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Biomedical subjects

G Melvill Jones

Publications and source records attributed to G Melvill Jones.

14 recordsLinked to original sources
PubMed2005

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 (tau=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 degrees /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.

Biomechanical Phenomena↗
PubMed2002

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.

Adult↗
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↗
PubMed1998

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 degrees/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 degrees/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 1 st 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.

Adaptation, Physiological↗
PubMed1995

Adaptive plasticity in the control of locomotor trajectory.

Eight human subjects were exposed to 2 h of walking on the perimeter of a horizontally rotating disc with the body remaining still in space. After adaptation to this experience subjects were blindfolded and asked to walk straight ahead on firm ground. When doing so all subjects generated curved walking trajectories of radii ranging from 65 to 200 inches and angular velocities from 7 to 20 deg/s. Subsequent trials over the next half hour revealed retained, but decreasing, trajectory curvature. Angular velocities associated with these trajectories were well above vestibular sensory threshold, yet all subjects consistently perceived themselves as walking straight ahead. The blindfolded subjects were also asked to propel themselves in a straight line in a wheel chair. Post-adaptation wheel chair trajectories showed no change from those before adaptation. Hence we infer that it was the relation between somatosensory/motor elements of gait and the perception of trunk rotation that had been remodelled during walking on the turning disc. This novel form of adaptive plasticity presumably serves to maintain optimal values of central neural parameters that control the trajectory of locomotion. The findings may have significant implications for the diagnosis and rehabilitation of locomotor and vestibular disorders.

Adaptation, Physiological↗
PubMed1991

Adaptive plasticity in the gaze stabilizing synergy of slow and saccadic eye movements.

When a normal human subject is briefly turned in total darkness while trying to "look" at a spatially fixed target, the vestibulo-ocular reflex (VOR) produces slow-phase compensatory eye movements tending to hold the eyes on target. However, slow-phase compensation per se is generally inadequate in these circumstances. Nevertheless it has recently been found, that even in the dark, this inadequacy tends to be corrected by supplementary saccades usually acting in the compensatory direction. The present study further investigates this phenomenon by measuring the respective contributions of saccadic, slow-phase and overall net compensation in 9 subjects tested before and after 30% adaptive attenuation of VOR slow-phase gain. In each test series, subjects attempted to stabilize their gaze on a previously seen target during each of 40 brief (approximately 0.5 s) whole body rotations (40 degrees/s, 20 degrees amp) conducted in complete darkness. The adaptive experience comprised 2 h of full-field visual suppression of the VOR during sinusoidal rotation of subject and surround at 1/6 Hz and 40 degrees/s velocity amplitude. Before adaptation, the cumulative slow-phase and cumulative saccadic components produced on average 78% and 14% respectively of the ideal (100%) compensation, thus yielding an overall net compensation which was 92% of the desired value. After adaptation, the corresponding values in the same population were 53%, 18% and 71% respectively. Thus after adaptation, the combined saccadic-slow-phase response brought the final gaze position to a point in space that was systematically shifted in the direction of head rotation (i.e. undercompensation). Subjects re-exposed to 30 min of normal visual-vestibular interaction displayed a variety of recovery patterns using different combinations of slow and saccadic eye movements. However, there was a consistent "synergistic" tendency for saccadic eye movements to improve slow-phase performance, regardless of the subject's adaptive state. In one subject, compensatory saccadic eye movements corrected a consistent directional asymmetry in the slow-phase response. It is suggested that a conscious vestibular percept of self-rotation might underlie the combined saccadic-slow-phase response, and that the net under performance after adaptation might reflect attenuation of this percept relative to the actual rotational stimulus.

Acclimatization↗
PubMed1991

Adaptive modification of vestibularly perceived rotation.

