Reduced laterality among older adults: building on the HAROLD model
Evidence of hemispheric asymmetry reduction among older adults (HAROLD) was investigated concerning four cognitive processes. Additionally, evidence was examined for two hypotheses which attempt to explain changes consistent with the HAROLD model; the dedifferentiation hypothesis suggests these changes are merely a byproduct of cognitive decline, and the compensation hypothesis suggests these changes are functional adaptations to cognitive decline. Results for the HAROLD model failed to reject the null hypothesis of no significant difference between younger and older adults concerning hemispheric lateralization across four cognitive processes. Correlational analyses were explored and found to provide mixed support for the compensation hypothesis and evidence against the dedifferentiation hypothesis. Specifically, the correlations between measures of asymmetry showed increased separation among older adults in comparison to younger adults. This stands in contrast to dedifferentiation which predicts a general increase in correlations among measures of asymmetry, via a broad increase in bilaterality. Statistical support for compensation was found for one cognitive process in the association of lateralization with performance only for younger adults. Further support for compensation was found in the association of cognitive status with lateralization among older adults, with normal cognition older adults showing greater asymmetry reduction compared to mildly impaired cognition older adults. There was also some support for an association of lateralization with performance, significant for one of the four cognitive processes, supporting compensation. Limitations of particular concern were sample size and a potential confounding variable of sex among older adults. Further research is needed to validate these results, particularly given the potential role of sex in obscuring changes consistent with the HAROLD model among older adults. It is possible that the HAROLD model may not generalize to auditory linguistic, auditory emotional, spatial quantitative, or spatial attentive processing.