P 198. Relationship between functional connectivity and interhemispheric inhibition in older adults

Question Brain hemisphere co-activation or selection through inhibition of the contralateral hemisphere is implicit in the hemispheric asymmetry reduction in older adults (HAROLD) model and functional connectivity (FC) and interhemispheric inhibition (IHI) have been applied to operationalize these concepts. FC and IHI have never been directly compared in older adults. FC between motor cortices is demonstrated when fMRI signal intensity is correlated between homologous motor cortices even in subjects at rest (Biswal et al., 1997). IHI is demonstrated and reflects the degree to which the hand contraction evoked from a TMS pulse over motor cortex is inhibited when proceeded by a conditioning pulse in the opposite hemisphere. IHI is also believed to be mediated by the corpus callosum and has been previously related to IHI (Wahl et al., 2007). Older adults have previously been found to have decreases in long (40ms interpulse interval) but not short (10ms) IHI and under conditions related to the contraction state of the muscle (Talelli et al., 2008). Methods Resting state FC, resting muscle IHI, and diffusion tensor imaging (DTI) were collected in 16 older adults. IHI was measured with the right motor cortex (left hand) as the target. DTI was analyzed by placing a seed voxel in the motor hand area (based on structural MRI) and performing tractography projecting to the callosum and measuring fractional anisotropy. Long IHI was correlated with FC and it was also correlated with FA. Results Older adults demonstrated a significant correlation between long, IHI and FC ( r =−0.53, p r =−0.44, p Conclusions Resting IHI is related to resting state FC and callosum integrity in older adults. Implications for the HAROLD model and stroke-related functional compensation will be discussed.