APOE and Alzheimer disease: However, other functional MRI studies using blood-oxygenation-level-dependent (BOLD) contrast have reported similar BOLD activity in both ɛ4 and ɛ2 carriers, despite the expectation that the high-risk ɛ4 carriers would have an opposite activation to low risk ɛ2 carriers; it is thus necessary to consider more than the functional MRI signal to determine the relationship between APOE, AD risk, and brain function (Suri et al., 2015).