating spatially nonuniform membrane resistances with higher values in the dendrite. As in the experiments, HS models responded to high-frequency velocity modulation with a higher amplitude than did CH models.

Supralinear summation of synaptic inputs by an invertebrate neuron: dendritic gain is mediated by an "inward rectifier" K(+) current

Dendritic processing of glutamatergic synaptic inputs was investigated in the anterior pagoda cell of leech. We observed that below spike threshold, the amplitude of individual EPSPs decreased with hyperpolarization and that simultaneous stimulation of pairs of synaptic inputs leads to the supralinear summation of EPSPs. Voltage-clamp measurements revealed a hyperpolarization-activated, Ba(2+)-sensitive, fast, noninactivating K(+) conductance that depends on the external [K(+)]. These features are those of an "inward rectifier," Kir. Microsurgery experiments, in combination with electrophysiological measurements, revealed an inhomogeneous spatial distribution of the Kir conductance. Furthermore, on surgical removal of the neurites that contain the Kir conductance, the amplitude of EPSPs from the remaining synaptic inputs increased with hyperpolarization. A model cell, with the Kir conductance as the sole voltage-dependent conductance, reproduced qualitatively the observed voltage dependence of individual EPSPs as well as the supralinear summation of EPSP pairs.

Response attenuation during coincident afferent excitatory inputs

The linearity of the synaptic summation of two unitary excitatory synaptic events was investigated during whole cell recordings from retinal target neurons in an eye-attached isolated brain stem preparation. Pairs of unitary excitatory postsynaptic potentials (EPSPs) were evoked by bipolar stimulation electrodes that were directed to two distinct foci on the retinal surface based on the visual receptive field boundaries. The interval between stimulation of each retinal site was incremented by 0.5-1 ms to quantify the time course of nonlinear summation using an exponential fit. Response facilitation was never observed; however, the coincident arrival of synaptic inputs caused a response attenuation in 26 of the 37 pairs studied. Twelve of the 26 pairs had time constants of their attenuation that were similar to the time constants of the decaying phases of the first EPSPs of each pair. This suggests that the attenuation of these 12 pairs may be entirely due to voltage-dependent mechanisms, such as a reduction in driving force or a change of the activity of voltage-sensitive channels. On the other hand, the 14 other pairs had their time constant of attenuation shorter than the time constants of the decaying phase of the first EPSP. In fact, the attenuation time constants were often closer to the time constants of the decaying phases of the first excitatory postsynaptic currents of each pair. This finding suggests that the attenuation of these 14 pairs involve a shunting mechanism due to the opening of synaptic channels. The presence of this conductance-dependent mechanism is supported by the finding of asymmetric effects on the time course of attenuation when the stimulation sequence was reversed. These results are discussed in terms of the processing by neurons of coincident excitatory inputs onto spatially distinct points of their dendritic trees.

Spatial response properties of contralateral inhibited lobula plate tangential cells in the fly visual system

This study describes the spatial response properties of a particular group of motion-sensitive and directionally selective neurons located in the lobula plate of the fly visual system. Their preferred motion direction is front-to-back (depolarization), and their null direction is back-to-front (hyperpolarization). They receive inhibitory input from the contralateral eye during pattern motion from back to front (regressive). In this study, we call these neurons regressive contralateral inhibited lobula plate tangential cells (rCI-LPTCs). Three physiologic groups of rCI-neurons (rCI-I, rCI-IIa, and rCI-IIb) can be distinguished on the basis of their ipsilateral pattern size dependence and their inhibitory contralateral input. rCI-I neurons depolarize during the motion of small ipsilateral patterns from front to back, but they become hyperpolarized by large ipsilateral patterns moving in the same direction. rCI-IIa and rCI-IIb neurons receive bidirectional inhibitory input from the contralateral eye. rCI-IIa neurons respond best to small ipsilateral pattern sizes, but unlike rCI-I neurons, their net response to large patterns is positive. rCI-IIb neurons respond best to large ipsilateral patterns. The anatomical and physiologic variability of the rCI-neurons suggests that more than three types of rCI-neurons exist. The suggested physiologic groups might be preliminary. We recorded one neuron that could mediate the bidirectional inhibitory input that rCI-IIa and rCI-IIb neurons receive from the contralateral eye. In the case of the rCI-IIa neurons at least one further contralateral inhibitory element has to be assumed. The tuning of rCI-IIa neurons to small ipsilateral pattern sizes is likely to be based on an on-center/off-surround organization of their synaptic input.

