AGE AND BIMANUAL MOTOR COORDINATION: IMPLICATIONS FOR CALLOSAL DECLINES Ashley S. Bangert 1, Christine M. Walsh 2, Anna E. Boonin 1, 3, Emily Anderson 3, Daniel J. Goble 3, Patricia A. Reuter-Lorenz 1, 2, & Rachael D. Seidler 1, 2, 3, 4 1 Department of Psychology, 2 Neuroscience Program, 3 Division of Kinesiology, 4 Institute of Gerontology, University of Michigan Introduction  Older adults show impairments in continuous bimanual coordination (Swinnen et al., 1998), but it remains unclear whether the same is true for discrete bimanual tasks.  Studies of callosotomy patients and normal controls suggest that coordination of discrete, repetitive movements relies on subcortical mechanisms, while continuous movements are mediated cortically via the corpus callosum (Ivry & Hazeltine, 1999; Kennerley et al., 2002).  Temporal consistency on unimanual and bimanual discrete and continuous tasks is correlated within, but not between, task types (Roberston et al. 1999; Zelaznik et al., 2002). This provides additional support for task-dependent coordination mechanisms.  We exploited the discrete/continuous distinction to determine whether aging has pervasive global or differential effects on cortical versus subcortical substrates of motor control. Methods  12 older (65-80 years) and 13 younger (18-30 years) right-handed adults participated in tasks administered over 2 testing sessions.  Discrete Repetitive Tapping: Tapped with index fingers in time to visually-presented targets Responses made on a button box situated under each hand 3 Speeds (ITI = 800 ms, 1000 ms, 1200 ms) 5 Conditions Right Hand Only Left Hand Only Simultaneous (0 ms lag between hands) Right leads Left (180 ms lag between hands) Left leads Right (180 ms lag between hands) Dependent Variables All conditions = Within-Hand Standard Deviation Bimanual Conditions = Between-Hand Standard Deviation  Continuous Circle Drawing: Participants drew circles with joysticks 2 Speeds - preferred & maximum rate 4 Conditions – See Figure 1 2 Mirror-Symmetric (symmetric with regard to the body midline) 2 Parallel (asymmetric with regard to the body midline) Dependent Variable = Average Absolute Time Lag Between Hands Figure 2. Bimanual tapping results. The complex condition encompasses the two hand lag tapping conditions: Right leads Left, and Left leads Right. Older adults did not show significant intertask correlations between easy conditions, countering the idea of global declines in the bimanual coordination mechanisms subserving these tasks. Older adults displayed higher overall intertask correlations than younger adults, especially for hard tasks. This may indicate an increased reliance on common resources when dealing with increasing difficulty on otherwise diverse motor tasks. Acknowledgements This work was supported by NIA AG18286 (PARL), UM OAIC and OVPR (RDS) grants. Results – Tapping References  Ivry, R. B. & Hazeltine, E. (1999). Human Movement Science, 18,  Kennerley et al. (2002). Nature Neuroscience, 5,  Robertson et al. (1999). Journal of Experimental Psychology: Human Perception and Performance, 25,  Swinnen et al. (1998). Cognitive Neuropsychology, 15,  Zelaznik et al. (2002). Journal of Experimental Psychology: Human Perception and Performance, 28, Tapping Condition x Age: F(1,23) = 6.91, p <.05 Main Effect of Tapping Condition: F(1,23) = , p <.001 Main Effect of Age: F(1,23) = 4.68, p <.05 Tapping Condition x Tapping Speed: F(2,46) = 13.80, p <.001 Main Effect of Speed: F(1,46) = 30.83, p <.001 Figure 3. Although the finding did not reach significance within either age group, there was a trend across all subjects for smaller within-hand SD for simultaneous in comparison to single hand tapping (p <.07). It is important to note that callosotomy patients also show this bimanual advantage with tapping. Figure 5. Circling results when lag between hands is normalized by circling speed. This analysis was used to correct for slower overall circling evidenced by older adults. Main Effect of Circling Condition: F(1,23) = 45.75, p <.001 L is CCW, R is CW L is CW, R is CCW Mirror -Symmetric Both Hands CCW Both Hands CW Parallel Figure 1. A schematic of the 4 bimanual circling conditions. Predictions  If aging has global effects on subcortical and cortical substrates; Temporal consistency between hands for both discrete and continuous tasks should be worse for older adults over all levels of task complexity. Performance measures for older adults on all tasks should positively correlate with each other.  If aging has differential effects: Older adults’ performance should dissociate on the two tasks. Tap Simultaneous Tap Complex Circle Symmetric Table 2. Correlations Between Tasks for Older Adults Tap Circle Circle Complex Symmetric Parallel.711*** *.584**.637**.464* *Significant at the p <.10 level (one-tailed) **Significant at the p <.05 level (one-tailed) ***Significant at the p <.01 level (one-tailed) Results – Circle Drawing Complex Conditions Figure 4. Circling results. The mirror-symmetric condition included tasks where the hands circled in opposite directions; the parallel condition included tasks where the hands circled in the same direction. Circling Condition x Age Interaction: F(1,23) = 5.13, p <.05 Main Effect of Circling Condition: F(1,23) = 75.81, p <.001 Marginal Main Effect of Age: F(1,23) = 3.11, p =.091 Discussion  Although minimal on easier tasks, age differences become evident with increasing temporal and spatial coordination demands.  Intermanual synchrony appears harder to maintain at slower speeds for all participants, especially older adults.  Motor slowing in older adults may, therefore, be a source of compromised intermanual temporal synchrony.  There are no indications of specific declines on conditions that (putatively) require the corpus callosum.  The preserved benefit for bimanual relative to unimanual tapping and age- equivalent circling lags (normalized data) argue against global motor declines with age. Tap Simultaneous Tap Complex Circle Symmetric Table 1. Correlations Between Tasks for Younger Adults Tap Circle Circle Complex Symmetric Parallel Results – Task Correlations Older adults had a slower circling rate than young adults t(1, 15.6) = 1.77, p <.05, one-tailed. Lag was positively correlated with circling rate for both age groups, p <.001.