2005). Supporting this suggestion, inspection time is partly heritable, and has a sizeable genetic correlation with higher cognitive abilities in children and in adults \cite{Edmonds_inpress, Luciano_2001, Luciano_2004}. The functional anatomy of inspection time in young adults has been described using block- and event-related fMRI study designs \cite{Deary_2001, Deary_2004a}. In the event-related fMRI study, subjects performed an inspection time task while fMRI took place. The BOLD activation–deactivation pattern was mapped in relation to a linear function of the inspection time stimulus duration. Task difficulty was associated with bilateral activation in the inferior fronto-opercular cortex, superior/medial frontal gyrus, and anterior cingulate gyrus, and bilateral deactivation in the posterior cingulate gyrus and precuneus. Therefore, in young adults, functional imaging has already provided information about brain areas which might be sufficient for task performance \cite{Deary_2004a}.

In summary, there is good evidence that retaining efficient information processing is a key aspect of successful cognitive ageing. The inspection time technique has many findings that make it an attractive procedure—biomarker or endophenotype—for further exploring the neural foundations of successful cognitive ageing. The functional anatomy of efficient information processing with the inspection time task has been studied, to date, solely in younger people. Here, for the first time, we examine whether successful lifetime cognitive ageing, based on mental test scores taken over 50 years apart, is associated with the retention or loss of those functional brain networks that support efficient information processing in younger people (cf. \cite{Deary_2004a}). There are four theoretical models that attempt to account for how the average older brain operates when it tackles the same mental material as the average younger brain, viz. compensatory reorganization, dedifferentiation, computational capacity limitation, and neural inefficiency \cite{Zarahn_2007}. The first two models make opposite predictions about the brain activation patterns of higher and lower mentally functioning older people when compared with younger people. Compensatory reorganization hypothesizes that the lower functioning older people will more closely resemble younger people, and dedifferentiation hypothesizes that it is the higher functioning older people who would have brain activation patterns more like the young \cite{Cabeza_2002, Cabeza_2004, Zarahn_2007}. The present study provides evidence relevant to deciding between these hypotheses in the context of inspection time performance.

We examined individuals within a uniquely valuable cohort whose cognitive ability was assessed at age 11 and then again in their mid-to-late 60s: the Aberdeen Birth Cohort 1936 (e.g. \cite{Deary_2004b, Whalley_2005}). Briefly, the design of the experiment was as follows. We compared two groups of non-demented 70-year-olds who, at age 11, had relatively similar general cognitive ability but who, in old age, had diverged, with one group demonstrating relatively successful, and the other unsuccessful, cognitive ageing. We examined whether, in older people with relatively successful cognitive ageing, their BOLD activation–deactivation pattern while they performed an inspection time task was more or less similar to those of healthy younger individuals than older people with relatively unsuccessful cognitive ageing.

\section{Methods}

\subsection{Participants and mental tests}

Written informed consent for the study was obtained and the study was approved by the Grampian Research Ethics Committee. Subjects in the study were surviving participants of the Scottish Mental Survey 1947 (SMS1947; \cite{SCRE_1949}). This was a nationwide exercise including almost all school children born in 1936 and attending Scottish schools on June 4th 1947 (N = 70,805). They sat a version of the Moray House Test No. 12 (MHT). This is a group-administered mental test with a time limit of 45 min, a maximum score of 76, and a range of questions including verbal, numerical and spatial reasoning, as well as other types of item. It has a very high correlation (\textasciitilde0.8) with the individually-administered Stanford–Binet mental test \cite{SCRE_1949}. Data from the SMS1947 were retained by the SCRE. With ethical approval, 508 Aberdeen residents who had taken part in the SMS1947 at age 11 were recruited into a prospective, longitudinal study of brain ageing and health from 1999. They are known as the Aberdeen Birth Cohort 1936 (ABC1936). They have undertaken a series of substantial medical and cognitive assessments \cite{Deary_2004b}. By the time the current fMRI study was conducted, members of the ABC1936 had already been assessed for the third time within older age. The three waves of testing took place around 2000, 2002, and 2004, when cohort members were about 64, 66, and 68 years old, respectively. The cognitive assessment on each of these occasions within older age included a test of non-verbal reasoning (Raven's Standard Progressive Matrices [RPM]; \cite{Raven_1977}). This is a 60-item test in which subjects examine abstract patterns arrayed as a 3 $\times$ 3 matrix. In each item the bottom, right hand component of the matrix is missing. The subject's task is to examine the rest of the matrix and, by inducing and then applying the logical rules underlying the rest of the pattern, decide which of the answer options correctly completes the pattern. The RPM loads highly on the general cognitive ability factor, making it a good indicator of general mental ability \cite{Carroll_1993}. RPM scores obtained in old age correlate moderately and significantly with the MHT scores from age 11 \cite{Deary_2004b}. In the present, ABC1936, sample, the two-tailed Pearson correlation between MHT and RPM at age about 68 (wave 3, N = 294) was 0.56 (p < 0.001).

\subsection*{Participants with relatively successful and unsuccessful cognitive ageing}

The availability of MHT (at age 11) and RPM (from the mid-sixties, onwards) scores for members of the ABC1936 meant that subgroups of subjects could be identified with more or less successful cognitive ageing in the long period between age 11 and the middle-to-late 60s. The aim was to select two subgroups of subjects with relatively similar MHT scores at age 11, but divergent RPM scores in later life. The Moray House Test score at age 11 was used as a measure of prior ability. It provided the baseline from which relatively successful and unsuccessful cognitive change in older age was determined. From the ABC1936 sample, 58 individuals (aged 69–70 years then; 28 female) with an age 11 IQ (calculated from MHT raw scores) between 85 and 115 were invited for fMRI scanning; i.e., at age 11 they were within 1 SD of the sample mean. The age 11 IQ score was calculated by first transforming the ABC1936 sample's age 11 MHT raw scores to a standard IQ-type scale with a mean of 100 and SD of 15. Thereafter, only subjects with integer IQs greater than 84 and less than 116 were considered for the present study. Participants within this 'normal' childhood IQ band were grouped, according to RPM scores in later life, into relative cognitive sustainers and decliners.

To form a metric for cognitive 'decliners' and 'sustainers', the ABC wave 3 testing data were used. First, to ensure that subjects had similar experience with the RPM test, the ABC1936 participants used in this exercise had to have taken RPM at wave 1 and wave 3 of