Grasping in the pigeon: mechanisms of motor control

Object manipulation without hands

Our current understanding of manipulation is based on primate hands, resulting in a detailed but narrow perspective of ways to handle objects. Although most other animals lack hands, they are still capable of flexible manipulation of diverse objects, including food and nest materials, and depend on dexterity in object handling to survive and reproduce. Birds, for instance, use their bills and feet to forage and build nests, while insects carry food and construct nests with their mandibles and legs. Bird bills and insect mandibles are much simpler than a primate hand, resembling simple robotic grippers. A better understanding of manipulation in these and other species would provide a broader comparative perspective on the origins of dexterity. Here we contrast data from primates, birds and insects, describing how they sense and grasp objects, and the neural architectures that control manipulation. Finally, we outline techniques for collecting comparable manipulation data from animals with diverse morphologies and describe the practical applications of studying manipulation in a wide range of species, including providing inspiration for novel designs of robotic manipulators.

Chickens selected for feather pecking can inhibit prepotent motor responses in a Go/No-Go task

Repetitive feather pecking (FP) where birds peck and pull out feathers of conspecifics could reflect motor impulsivity through a lack of behavioural inhibition. We assessed motor impulsivity in female chickens (n = 20) during a Go/No-Go task where birds had to peck (Go) or inhibit pecks (No-Go) appropriately to obtain a food reward, depending on visual cues in an operant chamber. Birds were selected to show divergent FP performance based on their genotype (high predisposition for FP or unselected control line) and phenotype (peckers or non-peckers). Genotype, phenotype, and its interaction did not affect the number of pre-cue responses, percentage of responses during No-Go cues (false alarms), or efficiency (number of rewards over number of responses). We present the first documentation of a Go/No-Go task to measure the ability of birds genetically and phenotypically selected for FP activity to inhibit a prepotent motor response. Results indicate that the repetitive motor action of FP does not reflect impulsivity and is not genetically linked to a lack of behavioural inhibition as measured in a Go/No-Go task.

Control of bill-grasping aperture with varying food size in crows

Grasping movement in primates is known to be a visually guided behavior and the aperture of hand opening is adjusted to the target size on the basis of visual information. The analogous behavior can be found in birds, called 'pecking', consisting of head-reaching and bill-grasping. Bill-grasping has been investigated mainly in pigeons and an aperture adjustment as seen in primates has been reported. This study focused on kinematics of pecking in crows, known to possess dexterous visuomotor skills, to examine whether crows adjust the grasping aperture to food diameter with a kinematic mechanism similar to that in pigeons. The pecking at a small piece of food was video recorded to analyze the grasping aperture. The results showed that the grasping aperture was proportional to food diameter. Kinematic analysis showed that the aperture adjustment was mediated by grasping velocity and grasping duration, which is consistent with the findings of previous research on pecking in pigeons. However, the relative contribution of grasping velocity was much higher than that of grasping duration. Our findings suggest the different sensorimotor mechanisms to control bill-grasping between the avian species with different foraging ecology.

Flexible motor adjustment of pecking with an artificially extended bill in crows but not in pigeons

The dextrous foraging skills of primates, including humans, are underpinned by flexible vision-guided control of the arms/hands and even tools as body-part extensions. This capacity involves a visuomotor conversion process that transfers the locations of the hands/arms and a target in retinal coordinates into body coordinates to generate a reaching/grasping movement and to correct online. Similar capacities have evolved in birds, such as tool use in corvids and finches, which represents the flexible motor control of extended body parts. However, the flexibility of avian head-reaching and bill-grasping with body-part extensions remains poorly understood. This study comparatively investigated the flexibility of pecking with an artificially extended bill in crows and pigeons. Pecking performance and kinematics were examined when the bill extension was attached, and after its removal. The bill extension deteriorated pecking in pigeons in both performance and kinematics over 10 days. After the bill removal, pigeons started bill-grasping earlier, indicating motor adaptation to the bill extension. Contrastingly, pecking in crows was deteriorated transiently with the bill extension, but was recovered by adjusting pecking at closer distances, suggesting a quick adjustment to the bill extension. These results indicate flexible visuomotor control to extended body parts in crows but not in pigeons.

Pigeons use distinct stop phases to control pecking

Pecking at small targets requires accurate spatial coordination of the head. Goodale (1983a) suggested that planning of the peck happens during two distinct stop phases, but although this idea has now been around for a long time, the specific functional roles of these stop phases remain unsolved. Here, we investigated the characteristics of the two stop phases using high-speed motion capture and examined their functions with two experiments. In Experiment 1, we tested the hypothesis that the second stop phase is used to pre-program the final approach to a target and analyzed head movements while pigeons (Columba livia) pecked at targets of different size. Our results show that the duration of both stop phases significantly increased as stimulus size decreased. We also found significant positive correlations between stimulus size and the distances of the beaks to the stimulus during both stop phases. In Experiment 2, we used a two-alternative forced choice task with different levels of difficulty to test the hypothesis that the first stop phase is used to decide between targets. The results indicate that the characteristics of the stop phases do not change with an increasing difficulty between the two choices. Therefore, we conclude that the first stop phase is not exclusively used to decide upon a target to peck at, but also contributes to the function of the second stop phase, which is improving pecking accuracy and planning the final approach to the target.

