1. Normal dogs were trained to stand quietly upon a movable platform. The animals' postural adjustments in response to sinusoidal motion of the platform were assessed in both the normal and the blindfolded condition. The induced postural adjustments were described by the Fourier coefficients for the fundamental of the input together with the coefficients which were integral multiples of the fundamental.2. All of the output variables, including motion at the hind limb joints and of the body, showed modifications when the dogs were blindfolded. The body motion increased in amplitude, as measured by the fundamental Fourier coefficient, and further lagged the input motion. Motion at the knee and phalangeal joints also increased in amplitude and the timing of these movements shifted from the normal in characteristic patterns for different frequencies of input motion.3. The modifications of the postural response due to blindfolding were most pronounced at the lower input frequencies: 0·2-1·0 Hz. At 1·4 Hz or above the fundamental coefficients of most of the output measures resembled those of the normal condition. Even at the higher input frequencies, however, the postural response of the blindfolded dog was more distorted than that of the normal dog.4. It is hypothesized that relatively slow movement of the visual surround provides the dog with information which is essential to normal operations of the postural control system.
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