Results from Bloomberg et al. (1991) led to the hypothesis that saccades which accompany the dark-tested vestibulo-ocular reflex (VOR) tend to move the eyes towards a vestibularly derived percept of an intended oculomotor goal: also that this is so even when that percept has been adaptively modified by suitably prolonged visual-vestibular conflict. The present experiments investigate these implications by comparing the combined VOR + saccade performance with a presumed "motor readout" of the normal and adaptively modified vestibular percept. The methods employed were similar to those of an earlier study Bloomberg et al. (1988) in which it was found that after cessation of a brief passive whole body rotation in the dark, a previously seen earth-fixed target can be accurately located by saccadic eye movements based on a vestibular memory of the preceding head rotation; the so-called "Vestibular Memory-Contingent Saccade" (VMCS) paradigm. The results showed that the vestibular perceptual response, as measured after rotation by means of the VMCS paradigm, was on average indistinguishable from the combined VOR + saccade response measured during rotation. Furthermore, this was so in both the normal and adapted states. We conclude that these findings substantiate the above hypothesis. The results incidentally reaffirm the adaptive modifiability of vestibular perception, emphasing the need for active maintenance of its proper calibration according to behavioural context.

Acclimatization↗
PubMed1988

Changing patterns of eye-head coordination during 6 h of optically reversed vision.

1) This study investigates the early development of adaptive changes in oculomotor function associated with coordinated eye-head tracking of the optically reversed image of an earth-fixed target seen through horizontally reversing dove prism goggles attached to the skull. 2) Two tasks comprised a) fixation of a single target during head rotation which causes the seen target's image to move in the direction of head motion by an amount exactly equal to the head movement itself (the 1-Target task), and b) change of gaze onto a displaced target with head free to move (2-Target task). 3) The 1-Target task requires the eyes to move in a direction opposite to that of the normal vestibulo-ocular reflex (VOR). The 2-Target task is identical, except that reorientation onto the new target calls for an initial saccadic eye movement in a direction opposite to that of the ensuing head movement, which is contrary to the normal pattern of eye-head coordination during gaze shifts. 4) Eye (EOG) and head (potentiometer) movements were continuously recorded (0-250 Hz) in an apparatus which permitted sudden, unexpected, electromagnetic braking of the head movement, either just before or during the intended manoeuvre. 5) Early adaptive strategies employed reduction of VOR gain, rearrangement of timing, amplitude and shape of "catch-up" saccades and the introduction of centrally programmed eye movements uncovered by the braking manoeuvres. 6) All of these phenomena were detectable in an initial series of 60 trials, in which the total exposure to visual-vestibular conflict was less than 30 s. They became more systematized and more marked after 6 h of active reversed vision experience. 7) Specifically, mean VOR gain, measured within the first 80 ms of head movement (deemed free of visuomotor influence), became markedly attenuated (25% in the first test series; 66% after 6 h of active vision-reversed exercise). In addition (not included in the above percentages) there were numerous occasions of complete absence of measurable VOR during head rotation, in both the first and final test series. 8) In the 1-Target task, the latency of the first "catch-up" saccade (re onset of head movement) tended to offset residual VOR by becoming shortened to the point of synchrony with head movement onset. This saccade (not present in control tests) continued to occur on those occasions when the head was unpredictably prevented from moving, and when head movements were made in the dark.(ABSTRACT TRUNCATED AT 400 WORDS)

Adaptation, Physiological↗
PubMed1982

Oculomotor response to rapid head oscillation (0.5-5.0 Hz) after prolonged adaptation to vision-reversal. "Simple" and "complex" effects.

This study examined long-term (up to 27 days) effects of maintained vision reversal on (i) smooth visual tracking with head still, (ii) oculomotor response to actively, generated head oscillation and (iii) "spontaneous" saccades. Dove prism goggles produced horizontal, but not vertical (sagittal plane), vision reversal. Eye movements were recorded by EOG; head movements by an electro-magnetic search coil. Both visual tracking and saccade dynamics remained unchanged throughout. In contrast, both the ocular response to active head oscillations (goggles off and subject looking at a stationary target) and associated retinal image blur showed substantial and retained adaptive changes, akin to those previously found in the vestibulo-ocular reflex as tested in darkness at 0.17 Hz. However, several addition unexpected results emerged. First, in the fully adapted state smooth eye movements tended to be of reversed phase in the range 0.5-1.0 Hz (in spite of normal vision during tests), but of normal phase from about 2 Hz and above (in spite of negligible visual tracking in this upper range). Second, after permanent removal of the inverting goggles, this peculiar frequency response of the fully adapted state quickly (36 h) reverted to a dynamically simpler condition manifest as retained (2-3 weeks) attenuation of gain (eye vel./head vel.) which, as in control conditions, was monotonically related to frequency. From these two findings it is inferred that the fully adapted state may have comprised two separate components: (i) A "simple element of monotonic and long-lasting gain attenuation and (ii) a "complex", frequency labile, element which could be quickly rejected. Dynamic characteristics of the putative "complex" element were estimated by vectorial subtraction of the "simple" one from that of the fully adapted condition. The outcome suggests that the inferred "complex" condition might represent a predictive element. Two further findings are reported: (i) Substantially different vectors of the adapted response were obtained with normal and reversed vision at 3.0 Hz head oscillation, indicating a novel visual tracking. (ii) During head oscillation in the vesicle sagittal plane (in which vision was not reversed) there was never any image blur, indicating high geometric specificity in the adaptive process.

