Transfer of podokinetic adaptation from stepping to hopping

Following stepping in-place on the surface of a rotating circular treadmill, a subject attempting to step in-place or walk in a straight line across the floor without vision will rotate relative to space. This adaptation, termed podokinetic after-rotation (PKAR), transfers to backward walking following forward walking on the rotating disk. We asked whether adaptation obtained during stepping in-place on the rotating disk would transfer to hopping on both feet. We hypothesized that subjects would demonstrate PKAR during both hopping and stepping, adding support to the hypothesis that PKAR is a centrally mediated adaptation of general locomotor trajectory that is not specific to the form of locomotion used while on the rotating disk. Subjects demonstrated PKAR during both hopping and stepping after stepping in-place on the rotating disk. The time courses of PKAR during hopping and stepping were similar, although the angular velocity amplitude of PKAR was lower in hopping than in stepping. This difference in amplitude suggests an incomplete transfer of PKAR.     

Evidence for limb-independent control of locomotor trajectory

After stepping in place on a rotating treadmill, individuals exhibit involuntary turning in the direction opposite treadmill rotation when stepping in place on a stationary surface without vision. This response is called podokinetic after-rotation (PKAR). It remains unclear where the control center for PKAR is located and whether separate, independent podokinetic control centers exist for each lower limb. To better understand neural mechanisms underlying locomotor trajectory adaptation, this study asked whether PKAR transfers between lower limbs. Thirteen healthy adults underwent separate 15-min sessions where one (trained) leg or both legs stepped on the rotating surface. Afterward, all subjects exhibited PKAR during one-legged hopping on a stationary surface, whether hopping on the trained or untrained limb. There were no significant differences in mean turning velocity across conditions. Our results support the absence of independent podokinetic control centers for lower limbs, indicating that a single center may control locomotor trajectory.     

Does the cerebellum play a role in podokinetic adaptation?

After sustained stepping in-place on a rotating disc, healthy subjects will inadvertently turn in circles when asked to step in-place on a stationary surface with eyes closed. We asked whether the cerebellum is important for this adaptive phenomenon, called podokinetic after-rotation (PKAR). Subjects with cerebellar degeneration and age-matched control subjects performed 15 min of stepping in-place with eyes open on a rotating disc, then 30 min of attempting to step in-place with eyes closed on a stationary surface. Rotational velocity of PKAR was measured during this 30-min period. All control subjects demonstrated PKAR; average initial rotational velocity for control subjects was 16.4+/-3.5 degrees /s. Five of the eight cerebellar subjects demonstrated impaired PK adaptation, defined as PKAR with an initial velocity more than two standard deviations below the control mean initial velocity. Average initial rotational velocity for cerebellar subjects was 7.8+/-0.2 degrees /s. Impaired PK adaptation was not associated with impaired time constants of decay and was not correlated with variability of PKAR velocity. Our results suggest that the cerebellum is important for regulation of the amplitude of PK adaptation and that reduced PKAR amplitude is not likely the result of dyscoordination or variability of movement in the subjects tested.     

Oculomotor responses to on-axis rotational stepping in normal and adaptively altered podokinetic states

Previous studies investigated adaptive properties of a podokinetic (PK) system that senses and controls angular movement of the trunk relative to the stance foot when walking around a curved trajectory or during rotational stepping on the spot. In particular, after adaptively modifying the PK system by prolonged stepping-in-place over the axis of a horizontally rotating platform, blind-folded subjects could no longer step in place on firm ground. When trying to do so they invariably rotated themselves relative to space without perceiving their rotation, a phenomenon termed podokinetic after-rotation (PKAR). It is well known that normal rotational stepping generates a specifically podokinetic component of compensatory nystagmus which is independent of the VOR. The present study investigated whether during PKAR this podokinetic component of oculomotor activity follows the somatosensory correlate of actually stepping around, or the cognitive intent or percept of 'no rotation'. Experiments were conducted in two phases on five normal human subjects. In the first phase, the normal passively induced VOR was compared with the combined VOR and PK oculomotor response induced by intentional rotational stepping on the spot. In both cases the angular stimulus was a 2-min rotation at 15 degrees/s. Subtraction of the decaying VOR from the actively induced combined response revealed a constant podokinetic nystagmus with slow-phase velocity gain of about 0.4 maintained throughout this period. The PK and VOR response components appeared to sum linearly. In the second phase, we measured oculomotor activity during PKAR, when the blindfolded subjects involuntarily rotated themselves at around 15 degrees/s while attempting to step-in-place after the podokinetic adaptation procedure noted above. The striking result of the second phase of experiments was that, although an apparently normal decaying VOR was present, the maintained PK component of response was consistently absent, despite an essentially normal physical pattern of rotational stepping. Thus, in the adapted state, non-vestibular oculomotor activity followed the cognitive intent or percept of 'no rotation', rather than the prevailing somatosensory-motor activity of the lower limbs. The finding points to an important cognitive element in this form of oculomotor control.     

