Orientation tuning of motion-sensitive neurons shaped by vertical-horizontal network interactions

We measured the orientation tuning of two neurons of the fly lobula plate (H1 and H2 cells) sensitive to horizontal image motion. Our results show that H1 and H2 cells are sensitive to vertical motion, too. Their response depended on the position of the vertically moving stimuli within their receptive field. Stimulation within the frontal receptive field produced an asymmetric response: upward motion left the H1/H2 spike frequency nearly unaltered while downward motion increased the spike frequency to about 40% of their maximum responses to horizontal motion. In the lateral parts of their receptive fields, no such asymmetry in the responses to vertical image motion was found. Since downward motion is known to be the preferred direction of neurons of the vertical system in the lobula plate, we analyzed possible interactions between vertical system cells and H1 and H2 cells. Depolarizing current injection into the most frontal vertical system cell (VS1) led to an increased spike frequency, hyperpolarizing current injection to a decreased spike frequency in both H1 and H2 cells. Apart from VS1, no other vertical system cell (VS2-8) had any detectable influence on either H1 or H2 cells. The connectivity of VS1 and H1/H2 is also shown to influence the response properties of both centrifugal horizontal cells in the contralateral lobula plate, which are known to be postsynaptic to the H1 and H2 cells. The vCH cell receives additional input from the contralateral VS2-3 cells via the spiking interneuron V1.     

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.     

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.     

Visuomotor transformation in the fly gaze stabilization system

For sensory signals to control an animal's behavior, they must first be transformed into a format appropriate for use by its motor systems. This fundamental problem is faced by all animals, including humans. Beyond simple reflexes, little is known about how such sensorimotor transformations take place. Here we describe how the outputs of a well-characterized population of fly visual interneurons, lobula plate tangential cells (LPTCs), are used by the animal's gaze-stabilizing neck motor system. The LPTCs respond to visual input arising from both self-rotations and translations of the fly. The neck motor system however is involved in gaze stabilization and thus mainly controls compensatory head rotations. We investigated how the neck motor system is able to selectively extract rotation information from the mixed responses of the LPTCs. We recorded extracellularly from fly neck motor neurons (NMNs) and mapped the directional preferences across their extended visual receptive fields. Our results suggest that-like the tangential cells-NMNs are tuned to panoramic retinal image shifts, or optic flow fields, which occur when the fly rotates about particular body axes. In many cases, tangential cells and motor neurons appear to be tuned to similar axes of rotation, resulting in a correlation between the coordinate systems the two neural populations employ. However, in contrast to the primarily monocular receptive fields of the tangential cells, most NMNs are sensitive to visual motion presented to either eye. This results in the NMNs being more selective for rotation than the LPTCs. Thus, the neck motor system increases its rotation selectivity by a comparatively simple mechanism: the integration of binocular visual motion information.     

Motion sensitive interneurons in the optomotor system of the fly

The functional properties of the three horizontal cells (north horizontal cell, HSN; equatorial horizontal cell, HSE; south horizontal cell, HSS) in the lobula plate of the blowflyCalliphora erythrocephala were investigated electrophysiologically. 1. The receptive fields of the HSN, HSE, and HSS cover the dorsal, equatorial and ventral part of the ipsilateral visual field, respectively. In all three cells, the sensitivity to visual stimulation is highest in the frontal visual field and decreases laterally. The receptive fields and spatial sensitivity distributions of the horizontal cells are directly determined by the position and extension of their dendritic fields in the lobula plate and the dendritic density distributions within these fields. 2. The horizontal cells respond mainly to progressive (front to back) motion and are inhibited by motion in the reverse direction, the preferred and null direction being antiparallel. The amplitudes of motion induced excitatory and inhibitory responses decline like a cosine function with increasing deviation of the direction of motion from the preferred direction. Stimulation with motion in directions perpendicular to the preferred direction is ineffective. 3. The preferred directions of the horizontal cells show characteristic gradual orientation changes in different parts of the receptive fields: they are horizontally oriented only in the equatorial region and increasingly tilted vertically towards the dorsofrontal and ventrofrontal margins of the visual field. These orientation changes can be correlated with equivalent changes in the local orientation of the lattice of ommatidial axes in the pertinent compound eye. 4. The response amplitudes of the horizontal cells under stimulation with a moving periodic grating depend strongly on the contrast frequency of the stimulus. Maximal responses were found at contrast frequencies of 2–5 Hz. 5. The spatial integration properties of the horizontal cells (studied in the HSE) are highly nonlinear. Under stimulation with extended moving patterns, their response amplitudes are nearly independent of the size of the stimuli. It is demonstrated that this response behaviour does not result from postsynaptic saturation in the dendrites of the cells. The results indicate that the horizontal system is essentially involved in the neural control of optomotor torque responses performed by the fly in order to minimize unvoluntary deviations from a straight flight course.     

