Foundations

Sensory Integration

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In this module

  1. Sensory weighting and re-weightingFoundation · Trainee · Clinician
  2. Sensory conflict resolutionFoundation · Trainee · Clinician
  3. Ankle and hip postural strategiesFoundation · Trainee · Clinician
  4. Limits of stabilityTrainee · Clinician
  5. Predictive vs reactive postural controlClinician

Sensory weighting and re-weighting

Sensory weighting is the term for how much the brain trusts each balance input. In a healthy person standing on solid ground in good light, vision and somatosensation are weighted heavily because they're cheap and reliable; vestibular input is weighted more lightly, held in reserve.

When the environment changes — the lights go out, the ground softens — the brain re-weights within seconds. Vision gets discounted in the dark; somatosensation gets discounted on a foam pad; vestibular input picks up the slack. Healthy adults do this re-weighting unconsciously and continuously.

When the re-weighting fails — either because the alternative input is damaged (vestibular loss) or because the central re-weighting machinery is broken (some central disorders, or PPPD's hyper-reliance on vision) — the patient cannot smoothly adapt to changing sensory environments. They feel unsteady in dim corridors, busy supermarkets, or on uneven ground.

Sensory conflict resolution

When two sensory streams disagree — say, vision tells the brain the body is upright while vestibular input says it's tilting — the brain must resolve the conflict. Healthy resolution involves down-weighting the stream judged less reliable in the current context.

On condition 3 of the SOT, the surround sways with the patient. Vision now reports that the body is stationary (because the visual world moves in lock-step), while vestibular and somatosensory input correctly report movement. A healthy subject down-weights vision and stays upright. A subject with visual preference — hyper-reliance on vision — cannot down-weight, and sways in line with the moving surround.

This is the physiological basis of the PREF ratio derived from the SOT. Elevated PREF identifies patients whose central re-weighting is biased toward vision, a pattern characteristic of PPPD and of some recovering peripheral vestibulopathies.

Ankle and hip postural strategies

Two postural strategies handle perturbations. The ankle strategy uses ankle torque to keep the centre of mass over the base of support; the body rotates about the ankle joints, with the rest of the body kept relatively straight. It works well for small, slow perturbations on a stable surface.

The hip strategy recruits hip torque, bending the body at the waist. The trunk and arms move while the legs stay relatively still. It is recruited for larger or faster perturbations, or when the support surface is too narrow or too soft for ankle torque to suffice.

Healthy adults transition smoothly between the two. Loss of the ankle strategy — for example after lower-leg amputation or severe peripheral neuropathy — forces reliance on hip strategy and shows up as elevated sway on CDP. Loss of the hip strategy is rarer but can occur with hip pathology or with central lesions affecting trunk control.

Limits of stability

The limit of stability (LOS) is the boundary of the cone of sway within which the body can remain upright without taking a step. For a typical adult on a stable surface, this is about 12.5 degrees anteriorly, 6.5 degrees posteriorly, and 8 degrees laterally — though the anterior–posterior asymmetry is what CDP scoring uses.

Within the LOS, the patient can correct by ankle or hip torque alone. Exceed it and a step (or a fall) becomes inevitable. CDP scoring expresses sway as a fraction of the LOS, with an equilibrium score of 100 meaning negligible sway and 0 meaning the cone was breached.

The 12.5 degree figure is an idealised anterior limit. Real-world LOS depends on the patient's height, stance width, footwear, and lower-limb strength. Most commercial systems use the 12.5 degree default, accepting some imprecision in exchange for cross-patient comparability.

Predictive vs reactive postural control

Postural control has two distinct modes. Reactive control responds to a perturbation that has already happened — the MCT measures it. Predictive control anticipates a perturbation that is about to happen, and pre-tunes muscle tone and posture accordingly.

Predictive control depends on cortical, cerebellar, and basal-ganglia circuits that learn from prior experience. Parkinson's disease classically impairs predictive control: patients can react to a push, but they don't prepare for an expected one, producing the characteristic backward fall when expecting a pull.

Standard CDP does not isolate predictive control; the test paradigm is largely reactive. Adaptive paradigms in research use repeated perturbations with predictable cues to probe predictive control specifically, but these are not yet part of routine clinical protocols.