Conditioned 'prehension' in the pigeon: kinematics, coordination and stimulus control of the pecking response

Like human prehensile behavior, the pigeon's ingestive pecking response is elicited by visual stimuli conveying information about the location and size of the target. This information is used to generate localized ingestive pecks whose gapes are amplitude-scaled to seed size, prior to contact. We employed high-resolution, 'real-time' monitoring of head acceleration, jaw movements and terminal peck location to examine the kinematics, coordination and stimulus control of conditioned pecking. Conditioning procedures were used to bring pecking under the control of visual targets whose stimulus properties (size, location) were independently varied, while simultaneously monitoring pecking response parameters. Stimulus control of the transport component (peck localization) is extremely precise, even in the absence of a specific localization-dependent reinforcement contingency. Subjects also showed amplitude-scaling of gape size to the size of a visual target, but over a more restricted range than shown to food pellets of comparable sizes. Comparison of the kinematic profiles of conditioned and ingestive pecks suggests that conditioned pecking is functionally analogous to human 'pointing' rather than 'grasping' behavior.

Systematic changes in gaping during the ontogeny of pecking in ring doves (Streptopelia risoria)

Food pecking in the ring dove is a skilled prehensile response that is similar to, but simpler than, many other prehensile responses. Previous work has shown that this response is initially poorly executed and requires experience for its accurate direction and coordination. The response involves two components: the thrusting of the bird's head toward food, and the opening and closure of the beak around food. Here, this second component, called gape, is followed through development with a precise measurement system. Four squabs moved through a similar sequence of three gape topographies, each of which is more efficient in picking up seed, during development. The present outcome, together with other work, argues for a substantial contribution of experience with pecking to the development of food pecking. We discuss the implications of these findings for understanding the ontogeny of motor control and for understanding how experience affects behavioral development.

Integration of pecking, filter feeding and drinking mechanisms in waterfowl

This paper is one of several contributions in a series, illustrating the application of a specific deductive methodology to explain diversity of form. The methodology facilitates the explanation of feeding morphologies in various ducks as a transformation of the mallard's feeding design maximized for specific proportions of performance that are contributed by pecking and filter feeding mechanisms. The earlier described anatomy and formal analyses of the three mechanisms in the mallard served as the initial conditions used in simulation models. Four elements of the feeding system were chosen that play a major role in all three mechanisms. For each element, the main parameter was selected: storage capacity of the rostral mouth cavity, transport capacity of the rostral mouth tube, storage capacity of the caudal mouth cavity and transport capacity of the caudal mouth tube. The boundary conditions for the simulation were determined from internal organismic constraints. The total food uptake of the mallard was regarded as the function to be maximized. This 'object' function is the summation of the food uptake by one second of pecking and one second of filter feeding. The drinking mechanism was shown not to interfere, since that mechanism operates sufficiently whenever the pumping mechanism works properly. The 'object' function, made up by the pecking and filter feeding performances was graphed. From these graphs a morphospace was developed: the region within which modifications of the mouth design are feasible. This procedure allowed examination of the general hypothesis that different modifications of one design for a complex multi-role system are explainable from differences in proportions of the functional performance contributed by each of the roles. Two predictions were evaluated more specifically: 1) If filter feeding performance must increase for a specific change in total food uptake, the volume of the rostral mouth cavity must increase; this requires widening and lengthening of the rostral maxillar portion and also a phase shift in jaw and lingual motion patterns, increasing the stroke volume. 2) If pecking performance must increase, the transport capacity of the rostral mouth tube must increase; this requires shortening of the maxillar mid portion. These two predictions regarding change in mouth morphology were borne out by shovelers and tufted ducks, respectively.(ABSTRACT TRUNCATED AT 400 WORDS)

Visuomotor feeding perturbations after lateral telencephalic lesions in pigeons

Pigeons with lesions of the lateral part of the telencephalon, visual Wulst, and fronto-archistriatal tract were compared with sham-operated controls in 2 procedures. In one of them the time it took the pigeons to grasp and eat a certain number of grains was recorded. In the other experiment the number of grains was counted that the pigeons consumed out of a mixture of grains and pebbles within a fixed time interval. Only the pigeons with lateral telencephalic lesions were impaired. While in the first experiment the lateral ablated birds improved with time there was no recovery of performance in the second experiment.

Cerebellar connections of the trigeminal system in the pigeon (Columba livia)

Wheat germ-agglutinin conjugated horseradish peroxidase (WGA-HRP) was used to delineate trigeminocerebellar connections in the pigeon. Subnucleus oralis of the nucleus of the descending trigeminal tract (nTTD) is the exclusive origin of trigeminal mossy fibers, which terminate in lobules VIII and IXa. The trigemino-olivary projection originates from subnucleus interpolaris of nTTD, but the existence of an additional pathway relaying in the adjacent lateral reticular formation (i.e. the plexus of Horsley) cannot be excluded. Structures linking the trigeminal cerebellar projections to jaw motoneurons were identified within the cerebellar cortex, the deep cerebellar nuclei and the lateral medullary reticular formation, completing a trigeminocerebellar sensorimotor circuit for the jaw.

Grasping in the pigeon (Columba livia): final common path mechanisms

A combination of cinematographic and denervation procedures were used to analyse the mechanisms involved in the adjustment of gape size during grasping in the pigeon. Gape size was found to vary directly with seed size and to reflect the operation of two variables, jaw opening velocity and jaw opening duration. Effects upon duration are mediated, indirectly, by the effect of seed size upon head height, which, in turn, controls the velocity of head descent. The data suggest that the control of gape during grasping may involve two different effector systems (jaw muscles, neck muscles). Analysis of the displacement of individual jaws (maxilla, mandible) during grasping indicates that both opener muscles take part in the control of gape. Denervation experiments (motor nerve section) identified these opener motoneurons as contributors to the final common path for the opening phase of grasping. A comparison of the kinematics of pecking/grasping in pigeons and reaching/grasping in humans reveals a number of similarities in the topography and spatiotemporal organization of these behaviors.