To determine the effect of citicoline supplementation on cortical structure and function in healthy middle-aged adults. Test 1: The effect of citicoline (500 mg/d) compared with placebo on event-related anterior cingulate cortex (ACC) activation during sustained attention by fMRI. Test 2: The effect of citicoline (500 mg/d) compared with placebo on reaction time and increasing accuracy on the sustained attention task (CPT-IP). Test 3: The effect of citicoline compared with placebo on phosphodiester (PDE) and b-NTP levels in the ACC by 31P-MRS. Test 4: ACC activation and CPT-IP performance (fMRI) and ACC PDE levels (31P-MRS) following high-dose citicoline (500 mg/d) compared with placebo at Week 3. Test 5: The effect of low-dose citicoline (250 mg/d) compared with placebo on an ACC activation and CPT-IP performance (fMRI) and ACC PDE levels (31P-MRS). Test 6: Among all subjects (n=60), the relationship between baseline red blood cell (RBC) omega-3 fatty acid (EPA+DHA) levels and ACC activation, ACC PDE levels, or the effects of citicoline.