Association of retinal nerve fiber layer thinning with current and future cognitive decline: a study using optical coherence tomography by Ko, F, Muthy, Z, Gallacher, J, Sudlow, C, Rees, G, Yang, Q, Keane, P, Petzold, A, Khaw, P, Reisman, C, Strouthidis, N, Foster, P, Patel, P, UK Biobank Eye & Vision Consortium