Inspection time is a psychophysical measure of the efficiency of early visual processing. In meta-analyses inspection time correlates moderately highly with higher-level cognitive ability test scores, especially with those tests that are known to be sensitive to ageing [45]. It is sensitive to normal human ageing and there is evidence that it mediates some of the effect of age on scores on psychometric tests of higher cognitive functions [46]. It is slower in people with mild cognitive impairment [47] and dementia [48]. In genetic covariance analyses inspection time has some shared, additive genetic effects with higher mental test scores [49]. Inspection time will be used in the Lothian Birth Cohort 1936 as a sensitive, early indicator of more general cognitive decrements. Reaction time has also been conceived as a lower-level indicator of processing efficiency that can mediate effects of age on higher cognitive functions [42]. Reaction time has the additional advantage of providing measures of both speed and variability; both are sensitive to ageing from the 30s onwards [50]. In the present study the research team will be able to
