1. letter search task Task, Environmental structure, and Illumination Influences on Posture Cedrick T. Bonnet 1, Claudia Carello 1, Jeffrey M. Kinsella-Shaw 1,2, Deborah Bubela 1,2, and M. T. Turvey 1 1 CESPA 2 Department of Physical Therapy, University of Connecticut, Storrs, CT, U.S.A. INTRODUCTION The Problem: Old fall more than young Relationships between postural fluctuation and incidence of fall have been found (Campbell, Borrie, & Spears, 1989). 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)? Environment Task Organism Postural activity can be understood in the context of three sources of constraint: organism, task, and environment (e.g., Slobounov & Newell, 1994). The Context Experiment 1 (E1): grating array vs. blank background Experiment 2 (E2): low vs. high level of illumination 440 lux 3 lux Environment Task Organism Two Experiments Young vs. Old vs. Visual Search (with a text target) Visual Fixation (with a blank target) METHOD 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  14 o ) 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). (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. RESULTS—VISUAL SEARCH 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) > -2.33, p <.05, indicating faster reading, the mean proportion correct did not differ for the two age groups in either experiment, t(22) < 1. Table 1. Visual Search Task Mean (with SD) for Reading Speed and Accuracy Old AdultsYoung Adults E1speed 18.30 (3.33)21.92 (4.21) accuracy 34.38% (22%) 32.29% (33%) E2speed 11.74 (2.52) 14.70 (2.64) accuracy 22.92% (23.13%) 22.92% (23.74%) RESULTS—POSTURE 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 <.05). 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 (SD AP ), standard deviation of fluctuation in the medial-lateral direction (SD ML ), path length of the COP excursion, and ellipse area traced by the COP. Nonlinear measures included detrended fluctuation analysis DFA AP and DFA ML and, from the recurrence quantification analysis, the percentage of recurrent points (%REC AP and %REC ML ) and the longest string of recurrent points (MAX AP and MAX ML ) SD AP SD ML Path length Ellipse areaDFA AP DFA ML % REC AP % REC ML MAX AP MAX ML Age Old 0.44* (0.14) 0.32* (0.21) 40.39* (12.98) 1.69* (1.33) 1.51 (0.10) 1.46 (0.13) 3.02 (3.03) 3.96 (4.40) 830.60 (566.55) 909.21 (664.26) Young 0.33* (0.15) 0.18* (0.10) 31.48* (6.66) 0.78* (0.74) 1.51 (0.08) 1.36 (0.13) 3.28 (2.49) 2.90 (2.83) 898.49 (581.28) 549.29 (530.85) Task Old 0.42* (0.17) 0.21* (0.12) 36.02 (8.42) 1.12 (1.02) 1.51 (0.09) 1.39* (0.14) 3.03 (2.17) 2.57* (2.61) 854.56 (508.80) 616.98* (671.83) Young 0.36* (0.13) 0.29* (0.22) 35.85 (13.51) 1.35 (1.30) 1.50 (0.09) 1.43* (0.13) 3.27 (3.27) 4.30* (4.41) 874.53 (634.13) 841.53* (558.65) Blank Old 0.45 (0.16) 0.25+ (0.14) 39.38* (8.78) 1.34 (1.17) 1.51 (0.10) 1.43+ (0.14) 2.35 (1.61) 2.80 (2.64) 711.5 (466.7) 795.6+ (764.9) Young 0.40+ (0.17) 0.17 (0.10) 32.66*+ (6.65) 0.90 (0.82) 1.52+ (0.07) 1.34 (0.14) 3.71 (2.45) 2.34 (2.62) 997.7 (518.1) 438.3+ (520.4) Text Old 0.44* (0.11) 0.40*+ (0.25) 41.40* (16.28) 2.05* (1.42) 1.52 (0.09) 1.49*+ (0.11) 3.70 (3.90) 5.13 (5.42) 949.7 (639.1) 1022.8+ (538.3) Young 0.28*+ (0.11) 0.19* (0.10) 30.29*+ (6.61) 0.66* (0.65) 1.49+ (0.09) 1.37* (0.12) 2.85 (2.51) 3.46 (2.98) 799.3 (633.6) 660.2+ (528.6) Table 2. Statistical Evaluation of Posture Measures in Experiment 1. Postural fluctuation, E1. ANOVAs revealed significant main effects of age and task as well as their interaction, p <.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). (a ) (b ) (c ) (d ) Figure 2. The search task reduced SD AP for younger adults (a) but increased SD ML for 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. Postural fluctuation, E2. ANOVAs revealed significant main effects of age, task, and illumination, p <.05, as well as interactions of task with age (Figure 3 a and b) and task with illumination interaction (Figure 3c), p <.05. For the main effect of illumination, SD AP and MAX ML were lower with high illumination than low illumination. The influence of task depended on measure: The search task decreased SD AP and COP path length but increased DFA ML. (a) (b) (c) Figure 3. For younger adults, the search task reduced ellipse area (a) and MAX AP (b). The search task decreased DFA AP under high illumination only. SUMMARY AND CONCLUSION 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). 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