The centrifugal horizontal cells in the lobula plate of the blowfly, Phaenicia sericata

Intracellular recordings combined with iontophoretic injection of Procion Yellow M4RAN were used to study the anatomy and physiology of the centrifugal horizontal cells (CH-cells) in the lobula plate of the blowfly, Phaenicia sericata.Anatomy: The CH-cells comprise a set of two homolateral, giant visual interneurones (DCH, VCH) at the rostral surface of each lobula plate. Their extensive arborizations in the lobula plate possess bulbous swellings (boutons terminaux). The arborization of one cell (DCH) covers the dorsal, and the arborization of the other cell (VCH) the ventral half of the lobula plate. Their axons run jointly with those of the horizontal cells through the chiasma internum and the optic peduncle. Their protocerebral arborization possesses spines; they form a dense network together with the axonal arborization of the horizontal cells, a second type of giant homolateral cell most sensitive to horizontal motion. The protocerebral arborization of the CH-cells gives rise to a cell body fibre which traverses the protocerebrum dorsally to the oesophageal canal. The cell body lies on the contralateral side laterally and slightly dorsally to the oesophageal canal in the frontal cell body layer.Physiology: The CH-cells respond with graded potentials to rotatory movements of their surround. Cells in the right lobula plate respond with excitation (excitatory postsynaptic potentials, membrane depolarization) to clockwise motion (contralateral regressive, ipsilateral progressive), and with inhibition (inhibitory postsynaptic potentials, membrane hyperpolarization) to counterclockwise motion in either or both receptive fields; CH-cells respond to motion presented to the ipsilateral and/or contralateral eye. Cells of the left lobula plate respond correspondingly to the reverse directions of motion. Vertical pattern motion and stationary patterns are ineffective.The heterolateral H1-neurone elicits excitatory postsynaptic potentials in the DCH-cell; these postsynaptic potentials are tightly correlated 1:1 to the preceding H1-action potentíal. The delay between the peak of the action potential and the beginning of the DCH-postsynaptic potential is 1.15 msec, agreeing very well with the value reported previously for the blowfly, Calliphora (Hausen, 1976a). The synaptic input and output connections of the CH-cells are discussed.     

Lobula plate and ocellar interneurons converge onto a cluster of descending neurons leading to neck and leg motor neuropil in Calliphora erythrocephala

Summary In the fly, Calliphora erythrocephala, a cluster of three Y-shaped descending neurons (DNOVS 1–3) receives ocellar interneuron and vertical cell (VS4–9) terminals. Synaptic connections to one of them (DNOVS 1) are described. In addition, three types of small lobula plate vertical cell (sVS) and one type of contralateral horizontal neuron (Hc) terminate at DNOVS 1, as do two forms of ascending neurons derived from thoracic ganglia. A contralateral neuron, with terminals in the opposite lobula plate, arises at the DNOVS cluster and is thought to provide heterolateral interaction between the VS4–9 output of one side to the VS4–9 dendrites of the other. DNOVS 2 and 3 extend through pro-, meso-, and metathoracic ganglia, branching ipsilaterally within their tract and into the inner margin of leg motor neuropil of each ganglion. DNOVS 1 terminates as a stubby ending in the dorsal prothoracic ganglion onto the main dendritic trunks of neck muscle motor neurons. Convergence of VS and ocellar interneurons to DNOVS 1 comprises a second pathway from the visual system to the neck motor, the other being carried by motor neurons arising in the brain. Their significance for saccadic head movement and the stabilization of the retinal image is discussed.     

