Involvement of a midbrain vocal nucleus in the production of both the acoustic and postural components of crowing behavior in Japanese quail.

Many bird species produce vocalizations which are accompanied by distinctive postural displays, but the neural mechanisms that allow such integrated production of vocal and postural motor patterns are not understood. In the crowing behavior of Japanese quail, a characteristic vocal pattern is accompanied by and coordinated with a postural display that consists of a sequence of rapid, patterned head movements. The purpose of the present study was to investigate the role of a midbrain vocal nucleus, the nucleus intercollicularis, in the production of the acoustic and postural components of crowing in quail. Brief electrical stimuli were applied to the nucleus intercollicularis during spontaneously emitted crows in quail with chronically implanted electrodes, to determine if perturbing neural activity in the nucleus intercollicularis resulted in a disruption of ongoing crowing behavior. The most common effect of such stimuli was a concurrent, premature termination of both the acoustic and head movement components of the crow. These results imply that the nucleus intercollicularis plays a role in the production of both the acoustic and postural components of crowing in quail.

Relative roles of the S cell network and parallel interneuronal pathways in the whole-body shortening reflex of the medicinal leech.

The whole-body shortening reflex of the medicinal leech Hirudo medicinalis is a withdrawal response produced by anterior mechanical stimuli. The interneuronal pathways underlying this reflex consist of the S cell network (a chain of electrically coupled interneurons) and a set of other, parallel pathways. We used a variety of techniques to characterize these interneuronal pathways further, including intracellular stimulation of the S cell network, photoablation of the S cell axon, and selective lesions of particular connectives (the axon bundles that link adjacent ganglia in the leech nerve cord). These experiments demonstrated that the S cell network is neither sufficient nor necessary for the production of the shortening reflex. The axons of the parallel pathways were localized to the lateral connectives (whereas the S cell axon runs through the medial connective). We used physiological techniques to show that the axons of the parallel pathways have a larger diameter in the anterior connective and to demonstrate that the parallel pathways are activated selectively by anterior mechanosensory stimuli. We also presented correlative evidence that the parallel pathways, along with activating motor neurons during shortening, are responsible for inhibiting a higher-order "command-like" interneuron in the neuronal circuit for swimming, thus playing a role in the behavioral choice between swimming and shortening.

Behavioral hierarchy in the medicinal leech, Hirudo medicinalis: feeding as a dominant behavior.

The effect of feeding behavior on other behaviors (swimming, crawling and shortening) was investigated in the leech, Hirudo medicinalis. The stimulus locations and intensities required to produce mechanically elicited behaviors were first determined in the non-feeding leech. Stimuli were delivered while the leech was in various body positions to determine whether stimulus location affected behavioral response. Response thresholds were determined for the mechanically elicited behaviors. The same stimuli were then applied to feeding leeches to determine if response thresholds had changed. A solution with NaCl and arginine was used to elicit feeding. The same sets of stimuli were applied at intervals for an hour after feeding, to determine the duration of feeding-induced changes in behavior. Depending on the body position and stimulus location, stimuli produced different combinations of behaviors that included shortening, swimming and crawling. Anterior stimuli generally elicited shortening, whereas posterior stimuli generally elicited crawling and swimming, with swimming more likely to ventral stimulation than to dorsal stimulation. Having the front sucker attached changed these behavioral patterns. During feeding, the response thresholds changed dramatically, from 3-5 V to greater than 9 V. This increase in threshold began with the start of feeding, even before ingestion commenced. Suppression of the behaviors lasted up to 1 h after the end of feeding, with the effect on swimming being the most pronounced and longest lasting.

The neuronal basis of the behavioral choice between swimming and shortening in the leech: control is not selectively exercised at higher circuit levels.