Motor learning in the “podokinetic” system and its role in spatial orientation during locomotion

The present study characterizes a previously reported adaptive phenomenon in a somatosensory-motor system involved in directional control of locomotor trajectory through foot contact with the floor. We call this the “podokinetic” (PK) system. Podokinetic adaptation was induced in six subjects by stepping in-place over the axis of a horizontally rotating disc over a range of disc angular velocities (11.25–90°/s) and durations (7.5–60 min). After adaptation, subjects were blindfolded and attempted to step in-place on the floor without turning. Instead they all rotated relative to space. The rate of the “podokinetic afterrotation” (PKAR) was linearly related to stimulus amplitude up to 45°/s, and the ratio of initial PKAR velocity to that of the adaptive stimulus was approximately 1:3. PKAR exhibited exponential decay, which was composed of “short-” and “long-term” components with “discharging” time constants on the order of 6–12 min and 1–2 h, respectively. The effect of stimulus duration on PKAR revealed a “charging” time constant that approximated that of the short-term component. A significant suppression of PKAR occurred during the 1st min of the postadaptive response, suggesting functional interaction between the PK and vestibular systems during the period of vestibular stimulation. During PKAR subjects perceived no self-rotation, indicating that perception as well as locomotor control of spatial orientation were remodeled by adaptation of the PK system. Received: 4 August 1997 / Accepted: 19 November 1997     

Effects of bilateral vestibular loss on podokinetic after-rotation

We asked what the role of the vestibular system is in adaptive control of locomotor trajectory in response to walking on a rotating disc. Subjects with bilateral vestibular loss (BVL) were compared to age- and gender-matched controls (CTRL). Subjects walked in place on the surface of a rotating disc for 15 min and then attempted to step in place without vision on a stationary surface for 30 min. CTRL subjects demonstrated podokinetic after-rotation (PKAR), involuntarily and unknowingly turning themselves in circles while attempting to step in place. PKAR in CTRLs was characterized by a rapid rise in turning velocity over the first 1–2 min, followed by a gradual decay over the remaining 28 min. Subjects with BVL also demonstrated PKAR and had no knowledge of their turning. However, PKAR in BVLs was characterized by an extremely rapid, essentially instantaneous rise. Subjects with BVL immediately turned at maximum velocity and exhibited a gradual decay throughout the entire 30 min period. Despite this difference in the initial portion of PKAR in BVLs, their responses were not significantly different from CTRLs during minutes 2 to 30 of the response. These results suggest that vestibular inputs normally suppress PKAR velocity over the first 1–2 min of the response, but do not greatly influence PKAR decay. PKAR is therefore a process mediated primarily by somatosensory information and vestibular inputs are not required for its expression. Additionally, the absence of vestibular inputs does not result in increased somatosensory sensitivity that alters podokinetic intensity or decay time constants.     

