The Role of Central Complex Neurons in Prey Detection and Tracking in the Freely Moving Praying Mantis (Tenodera sinensis).

Complex tasks like hunting moving prey in an unpredictable environment require high levels of motor and sensory integration. An animal needs to detect and track suitable prey objects, measure their distance and orientation relative to its own position, and finally produce the correct motor output to approach and capture the prey. In the insect brain, the central complex (CX) is one target area where integration is likely to take place. In this study, we performed extracellular multi-unit recordings on the CX of freely hunting praying mantises (Tenodera sinensis). Initially, we recorded the neural activity of freely moving mantises as they hunted live prey. The recordings showed activity in cells that either reflected the mantis's own movements or the actions of a prey individual, which the mantises focused on. In the latter case, the activity increased as the prey moved and decreased when it stopped. Interestingly, cells ignored the movement of the other prey than the one to which the mantis attended. To obtain quantitative data, we generated simulated prey targets presented on an LCD screen positioned below the clear floor of the arena. The simulated target oscillated back and forth at various angles and distances. We identified populations of cells whose activity patterns were strongly linked to the appearance, movement, and relative position of the virtual prey. We refer to these as sensory responses. We also found cells whose activity preceded orientation movement toward the prey. We call these motor responses. Some cells showed both sensory and motor properties. Stimulation through tetrodes in some of the preparations could also generate similar movements. These results suggest the crucial importance of the CX to prey-capture behavior in predatory insects like the praying mantis and, hence, further emphasize its role in behaviorally and ecologically relevant contexts.

Extracellular wire tetrode recording in brain of freely walking insects.

Increasing interest in the role of brain activity in insect motor control requires that we be able to monitor neural activity while insects perform natural behavior. We previously developed a technique for implanting tetrode wires into the central complex of cockroach brains that allowed us to record activity from multiple neurons simultaneously while a tethered cockroach turned or altered walking speed. While a major advance, tethered preparations provide access to limited behaviors and often lack feedback processes that occur in freely moving animals. We now present a modified version of that technique that allows us to record from the central complex of freely moving cockroaches as they walk in an arena and deal with barriers by turning, climbing or tunneling. Coupled with high speed video and cluster cutting, we can now relate brain activity to various parameters of the movement of freely behaving insects.

Deciding which way to go: how do insects alter movements to negotiate barriers?

Animals must routinely deal with barriers as they move through their natural environment. These challenges require directed changes in leg movements and posture performed in the context of ever changing internal and external conditions. In particular, cockroaches use a combination of tactile and visual information to evaluate objects in their path in order to effectively guide their movements in complex terrain. When encountering a large block, the insect uses its antennae to evaluate the object's height then rears upward accordingly before climbing. A shelf presents a choice between climbing and tunneling that depends on how the antennae strike the shelf; tapping from above yields climbing, while tapping from below causes tunneling. However, ambient light conditions detected by the ocelli can bias that decision. Similarly, in a T-maze turning is determined by antennal contact but influenced by visual cues. These multi-sensory behaviors led us to look at the central complex as a center for sensori-motor integration within the insect brain. Visual and antennal tactile cues are processed within the central complex and, in tethered preparations, several central complex units changed firing rates in tandem with or prior to altered step frequency or turning, while stimulation through the implanted electrodes evoked these same behavioral changes. To further test for a central complex role in these decisions, we examined behavioral effects of brain lesions. Electrolytic lesions in restricted regions of the central complex generated site specific behavioral deficits. Similar changes were also found in reversible effects of procaine injections in the brain. Finally, we are examining these kinds of decisions made in a large arena that more closely matches the conditions under which cockroaches forage. Overall, our studies suggest that CC circuits may indeed influence the descending commands associated with navigational decisions, thereby making them more context dependent.

An implantable biofuel cell for a live insect.

A biofuel cell incorporating a bienzymatic trehalase|glucose oxidase trehalose anode and a bilirubin oxidase dioxygen cathode using Os complexes grafted to a polymeric backbone as electron relays was designed and constructed. The specific power densities of the biofuel cell implanted in a female Blaberus discoidalis through incisions into its abdomen yielded maximum values of ca. 55 µW/cm(2) at 0.2 V that decreased by only ca. 5% after ca. 2.5 h of operation.

Neural activity in the central complex of the insect brain is linked to locomotor changes.

Animals negotiating complex natural terrain must consider cues around them and alter movement parameters accordingly. In the arthropod brain, the central complex (CC) receives bilateral sensory relays and sits immediately upstream of premotor areas, suggesting that it may be involved in the context-dependent control of behavior. In previous studies, CC neurons in various insects responded to visual, chemical, and mechanical stimuli, and genetic or physical lesions affected locomotor behaviors. Additionally, electrical stimulation of the CC led to malformed chirping movements by crickets, and pharmacological stimulation evoked stridulation in grasshoppers, but no more precise relationship has been documented between neural activity in the CC and movements in a behaving animal. We performed tetrode recordings from the CC of cockroaches walking in place on a slippery surface. Neural activity in the CC was strongly correlated with, and in some cases predictive of, stepping frequency. Electrical stimulation of these areas also evoked or modified walking. Many of the same neural units responded to tactile antennal stimulation while the animal was standing still but became unresponsive during walking. Therefore, these CC units are unlikely to be reporting only sensory signals, but their activity may be directing changes in locomotion based on sensory inputs.

