Swimming behavior of the nudibranch Melibe leonina

Swimming in the nudibranch Melibe leonina consists of five types of movements that occur in the following sequence: (1) withdrawal, (2) lateral flattening, (3) a series of lateral flexions, (4) unrolling and swinging, and (5) termination. Melibe swims spontaneously, as well as in response to different types of aversive stimuli. In this study, swimming was elicited by contact with the tube feet of the predatory sea star Pycnopodia helianthoides, pinching with forceps, or application of a 1 M KCl solution. During an episode of swimming, the duration of swim cycles (2.7 +/- 0.2 s [mean +/- SEM], n = 29) and the amplitude of lateral flexions remained relatively constant. However, the latency between the application of a stimulus and initiation of swimming was more variable, as was the duration of an episode of swimming. For example, when touched with a single tube foot from a sea star (n = 32), the latency to swim was 7.0 +/- 2.4 s, and swimming continued for 53.7 +/- 9.4 s, whereas application of KCl resulted in a longer latency to swim (22.3 +/- 4.5 s) and more prolonged swimming episodes (174.9 +/- 32.1 s). Swimming individuals tended to move in a direction perpendicular to the long axis of the foot, which propelled them laterally when they were oriented with the oral hood toward the surface of the water. The results of this study indicate that swimming in Melibe, like that in several other molluscs, is a stereotyped fixed action pattern that can be reliably elicited in the laboratory. These characteristics, along with the large identifiable neurons typical of many molluscs, make swimming in this nudibranch amenable to neuroethological analyses.     

Motor Responses to Polarized Light and Gravity Sensing in Euglena gracilis*†

SYNOPSIS. Euglena gracilis strain Z has a motor response which results in orientation with respect to the polarization of a light stimulus. Cells swim preferentially in a direction perpendicular to the plane of polarization of the stimulus. If 2 polarized stimuli are given from opposite directions, the preferred direction is, under certain circumstances, at right angles to the directions of both stimuli. Euglena also preferentially assumes an orientation that is at right angles to the force of gravity. The relationships between these responses and phototactic movements oriented with respect to the direction of the stimulus are discussed.     

Phototropism and Local Adaptation in Phycomyces Sporangiophores

Phototropic responses to unilateral ultraviolet stimuli were studied to determine whether the response of one side of the cell is affected by the previous exposure of the opposite side to ultraviolet. It has been found that the direction of bending is not parallel to the stimulus direction, but is along a straight line rotated 17° clockwise from the stimulus direction. This deviation indicates that the photoreceptors may be in a state of continual clockwise rotation. If before the stimulus the cell is exposed briefly to ultraviolet and rotated through 90°, the response is not along the 17° line, but is deviated a greater or lesser amount, depending on whether the 90° rotation is clockwise or counterclockwise. This difference is evidence that the first ultraviolet exposure leaves a persistent patch of light-adapted receptors and the shaded part of the cell remains dark adapted. The phototropic stimulus straddles the edge between light- and dark-adapted regions, and the differing responses of the two regions affects the direction of phototropic bending. A phototropic mechanism is proposed which combines the features of local adaptation and photoreceptor rotation.     

The capacity of human subjects to process directional information provided at two skin sites

The ability of human subjects to discriminate direction of tactile stimulus motion on the dorsum of the hand was determined (1) in the absence and (2) in the presence of a moving stimulus delivered to a second skin site on the ipsilateral or contralateral forelimb. When the two skin sites were simultaneously contacted by stimuli moving in the same direction, directional sensitivity was typically below that predicted for a hypothetical subject who could independently process the information provided at each of the two skin sites. Even when the stimulus delivered to a second site was deliberately ignored, it could still alter a subject's perception of stimulus direction on the dorsal hand. Moreover, its influence was greatest whenever it moved in a direction opposite to that of the attended stimulus. Whenever the two moving stimuli were delivered nonsimultaneously to two skin sites, directional sensitivity rarely matched the levels predicted for a hypothetical subject who could independently process the information provided at each site. This, in part, resulted from the subjects' utilization of "long-range" cues provided by the temporal order of stimulation. Subjects frequently failed to distinguish these cues from the sensation of stimulus direction provided at each skin site.     

Triggering and gating of motor responses by sensory stimulation: behavioural selection in Xenopus embryos.

