Access Type

Open Access Dissertation

Date of Award

January 2017

Degree Type


Degree Name



Psychiatry and Behavioral Neurosciences

First Advisor

Naftali Raz

Second Advisor

Jeffrey A. Stanley


The decline of cognition with age is one of the most feared aspects of aging, while the slowing of responses, or reduced processing speed, is one of the most reliable aspects of aging. Slowing of processing has been hypothesized to affect other domains of cognition as well. Despite the well-known slowing-age relationship and central position processing speed plays in theories of cognitive aging the neurobiological mechanisms which underpin slowing is unclear. If we could identify the biology associated with processing speed we could then attempt to develop interventions to mitigate the effects of age on those variables. In turn we could test whether “preventing” or reducing the decline of processing speed helps alleviate the decline in other cognitive domains. Not only would this provide basic knowledge about aging, cognition, and their relationship but it may also have a broader societal impact.

In this project, we tested whether the amount of myelin content, indexed by MWF, in white matter tracts hypothesized to be important for performing a choice reaction time task, could explain the variance in processing speed or in change of processing speed after controlling for age effects. While MWF did not explain any additional variance in our variables of interest we did found that the geomT2-IEW, a measure thought to be related to axonal density, in the genu was negatively associated with change in processing speed. These results suggest that axons maybe an important structure supporting processing speed and that future investigations should look at both myelin and axonal changes as potential substrates sub serving processing speed.