Podokinetic after-rotation in patients with compensated unilateral vestibular ablation

Previous studies showed that after stepping-in-place on a rotating turntable, blindfolded subjects cannot step-in-place on firm ground. Instead they involuntarily turn themselves relative to space in the same direction as they were turning relative to the rotating turntable. This phenomenon has been termed podokinetic after-rotation (PKAR). PKAR comprises a brief exponentially rising phase of response during the first 2 min followed by a prolonged second phase of slow exponential decline during the next 28 min. Here we ask whether PKAR is modified in patients with compensated unilateral vestibular loss. Eleven patients who had previous vestibular ablation underwent (1) a Fukuda-like control stepping test, (2) podokinetic adaptation to 30 min of stepping in place on the centre of a turntable rotating at 45 deg/s and (3) PKAR. Control tests showed that the blindfolded patients had no significant rotational bias while stepping-in-place on the ground for 1 min. After 30 min of adaptation, the 2-min rising phase of PKAR was indistinguishable from normal. In contrast, the subsequent 28-min phase of exponential decline showed a lesion-dependent asymmetry. PKAR had significantly higher mean velocities toward the side of the lesion than away from the lesion. The observed PKAR asymmetry may signify occult residual static vestibular imbalance. Electronic Publication     

Forward versus backward walking: transfer of podokinetic adaptation

We asked whether podokinetic adaptation to walking on a circular treadmill transfers to different forms of locomotion. Subjects were blindfolded and asked to walk straight across the floor, in the forward and backward directions, following podokinetic (PK) stimulation that consisted of 30 min of forward walking-in-place on the perimeter of a disk rotating in the clockwise direction. During both forward and backward walking following forward-walking PK stimulation, subjects involuntarily walked along curved trajectories at angular velocities well above vestibular threshold, although they perceived that they were walking along straight paths. The curved paths of forward and backward walking were indistinguishable from one another. Transfer of PK adaptations acquired during forward walking on the turntable to backward walking trials suggests that the PK system controls general locomotor trajectory. Adaptation of the system thus influences forms of locomotion other than that used during acquisition of the adaptation. This transfer also supports the concept that forward and backward walking are controlled by neural networks that share common elements. An interesting feature of the transfer of PK adaptation is that for both forward and backward walking, subjects turned in a counterclockwise direction. As such, the direction of relative rotation between the trunk and feet was maintained for both forward and backward walking. However, the relationship of the lower extremities to the center of rotation was not preserved. The left limb was the inner leg during PK stimulation and forward walking after adaptation, but the left leg was the outer leg during backward walking. These results suggest that PK adaptation affects general locomotor trajectory via a remodeling of the rotational relationship between the trunk and the feet.     

Postural reorientation does not cause the locomotor after-effect following rotary locomotion

After a period of stepping on a rotating platform, blindfolded subjects demonstrate a tendency to unconsciously turn when stepping in place, an after-effect known as podokinetic after-rotation (PKAR). Recent studies have also reported a change in postural orientation following the adaptive period and have suggested that this is causally related to PKAR. Here, we assess changes in trunk orientation following platform adaptation and determine their relationship to PKAR. Specifically, we determine whether a reorganized standing posture causes PKAR. Ten subjects stepped on a platform rotating at 60°/s for 10?min, with a cadence of 100 steps/min. Following adaptation, a significant PKAR response was seen, with a mean yaw rotation velocity of 6.0?±?2.2°/s. In addition to this dynamic after-effect, there was a significant twist of the trunk with respect to the feet when standing still (6.9°?±?4.5°; mean?±?SD), confirming the presence of a postural reorientation after-effect. However, the magnitudes of the two after-effects did not correlate (r?=?0.06, p?=?0.87). Furthermore, in a second experiment, a prolonged passive twist of the trunk was used to induce postural reorientation. However, in this case, PKAR was not induced. These results demonstrate that PKAR is not an automatic consequence of reorganized standing posture.     

Vestibular–Podokinetic interaction without vestibular perception

After prolonged stepping in place relative to space over the center of a rotating turntable, blindfolded subjects cannot step on the stationary platform without unknowingly turning themselves relative to space, a phenomenon termed podokinetic after rotation (PKAR). We asked what role the resulting vestibular stimulation might play in the expression of this form of PKAR. A method of servo-stabilizing the body relative to space during PKAR was devised to compare PKAR expression with and without significant vestibular stimulation. Simulated estimates of average central vestibular response profiles were obtained by passing the averaged unidirectional body angular velocity profiles relative to space through a first-order model of the canal system (τ=15 s). Such simulation results suggested that during normal PKAR performed on a stationary platform, the average central vestibular response would likely rise to peak levels equivalent to that induced by about 9°/s within the frequency range of natural head movement. In the servo-stabilized condition, the simulated response was reduced to insignificant levels. Experimental results demonstrated that in the unstabilized condition the rate of rise of PKAR angular velocity was roughly four times slower than in the stabilized condition. We conclude that the normal expression of PKAR conducted on a stationary platform tends to be substantially slowed by interaction with an unperceived vestibular response.     

