Grasping in the pigeon (Columba livid): Stimulus control during conditioned and consummatory responses

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.

Rapid adjustment of pecking trajectory to prism-induced visual shifts in crows as compared with pigeons

Pecking in birds is analogous to reaching and grasping movements in primates. Earlier studies on visuomotor control in birds, which were conducted mostly in pigeons, suggested that avian pecking is controlled feedforwardly, and is out of the control of visual guidance during movement. However, recent studies using crows suggested a role of vision in pecking control during movement. To unveil what visuomotor mechanisms underlie the flexibility of pecking in crows, we examined whether pigeons and crows adjust their pecking to the visual distortion induced by prisms. Because prisms induce visual shifts of object positions, birds were required to adjust their movements. Pecking kinematics were examined before and after attaching prisms in front of the birds' eyes. Analysis of lateral deviation caused by the prisms showed that crows rapidly adjusted their pecking trajectories, but pigeons did so slowly. Angular displacement also increased in pigeons after attachment of the prism, but decreased in crows. These responses to prisms were consistent among individuals in pigeons but varied in crows, though the adjustment of pecking commonly succeeded in crows. These results suggest that pecking in pigeons predominantly involves feedforward control and that the movement is determined depending on the visual information available before the initiation of pecking. In contrast, the results from crows suggest that their pecking trajectories are corrected during the movement, supporting on-line visual control. Our findings provide the first evidence to suggest the on-line visual control of pecking in birds.

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.

Species-specific response-topography of chickens' and pigeons' water-induced autoshaped responding

Four pigeons and eight chickens received autoshaping training where a keylight (conditioned stimulus) signaled response-independent deliveries of water (unconditioned stimulus). Pigeons drink while keeping their beaks submerged in water and moving their beaks to create suction ("mumbling"), whereas chickens drink by trapping a small amount of water in their mouths and then lifting their heads so the water trickles down. This experiment tested whether these and other species-specific differences in drinking and related behaviors of pigeons and chickens would be reflected in the form of conditioned (autoshaped) responding. Touchscreens and videotapes were used for data recording. Results showed that chickens moved their heads more than pigeons when drinking (unconditioned response). The birds also differed in conditioned responding in the presence of the keylight: Pigeons produced more keyswitch closures and mumbled at the keylight more than chickens whereas chickens scratched more than pigeons. In conclusion, with this unique comparative method that employed identical contingencies and comparable deprivation levels, species-specific differences in unconditioned responses and, more importantly, differences in their corresponding conditioned responses were observed.

Ontogeny has a phylogeny: background to adjunctive behaviors in pigeons and budgerigars

Animals coping with operant conditioning tasks often show behaviors that are not recorded by keys, levers and similar response transducers. Nevertheless, these adjunctive behaviors should not be disposed of by classifying them as incidental. Often they are found to be at least partially influenced by the experimentally programmed contingencies, and under certain conditions they can in turn influence conditioned behaviors. Here we describe the occurrence and characteristics of two such behaviors, stimulus grasping in operantly key-pecking pigeons and intra-delay stereotypies in a delayed matching-to-sample task with budgerigars. It is argued that for a proper account of these behaviors it is necessary to refer to a behavioral systems approach that appeals to longer ranging ontogenetic and phylogenetic histories than is usually considered in the psychological literature. The gaping towards on-key stimuli by pigeons is attributed to the hypothesis that operantly conditioned key-pecks probably relate to a grasp-pecking response that is normally executed towards non-edible items covering food. The intra-delay behaviors shown by the budgerigars are assumed to have originated from stress-induced displacement responses that adventitiously came under the influence of differential reinforcement contingencies. Finally, we discuss what kinds of evidence are needed to put these hypothetical explanations on a more certain footing.

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.