Neural mechanisms underlying selection of motor responses are largely unknown in vertebrates. This study shows that in immobilized Xenopus embryos, brief mechanical or electrical stimulation of the trunk skin can trigger sustained fictive swimming, whereas sustained pressure or repetitive electrical stimulation can evoke fictive struggling. These two rhythmic motor patterns are distinct: alternating single motor root spikes propagate from head to tail during swimming; alternating motor root bursts propagate from tail to head during struggling. As both motor patterns can be evoked in embryos with the CNS transected caudal to the cranial roots, the sensory pathway responsible must have direct access to the spinal cord. Rohon-Beard sensory neurons provide the only such pathway known. They respond appropriately to brief stimuli applied to the trunk skin, and also to repetitive electrical stimuli and sustained pressure. The results suggest that Rohon-Beard sensory neurons can both trigger sustained swimming and 'gate in' struggling motor patterns, and thus effect behavioural selection according to their pattern of activity.     

Dynamic responses of tibial campaniform sensilla studied by substrate displacement in freely moving cockroaches

Responses of the tibial campaniform sensilla, receptors that encode strains in the exoskeleton, were characterized by recording sensory activities during perturbations in freely standing cockroaches. The substrate upon which the animal stood was displaced horizontally using ramp and hold stimuli at varied rates. The sensilla showed short latency responses that were initiated in the first 30 ms of platform movement. Responses of individual receptors depended upon the direction of displacement and the orientation of their cuticular cap. Proximal receptors, whose caps are perpendicular to the long axis of the tibia, responded to displacements directed from the contralateral side of the body and from the head toward the abdomen. The distal sensilla, oriented parallel to the tibia, discharged at longer latency to displacements in opposite directions. Plots of receptor activity versus displacement direction showed that proximal and distal sensilla are activated in non-overlapping ranges of movement direction. Afferent responses also increased as the platform was displaced more rapidly. These results are consistent with a model in which displacements produce forces that result in bending of the tibia. This information could be utilized to detect the direction and rate of forces that occur during leg slipping or in walking on unstable terrains.     

Neural correlates of swimming behavior in Melibe leonina

The nudibranch Melibe leonina swims by rhythmically bending from side to side at a frequency of 1 cycle every 2-4 s. The objective of this study was to locate putative swim motoneurons (pSMNs) that drive these lateral flexions and determine if swimming in this species is produced by a swim central pattern generator (sCPG). In the first set of experiments, intracellular recordings were obtained from pSMNs in semi-intact, swimming animals. About 10-14 pSMNs were identified on the dorsal surface of each pedal ganglion and 4-7 on the ventral side. In general, the pSMNs in a given pedal ganglion fired synchronously and caused the animal to flex in that direction, whereas the pSMNs in the opposite pedal ganglion fired in anti-phase. When swimming stopped, so did rhythmic pSMN bursting; when swimming commenced, pSMNs resumed bursting. In the second series of experiments, intracellular recordings were obtained from pSMNs in isolated brains that spontaneously expressed the swim motor program. The pattern of activity recorded from pSMNs in isolated brains was very similar to the bursting pattern obtained from the same pSMNs in semi-intact animals, indicating that the sCPG can produce the swim rhythm in the absence of sensory feedback. Exposing the brain to light or cutting the pedal-pedal connectives inhibited fictive swimming in the isolated brain. The pSMNs do not appear to participate in the sCPG. Rather, they received rhythmic excitatory and inhibitory synaptic input from interneurons that probably comprise the sCPG circuit.     

Representation of moving stimuli by somatosensory neurons.

The findings obtained in neurophysiological and psychophysical investigations using tactile stimuli that move at constant velocity across the skin are reviewed. For certain neurons in the postcentral gyrus of the cerebral cortex (S-I) of macaque monkeys, direction of stimulus motion is a "trigger feature" i.e., moving tactile stimuli evoke vigorous discharge activity in these neurons only if the stimuli are moved in a particular direction across the receptive field. This directional selectivity is maximal when stimulus velocity is between 5 and 50 cm/sec, and falls off rapidly at lower or higher velocities. The capacity for human subjects to correctly identify the direction of stimulus motion on the skin exhibits a similar dependence on stimulus velocity. The similar effects of velocity on neural and psychophysical measures of directional sensitivity support the idea that direction of stimulus motion on the skin can only be recognized if the moving stimulus optimally activates the group of S-I neurons for which that directions of simulus motion is the trigger feature.     

