Fast visual prediction and slow optimization of preferred walking speed

Abstract

O'Connor SM, Donelan JM. Fast visual prediction and slow optimization of preferred walking speed. J Neurophysiol 107: 2549-2559, 2012. First published February 1, 2012 doi:10.1152/jn.00866.2011.-People prefer walking speeds that minimize energetic cost. This may be accomplished by directly sensing metabolic rate and adapting gait to minimize it, but only slowly due to the compounded effects of sensing delays and iterative convergence. Visual and other sensory information is available more rapidly and could help predict which gait changes reduce energetic cost, but only approximately because it relies on prior experience and an indirect means to achieve economy. We used virtual reality to manipulate visually presented speed while 10 healthy subjects freely walked on a self-paced treadmill to test whether the nervous system beneficially combines these two mechanisms. Rather than manipulating the speed of visual flow directly, we coupled it to the walking speed selected by the subject and then manipulated the ratio between these two speeds. We then quantified the dynamics of walking speed adjustments in response to perturbations of the visual speed. For step changes in visual speed, subjects responded with rapid speed adjustments (lasting Ͻ2 s) and in a direction opposite to the perturbation and consistent with returning the visually presented speed toward their preferred walking speed, when visual speed was suddenly twice (one-half) the walking speed, subjects decreased (increased) their speed. Subjects did not maintain the new speed but instead gradually returned toward the speed preferred before the perturbation (lasting Ͼ300 s). The timing and direction of these responses strongly indicate that a rapid predictive process informed by visual feedback helps select preferred speed, perhaps to complement a slower optimization process that seeks to minimize energetic cost. visual feedback; energetics; locomotion OF ALL THE POSSIBLE WAYS TO WALK, people prefer specific patterns. For example, rather than use our full range of possible walking speeds, we prefer a narrow speed range and will switch to a running gait if we have to move much faster

Fast and slow processes underlie the selection of both step frequency and walking speed

“…The specific values depend on many details, including the experimental design we used to generate the input data, the assumptions we made regarding the underlying system and the optimization process we used to identify the best-fit system parameters. We have been refining our approach over the course of our research in this area (O'Connor and Donelan, 2012;Snaterse et al, 2011;Snyder et al, 2012). Consequently, there are important differences between the various approaches that render a detailed quantitative comparison of system parameters between experiments useless.…”

“…Recent work by O'Connor and Donelan (O'Connor and Donelan, 2012) tentatively suggests that the same fast prediction and slow optimization processes may underlie the control of walking speed. The authors applied visual perturbations to subjects walking on a self-paced treadmill to convey a visual perception of suddenly walking much slower or faster than their normally preferred speed.…”

Dynamic modulation of visual and electrosensory gains for locomotor control

“…In other words, a model fitted to these data would be underconstrained. Furnishing a predictive dynamical model requires broadband stimuli such as sums of sines [23], band-limited noise [22], chirps [24] and step functions [37]. Determining how the dynamics depend on sensory salience also requires an independent set of perturbations to the quality of the sensory cues themselves.…”

Domestic cat walking parallels human constrained optimization: Optimization strategies and the comparison of normal and sensory deficient individuals

“…determine the shape of the cost surface for locomotion, but results such as reported here and for humans (Bertram, 2005;Kuo, 2001;O'Connor and Donelan, 2012;Snaterse, Ton, Kuo, & Donelan, 2011) suggest that it may have substantial influence on gait strategy and implementation. This conclusion is also supported by the observation that the speed-frequency relationship selected during step length constrained walking is substantially different from that selected during speed motivated walking (Table 1, Fig.…”

“…This results in an immediate 'fast' adaptation response, on the order of a few seconds. This is later more precisely adjusted using a 'slow' adaptive response, in the order of tens of seconds (O'Connor and Donelan, 2012;Snaterse et al, 2011). The basic patterns observed in the sensory deficit animals may indicate that, in spite of their deficit, they remain capable of implementing the task anticipation 'fast' response but have a compromised ability to 'tune' their movement pattern through the feedback dependent 'slow' response.…”