Postural demands modulate tactile perception in the lower limb in young and older adults

Abstract

Balance control requires the continuous integration of feedback signals from several sensory organs with feedforward estimates about the state of the body. Such feedback signals are important for standing upright, as shown in increased and more variable sway patterns when sensory feedback is compromised, for instance when standing with eyes closed or on unstable surfaces that make cutaneous signals from the foot less reliable. Poorer sensory processing is also considered to arise during healthy aging due to a decrease of the reliability and transmission rate of feedback signals. Here, we are interested in how processing of tactile signals from the lower leg is modulated when balance control is challenged and how this interacts with age-related sensorimotor changes. We examined tactile sensitivity on the lower leg during sitting, standing on stable ground, and standing on unstable ground (foam). We quantified the center of pressure during the two standing conditions by determining the area of a 95% confidence interval ellipse as well as the total displacement of the center of pressure. Tactile sensitivity was assessed by asking participants to detect brief vibrotactile probes of various intensities to the lower leg. As expected, postural sway increased when standing on foam than stable ground for both age groups. When postural demands were minimal (sitting), tactile sensitivity was overall poorer in older than younger adults. Tactile perception was also poorer when standing on foam than on the stable ground, for both age groups. We conclude that increased postural demands reduce reliance on tactile signals from the lower limb in both young and older adults.

Keywords: Aging; Balance control; Posture; Tactile modulation.

Fig. 1

Experimental setup and psychometric functions. From left to right in the upper panel, pictures of the tactor placement, the fixation location (red circle), the sitting, the stable standing, and the unstable standing condition. In the lower panel there are 3 exemplary psychometric functions for the three conditions from the first participant. Blue dots represent the ratio with which stimuli of different intensities were perceived. The units are arbitrary. The vertical line indicates the estimated detection threshold as the 50% level.

Fig. 2

Kinetic data preparation. The panels show the statokinesiogram of the anterior-posterior position (top-left) and the medio-lateral position (bottom-right) of the center of pressure within one trial. On the top-right is the stabilogram of that same trial. Blue lines indicate the 2 s truncated data that went into the analysis, while the green lines are the discarded rest of the trial. The red ellipse corresponds to the sway area analysis.

Fig. 3

Kinetic modulation. Comparison between postural demands and age group for (a) sway area, as indicated by the 95% Confidence Interval of the COP, and (b) sway length, as indicated by the total length of the COP. Means and single subject data for the young (circles) and older adults (diamonds) are depicted with standard error as error bars.

Fig. 4

Age effects on tactile perception during sitting. Comparison between the age groups for (a) detection thresholds, and (b) slope of the psychometric function in the baseline, sitting condition. Means and single subject data for the young (circles) and older adults (diamonds) are depicted with standard error as error bars. * p < 0.05.

Fig. 5

Effects of age and postural demands on tactile modulation. Comparison between postural demands and age group for (a) normalized detection thresholds, and (b) normalized slopes. Means and single subject data for the young (circles) and older adults (diamonds) are depicted with standard error as error bars. Horizontal dotted lines represent values at sitting (baseline).