Podokinetic after-rotation does not depend on sensory conflict

Humans who have been stepping for 10 min or more about their vertical axis on a counterrotating platform while fixating on a stationary visual scene continue to circle in the same direction when they attempt, thereafter, to step on firm ground in darkness without turning (”podokinetic after-rotation”: PKAR). In the present report, we investigate whether PKAR is due to: (1) a sensory reinterpretation triggered by the conflict between the visual signal of stationarity and the somatosensory message of feet-on-platform rotation, or (2) an adaptation of the somatosensory afferents to prolonged unilateral stimulation irrespective of visual stimulation. Subjects (Ss) circled for 10 min about their vertical axis on an either stationary or counterrotating platform while they were either in darkness, or exposed to an optokinetic stimulus, or to a ”head-fixed” stationary pattern. Thereafter, Ss first stood motionless in darkness for 30 s, allowing vestibular after-effects to decay, and then tried (still without vision) to step in place on the stationary platform without turning while their body rotation was recorded by a potentiometer coupled to the head. All conditions involving podomotor activity without concomitant optokinetic stimulation evoked similar PKAR. With optokinetic stimulation, PKAR became larger, apparently because it was summed with an optokinetically induced after-rotation (oPKAR). This oPKAR could be demonstrated in isolation when Ss were passively rotated in front of the OKN-pattern instead of actively circling. PKAR could not be ”dumped”; it reappeared after 30 s of straight stepping under visual control. We suggest that PKAR is caused by adaptation of the somatosensory channel and not by a sensory conflict. Received: 16 April 1999 / Accepted: 30 June 1999     

Interstitial nucleus of cajal encodes three-dimensional head orientations in Fick-like coordinates

Two central, related questions in motor control are 1) how the brain represents movement directions of various effectors like the eyes and head and 2) how it constrains their redundant degrees of freedom. The interstitial nucleus of Cajal (INC) integrates velocity commands from the gaze control system into position signals for three-dimensional eye and head posture. It has been shown that the right INC encodes clockwise (CW)-up and CW-down eye and head components, whereas the left INC encodes counterclockwise (CCW)-up and CCW-down components, similar to the sensitivity directions of the vertical semicircular canals. For the eyes, these canal-like coordinates align with Listing's plane (a behavioral strategy limiting torsion about the gaze axis). By analogy, we predicted that the INC also encodes head orientation in canal-like coordinates, but instead, aligned with the coordinate axes for the Fick strategy (which constrains head torsion). Unilateral stimulation (50 microA, 300 Hz, 200 ms) evoked CW head rotations from the right INC and CCW rotations from the left INC, with variable vertical components. The observed axes of head rotation were consistent with a canal-like coordinate system. Moreover, as predicted, these axes remained fixed in the head, rotating with initial head orientation like the horizontal and torsional axes of a Fick coordinate system. This suggests that the head is ordinarily constrained to zero torsion in Fick coordinates by equally activating CW/CCW populations of neurons in the right/left INC. These data support a simple mechanism for controlling head orientation through the alignment of brain stem neural coordinates with natural behavioral constraints.     

Pelvic kidney with an unusual blood supply angiographic findings

A right pelvic kidney was observed in a patient, who presented with hypertension. On angiograms, the left kidney was normally positioned and had a single renal artery, whereas the right pelvic kidney received three arteries, which arose from bilateral common iliac arteries and from ipsilateral internal iliac artery. The renal arteries from the ipsilateral internal iliac artery and the contralateral common iliac artery supplied the medial half of the pelvic kidney. In the present case, the blood supply from both the right and left sides appeared to be related to the medial position of the right pelvic kidney. As the incidence of unilateral renal ectopia is not extremely low, it is possible to encounter in a surgical or cancer treatment case. Variations in the positional anatomy of the kidney and its vascular supply are of clinical importance and our case illustrates a different kind of blood supply that a pelvic kidney may possess.     