Coordinates of Human Visual and Inertial Heading Perception

Heading estimation involves both inertial and visual cues. Inertial motion is sensed by the labyrinth, somatic sensation by the body, and optic flow by the retina. Because the eye and head are mobile these stimuli are sensed relative to different reference frames and it remains unclear if a perception occurs in a common reference frame. Recent neurophysiologic evidence has suggested the reference frames remain separate even at higher levels of processing but has not addressed the resulting perception. Seven human subjects experienced a 2s, 16 cm/s translation and/or a visual stimulus corresponding with this translation. For each condition 72 stimuli (360° in 5° increments) were delivered in random order. After each stimulus the subject identified the perceived heading using a mechanical dial. Some trial blocks included interleaved conditions in which the influence of ±28° of gaze and/or head position were examined. The observations were fit using a two degree-of-freedom population vector decoder (PVD) model which considered the relative sensitivity to lateral motion and coordinate system offset. For visual stimuli gaze shifts caused shifts in perceived head estimates in the direction opposite the gaze shift in all subjects. These perceptual shifts averaged 13 ± 2° for eye only gaze shifts and 17 ± 2° for eye-head gaze shifts. This finding indicates visual headings are biased towards retina coordinates. Similar gaze and head direction shifts prior to inertial headings had no significant influence on heading direction. Thus inertial headings are perceived in body-centered coordinates. Combined visual and inertial stimuli yielded intermediate results.     

Analysis of surface wave direction by the lateral line system of Xenopus: Source localization before and after inactivation of different parts of the lateral line

The turning responses of clawed toads (Xenopus laevis) to surface waves were examined in animals with an intact lateral line or with different combinations of lateral lines reversibly inactivated by CoCl₂. The responses were characterized with respect to response frequency, turning accuracy, turning side, response time, and swim distance. After the inactivation most animals still responded to surface waves but the responses were different from those of animals with an intact lateral line. They also differed according to the combination of inactivated lines. In all experiments the responses for stimuli in some sectors of the surface did not differ from controls. The location of these sectors co-varied with the position of the intact lines, i.e., normal responses were found for frontal stimulus directions when head lines were intact and for caudolateral stimulus directions when trunk lines were intact. Their size was larger when lines on both sides of the body were intact and smaller when only lines on one side were intact. When the number of functional lines was reduced to one or two on one side of the body the turning angles shown within the sector of normal responses were maintained for stimulus directions outside these sectors. These results can be interpreted as indicating that head and trunk lines represent different position values. When only a single line was functional the toads still turned towards the stimulus source more often than by chance.It is hypothesized that Xenopus uses two mechanisms to determine the direction of surface waves. One uses the position values of head and trunk lines; this mechanism is comparable to the place value postulated for individual head neuromasts of surface feeding fish. The other uses the information encoded in the activity pattern that is elicited in one line when the surface wave travels over the line. This second mechanism yields information about stimulus side but not about stimulus angle.     

Interspecific differences in crustacean homologous behavior: Neural mechanisms underlying the reversal of uropod steering movement

Summary Rolling of the body in the same direction induces an asymmetrical steering movement of uropods in the opposite directions in two closely related species of crayfish. InProcambarus clarkii the uropod on the upper side is spread out and closed on the lower side, whereas inCambaroides japonicus the uropod moves in the opposite direction. The stimulus detector, the statocysts, the effector, the uropod musculature, are neither structurally nor functionally significantly different in the two species. The results indicate that the opposite responses could be ascribed to differences in the interneuronal connections within the central nervous system of these two species.     

Responses of neurons in the primary auditory cortex of the cat to the auditory motion stimuli with variable interaural delay

Unit responses in the primary auditory cortex of anesthetized cats to stationary and apparently moving stimuli resulted from a static and dynamically varying interaural delay (ITD) were recorded. The static stimuli consisted of binaurally presented tones and clicks. The dynamic stimuli were produced by in-phase and out-of-phase binaurally presented click trains with time-varying ITD. Sensitivity to ITDs was mostly seen in responses of the neurons with low characteristic frequency (below 2.8 kHz). All cells sampled with static stimuli responded to simulated motion. A motion effect could take the form of a difference in response magnitude depending on the direction of stimulus motion and a shift in the ITD-function opposite the direction of motion. The magnitude of motion effects was influenced by the position of motion trajectory relative to the ITD-function. The greatest motion effect was produced by motion crossing the ITD-function slopes.     

