Wind direction coding in the cockroach escape response: winner does not take all.

Cockroaches respond to the approach of a predator by turning away and then running. Three bilateral pairs of giant interneurons are involved in determining the direction of the sensory stimulus and setting the turn direction. Each of these six interneurons has a different directional response to wind stimuli. We have tested whether these six cells use a winner-take-all mechanism to perform this directional determination: that is, each of these cells suppressing the motor response that each of the other cells promotes. Such a mechanism is found in similar behaviors of some other animals. By adding spikes to identified giant interneurons through intracellular stimulation during the sensory-induced behavior and analyzing the resulting directional leg movements, we find that a winner-take-all is not used in this system. Rather, directional determination appears to be based on collaborative calculation of direction by the giant interneurons as a group.

Population vector coding by the giant interneurons of the cockroach.

We tested two alternative models of integration among the cockroach giant interneurons (GIs) for determining the directions of wind-evoked escape turns. One model, called steering wheel, pits contralateral GIs against one another; the other, called population vector model, involves a vector computation among the GIs. In testing each model theoretically, the population vector was found to account far better for the actual behavior. Both models could account for the results of previous behavioral-physiological experiments in which spikes had been added to the right GI3 together with wind stimuli from the right side. The two models revealed a critical behavioral-physiological experimental test that we then performed; namely, when delivering wind from the right side, adding spikes experimentally to the right GI2 should increase turn size according to the steering wheel model but should decrease turn size according to the population vector model. The latter result was obtained. The population vector, but not the steering wheel, model also could account for previous behavioral-physiological experiments in which spikes were added experimentally to a GI contralateral to the wind stimuli. The results support the population vector model as accounting for direction determination among the cockroach GIs.

Right-left discrimination in a biologically oriented model of the cockroach escape system.

We present a biologically oriented model that accounts for left-right discrimination in the cockroach's escape behavior. The model includes the main groups of neurons found to be involved in the escape response. Each one of the included neurons is described by the actual processes taking place in an individual neuron (formation of an action potential, transmitter release, conductance changes, etc.). Furthermore, realistic chemical synapses (excitatory or inhibitory and able to undergo different types of modulation) connect the various neurons. With this model, we ere able to achieve, for a wide range of inputs representing different wind directions, behavior which resembles that found experimentally. The model indicates that several synaptic properties, in particular postsynaptic inhibition and presynaptic facilitation, play a key role in the discrimination of wind direction.

High-frequency steering maneuvers mediated by tactile cues: antennal wall-following in the cockroach.

Cockroaches (Periplaneta americana) use their antennae to detect a wall and to maintain a constant distance from it as they walk or run along it. The faster they run, the closer they position themselves to the wall. They also use their antennae to detect and follow multiple accordion-like projections in the wall. They can make up to 25 body turns s-1 for short periods during rapid running to follow such wall projections. Each turn apparently involves a change in stepping direction. These turns help to avoid collisions with the outward projections, while keeping the body close to the wall. Sensory inputs from the flagellum of the antenna, and not from its base, appear to evoke the turns in response to wall projections. These flagellar inputs appear to report the position along the antenna of its contact with the wall and/or the position of the consequent antennal bend. This flagellar information constitutes a one-dimensional sensory map, with location along the map indicating the distance to the wall.

Cartesian representation of stimulus direction: parallel processing by two sets of giant interneurons in the cockroach.

The cockroach Periplaneta americana responds to wind puffs by turning away, both on the ground and when flying. While on the ground, the ventral giant interneurons (ventrals) encode the wind direction and specify turn direction, whereas while flying the dorsal giant interneurons (dorsals) appear to do so. We report here on responses of these cells to controlled wind stimuli of different directions. Using improved methods of wind stimulation and of positioning the animal revealed important principles of organization not previously observed. All six cells of largest axonal diameter on each side respond preferentially to ipsilateral winds. One of these cells, previously thought to respond non-directionally (giant interneuron 2), was found to have a restricted directional response (Fig. 3). The organization of directional coding among the ventral giant interneurons is nearly identical to that among the dorsals (Fig. 2). Each group contains, on each side, one cell that responds primarily to wind from the ipsilateral front, another primarily in the ipsilateral rear, and a third responding more broadly to ipsilateral front and rear. These results are discussed in terms of the mechanisms of directional localization by the assembly of giant interneurons.

Critical parameters of the spike trains in a cell assembly: coding of turn direction by the giant interneurons of the cockroach.