Unilateral testing of utricular function

A modified rotatory chair test is reported in which radial acceleration, generated by eccentric displacement of the subject during constant angular velocity, is exploited as a unilateral stimulation to the otolith organs. During constant angular rate rotation, the test subject is displaced laterally on the rotating turntable by 3.5 cm, so that one labyrinth becomes aligned with the rotatory axis while the second – eccentric – labyrinth is solely exposed to the altered gravito-inertial acceleration (GIA). Previously reported results showed that the direction of the response is independent of the direction of turntable rotation, ruling out any canal influence, and indicated that in a normal population the response, measured in one eye, was symmetrical for displacement of the left and right labyrinths. This mode of stimulus thus appears to elicit a unilateral otolith-ocular response (OOR). Examination of this unilateral OOR was extended in the present study; comparative testing with head-tilt to gravity, i.e. involving bilateral stimulation to the otolith organs, was carried out. Movements of both eyes were recorded (by three-dimensional video-oculography), in order to examine response conjugacy. To verify the specificity of the unilateral stimulus, tests were performed with patients who had previously undergone unilateral section of the vestibular nerve as treatment for acoustic neuroma. The eccentric displacement profile (EDP) and head-tilt stimulus each included ten cycles of left-right oscillation in order to permit signal averaging. In the normal subjects (n=12) the torsional component of the OOR proved to be both labyrinth-symmetrical and conjugate, during both bilateral and unilateral otolith stimulation. OOR gain (ocular torsion/GIA tilt) was higher for bilateral than unilateral stimulation. Bilateral OORs, obtained from three of the five unilaterally deafferented patients, proved less symmetrical and conjugate than in the normals. Unilateral OORs in all five patients were characteristically asymmetrical, with little or no response during stimulation of the diseased labyrinth. Received: 28 July 1997 / Accepted: 3 February 1998     

Podokinetic stimulation causes shifts in perception of straight ahead

Podokinetic after-rotation (PKAR) is a phenomenon in which subjects inadvertently rotate when instructed to step in place after a period of walking on a rotating treadmill. PKAR has been shown to transfer between different forms of locomotion, but has not been tested in a non-locomotor task. We conducted two experiments to assess effects of PKAR on perception of subjective straight ahead and on quiet standing posture. Twenty-one healthy young right-handed subjects pointed to what they perceived as their subjective straight ahead with a laser pointer while they were recorded by a motion capture system both before and after a training period on the rotating treadmill. Subjects performed the pointing task while standing, sitting on a chair without a back, and a chair with a back. After the training period, subjects demonstrated a significant shift in subjective straight ahead, pointing an average of 29.1 ± 10.6° off of center. The effect was direction-specific, depending on whether subjects had trained in the clockwise or counter-clockwise direction. Postures that limited subjects’ ability to rotate the body in space resulted in reduction, but not elimination, of the effect. The effect was present in quiet standing and even in sitting postures where locomotion was not possible. The robust transfer of PKAR to non-locomotor tasks, and across locomotor forms as demonstrated previously, is in contrast to split-belt adaptations that show limited transfer. We propose that, unlike split-belt adaptations, podokinetic adaptations are mediated at supraspinal, spatial orientation areas that influences spinal-level circuits for locomotion.     