Multi-unit recording of antennal mechano-sensitive units in the central complex of the cockroach, Blaberus discoidalis.

The central complex (CC) is a group of midline neuropils in the protocerebrum of all insects (Williams, J Zool, 176:67-86, 1975; Strausfeld, Prog Brain Res, 123:273-284, 1999). Its columnar organization coupled with the anatomical tracts to and from this region suggests that the CC may supervise various forms of locomotion. In cockroach, lesions of the CC affect turning and controlled climbing over blocks (Ridgel et al., J Comp Physiol A, 193:385-402, 2007). Since these behaviors are largely directed by tactile cues detected by antennae, we predicted that some neurons in the CC respond to mechanical antennal stimulation. We used 16-channel probes to record from broad regions within the CC, while mechanically stimulating one or the other antenna. Using cluster cutting procedures, we examined 277 units in 31 preparations. Many of these units responded to mechanical stimulation of the antennae, and of these, most responded equally well to medial or lateral stimulation of either antenna. However, several units either responded to only one antenna or responded significantly more strongly to one of them. Most of the units responding to antennal stimulation were sensitive to changes in the velocity as well as changes in light. Our data reveal a large population of mult-sensory neurons in the CC that could contribute to locomotion control.

Descending control of body attitude in the cockroach Blaberus discoidalis and its role in incline climbing.

Unlike intact animals, cockroaches with bilateral circumoesophageal connective lesions have difficulty climbing up smooth inclines. Typically, they slipped badly or even fell over backward before traveling more than 1.5 body lengths up the incline. The major problem involves increased slippage of the front and sometimes middle legs. Periods of front leg slipping are correlated with excess body elevation that pushes the height of the head to 11 mm above the substrate. Intact animals control body attitude very well on the incline, only rarely rearing above 11 mm. Cockroaches with bilateral circumoesophageal connective lesions spend considerable time above this critical amplitude and slipping increases with amplitude above that value. We conclude that circumoesophageal lesion compromises the insect's ability to control body attitude on the incline and this deficit contributes to the inability of lesioned cockroaches to climb steep inclines. A separate body attitude deficit was noted on steps. Upon climbing to the top of a barrier, intact animals bend their body between the first and second thoracic segments. This action maintains good leg mechanics throughout the climb. Cockroaches with bilateral circumoesophageal connective lesions fail to perform this downward flexion. A beneficial role for similar body flexion in simple robots is described in the Discussion.

Control of obstacle climbing in the cockroach, Blaberus discoidalis. I. Kinematics.

An advantage of legged locomotion is the ability to climb over obstacles. We studied deathhead cockroaches as they climbed over plastic blocks in order to characterize the leg movements associated with climbing. Movements were recorded as animals surmounted 5.5-mm or 11-mm obstacles. The smaller obstacles were scaled with little change in running movements. The higher obstacles required altered gaits, leg positions and body posture. The most frequent sequence used was to first tilt the front of the body upward in a rearing stage, and then elevate the center of mass to the level of the top of the block. A horizontal running posture was re-assumed in a leveling-off stage. The action of the middle legs was redirected by rotations of the leg at the thoracal-coxal and the trochanteral-femoral joints. The subsequent extension movements of the coxal-trochanteral and femoral-tibial joints were within the range seen during horizontal running. The structure of proximal leg joints allows for flexibility in leg use by generating subtle, but effective changes in the direction of leg movement. This architecture, along with the resulting re-direction of movements, provides a range of strategies for both animals and walking machines.

Control of climbing behavior in the cockroach, Blaberus discoidalis. II. Motor activities associated with joint movement.

Deathhead cockroaches employ characteristic postural strategies for surmounting barriers. These include rotation of middle legs to re-direct leg extension and drive the animal upward. However, during climbing the excursions of the joints that play major roles in leg extension are not significantly altered from those seen during running movements. To determine if the motor activity associated with these actions is also unchanged, we examined the electromyogram activity produced by the slow trochanteral extensor and slow tibial extensor motor neurons as deathhead cockroaches climbed over obstacles of two different heights. As they climbed, activity in the slow trochanteral extensor produced a lower extension velocity of the coxal-trochanteral joint than the same frequency of slow trochanteral extensor activity produces during horizontal running. Moreover, the pattern of activity within specific leg cycles was altered. During running, the slow trochanteral extensor generates a high-frequency burst prior to foot set-down. This activity declines through the remainder of the stance phase. During climbing, motor neuron frequency no longer decreased after foot set-down, suggesting that reflex adjustments were made. This conclusion was further supported by the observation that front leg amputees generated even stronger slow trochanteral extensor activity in the middle leg during climbing movements.