On the mechanisms underlying appearance of responses to movement,directional sensitivity and velocity tuning of the cat's striate cortical neurons

The responses to moving and stationary stimuli of 27 cat's striate cortical units were studied. Two stationary light bars located in different parts of the receptive field were used. The order of presentation and the time-interval between the stimuli varied; so, the presentation of a pair of stationary stimuli was an analogue of a moving stimulus.It was shown that responses occurred in neurons previously unresponsive to stationary stimuli when two stationary stimuli were presented successively in certain order. In the direction-sensitive units an asymmetry of the temporal course of the inhibitory processes was observed. The inhibitory zone located on the side of the preferred direction of movement was characterized by an early inhibitory phase followed by a phase of disinhibition and by a second inhibitory phase. For the inhibitory zone located on the side of the null direction no disinhibitory phase was demonstrated.The significance of the spatial and temporal characteristics of the receptive field for the appearance of responses to movement, the directional sensitivity and the velocity tuning in striate neurons is discussed.     

Visually guided avoidance in the chameleon (Chamaeleo chameleon): response patterns and lateralization

The common chameleon, Chamaeleo chameleon, is an arboreal lizard with highly independent, large-amplitude eye movements. In response to a moving threat, a chameleon on a perch responds with distinct avoidance movements that are expressed in its continuous positioning on the side of the perch distal to the threat. We analyzed body-exposure patterns during threat avoidance for evidence of lateralization, that is, asymmetry at the functional/behavioral levels. Chameleons were exposed to a threat approaching horizontally from the left or right, as they held onto a vertical pole that was either wider or narrower than the width of their head, providing, respectively, monocular or binocular viewing of the threat. We found two equal-sized sub-groups, each displaying lateralization of motor responses to a given direction of stimulus approach. Such an anti-symmetrical distribution of lateralization in a population may be indicative of situations in which organisms are regularly exposed to crucial stimuli from all spatial directions. This is because a bimodal distribution of responses to threat in a natural population will reduce the spatial advantage of predators.     

Integration of photosensory signals in Halobacterium halobium.

Stimulation of Halobacterium halobium through its sensory photosystems, PS 370 and PS 565, leads either to a prolonged or to a shortened interval between two reversals of the swimming direction of the cell, the attractant or repellent response. Stimuli are integrated to yield the same response regardless through which photosystem they are given. Simultaneously elicited attractant and repellent signals cancel each other at any time during a reversal interval, even in the period of refractoriness shortly after a reversal, when the cell is insensitive to repellent stimuli. Successively applied stimuli are less completely integrated. The net response depends on the moment of stimulation during the interval, on the sequence of stimuli, and on the delay between them. Integration of successively applied effective stimuli (after refractoriness) is to a great extent explained in terms of a cellular oscillator (A. Schimz and E. Hildebrand, Nature [London] 317:641-643, 1985) which is changed in opposite directions by attractant and repellent signals. Some conclusions on the shape of the oscillator after its disturbance by a stimulus can be made. Integration of signals during refractoriness leads us to postulate an additional step before the oscillator in the sensory pathway. Cancelling of simultaneous opposite signals is thought to proceed at this integrator. It also takes part in the integration of successively evoked signals. At this step signals rapidly decline within 10 ms, and the total life time (at least of repellent signals) does not exceed 1.2 s.     

Behavioural Stress Responses Predict Environmental Perception in European Sea Bass (Dicentrarchus labrax)

Individual variation in the response to environmental challenges depends partly on innate reaction norms, partly on experience-based cognitive/emotional evaluations that individuals make of the situation. The goal of this study was to investigate whether pre-existing differences in behaviour predict the outcome of such assessment of environmental cues, using a conditioned place preference/avoidance (CPP/CPA) paradigm. A comparative vertebrate model (European sea bass, Dicentrarchus labrax) was used, and ninety juvenile individuals were initially screened for behavioural reactivity using a net restraining test. Thereafter each individual was tested in a choice tank using net chasing as aversive stimulus or exposure to familiar conspecifics as appetitive stimulus in the preferred or non preferred side respectively (called hereafter stimulation side). Locomotor behaviour (i.e. time spent, distance travelled and swimming speed in each tank side) of each individual was recorded and analysed with video software. The results showed that fish which were previously exposed to appetitive stimulus increased significantly the time spent on the stimulation side, while aversive stimulus led to a strong decrease in time spent on the stimulation side. Moreover, this study showed clearly that proactive fish were characterised by a stronger preference for the social stimulus and when placed in a putative aversive environment showed a lower physiological stress responses than reactive fish. In conclusion, this study showed for the first time in sea bass, that the CPP/CPA paradigm can be used to assess the valence (positive vs. negative) that fish attribute to different stimuli and that individual behavioural traits is predictive of how stimuli are perceived and thus of the magnitude of preference or avoidance behaviour.     