Human spatial orientation in non-stationary environments: relation between self-turning perception and detection of surround motion

We investigated the relative weighting of vestibular, optokinetic and podokinetic (foot and leg proprioceptive) cues for the perception of self-turning in an environment which was either stationary (concordant stimulation) or moving (discordant stimulation) and asked whether cue weighting changes if subjects (Ss) detect a discordance. Ss (N = 18) stood on a turntable inside an optokinetic drum and turned either passively (turntable rotating) or actively in space at constant velocities of 15, 30, or 60°/s. Sensory discordance was introduced by simultaneous rotations of the environment (drum and/or turntable) at ±{5, 10, 20, 40, 80}% of self-turning velocity. In one experiment, Ss were to detect these rotations (i.e. the sensory discordance), and in a second experiment they reported perceived angular self-displacement. Discordant optokinetic cues were better detected, and more heavily weighted for self-turning perception, than discordant podokinetic cues. Within Ss, weights did not depend on whether a discordance was detected or not. Across Ss, optokinetic weights varied over a large range and were negatively correlated with the detection scores: the more perception was influenced by discordant optokinetic cues, the poorer was the detection score; no such correlation was found among the podokinetic results. These results are interpreted in terms of a "self-referential" model that makes the following assumptions: (1) a weighted average of the available sensory cues both determines turning perception and serves as a reference to which the optokinetic cue is compared; (2) a discordance is detected if the difference between reference and optokinetic cue exceeds some threshold; (3) the threshold value corresponds to about the same multiple of sensory uncertainty in all Ss. With these assumptions the model explains the observed relation between optokinetic weight and detection score.     

Symmetry of oculomotor burst neuron coordinates about Listing's plane.

1. The purpose of this investigation was to determine the axes of eye rotation generated by oculomotor burst neuron populations and the coordinate system that they collectively define. In particular, we asked if such coordinates might be related to constraints in the emergent behavior, i.e., Listing's law for saccades. 2. The mesencephalic rostral interstitial nucleus of the medial longitudinal fasciculus (riMLF) was identified in four monkeys with the use of single-unit recording, and then explored with the use of electrical microstimulation and pharmacological inactivation with the inhibitory gamma-aminobutyric acid (GABA) agonist muscimol. Three-dimensional (3-D) eye positions and velocities were recorded in one or both eyes while alert animals made eye movements in response to visual stimuli and head rotation. 3. Unilateral stimulation of the riMLF (20 microA, 200 Hz, 300-600 ms) produced conjugate, constant velocity eye rotations, which then stopped abruptly and held their final positions. This is expected if the riMLF produces phasic signals upstream from the oculomotor integrator. 4. Units that burst before upward or downward saccades were recorded intermingled in each side of the riMLF. Unilateral stimulation of the same riMLF sites produced eye rotations about primarily torsional axes, clockwise (CW) during right riMLF stimulation and counterclockwise (CCW) during left stimulation. Only small and inconsistent vertical components were observed, supporting the view that the riMLF carries intermingled up and down signals. 5. The torsional axes of eye rotation produced by riMLF stimulation did not correlate to external anatomic landmarks. Instead, stimulation axes from both riMLF sides aligned with the primary gaze direction orthogonal to Listing's plane of eye positions recorded during saccades. 6. Injection of muscimol into one side of the riMLF produced a conjugate deficit in saccades and quick phases, including a 50% reduction in all vertical velocities and complete loss of one torsional direction. CW was lost after right riMLF inactivation, and CCW was lost after left inactivation. 7. The plane that separated the intact torsional axes from the missing axes correlated with the orientation of Listing's plane. Thus, during left or right riMLF inactivation, the vertical axes of intact horizontal saccades were abnormally aligned with Listing's plane. The orientation of these axes was not correlated with external anatomic landmarks. 8. As suggested by their alignment with Listing's plane, the intact vertical axes of horizontal saccades following riMLF inactivation were orthogonal to torsional riMLF stimulation axes.(ABSTRACT TRUNCATED AT 400 WORDS)     

Bilateral motor tasks involve more brain regions and higher neural activation than unilateral tasks: an fMRI study

