APOE and Alzheimer disease: Learning ability was not correlated with the number of parvalbumin-positive interneurons in the hilus for either male or female apoE-KI mice at 16 months of age (females, R2 = 0.0235 for apoE3-KI mice, R2 = 0.2361 for apoE4-KI mice, p>0.05; males, R2 = 0.3127 for apoE3-KI mice, R2 = 0.1557 for apoE4-KI mice, p>0.05), suggesting that these changes occur independently of AD-related pathogenesis.