Acoustic Stimulation↗
PubMed1981

Adaptability of the vestibulo-ocular reflex to vision reversal in strobe reared cats.

Optical reversal of vision brings about adaptive changes in the vestibulo-ocular reflex (VOR) tending to reduce retinal image slip during head movement. The present experiments investigated this form of adaptation in cats whose complement of direction sensitive central visual cells had been substantially reduced by rearing in 8 Hz stroboscopic light. Horizontal vision reversal was produced by dove prisms carried in a skull-mounted mask. A scleral eye coil was used to measure horizontal eye movements. VOR gain and phase were measured in the dark during sinusoidal rotation using test stimuli of 1/8 Hz and 5- or 20 degrees/sec velocity amplitude. Initially, strobe reared cats produced virtually normal VOR in the dark, except for slight but significant exaggeration of the normal phase advancement to be expected at 1/8 Hz. Addition of their familiar strobe illumination produced almost perfect oculomotor compensation. Maintained vision reversal in both strobe and normal illumination produced similar patterns of adaptive change in normal and strobe reared subjects, i.e. all animals exhibited an initial fast, and subsequent much slower, stage of gain attenuation, with similar changes in phase. Thus, strobe rearing did not prevent the development of an essentially normal VOR, nor did it interfere significantly with the ability to adapt in response to vision reversal. Since strobe rearing depletes direction selective visual movement detectors in the cortex and superior colliculi, it is inferred that signals responsible for activating the adaptive process are probably carried mainly in the accessory optic, rather than cortical and collicular, visual system.

Animals↗
PubMed1981

Eye oscillations in strobe reared cats.

Cats reared from birth in stroboscopic illumination develop abnormal spontaneous eye oscillations of low amplitude. The present experiments were undertaken to define these eye movements as recorded in the dark, in stroboscopic light of various frequencies, after exposure to normal light and after attenuation of the vestibulo-ocular reflex (VOR) gain by optical reversal of vision. The interaction of spontaneous eye oscillations with voluntary saccadic eye movements, and optokinetic tracking (OKN), were also studied. Two cats, reared from birth to 18 months in 8 Hz strobe light, and one normally reared control animal, were used. Horizontal movement of the right eye was measured by the scleral eye coil method. The frequency content of eye movement records was determined by power spectral analysis. VOR gain was estimated in the dark, by rotating the animals sinusoidally at 1/8 Hz and 5 degrees/sec velocity amplitude. In the dark, both strobe reared cats had abnormal spontaneous eye oscillations at a frequency close to 8 Hz, with peak-to-peak amplitudes of 0.5--1.0 degrees. These abnormal eye movements did not interfere with, nor were they abolished by, normal oculomotor activity. The introduction of strobe light modified the spontaneous eye movements by entraining the oscillations at a given 'forcing' frequency, and by producing a number of harmonics or sub-harmonics. In one of the strobe reared animals, the effect of normal light was to reduce the characteristic 'dark' value of 9 Hz, to a new maintained 'light' value of 2.7 Hz. Adaptive attenuation of the VOR gain caused the abolition of regular spontaneous eye oscillations in the dark; nevertheless, transient oscillations to single strobe flashes could still be elicited in the VOR adapted condition. The results are interpreted as representing an organised attempt by the developing oculomotor system to attain the goal of stable visual perception in a new visual environment.

Animals↗