Movements that involve simultaneous coordination of muscles of the right and left lower limbs form a large part of our daily activities (e.g., standing, rising from a chair). This study used functional magnetic resonance imaging to determine which brain areas are used to control coordinated lower-limb movements, specifically comparing regions that are activated during bilateral exertions to those performed unilaterally. Plantarflexor exertions were produced at a target force level of 15 % of the participants’ maximum voluntary contraction, in three conditions, with their right (dominant) foot, with their left foot, and with both feet simultaneously. A voxel-wise analysis determined which regions were active in the bilateral, but not in the unilateral conditions. In addition, a region of interest (ROI) approach was used to determine differences in the percent signal change (PSC) between the conditions within motor areas. The voxel-wise analysis showed a large number of regions (cortical, subcortical, and cerebellar) that were active during the bilateral condition, but not during either unilateral condition. The ROI analysis showed several motor regions with higher activation in the bilateral condition than unilateral conditions; further, the magnitude of bilateral PSC was more than the sum of the two unilateral conditions in several of these regions. We postulate that the greater levels of activation during bilateral exertions may arise from interhemispheric inhibition, as well as from the greater need for motor coordination (e.g., synchronizing the two limbs to activate together) and visual processing (e.g., monitoring of two visual stimuli).     

Does unilateral basal ganglia activity functionally influence the contralateral side? What we can learn from STN stimulation in patients with Parkinson's disease

In humans, the control of voluntary movement, in which the corticobasal ganglia (BG) circuitry participates, is mainly lateralized. However, several studies have suggested that both the contralateral and ipsilateral BG systems are implicated during unilateral movement. Bilateral improvement of motor signs in patients with Parkinson's disease (PD) has been reported with unilateral lesion or high-frequency stimulation (HFS) of the internal part of the globus pallidus or the subthalamic nucleus (STN-HFS). To decipher the mechanisms of production of ipsilateral movements induced by the modulation of unilateral BG circuitry activity, we recorded left STN neuronal activity during right STN-HFS in PD patients operated for bilateral deep brain stimulation. Left STN single cells were recorded in the operating room during right STN-HFS while patients experienced, or did not experience, right stimulation-induced dyskinesias. Most of the left-side STN neurons (64%) associated with the presence of right dyskinesias were inhibited, with a significant decrease in burst and intraburst frequencies. In contrast, left STN neurons not associated with right dyskinesias were mainly activated (48%), with a predominant increase 4-5 ms after the stimulation pulse and a decrease in oscillatory activity. This suggests that unilateral neuronal STN modulation is associated with changes in the activity of the contralateral STN. The fact that one side of the BG system can influence the functioning of the other could explain the occurrence of bilateral dyskinesias and motor improvement observed in PD patients during unilateral STN-HFS, as a result of a bilateral disruption of the pathological activity in the corticosubcortical circuitry.     

Turning strategies during human walking.

The mechanisms involved in rapidly turning during human walking were studied. Subjects were asked to walk at a comfortable speed and to turn toward the instructed direction as soon as they felt an electrical stimulus to the superficial peroneal nerve. Stimuli were presented repeatedly at random over 10- to 15-min periods of walking for turning in both directions. Electromyograms (EMGs), joint angular movements of the right leg, and forces under both feet were recorded. The step cycle was divided into 16 parts, and the responses to stimuli in each part were analyzed separately. Two turning strategies were used, depending on which leg was placed in front for braking. For example, to turn to the right when the right foot was placed in front, subjects generally altered direction by spinning the body around the right foot (spin turn). To turn left when the right foot was in front, subjects shifted weight to the right leg, externally rotated the left hip, stepped onto the left leg, and continued turning until the right leg stepped in the new direction (step turn). The step turn is easy and stable because the base of support during the turn is much wider than in the spin turn, so some subjects used it in all parts of the cycle. Initially, the deceleration of walking is similar to a rapid stopping task, which has been previously examined. The deceleration mechanism involves a sequence of distal-to-proximal activation of muscles on one side of the body (soleus, biceps femoris, and erector spinae). This pattern is similar to the "ankle strategy" used in postural control during forward sway. The control of foot placement in the swing leg and muscle activities for rotating the trunk in the stance leg occurred within a step after the cue. The action of ankle inverters and elevation of the pelvis by activity of gluteus medius may contribute to the control of trunk rotation. This activity was closely related to the timing of the opposite foot strike, independent of the part of the step cycle when the stimulus was applied. In most subjects, the turn was completed without resetting the underlying walking rhythm. This first EMG analysis of rapid turning shows how common strategies for postural sway and stopping can be combined with one of two turning strategies. This simplifies the complex task of turning at a random time in the step cycle.