Cockroaches (Periplaneta americana) respond to air displacement produced by an approaching predator by turning and running away. A set of 4 bilateral pairs of ventral giant interneurons is important in determining turn direction. Wind from a given side is known to produce more spikes, an earlier onset of the spike trains, and different fine temporal patterning, in the ipsilateral vs the contralateral set of these interneurons. Here we investigate which of these spike train parameters the cockroach actually uses to determine the direction it will turn. We delivered controlled wind puffs from the right front, together with intracellular injection of spike trains in a left ventral giant interneuron, under conditions where the animal could make normally directed turning movements of the legs and body. In trials where our stimuli caused the left side to give both the first spike and more total spikes than the right, but where our injected spike train included none of the normal fine temporal patterning, 92% of the evoked turns were to the right-opposite of normal (Figs. 4-6). In trials where the left side gave the first spike, but the right side gave more spikes, 100% of the turns were to the left--the normal direction (Figs. 8, 9). Comparable results were obtained when each of the left giant interneurons 1, 2 or 3 were electrically stimulated, and when either weak or stronger wind puffs were used. Stimulating a left giant interneuron electrically in the absence of a wind puff evoked an escape-like turn on 9% of the trials, and these were all to the right (Fig. 9). These results indicate that fine temporal patterning in the spike trains is not necessary, and information about which side gives the first spike is not sufficient, to determine turn direction. Rather, the key parameter appears to be relative numbers of action potentials in the left vs the right group of cells. These conclusions were supported by similar experiments in which extracellular stimulation of several left giant interneurons was paired with right wind (Figs. 11, 12).

Neural mechanisms of behavior.

Work during the past year has revealed increasing diversity and complexity of sensory inputs that trigger behavior, and correspondingly wide-ranging adaptations of sensory receptors and receptor organs. Many of these have been discovered by taking careful account of the animal in its natural habitat. Studies on central processing have made major strides in the understanding of neural maps and assembly codes. Studies that are primarily experimental, as opposed to theoretical, continue to produce most of the significant advances in this field. Perhaps the single most dominant theme has been the continuing erasure of the line separating invertebrate and vertebrate neurobehavioral systems.

Photoablating neuronal groups using an intracellularly aggregating dye.

We report here on a new method to kill several selected neurons, using a modified fluorescence photoablation technique that does not require impaling any cells. Rather, we injected extracellularly the dye rhodamine 6-G, which is taken up by the cells. We made our injections into one side of a cockroach ganglion, after which the nearby cell bodies of individually known giant interneurons (GIs), as well as other cell bodies and many axons, became highly fluorescent. After this dye accumulation, we irradiated either the entire ganglionic region of the accumulated dye, or specifically the cell bodies of two identified GIs. After allowing over one week for axonal degeneration, both histological and behavioral tests confirmed that specifically the axons of the targeted illuminated cells had been killed.

Using fluorescence photoablation to study the regeneration of singly cut leech axons.

The regeneration of the axons of leech Retzius cells was compared following two different methods of axonal severing: (1) a crush of the whole connective that includes the Retzius axon; and (2) photoablation of a small segment of only the Retzius axon. The photoablation was carried out after filling the Retzius cell with Lucifer Yellow (LY). Several tests were carried out to determine whether the photoablation actually severed the axon. These included (1) using the lipophilic membrane probe DiI as an indicator of membrane severance (2) electron microscopic examination of the photoablated axon after filling it with horseradish peroxidase (HRP); and (3) filling the Retzius cell first with HRP, then photoablating, and looking for the disappearance of the HRP in the photoablated region. These and other observations indicated that the photoablated axon was actually severed. Two differences were seen in the regeneration of the Retzius axon after crush versus after photoablation. First, the sprouting following crush was far more disorganized, and included significantly more lateral spread. Second, after photoablation, over 70% of the axons, upon refilling with LY after 3 days or more, showed the newly introduced LY suddenly extending far down the distal segment, indicating that the proximal and distal segments had become reconnected. This was never seen following a crush. The photoablated axons did not pass HRP into the distal segment, suggesting that the reconnection was not by fusion, but perhaps by a gap junction. The results show that axonal regeneration can take a dramatically different form than it does following a standard crush procedure if, instead, the axon is severed in a way that preserves the structural integrity of the surrounding tissue.

Different effects of the biogenic amines dopamine, serotonin and octopamine on the thoracic and abdominal portions of the escape circuit in the cockroach.