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.

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).

Dendritic computation of direction selectivity and gain control in visual interneurons

The extraction of motion information from time varying retinal images is a fundamental task of visual systems. Accordingly, neurons that selectively respond to visual motion are found in almost all species investigated so far. Despite its general importance, the cellular mechanisms underlying direction selectivity are not yet understood in most systems. Blocking inhibitory input to fly visual interneurons by picrotoxinin (PTX), we demonstrate that their direction selectivity arises largely from interactions between postsynaptic signals elicited by excitatory and inhibitory input elements, which are themselves only weakly tuned to opposite directions of motion. Their joint activation by preferred as well as null direction motion leads to a mixed reversal potential at which the postsynaptic response settles for large field stimuli. Assuming the activation ratio of these opponent inputs to be a function of pattern velocity can explain how the postsynaptic membrane potential saturates with increasing pattern size at different levels for different pattern velocities ("gain control"). Accordingly, we find that after blocking the inhibitory input by PTX, gain control is abolished.

Encoding of visual motion information and reliability in spiking and graded potential neurons

We investigated the information about stimulus velocity inherent in the membrane signals of two types of directionally selective, motion-sensitive interneurons in the fly visual system. One of the cells, the H1-cell, is a spiking neuron, whereas the other, the HS-cell, encodes sensory information mainly by a graded shift of its membrane potential. Using a pseudo-random velocity waveform by which a visual grating is moving along the horizontal axis of the eye, both cell types follow the stimulus velocity at higher precision than in response to a step-like velocity function. To measure how much information about the stimulus velocity is preserved in the cellular responses, we calculated the coherence between the stimulus and the neural signals as a function of stimulus frequency. At frequencies up to approximately 10 Hz motion information is well contained in the electrical signals of HS- and H1-cells: For HS-cells the coherence value amounts to approximately 70%, and for H1-cells this value is approximately 60%. Comparing these values with the coherence expected from a linear encoding reveals that the fidelity of the original stimulus is deteriorated in the neural signal partly by neural noise and partly by the nonlinearity inherent in the process of visual motion detection. The degree to which this nonlinearity contributes to the decrease in coherence depends on the maximum velocity used in the experiments; the smaller the stimulus amplitude, the higher the coherence and, thus, the smaller the nonlinearity in encoding of stimulus motion. All these results are in agreement with model simulations in which visual motion is processed by an array of local motion detectors, the spatially integrated output of which is considered the equivalent of the neural signals of HS- and H1-cells.

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).

Optomotor-blind of Drosophila melanogaster: a neurogenetic approach to optic lobe development and optomotor behaviour

The gene optomotor-blind (omb) plays a crucial role in Drosophila optic lobe development. Various mutations in omb lead to different structural defects in the adult optic lobes with correlated behavioural phenotypes. Molecular analysis of omb allows one to trace back behavioural defects to the spatio-temporal misexpression of the gene in mutant development.

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.

Mechanisms of dendritic integration underlying gain control in fly motion-sensitive interneurons

In the compensatory optomotor response of the fly the interesting phenomenon of gain control has been observed by Reichardt and colleagues (Reichardt et al., 1983): The amplitude of the response tends to saturate with increasing stimulus size, but different saturation plateaus are assumed with different velocities at which the stimulus is moving. This characteristic can already be found in the motion-sensitive large field neurons of the fly optic lobes that play a role in mediating this behavioral response (Hausen, 1982; Reichardt et al., 1983; Egelhaaf, 1985; Haag et al., 1992). To account for gain control a model was proposed involving shunting inhibition of these cells by another cell, the so-called pool cell (Reichardt et al., 1983), both cells sharing common input from an array of local motion detectors. This article describes an alternative model which only requires dendritic integration of the output signals of two types of local motion detectors with opposite polarity. The explanation of gain control relies on recent findings that these input elements are not perfectly directionally selective and that their direction selectivity is a function of pattern velocity. As a consequence, the resulting postsynaptic potential in the dendrite of the integrating cell saturates with increasing pattern size at a level between the excitatory and inhibitory reversal potentials. The exact value of saturation is then set by the activation ratio of excitatory and inhibitory input elements which in turn is a function of other stimulus parameters such as pattern velocity. Thus, the apparently complex phenomenon of gain control can be simply explained by the biophysics of dendritic integration in conjunction with the properties of the motion-sensitive input elements.

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.