Approximately two-thirds of the population is right-eye dominant and one-third left-eye dominant;[1][4][5][6] however in a small portion of the population neither eye is dominant.[7] Dominance does appear to change depending upon direction of gaze[2][8] due to image size changes on the retinas.[9] There also appears to be a higher prevalence of left-eye dominance in those with Williams–Beuren syndrome,[10] and possibly in migraine sufferers as well.[11] Eye dominance has been categorized as "weak" or "strong";[12] highly profound cases are sometimes caused by amblyopia or strabismus.