Objective Assessment of cognitive impairment in rural areas of developing countries is complicated by illiteracy and cross‐cultural factors. A better way to estimate the usefulness of cognitive screening instruments is to evaluate their correlation with imaging biomarkers. The bicaudate index (a marker of central atrophy) correlates with cognitive performance. We assessed the relationship of the bicaudate index with the MoCA to estimate the usefulness of this test to detect individuals with cognitive decline in these regions. Methods Atahualpa residents aged ≥60 years identified during door‐to‐door surveys were evaluated with the MoCA and invited to undergo brain MRI. Using generalized linear models, we estimated whether the bicaudate index correlates with MoCA scores, after adjusting for demographics and relevant clinical and neuroimaging confounders. Results Out of 385 eligible persons, 290 (75%) were enrolled. Mean bicaudate index was 0.14 ± 0.03, and mean total MoCA score was 19 ± 5 points. Locally weighted scatterplot smoothing showed a nearly linear inverse relationship between the bicaudate index and the total MoCA score. In the fully adjusted generalized linear model, the bicaudate index was inversely associated with the total MoCA score (p < 0.001), which dropped by 5.3% (95% C.I.: 1.7%–8.8%) for every standard deviation of the bicaudate index. In addition, most domain‐specific MoCA scores were inversely associated with the bicaudate index. Conclusions The inverse relationship between the bicaudate index and the MoCA score provides evidence that the MoCA is reliable to detect structural brain damage and useful to assess cognitive performance in less educated individuals. Copyright © 2016 John Wiley & Sons, Ltd.