Task, Environmental structure, and Illumination Influences on Posture

1
Task, Environmental structure, and Illumination
Influences on Posture Cedrick T. Bonnet1,
Claudia Carello1, Jeffrey M. Kinsella-Shaw1,2,
Deborah Bubela1,2, and M. T. Turvey1 1CESPA
2Department of Physical Therapy, University of
Connecticut, Storrs, CT, U.S.A.
INTRODUCTION
RESULTSPOSTURE
Statistical Analyses of Posture. A variety of
linear dependent measures were evaluated in 2
(environment) ? 2 (task) ? 2 organism) ANOVAs
standard deviation of fluctuation in the
anterior-posterior direction (SDAP), standard
deviation of fluctuation in the medial-lateral
direction (SDML), path length of the COP
excursion, and ellipse area traced by the COP.
Nonlinear measures included detrended fluctuation
analysis DFAAP and DFAML and, from the recurrence
quantification analysis, the percentage of
recurrent points (RECAP and RECML) and the
longest string of recurrent points (MAXAP and
MAXML)
Table 2. Statistical Evaluation of Posture
Measures in Experiment 1.
Table 2 shows means (and standard deviations)
from E1. It also shows the results of the planned
t-tests, either independent t-tests (between
young and old) for both blank target and text
target conditions, or the paired t-tests (between
blank target and text target conditions) for both
old and young adults. ( indicates a significant
difference between age groups indicates a
significant difference between both tasks for a
particular group (p lt .05).
Will the goal-directedness of the supra-postural
visual search task interact with the influences
of passive visual factors and age (cf. Riccio
Stoffregen, 1988)?
METHOD
Postural fluctuation, E1. ANOVAs revealed
significant main effects of age and task as well
as their interaction, p lt .05. Generally, older
adults fluctuated more than younger adults. The
task effect depended on age, with the particular
Task ? Age interaction differing for different
measures (Figure 2).
(b) E1 2 (Background) ? 2 (Task)
(c) E2 2 (Illumination) ? 2 (Task)
(a) Posture for E1 and E2
Figure 1. Participants stood barefoot with left
and right feet on separate force platforms (a).
In E1, a text or blank target was mounted on the
front of a dowel array or a large white board
(b). In E2, the text or blank target was mounted
on the front of a large white board in bright or
dim illumination.
Figure 2. The search task reduced SDAP for
younger adults (a) but increased SDMLfor older
adults (b). Two nonlinear measures, REC (c) and
MAXLINE (d), paralleled each other an increase
for older adults and a decrease for younger
adults.
Organism 12 younger adults (college students) and
12 older adults (65 years of age) were enclosed
by three white sheet walls (Figure 1 a).
Environment E1 Background. Three rows of nine
rods (92 cm high) comprised the grating array
(Figure 1b, left) white poster board provided a
blank background (Figure 1b, right). E2
Illumination. High illumination (440 lux) was at
a level comparable to normal room lighting
(Figure 1b, left) low illumination (3 lux)
approximated the functional limit of recognition
vision (Figure 1c, right). Task A text or blank
target (visual angle 13 ? 14o) was mounted on
the front of the grating or white board. For the
visual search task, participants reported how
many times a specified letter appeared for the
visual fixation task, participants simply fixated
the smaller blank card (cf. Stoffregen,
Pagulayan, Bardy, Hettinger, 2000). A trial
lasted 35 s faster readers should get further in
the passage (and, consequently, a higher absolute
letter score).
Postural fluctuation, E2. ANOVAs revealed
significant main effects of age, task, and
illumination, p lt .05, as well as interactions of
task with age (Figure 3 a and b) and task with
illumination interaction (Figure 3c), p lt .05.
For the main effect of illumination, SDAP and
MAXML were lower with high illumination than low
illumination. The influence of task depended on
measure The search task decreased SDAP and COP
path length but increased DFAML.
Figure 3. For younger adults, the search task
reduced ellipse area (a) and MAXAP (b). The
search task decreased DFAAP under high
illumination only.
RESULTSVISUAL SEARCH
SUMMARY AND CONCLUSION
Both speed (number of letters found in a 35 s
trial) and accuracy (proportion of letters found
given how much a participant read) were evaluated
(Table 1). Although the number of letters counted
was significantly higher for young people, t(22)
gt -2.33, p lt .05, indicating faster reading, the
mean proportion correct did not differ for the
two age groups in either experiment, t(22) lt 1.
Overall, older adults fluctuated more than
younger adults. The structure of the environment
surrounding the target did not influence postural
fluctuation (E1) for either age group. The
influence of level of illumination (E2) was
age-specific Older adults fluctuated more when
the level of illumination was low (replicating
Kinsella-Shaw, Harrison, Colon-Semenza, Turvey,
2006). The supra-postural visual search task was
the most important constraint in reducing
postural fluctuation in younger adults (Prado,
Duarte, Stoffregen, 2007 Stoffregen et al.,
2000). Given that this was not the case for older
adults, it suggests that they may engage in
different postural behaviors than younger adults.
This research was supported by a grant from the
Provost of the University of Connecticut to the
Collaboratory of Rehabilitation Research
REFERENCES
Campbell, A. J., Borrie, M. J., Spears, G. F.
(1989). Risk factors for falls in a
community-based prospective study of people 70
years and older. Journal of Gerontology, 44,
M112-M117. Kinsella-Shaw, J. M., Harrison, S.
J., Colon-Semenza, C., Turvey, M. T. (2006).
Effects of visual environment on quiet standing
by young and old adults. Journal of Motor
Behavior, 38, 251-264. Prado, J. M., Duarte, M.,
Stoffregen, T. A. (2006). Postural sway during
dual tasks in young and elderly adults.
Riccio, G. E., Stoffregen, T. A. (1988).
Affordances as constraints on the control of
stance. Human Movement Science, 7,
265-300. Slobounov, S. M. Newell, K. M. (1994).
Postural dynamics as a function of skill level
and task constraints. Gait and Posture, 2,
85-93. Stoffregen, T. A., Pagulayan, R. J.,
Bardy, B. G., Hettinger, L. J. (2000).
Modulating postural control to facilitate visual
performance. Human Movement Science, 19, 203-230.