Effects of food-pellet size on rate, latency, and topography of autoshaped key pecks and gapes in pigeons

Four pigeons responded under autoshaping contingencies in which different conditional stimuli (red or green keylights) were associated with unconditional stimuli of different magnitudes (large or small food pellets) over successive trials within a session. Both topography (beak opening or gape) and strength (rates and latencies of key pecks and gapes) of responding during the conditional stimuli depended on the magnitude of the correlated unconditional stimulus. Key-peck and gape rates were higher and latencies were shorter in large-pellet trials than in small-pellet trials. Gape amplitudes varied directly with pellet size, although conditional and unconditional gapes were larger than either pellet. These findings were replicated when the key colors were presented either on one or two keys and after reversals of the color-size correlations. Because the unconditional stimulus was varied through pellet size, magnitude was not confounded with food-access duration or quality. These results demonstrate the effects of the magnitude of the unconditional stimulus, in that rates and latencies of both key pecks (which are directed movements toward the key) and gapes (which are independent of the bird's position and key properties) varied with pellet size. Gape measures were unique in that two dimensions (response strength and topography) of a single response class varied simultaneously with magnitude.

Autoshaping the pigeon's gape response: acquisition and topography as a function of reinforcer type and magnitude

The pigeon's key-pecking response is experimentally dissociable into transport (head movement) and gape (jaw movement) components. During conditioning of the key-pecking response, both components come under the control of the conditioned stimulus. To study the acquisition of gape conditioned responses and to clarify the contribution of unconditioned stimulus (reinforcer) variables to the form of the response, gape and key-contact responses were recorded during an autoshaping procedure and reinforcer properties were systematically varied. One group of 8 pigeons was food deprived and subgroups of 2 birds each were exposed to four different pellet sizes as reinforcers, each reinforcer signaled by a keylight conditioned stimulus. A second group was water deprived and received water reinforcers paired with the conditioned stimulus. Water- or food-deprived control groups received appropriate water or food reinforcers that were randomly delivered with respect to the keylight stimulus. Acquisition of the conditioned gape response frequently preceded key-contact responses, and gape conditioned responses were generally elicited at higher rates than were key contacts. The form of the conditioned gape was similar to, but not identical with, the form of the unconditioned gape. The gape component is a critical topographical feature of the conditioned key peck, a sensitive measure of conditioning during autoshaping, and an important source of the observed similarities in the form of conditioned and consummatory responses.

Dissecting the conditioned pecking response: an integrated system for the analysis of pecking response parameters

The conventional pecking response key, although an excellent transducer of response rate, can provide minimal information on the topography, coordination, or localization of conditioned pecking. We describe the hardware and software components of a system that, in addition to recording response rates, permits simultaneous "on-line" monitoring of head acceleration, jaw movement, terminal peck location, and duration of pecking response. Head movements are monitored with a miniature accelerometer, jaw movements with a magnetosensitive transducer, and peck location with modified touch screen technology. Initial experiments with the system suggest that it will be useful in studies of response differentiation, acquisition and maintenance of complex discriminations, and interaction of conditioned and unconditioned stimuli in the control of pecking response probability and response topography.

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.

Measurement and control of pecking response location in the pigeon

Experimental arrangements for the detection and control of pecking response location in the pigeon are described. Two different, commercially available detection devices are compared, one utilizing an array of infrared light emitters and detectors (infrared device), the other requiring contact with sheets of conductive film (mylar device) but both specifying a location as a set of X-Y coordinates. The utility of these devices for monitoring peck location was assessed in experiments involving pecking responses emitted under continuous reinforcement, fixed ratio, and response differentiation schedules. Both devices allow measurement of responses over areas much greater than those of the standard pecking response key, provide information about the location of all responses, whether on or off a designated stimulus area, and permit the programming of reinforcement contingencies based upon peck location. While the two devices differ with respect to their sensitivity and their resolution, they yield similar distributions of peck locations for the same animal tested under similar reinforcement schedules. Both devices have some limitations related to their design as detectors of human touch, and suggestions for minimizing these limitations are presented.

