In the Symbol Digit Substitution test, participants replace symbols with numbers using a substitution key. Strong IDP associations were found with two scores: Number of symbol digit matches made correctly and Number of symbol digit matches attempted in the time allowed (because subjects rarely make mistakes, these two scores are highly correlated, r=0.97). These scores correlated negatively with measures of water diffusivity in the corona radiata and superior thalamic radiation, and with FA in the posterior fornix (consistent with literature27, which as discussed above may reflect variations in tract thickness28). Finally, there was significant association with thalamus volume (right thalamic volume significant, left thalamic volume close to significance with r=0.10), consistent with literature29. These negative associations likely reflect lower cognitive performance with aging and pathology (increased diffusivity and atrophy).
