Hand dominance and age have interactive effects on motor cortical representations

Abstract

Older adults exhibit more bilateral motor cortical activity during unimanual task performance than young adults. Interestingly, a similar pattern is seen in young adults with reduced hand dominance. However, older adults report stronger hand dominance than young adults, making it unclear how handedness is manifested in the aging motor cortex. Here, we investigated age differences in the relationships between handedness, motor cortical organization, and interhemispheric communication speed. We hypothesized that relationships between these variables would differ for young and older adults, consistent with our recent proposal of an age-related shift in interhemispheric interactions. We mapped motor cortical representations of the right and left first dorsal interosseous muscles using transcranial magnetic stimulation (TMS) in young and older adults recruited to represent a broad range of the handedness spectrum. We also measured interhemispheric communication speed and bimanual coordination. We observed that more strongly handed older adults exhibited more ipsilateral motor activity in response to TMS; this effect was not present in young adults. Furthermore, we found opposing relationships between interhemispheric communication speed and bimanual performance in the two age groups. Thus, handedness manifests itself differently in the motor cortices of young and older adults and has interactive effects with age.

Figure 1. Ipsilateral MEP Traces.

Example ipsilateral MEP traces in both young (top row) and older (bottom row) adults with an amplitude of approximately 15 µVolts. All traces have been time locked to show 25 ms of activity prior to the onset of stimulation, with stimulation occurring at time 0. The ipsilateral MEPs are highlighted using a rectangle to distinguish them from the stimulation artifact. The peak-to-peak amplitude and latency of each MEP is also presented.

Figure 2. Relationships Between CUD and Handedness.

CUD plotted as a function of handedness measured with the Tapping Squares LI in both the young (black) and older (grey) adults. There is a significant correlation when the two age groups are combined (r₍₄₈₎ = .32, p<.05).

Figure 3. Relationship Between CUD and Bimanual Performance.

Correlation between performance on the assembly component of the Purdue pegboard and CUD for the older adults (gray) and the young adults (black). There is a significant correlation in the older adults (r₍₂₁₎ =  −.45, p<.05), though the correlation is not significant in the young adults (r₍₂₅₎ = .36, p = .07). The strength of these correlations are significantly different (z = −2.67, p<.01).

Figure 4. A. Ipsilateral MEPs and Handedness.

Total number of ipsilateral MEPs plotted as a function of degree of handedness as measured by the absolute value of the tapping squares measure for the older (gray) and young (black) adults. The Poisson regression models indicate statistically significant relationships for both age groups (older adults: chi-squared = 8.42, p<.01; young adults: chi-squared = 3.98, p<.05) though they are in opposite directions. Overlaid lines serve to highlight and differentiate the relationships. B. Contralateral MEPs and Handedness. Total number of contralateral MEPs plotted as a function of degree of handedness, as measured by the absolute value of the tapping squares measure for the older (gray) and young (black) adults. The Poisson regression models indicate a statistically significant relationship in the older adults (r₍₂₁₎ = .72, p<.001), though there is no relationship in the young adults (r₍₃₁₎ = .02, p>.8).

Figure 5. Representative MEP Distribution.

The MEP distribution of the dominant (left) hemisphere of a right-handed young adult is presented to illustrate the created distributions. The color bar indicates the average amplitude at each point (in µVolts).

Figure 6. CUD and MEP Distribution Symmetry.

Correlation between CUD and contralateral MEP distribution symmetry (# points dominant - # points non-dominant) for the older adults (gray) and the young adults (black). The correlations in both age groups are significant (young adults: r = −.53, p<.01; older adults: r = .49, p<.05), and the strength of these correlations is also significantly different (z = −3.72, p<.001).