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Biomedical subjects

Gammon M Earhart

Publications and source records attributed to Gammon M Earhart.

10 recordsLinked to original sources
PubMed2006

Podokinetic after-rotation in Parkinson disease.

Walking on a rotating platform for 15 min causes healthy subjects to involuntarily turn when walking without vision. This adaptive response, called podokinetic after-rotation (PKAR), uses the same kinematic patterns as voluntary turning suggesting that PKAR and voluntary turning share common mechanisms. The purpose of this study is to determine whether people with Parkinson disease (PD), a condition that produces substantial disability from turning difficulties, can adapt to the rotating platform. Initial testing of people with PD revealed that most were unable to step on the rotating platform for 15 continuous minutes. We thus tested a less intense version of the paradigm in eight healthy people. On one day, subjects walked on the platform for 15 continuous minutes; on another day, they walked on the platform for three 5-minute intervals separated by 5-minute rests. After both sessions, subjects rested for 5 min then walked in place for 30 min without vision, while we recorded rotational velocity of PKAR. Continuous and interval protocols effectively elicited robust PKAR. We then tested eight subjects with PD and matched controls using the 5-minute interval protocol and recorded PKAR responses for 10 min. There were no significant differences between the PD and control groups. We conclude that PD subjects can adapt to the rotating platform and develop PKAR from interval training. Future studies are needed to determine whether the rotating platform may act as a rehabilitative tool to reinforce motor patterns for turning and alleviate turning difficulties in people with PD.

Adaptation, Physiological↗
PubMed2006

Kinematics of podokinetic after-rotation: similarities to voluntary turning and potential clinical implications.

We examined the kinematics of voluntary turning in place at three different speeds and of inadvertent turning in place during attempts to step in place following stepping on a rotating disc (podokinetic after-rotation, PKAR). We hypothesized that voluntary turning in place, like online turning during walking, would be characterized by a top-down sequence of yaw rotations in the direction of the turn, i.e. the head would rotate first, followed by the trunk and then the foot. We also hypothesized that in place PKAR would be characterized by a bottom-up sequence of yaw rotations, i.e. the foot would rotate first, followed by the trunk and the head. The alternative possibility was that PKAR, like voluntary turning, would be initiated by the head and trunk and the foot would rotate last. As expected, voluntary turning in place was characterized by a top-down sequence similar to that noted previously during online turning in the midst of walking. Turning velocity did not alter the sequence of rotations in voluntary turning. In place PKAR was also characterized by a top-down sequence, indicating that PKAR may access the same neural circuits employed during voluntary turning. These data suggest that the rotating treadmill may be a useful training tool for addressing difficulties with turning that are experienced by individuals with Parkinson disease (PD).

Adult↗
PubMed2006

Walking and running on the circular treadmill: transition speed and podokinetic aftereffects.

The author compared 10 participants' self-selected walk-to-run transition speeds on a standard treadmill with those on a circular treadmill. The speed of the outer limb at walk-to-run transition on the circular treadmill and on the standard treadmill were very similar. Adaptive aftereffects from running and walking on the circular treadmill were also similar. When asked to step in place without vision, all participants inadvertently turned in circles following walking or running on the treadmill. The results of the present study suggest that the mechanisms controlling walk-to-run transitions are similar for the standard and circular treadmills and demonstrate the robust generalizability of locomotor aftereffects from running to walking. Adaptive control of speed, form, and direction may therefore share similar mechanisms for walking and running.

Adult↗
PubMed2005

Changes in perception of active but not passive turning following stepping on the rotating treadmill.

The purpose of this work was to determine whether walking on a rotating disc would cause changes in perception of active and passive turning. Ten healthy control subjects wore a blindfold and earplugs while completing trials of active and passive turning. For active conditions, subjects were told the direction and amplitude of the desired turn and then attempted to turn in place the specified amount via actively stepping. For passive conditions, subjects were told that the disc they stood on would turn and they were to press a button when they had traveled the specified amplitude. Subjects completed active and passive trials to the left and right with amplitudes of 90, 180, 270, and 360 degrees. Subjects then stepped in place for 15 min on a disc rotating clockwise at 90 degrees /s, after which they repeated the trials of active and passive turning. Following rotating treadmill stimulation, subjects asked to turn in the direction opposite disc rotation (i.e., to the left) consistently and significantly overshot their targets in active trials. There were no changes in accuracy of active turning to the right or passive turning in either direction. This indicates that adaptation to the rotating treadmill is expressed even when subjects have a conscious intent to turn. The positive after-effects of the rotating treadmill add to the intended active turning in the direction of the after-effect but there is no decrement in active turning in the direction opposite the after-effect.

Adult↗
PubMed2004

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.

Adaptation, Physiological↗
PubMed2004

Effects of cadence on the acquisition and expression of podokinetic after-rotation.

Podokinetic after-rotation (PKAR) occurs as blindfolded subjects inadvertently rotate when asked to step in-place following stepping in-place on a rotating surface. We examined the effects of using different cadences on PKAR to test the following hypothesis: the position signal indicating the amount of rotation between the trunk and the feet during each stance period of treadmill stimulation is used to determine the amount of rotation between the trunk and the feet that is expressed during each stance period of PKAR. Based on this hypothesis, we predicted that use of different cadences during treadmill stimulation would alter PKAR velocity because use of different cadences alters stance duration, thus changing the amount of limb rotation under the trunk during each stance phase of stimulation. We also predicted that use of different cadences during PKAR would alter PKAR velocity because the more steps that are taken in a given time the higher the velocity of PKAR given that the same rotation between trunk and feet occurs during each stance period. Use of different cadences during treadmill stimulation did not alter PKAR velocity, suggesting that PKAR velocity is not determined based upon a position signal regarding the relative rotation between the trunk and feet during stimulation. Use of different cadences during PKAR resulted in lower and higher velocities, respectively, than using a medium cadence. Based on these results, we now hypothesize that the PK system likely uses a velocity or acceleration signal present during stimulation to recalibrate the amount of relative rotation between the trunk and limbs that is expressed with each step during PKAR.

Adult↗
PubMed2002

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.

Adaptation, Psychological↗
PubMed2002

Podokinetic after-rotation following unilateral and bilateral podokinetic stimulation.

Previous studies demonstrated an aftereffect of walking on a rotating treadmill, involving inadvertent circular navigation with eyes closed [podokinetic after-rotation (PKAR)]. We compared PKAR following unilateral and bilateral podokinetic (PK) stimulation to determine whether the left and right legs could be independently adapted. Each subject performed two sessions of PK stimulation, stepping in place with one foot on either side of the axis of a rotating disk. Subjects experienced bilateral stimulation (i.e., both left and right feet stepped on the rotating disk) in one session and unilateral stimulation (i.e., the left foot stepped on the rotating disk and the right foot stepped on a stationary surface) in the other. Following stimulation, we recorded foot lift-off and touchdown times and pelvic angular velocity while subjects stepped in place on a stationary surface. PKAR velocity following unilateral stimulation was lower than that following bilateral stimulation. Following bilateral stimulation, pelvic rotation was in the counterclockwise (CCW) direction during single-limb support on both the left and right sides. Immediately following left unilateral stimulation, subjects demonstrated CCW pelvic rotation during left single-limb support but not during right single-limb support. Across the first 13 strides, the difference between left and right sides diminished; pelvic angular velocity was then CCW during single-limb support on both sides. This suggests that both the adapted left and the unadapted right limb influenced the final PKAR response with information from the two limbs being integrated over the first few strides.

Adaptation, Physiological↗
PubMed2002

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.

Adaptation, Physiological↗