1. The escape behavior of the cockroach, Periplaneta americana, is known to be modulated under various behavioral conditions (Camhi and Volman 1978; Camhi and Nolen 1981; Camhi 1988). Some of these modulatory effects occur in the last abdominal ganglion (Daley and Delcomyn 1981a, b; Libersat et al. 1989) and others in the thoracic ganglia (Camhi 1988). Neuromodulator substances are known to underlie behavioral modulation in various animals. Therefore, we have sought to determine whether topical application of putative neuromodulators of the escape circuit enhance or depress this circuit, and whether these effects differ in the last abdominal vs. the thoracic ganglia. 2. Topical application of the biogenic amines serotonin and dopamine to the metathoracic ganglion modulates the escape circuitry within this ganglion; serotonin decreases and dopamine enhances the response of leg motoneurons to activation of interneurons in the abdominal nerve cord by electrical or wind stimulation. 3. The neuropil of the thoracic ganglia contains many catecholamine-histofluorescent processes bearing varicosities, providing a possible anatomical substrate for dopamine release sites. 4. Topical application of octopamine to the terminal abdominal ganglion enhances the response of abdominal interneurons to wind stimulation of the cerci. In contrast, serotonin and dopamine have no effect at this site. 5. It is proposed that release of these biogenic amines may contribute to the known modulation of the cockroach escape response.

Multiple feedback loops in the flying cockroach: excitation of the dorsal and inhibition of the ventral giant interneurons.

1. In a tethered cockroach (Periplaneta americana) whose wings have been cut back to stumps, it is possible to elicit brief sequences of flight-like activity by puffing wind on the animal's body. 2. During such brief sequences, rhythmic bursts of action potentials coming from the thorax at the wingbeat frequency, descend the abdominal nerve cord to the last abdominal ganglion (A6). This descending rhythm is often accompanied by an ascending rhythm (Fig. 2). 3. Intracellular recording during flight-like activity from identified ascending giant interneurons, and from some unidentified descending axons in the abdominal nerve cord, shows that: (a) ventral giant interneurons (vGIs) remain silent (Fig. 3); (b) dorsal giant interneurons (dGIs) are activated at the onset of the flight-like activity and remain active sporadically throughout the flight sequence (Fig.4); (c) some descending axons in the abdominal nerve cord show rhythmic activity phase-locked to the flight rhythm (Fig. 5). 4. Also during such brief sequences, the cercal nerves, running from the cerci (paired, posterior, wind sensitive appendages) to the last abdominal ganglion, show rhythmic activity at the wingbeat frequency (Fig. 6). This includes activity of some motor axons controlling vibratory cercal movements and of some sensory axons. 5. More prolonged flight sequences were elicited in cockroaches whose wings were not cut and which flew in front of a wind tunnel (Fig. 1B). 6. In these more prolonged flight sequences, the number of ascending spikes per burst was greater than that seen in the wingless preparation (Fig. 8; compare to Fig. 2). Recordings from both ventral and dorsal GIs show that: in spite of the ongoing wind from both the tunnel and the beating wings, which is far above threshold for the vGIs in a resting cockroach, the vGIs are entirely silent during flight. Moreover, the vGIs response to strong wind puffs that normally evoke maximal GI responses is reduced by a mean of 86% during flight (Fig. 9). The dGIs are active in a strong rhythm (Figs. 11 and 12). 7. Three sources appear to contribute to the ascending dGI rhythm (1) the axons carrying the rhythmic descending bursts; (2) the rhythmic sensory activity resulting from the cercal vibration; and (3) the sensory activity resulting from rhythmic wind gusts produced by the wingbeat and detected by the cerci. The contribution of each source has been tested alone while removing the other two (Figs. 13 and 14). Such experiments suggest that all 3 feedback loops are involved in rhythmically exciting the dGIs (Fig. 15).

The code for stimulus direction in a cell assembly in the cockroach.

The cockroach Periplaneta americana responds to the approach of a predator by turning away. A gentle wind gust, caused by the predator's approach, excites cercal wind receptors, which encode both the presence and the direction of the stimulus. These cells in turn excite a group of giant interneurons (GI's) whose axons convey the directional information to thoracic motor centers. A given wind direction is coded not by a single GI functioning as a labeled line, but rather by some relationship among the spike trains in an assembly of GI's. This paper analyzes the code in this assembly. It is shown that all three pairs of GI's with the largest axonal diameters respond differentially to wind from left front vs. right front (Figs, 3, 4; Table 2). Each GI encodes these angles by both the time of its first action potential, and the number of action potentials, relative to its contralateral homolog. It is shown that the behavioral discrimination cannot rely solely upon the left-right differences in the time of the first action potential. A model of the assembly code is developed that involves a comparison of the numbers of action potentials in the left vs. the right group of giant interneurons. The model is shown to account for a large number of pre-existing experimental data on direction discrimination. The model requires, however, the involvement of additional cells in the left and right groups, besides the specific GI's whose role had been tested in prior experiments. The model is then tested by further experiments designed to verify the involvement of these added cells. These experiments support the model.