Response properties of visual interneurons to motion stimuli in the praying mantis,Tenodera aridifolia

Intracellular responses of motion-sensitive visual interneurons were recorded from the lobula complex of the mantis, Tenodera aridifolia. The interneurons were divided into four classes according to the response polarity, spatial tuning, and directional selectivity. Neurons of the first class had small, medium, or large receptive fields and showed a strong excitation in response to a small-field motion such as a small square moving in any direction (SF neurons). The second class neurons showed non-directionally selective responses: an excitation to a large-field motion of gratings in any direction (ND neurons). Most ND neurons had small or medium-size receptive fields. Neurons of the third class had large receptive fields and exhibited directionally selective responses: an excitation to a large-field motion of gratings in preferred direction and an inhibition to a motion in opposite, null direction (DS neurons). The last class neurons had small receptive fields and showed inhibitory responses to a moving square and gratings (I neurons). The functional roles of these neurons in prey recognition and optomotor response were discussed.     

Wide-field, motion-sensitive neurons and matched filters for optic flow fields

 The receptive field organization of a class of visual interneurons in the fly brain (vertical system, or VS neurons) shows a striking similarity to certain self-motion-induced optic flow fields. The present study compares the measured motion sensitivities of the VS neurons (Krapp et al. 1998) to a matched filter model for optic flow fields generated by rotation or translation. The model minimizes the variance of the filter output caused by noise and distance variability between different scenes. To that end, prior knowledge about distance and self-motion statistics is incorporated in the form of a “world model”. We show that a special case of the matched filter model is able to predict the local motion sensitivities observed in some VS neurons. This suggests that their receptive field organization enables the VS neurons to maintain a consistent output when the same type of self-motion occurs in different situations. Received: 14 June 1999 / Accepted in revised form: 20 March 2000     

Pattern-dependent response modulations in motion-sensitive visual interneurons--a model study

Even if a stimulus pattern moves at a constant velocity across the receptive field of motion-sensitive neurons, such as lobula plate tangential cells (LPTCs) of flies, the response amplitude modulates over time. The amplitude of these response modulations is related to local pattern properties of the moving retinal image. On the one hand, pattern-dependent response modulations have previously been interpreted as 'pattern-noise', because they deteriorate the neuron's ability to provide unambiguous velocity information. On the other hand, these modulations might also provide the system with valuable information about the textural properties of the environment. We analyzed the influence of the size and shape of receptive fields by simulations of four versions of LPTC models consisting of arrays of elementary motion detectors of the correlation type (EMDs). These models have previously been suggested to account for many aspects of LPTC response properties. Pattern-dependent response modulations decrease with an increasing number of EMDs included in the receptive field of the LPTC models, since spatial changes within the visual field are smoothed out by the summation of spatially displaced EMD responses. This effect depends on the shape of the receptive field, being the more pronounced--for a given total size--the more elongated the receptive field is along the direction of motion. Large elongated receptive fields improve the quality of velocity signals. However, if motion signals need to be localized the velocity coding is only poor but the signal provides--potentially useful--local pattern information. These modelling results suggest that motion vision by correlation type movement detectors is subject to uncertainty: you cannot obtain both an unambiguous and a localized velocity signal from the output of a single cell. Hence, the size and shape of receptive fields of motion sensitive neurons should be matched to their potential computational task.     

Global versus local adaptation in fly motion-sensitive neurons

Flies, like humans, experience a well-known consequence of adaptation to visual motion, the waterfall illusion. Direction-selective neurons in the fly lobula plate permit a detailed analysis of the mechanisms responsible for motion adaptation and their function. Most of these neurons are spatially non-opponent, they sum responses to motion in the preferred direction across their entire receptive field, and adaptation depresses responses by subtraction and by reducing contrast gain. When we adapted a small area of the receptive field to motion in its anti-preferred direction, we discovered that directional gain at unadapted regions was enhanced. This novel phenomenon shows that neuronal responses to the direction of stimulation in one area of the receptive field are dynamically adjusted to the history of stimulation both within and outside that area.     