Determinants of tonic and phasic reactions in transswitching

Forty-eight college students were assigned randomly to four groups in a 2 X 2 factorial arrangement of phasic conditional stimuli (same vs. different) and tonic conditional stimuli (same vs. different) to receive 2 days of classical conditioning with a transswitching procedure. Tonic stimuli were a 5-minute projected white triangle or circle; phasic stimuli were a 5-second red or green square superimposed over the tonic stimuli. There were six tonic stimulus segments each day, separated by 20-second periods of no stimulus, three containing six trials of the phasic stimulus paired with shock and three containing six trials of the phasic stimulus alone, in the counterbalanced order. Tonic responding at the onset of the tonic stimuli or during brief periods following its onset were recorded, along with phasic responses to the phasic stimuli. Responses included magnitude of skin conductance responses, frequency of unelicited skin conductance responses, and tonic heart rate. Both skin conductance measures of responding to the tonic stimuli differentiated significantly between positive and negative tonic segments during Day 2, but only in the group with two different tonic stimuli and one phasic stimulus ("standard" transswitching). This supported the hypothesis that tonic stimulus differentiation would be absent when two different phasic stimuli were present. The heart rate data did not support this hypothesis, showing tonic differentiation in both groups with two tonic stimuli. Phasic differentiation controlled by the different phasic stimuli was observed on Day 1; on Day 2, phasic differentiation was present only in the group with two tonic and one phasic stimuli and the group with one tonic and two phasic stimuli.(ABSTRACT TRUNCATED AT 250 WORDS)     

The role of cortical orientation in the control of the direction of ciliary beat in Paramecium

The swimming behavior of many ciliate protozoans depends on graded changes in the direction of the ciliary effective stroke in response to depolarizing stimuli (i.e., the avoiding reaction of Paramecium). We investigated the problem of whether the directional response of cilia with a variable plane of beat is related to the polarity of the cell as a whole or to the orientation of the cortical structures themselves. To do this, we used a stock of Paramecium aurelia with part of the cortex reversed 180 degrees. We determined the relation of the orientation of the kineties (ciliary rows) to the direction of beat in these mosaic paramecia by cinemicrography of particle movements near living cells and by scanning electron microscopy of instantaneously fixed material. We found that the cilia of the inverted rows always beat in the direction opposite to that of normally oriented cilia during both forward and backward swimming. In addition, metachronal waves of ciliary coordination were present on the inverted patch, travelling in the direction opposite to those on the normal cortex. The reference point for the directional response of Paramecium cilia to stimuli thus resides within the cilia or their immediate cortical surroundings.     

The effect of sound location in a song-discrimination task by Bengalese finches (<Emphasis Type="Italic">Lonchura striata</Emphasis> var. <Emphasis Type="Italic">domestica</Emphasis>)

To understand the relationship between song perception and stimulus location, Bengalese finches were trained to discriminate between a normally played conspecific song and the reversed song. Loudspeakers on the opposite side of the response key (experimental condition) and above the subject (control condition) were used to present stimuli. All control birds learned the task, whereas some experimental subjects did not. In general, controls learned the task more quickly than experimental animals. These results suggest that sound location may be closely associated with behavioral responses in this species. Bioacoustical investigations should be performed while taking these results into consideration.     

An automated assay for quantifying the swimming behavior of Paramecium and its use to study cation responses

Paramecium tetraurelia is a ciliated protist that alters its swimming behavior in response to various stimuli. Like the sensory responses of many organisms, these responses in Paramecium show adaptation to continued stimulation. For quantitative studies of the initial response to stimulation, and of the time course of adaptation, we have developed a computerized motion analysis assay that can detect deviations from the normal swimming pattern in a population of cells. The motion of an average of ten cells was quantified during periods ranging from 15 to 60 seconds, with a time resolution of 1/15 seconds. During normal forward swimming, the maximum deviation from a straight-line path was less than 17 degrees. Path deviations above this threshold value were defined as changes in swimming direction. The percentage of total path time that cells spent deviating from forward swimming was defined as percent directional changes (PDC). This parameter was used to construct dose-response curves for the behavioral effects of various externally added cations known to induce behavioral changes and also to show the time course of adaptation to a depolarizing K+ stimulus. This assay is a valuable tool for studies of chemoeffectors or mutations that alter the swimming behavior of Paramecium and may also be applicable to other motile organisms.