Connectivity pattern of the cercal-to-giant interneuron system of the American cockroach.

1. The pattern of connectivity between identified cercal afferents and the three largest giant interneurons (GIs 1, 2, and 3) of the American cockroach was investigated by intracellular methods. These three GIs all have different directional response sensitivities and appear to be especially important in initiating the short latency escape behavior of the American cockroach. 2. One of the interneurons, GI 1 responds to wind from all four quadrants of space about the animal. However, it clearly shows a greater ipsilateral versus contralateral (relative to the GI's axon within the nerve cord) wind sensitivity. In contrast, the directional sensitivity of GI 2 is more nearly bilaterally symmetrical. Both of these interneurons receive excitatory synaptic input from the sensory cells of the nine most prominent columns (a, d, g, f, h, i, k, l, and m) of filiform hairs of the ipsilateral cercus. 3. The nine ipsilateral inputs all made roughly equivalent strength excitatory connections with GI 1. The connectivity pattern to GI 2 was the same as that to GI 1 except that the connection strength for two of the nine columns, h and i, was substantially stronger to GI 2 than to GI 1. The remaining seven sensory columns all make equivalent strength connection with GI 2. 4. Only select columns of contralateral sensory cells made synaptic connection with GIs 1 and 2. All detectable connections produced subthreshold depolarizations. 5. The response curve of GI 3 is more sharply restricted in space than that of either GI 1 or 2 and this interneuron only responds to wind stimuli originating from in front of the animal. GI 3 received excitatory synaptic input only from ipsilateral columns d, f, g, i, and k, all of which have their best excitatory directions well within the boundaries of the response curve of GI 3. Columns a and l with best excitatory directions near the edges of the response curve of GI 3 made no detectable connection. The remaining two columns (h and m) with best excitatory directions well outside the boundaries of the response curve of GI 3 provided inhibitory input. 6. GI 3 received synaptic input from contralateral columns d, f, g, h, i, k, and m.(ABSTRACT TRUNCATED AT 400 WORDS)

Modulation of activity in sensory neurons and wind-sensitive interneurons by cercal displacement in the cockroach.

1. The cockroach Periplaneta americana can modify the sensory activity received by its central nervous system from the cerci, paired abdominal wind-responsive appendages. Medial displacement of the cerci produces a reduction in the number of sensory action potentials (AP's) elicited by a wind stimulus (Fig. 2) (Libersat et al. 1987; Golstein and Camhi 1988). This movement occurs naturally, for example during flying. 2. This sensory reduction is present when measured as the integral of extracellularly recorded activity as well as when counting the number of AP's larger than a threshold voltage just larger than the background noise (Fig. 2C). 3. Histological results confirm prior physiological experiments suggesting that the reduction may be produced by mechanical forces on the sensory nerve, rather than synaptically (Fig. 4). 4. The wind-response of interneurons is significantly diminished by the sensory reduction when measured either extra- or intracellularly (Figs. 5, 6). Cells affected include identified ventral and dorsal giant interneurons (GI's), which carry directional information about wind from the abdominal cerci to the more anterior portions of the nervous system, and are involved in flying (Camhi 1980; Ritzmann 1984; Comer 1985). 5. The reduction in the interneuronal response was unaffected by the elimination of input from descending central pathways, and input from a cercal chordotonal organ that senses cercal position and inhibits some of the GI's (Fig. 5). Thus, the reduction in wind-evoked sensory activity can itself account for the modulation of interneuron activity.(ABSTRACT TRUNCATED AT 250 WORDS)

Organization of a complex movement: fixed and variable components of the cockroach escape behavior.