The optic lobe of Drosophila melanogaster

Summary We present a quantitative evaluation of Golgiimpregnated columnar neurons in the optic lobe of wildtype Drosophila melanogaster. This analysis reveals the overall connectivity pattern between the 10 neuropil layers of the medulla and demonstrates the existence of at least three major visual pathways. Pathway 1 connects medulla layer M10 to the lobula plate. Input layers of this pathway are M1 and M5. Pathway 2 connects M9 to shallow layers of the lobula, which in turn are tightly linked to the lobula plate. This pathway gets major input via M2. Pathways 1 and 2 receive input from retinula cells R1-6, either via the lamina monopolar cell L1 (terminating in M1 and M5) or via L2 and T1 (terminating in M2). Neurons of these pathways typically have small dendritic fields. We discuss evidence that pathways 1 and 2 may play a major role in motion detection. Pathway 3 connects M8 to deep layers of the lobula. In M8 information converges that is derived either from M3 (pathway 3a) or from M4 and M6 (pathway 3b), layers that get their major input from L3 and R8 or L4 and R7, respectively. Some neurons of pathway 3 have large dendritic fields. We suggest that they may be involved in the computation of form and colour. Possible analogies to the organization of pathways in the visual system of vertebrates are discussed.During the final editing of this work our friend A.P.M. Dittrich was tragically killed in an accident. Without him this and the previous work would never have been completed     

Columnar cells necessary for motion responses of wide-field visual interneurons in <Emphasis Type="Italic">Drosophila</Emphasis>

Wide-field motion-sensitive neurons in the lobula plate (lobula plate tangential cells, LPTCs) of the fly have been studied for decades. However, it has never been conclusively shown which cells constitute their major presynaptic elements. LPTCs are supposed to be rendered directionally selective by integrating excitatory as well as inhibitory input from many local motion detectors. Based on their stratification in the different layers of the lobula plate, the columnar cells T4 and T5 are likely candidates to provide some of this input. To study their role in motion detection, we performed whole-cell recordings from LPTCs in Drosophila with T4 and T5 cells blocked using two different genetically encoded tools. In these flies, motion responses were abolished, while flicker responses largely remained. We thus demonstrate that T4 and T5 cells indeed represent those columnar cells that provide directionally selective motion information to LPTCs. Contrary to previous assumptions, flicker responses seem to be largely mediated by a third, independent pathway. This work thus represents a further step towards elucidating the complete motion detection circuitry of the fly.     

Golgi analysis of tangential neurons in the lobula plate of<Emphasis Type="Italic">Drosophila melanogaster</Emphasis>

The lobula plate (LP), which is the third order optic neuropil of flies, houses wide-field neurons which are exquisitely sensitive to motion. Among Diptera, motion-sensitive neurons of larger flies have been studied at the anatomical and physiological levels. However, the neurons ofDrosophila lobula plate are relatively less explored. AsDrosophila permits a genetic analysis of neural functions, we have analysed the organization of lobula plate ofDrosophila melanogaster. Neurons belonging to eight anatomical classes have been observed in the present study. Three neurons of the horizontal system (HS) have been visualized. The HS north (HSN) neuron, occupying the dorsal lobula plate is stunted in its geometry compared to that of larger flies. Associated with the HS neurons, thinner horizontal elements known as h-cells have also been visualized in the present study. Five of the six known neurons of the vertical system (VS) have been visualized. Three additional neurons in the proximal LP comparable in anatomy to VS system have been stained. We have termed them as additional VS AVS)-like neurons. Three thinner tangential cells that are comparable to VS neurons, which are elements of twin vertical system (tvs); and two cells with wide dendritic fields comparable to CH neurons of Diptera have been also observed. Neurons comparable to VS cells but with ‘tufted’ dendrites have been stained. The HSN and VS1-VS2 neurons are dorsally stunted. This is possibly due to the shape of the compound eye ofDrosophila which is reduced in the fronto-dorsal region as compared to larger flies     