Swimming and the whole-body shortening reflex are two incompatible behaviors performed by the medicinal leech Hirudo medicinalis. We set out to examine the neuronal basis of the choice between these behaviors, taking advantage of the fact that the neuronal circuit underlying swimming is relatively well understood. The leech swim circuit is organized hierarchically and contains three interneuronal levels, including two upper levels of "command-like" neurons. We tested the responses of the swim circuit neurons to stimuli that produced shortening, using reduced preparations in which neurophysiological recording could be performed while behaviors were elicited. We found that the majority of the swim circuit neurons, including most of the command-like cells and all of the cells at the highest hierarchical level of the circuit, were excited by stimuli that produced shortening as well as by stimuli that produced swimming. Only a subset of neurons, at levels below the top, were inhibited during shortening; these included one of the command-like cells and an oscillator cell (an interneuron that is part of the central pattern generator for swimming). These results imply that the control of the choice between swimming and shortening is not exercised selectively at the higher levels of the swim circuit.

Population coding and behavioral choice.

Many individual behavioral acts are produced by the combined activity of large populations of broadly tuned neurons, and the neuronal populations for different behaviors can overlap. Recent experiments monitoring and manipulating neuronal activity during behavioral decisions have begun to shed light on the mechanisms that enable overlapping populations of neurons to generate choices between categorically distinct behaviors.

The whole-body shortening reflex of the medicinal leech: motor pattern, sensory basis, and interneuronal pathways.

The leech whole-body shortening reflex consist of a rapid contraction of the body elicited by a mechanical stimulus to the anterior of the animal. We used a variety of reduced preparations - semi-intact, body wall, and isolated nerve cord - to begin to elucidate the neural basis of this reflex in the medicinal leech Hirudo medicinalis. The motor pattern of the reflex involved an activation of excitatory motor neurons innervating dorsal and ventral longitudinal muscles (dorsal excitors and ventral excitors respectively), as well as the L cell, a motor neuron innervating both dorsal and ventral longitudinal muscles. The sensory input for the reflex was provided primarily by the T (touch) and P (pressure) types of identified mechanosensory neuron. The S cell network, a set of electrically-coupled interneurons which makes up a 'fast conducting pathway' in the leech nerve cord, was active during shortening and accounted for the shortest-latency excitation of the L cells. Other, parallel, interneuronal pathways contributed to shortening as well. The whole-body shortening reflex was shown to be distinct from the previously described local shortening behavior of the leech in its sensory threshold, motor pattern, and (at least partially) in its interneuronal basis.

Coupling dynamics in interlimb coordination.

In 1:1 frequency locking, the interlimb phase difference phi is an order parameter quantifying the spatial-temporal organization of 2 rhythmic subsystems. Dynamical modeling and experimental analyses indicate that an intentional parameter phi psi (intended coordination mode, phi = 0 degrees or phi = 180 degrees) and 2 control parameters omega c (coupled frequency) and delta omega (difference between uncoupled eigen-frequencies) affect phi. An experiment was conducted on 1:1 frequency locking in which phi psi, omega c, and delta omega were manipulated using a paradigm in which a person swings hand-held pendulums. As delta omega deviated from 0, the observed phi deviated from the phi psi, indicating a displacement in the phi attractor point. The displacements were exaggerated by increasing omega c. The displacements were coordinated with a decrease in the stability of phi and with higher harmonics in power spectrum of phi. Implications of the results for modeling interlimb coordination are discussed.

Dynamical aspects of learning an interlimb rhythmic movement pattern.

Learning a bimanual rhythmic task is explored from the perspective that motor skill acquisition involves the successive reparameterization of a dynamical control structure in the direction of increasing stability, where the intentional process of reparameterization is itself dynamical. Subjects learned to oscillate pendulums held in the right and left hands such that the right hand frequency was twice that of the left (2:1 frequency lock). Over 12 learning sessions of 20 trials each, we interpreted the decreasing fluctuations in the frequency locking to be an index of the increasing concavity of the underlying potential, a measure of stability; the time required to achieve the 2: 1 pattern was interpreted as indexing the relaxation time of an intentional dynamic. Power spectral analyses of the phase velocity ratio exhibited two strategies for acquiring the interlimb movement pattern: (a) adding spectral peaks at integer multiples of the left hand frequency or (b) distributing power across many frequencies in a l/f-like manner. Results are discussed in terms of the promise of a dynamical approach to learning coordinated movements.