The escape behavior of the cockroach Periplaneta americana was studied by means of high speed filming (250 frames/s) and a computer-graphical analysis of the body and leg movements. The results are as follows: 1. The behavior begins with pure rotation of the body about the posteriorly located cerci, followed by rotation plus forward translation, and finally pure translation (Figs. 1, 2). 2. A consistent inter-leg coordination is used for the entire duration of the turn (Fig. 3A). At the start of the movement, five or all six legs execute their first stance phase (i.e. leg on the ground during locomotion) simultaneously. By the end of the turn the pattern has changed to the alternate 'tripod' coordination characteristic of insect walking. The change-over from all legs working together, to working alternately, occurs by means of a consistent pattern of delays in the stepping of certain legs. 3. The movements made by each leg during its initial stance phase are carried out using consistent movement components in the anterior-posterior (A-P) and the medial-lateral (M-L) axes (Fig. 4A). The movement at a particular joint in each middle leg is found to be diagnostic for the direction of turn. 4. The size and direction of a given leg's M-L movement in its initial stance phase depends on the same leg's prior A-P position (Fig. 5). No such feedback effects were seen among different legs. 5. Animals that are fixed to a slick surface on which they make slipping leg movements show the same inter-leg coordination (Fig. 3B), direction of initial stance movement (Fig. 4B) and dependence of the leg's initial M-L movement on its prior A-P position (Fig. 6), as did free-ranging animals. 6. Cockroaches that are walking at the moment they begin their escape reverse those ongoing leg movements that are contrary to escape movements. 7. These results are discussed in terms of the overall coordination of the complex movements, and in terms of the known properties of the neural circuitry for escape. Possibilities for neurobiological follow-up of certain of the findings presented here are also addressed.

Escape behavior in the cockroach: distributed neural processing.

Escape reactions are often considered to be among the simplest behaviors. The nerve circuits guiding these reactions are also generally thought to be simple. For instance, in several species a single interneuron is sufficient to trigger normal escape. The evasive response of the cockroach, however, appears to be more complex both behaviorally and physiologically. In this review, several complications of the behavior are pointed out, based on a recent computer-graphic analysis of the leg movements. Next described is the cooperative role of several interneurons--not just one--in evoking an escape turn away from the stimulus. A model of this multicellular code for stimulus direction is then presented that correctly predicts the turning behavior under many different experimental conditions. Finally, an overall scheme of the information processing for escape behavior is presented.

The role of afferent activity in behavioral and neuronal plasticity in an insect.

Cockroaches (Periplaneta americana) have been shown to adapt behaviorally, in about 1 month, to ablation of one cercus. Additionally, those giant interneurons (GIs) that normally receive their major input from the lesioned cercus become more responsive to stimulation of the intact side (Vardi and Camhi 1982a, b). To investigate the role of afferent activity in the behavioral and neuronal plasticity, we silenced wind-evoked activity in the intact cercus by immobilizing the sensory hairs. This was carried out during the last nymphal stage which lasts for about one month. The animals were tested behaviorally and physiologically after they had molted to adults and a fresh set of mobile hairs had appeared. These animals showed no behavioral correction (Fig. 3). The responses of the GIs on the ablated side were somewhat enhanced, but they were also significantly smaller than those in animals with long-term cercal ablations and no sensory deprivation (Fig. 5). A variety of controls (Figs. 8, 9, and 10) were used to show that sensory deprivation by itself did not decrease the responsiveness of the afferents or the GIs. Thus elimination of wind-evoked activity specifically decreases enhancement of the responses in the GIs.

Reduction of sensory activity produced by cercal displacement modifies response of wind-sensitive interneurons in the cockroach.

The cockroach can reduce the amount of sensory activity received by its CNS from the cerci, paired wind-responsive appendages. This reduction is produced by medial displacement of the cerci, a movement the animal performs naturally during flying. We demonstrate here that this sensory reduction significantly reduces activity of the postsynaptic wind-sensitive interneurons in the abdominal nerve cord, cells which carry the wind information to higher centers of the nervous system. In addition, we have found that the wind-evoked activity of two identified giant interneurons that are involved in escape and locomotion behaviors, is significantly reduced by cercal displacement.

Nonsynaptic regulation of sensory activity during movement in cockroaches.

Here we describe a nonsynaptic mechanism for filtering out potentially perturbing sensory feedback during locomotion. During flight, the cockroach moves its cerci, two abdominal sensory appendages, about their joint with the body and holds them in place. The cerci bear highly sensitive wind-receptive hairs, which would be strongly stimulated by flight wind. Such wind could cause habituation of the synaptic connections from these cercal receptors onto interneurons responsible for the running escape response to an approaching predator. We have found that the cercal displacement blocks one-third to one-half of the action potentials along the sensory nerve, possibly aiding in protection against such habituation. This block occurs if one experimentally displaces a cercus, and the block persists in the complete absence of any connections with the central nervous system. The block appears to be nonsynaptic and to result instead from mechanical pressure on the nerve near the joint. The results suggest that activity in peripheral nerves in other animals may also be affected by the position or movement of joints through which the nerves pass.