Retinotopic organization of small-field-target-detecting neurons in the insect visual system

BACKGROUND: Despite having tiny brains and relatively low-resolution compound eyes, many fly species frequently engage in precisely controlled aerobatic pursuits of conspecifics. Recent investigations into high-order processing in the fly visual system have revealed a class of neurons, coined small-target-motion detectors (STMDs), capable of responding robustly to target motion against the motion of background clutter. Despite limited spatial acuity in the insect eye, these neurons display exquisite sensitivity to small targets. RESULTS: We recorded intracellularly from morphologically identified columnar neurons in the lobula complex of the hoverfly Eristalis tenax. We show that these columnar neurons with exquisitely small receptive fields, like their large-field counterparts recently described from both male and female flies, have an extreme selectivity for the motion of small targets. In doing so, we provide the first physiological characterization of small-field neurons in female flies. These retinotopically organized columnar neurons include both direction-selective and nondirection-selective classes covering a large area of visual space. CONCLUSIONS: The retinotopic arrangement of lobula columnar neurons sensitive to the motion of small targets makes a strong case for these neurons as important precursors in the local processing of target motion. Furthermore, the continued response of STMDs with such small receptive fields to the motion of small targets in the presence of moving background clutter places further constraints on the potential mechanisms underlying their small-target tuning.     

The vertical-horizontal neurone (VH) in the lobula plate of the blowfly,Phaenicia

Summary The anatomy and physiology of a motion-sensitive neurone, the vertical-horizontal (VH-) cell in the third visual neuropil (lobula plate) of the blowfly,Phaenicia was studied by intracellular recordings combined with dye injection. The cell possesses two dendritic fields in different layers of the lobula plate. The axon runs jointly with those of the vertical cells along the caudal surface of the lobula plate and terminates in the central protocerebrum lateral to the esophageal canal. The receptive field of the VH-cell is subdivided into two physiologically different parts which correspond to the two dendritic fields: if the input reaches the dendritic field residing in a more caudal layer (V-layer), the cell responds maximally to vertical pattern motion; whereas if the input reaches the dendritic field residing in a more rostral layer (H-layer), the cell responds maximally to horizontal pattern motion. The VH-neurone responds maximally to a contrast frequency of approximately / 1.8 Hz which coincides with the contrast frequency dependence of optomotor (following) responses. It is, therefore, considered to be a likely candidate mediating the pitch response (Blondeau and Heisenberg 1982) in flies.     

Candidate glutamatergic neurons in the visual system of Drosophila

The visual system of Drosophila contains approximately 60,000 neurons that are organized in parallel, retinotopically arranged columns. A large number of these neurons have been characterized in great anatomical detail. However, studies providing direct evidence for synaptic signaling and the neurotransmitter used by individual neurons are relatively sparse. Here we present a first layout of neurons in the Drosophila visual system that likely release glutamate as their major neurotransmitter. We identified 33 different types of neurons of the lamina, medulla, lobula and lobula plate. Based on the previous Golgi-staining analysis, the identified neurons are further classified into 16 major subgroups representing lamina monopolar (L), transmedullary (Tm), transmedullary Y (TmY), Y, medulla intrinsic (Mi, Mt, Pm, Dm, Mi Am), bushy T (T), translobula plate (Tlp), lobula intrinsic (Lcn, Lt, Li), lobula plate tangential (LPTCs) and lobula plate intrinsic (LPi) cell types. In addition, we found 11 cell types that were not described by the previous Golgi analysis. This classification of candidate glutamatergic neurons fosters the future neurogenetic dissection of information processing in circuits of the fly visual system.     