Prehension in the pigeon. I. Descriptive analysis

Eating in the pigeon involves a series of jaw movements some of which serve a prehensile function; i.e., they are utilized in the grasping and manipulation of objects. These prehensile behaviors are extremely brief (30-80 ms), produce an adjustment of jaw opening amplitude to the size of the food object, are mediated by an effector system involving a relatively small number of muscles and are amenable to both "reflexive" and "voluntary" control. This combination of structural simplicity and functional complexity suggests that the pigeon's jaw movements may provide a useful "model system" for the study of motor control mechanisms in targeted movements. The present report provides a classification of jaw opening movements occurring during eating and a preliminary determination of the extent to which each movement class is scaled to the size of the food object. Jaw movements were monitored during responses to spherical food pellets of six different sizes (3.2-11.1 mm in diameter) using a transducing system which produces a continuous record of gape (i.e., interbeak distance). Assignment to movement classes was then carried out using a computer-assisted scoring program. Functions relating jaw opening amplitude to target size were determined for each movement class. Four jaw movement classes were identified: Prepecks (just prior to pecking), Grasps (opening movements made during pecking but prior to contact with the target), Mandibulations (movements serving to position and transport the object within the buccal cavity) and Swallows. For two of these movement classes (Grasps, Mandibulations) jaw opening amplitude is scaled to pellet size but the scaling functions differ in ways that reflect the functional requirements of the two behaviors.(ABSTRACT TRUNCATED AT 250 WORDS)

Prehension in the pigeon. II. Kinematic analysis

During eating, the pigeon's jaw functions as a prehensile organ, i.e., as an effector organ involved in the grasping and manipulation of objects. The preceding paper provided a descriptive account of the jaw opening movements associated with each phase of the eating behavior sequence. For two of these movements, Grasping and Mandibulation, the amplitude of jaw opening is adjusted to pellet size. In the present study a kinematic analysis of these movements was carried out to clarify the motor control mechanisms mediating these adjustments. The analysis was carried out within the conceptual framework provided by a "pulse-control" model of targeted movement. For each of the movements the extent to which opening amplitude, its first and second derivatives and its rise time are scaled to pellet size was determined. Relationships among these kinematic variables were then examined in order to distinguish between "pulse-height" and "pulse-width" strategies. In addition, the possibility that "corrective adjustments" to the trajectory are made during its execution was also explored using a multiple regression analysis developed by Gordon and Ghez (1987a, b). For both movements, peak opening amplitude, acceleration and velocity are scaled to pellet size and these variables account for most of the variance in opening amplitude. The kinematic analysis suggests that critical parameters of the trajectory are determined ("programmed") prior to its initiation. Moreover, pigeons, like cats and humans, appear to utilize a "pulse-height" strategy for the control of amplitude scaling during targeted movements.(ABSTRACT TRUNCATED AT 250 WORDS)

Depth resolution in the pigeon

Pigeons possess a binocular visual field and a retinal region of higher cellular density pointing to the center of this overlap. These features and the precision of pecking behavior suggest that in this lateral-eyed bird cues other than monocular ones might participate in depth judgements. Pigeons were trained with an operant procedure to discriminate between luminous points differing in depth which appeared to the observer as floating in the dark. The accuracy of depth judgements was found to be a function of the ratio between the interstimulus distance and the mean eyes-to-stimulus distance. In a first test (experiment I) no external binocular disparity cues were available, the animal only seeing one luminous point at a time (near or far). In a second test (experiment II) where binocular disparity cues were available, the animal having this time to discriminate a pair of points placed at equal depth from a pair placed at unequal depths, only one pair being visible at a time, depth resolution did not improve. This suggests that, at least within the range of distances explored, the pigeon has no stereoscopic vision. Notwithstanding this, binocular cues do play a role, since when tests were done comparing binocular with monocular viewing (experiment III), monocular depth resolution was significantly worse.

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.

Grasping in the pigeon: mechanisms of motor control

A quantitative analysis of grasping in the pigeon suggests important functional similarities between the visuomotor controls of the avian beak and the primate hand. Beak-opening (gape) during eating is directly proportional to target size and the adjustment is completed prior to contact. The control of gape size involves variations in both the velocity and duration of jaw opening and these parameters are mediated by different effector systems (jaw muscles, neck muscles). Nerve section experiments were used to identify jaw motoneurons which are components of the final common path for grasping. Grasping in the pigeon approximates the functional complexity of mammalian visuomotor behavior but is mediated by a relatively simple effector system.