A synergetic theory of environmentally-specified and learned patterns of movement coordination

By combining neuropharmacology and electrophysiology, we tried to determine whether the main neuronal mechanism responsible for direction-selective motion detection in the fly is based on an excitatory or an inhibitory synaptic interaction. By blocking inhibitory interactions with picrotoxinin, an antagonist of the inhibitory neurotransmitter GABA, we could abolish most of the directional selectivity of a large-field movement-sensitive neuron (H1-cell) in the lobula plate of the blowfly Calliphora erythrocephala. These modifications are similar to changes observed in the optomotor response of the fruitfly Drosophila melanogaster after application of picrotoxinin (Bülthoff and Bülthoff 1987a, b). Assuming a simplified logical model, these results are compatible with inhibitory synaptic interactions at the level of the elementary movement detectors. The picrotoxinin-induced changes in direction selectivity are not due to modifications of the peripheral visual processing in the retina and lamina. This was shown by simultaneous recordings of the electroretinogram and the H1-cell. The latencies between drug injections into various parts of the brain and their first effects on the H1-cell suggest that the inhibitory mechanism for motion detection is located in the medulla rather than in the lobula plate.     

The horizontal cells in the lobula plate of the blowfly,Phaenicia sericata

SummarySummary Combining intracellular recording and dye injection techniques, the horizontal cells of the blowfly,Phaenicia (= Lucilia) sericata, were studied.Anatomy In each lobula plate, one finds a set of three cells, termed NH-, EH- and SH-cell. EH occurs in two distinct anatomical forms, EH1 and EH2, differing in their respective branching patterns of the axon at the frontal surface of the lobula plate. Each cell's dendrite covers approximately a third of the surface of the lobula plate corresponding to a third of the visual field of the ipsilateral eye. These dendrites possess postsynaptic spines. The axons of all three cells pass along the frontal surface of the lobula plate within the inner chiasma; they cross the optic peduncle and enter the central protocerebrum where they form a second arborization, the axonal arborization consisting of dorsally extending collaterals. The axons terminate in the posterior slope of the ventrolateral protocerebrum. The axonal arborization as well as the axonal terminals possess telodendritic knobs. Ultrastructural investigations show that the lobula plate-dendrite possesses exclusively postsynaptic chemical synapses, and that the axonal arborisation and the axonal terminals possess pre- as well as postsynaptic chemical synapses. The very endings of the axons are exclusively presynaptic.Physiology The horizontal cells respond to stimulation within the ipsi- and/or contralateral receptive field. Regressive motion within the contralateral receptive field induces EPSPs and action potentials of small amplitude (10–35 mV); progressive motion is ineffective. Within the ipsilateral receptive field, regressive motion hyperpolarizes the cell membrane whereas progressive motion induces a strong depolarizing membrane potential-shift with superimposed fast potential changes of noisy appearance. Thus, the horizontal cells respond to rotational movement of the surround around the high axis of the animal: clockwise rotation excites the horizontal cells of the right lobula plate and counterclockwise motion those of the left lobula plate, respectively. However, this compound potential behaviour can only be recorded in the lobula plate-axon and main dendrites, whereas the horizontal cells respond tocontralateral regressive motion with action potentialsonly in their axonal terminals in the posterior slope; no graded potentials can be recorded in this cell region if stimulation occurs within theipsilateral receptive field. It is discussed that the previously described graded potentials for the axonal terminals (Hausen 1976b) can only be measured if the cells are already damaged. The probable cause of this change in response behaviour from action potentials to a compound potential behaviour (consisting of graded potentials and action potentials though of small amplitude) is discussed.This research was supported by the Deutsche Forschungsgemeinschaft through grants Ec56/1a + b and a Heisenberg stipend EC 56/3, funds from the SFB 114, and a grant from the National Science Foundation (NSF BMS 74-21712) awarded to the author and L.G. Bishop. I am indebted to Dr. A. Whittle and particularly to Prof. K. Meller for their invaluable help in ultrathin sectioning and to Mrs. B. Decker who introduced me to the technique of cutting serial semithin sections. Prof. K. Hamdorf helped with many stimulating discussions. I am most grateful to Dr. W. Broughton for kindly correcting the English style.