Inspection time as a biological marker for functional age

Abstract

Inspection Time (IT) is a speed measure that has been primarily investigated in the field of individual differences. However, Nettelbeck and Wilson (2004) proposed that IT could have promise as a biomarker for functional outcomes, particularly cognitive aging. The premise behind biomarker research is that chronological age is simply a proxy for the physiological and cognitive changes that occur in the body with advancing age. Biomarkers are measures that 'mark' the aging process and represent the biological age of an individual rather than the years since his/her birth. Speed of processing tasks offer promise as biomarkers because decline in speed of processing is one of the most robust findings in cognitive aging research. However, traditionally used tasks are problematic because they confound speed and accuracy and some are sensitive to cohort effects. Inspection time is a speed of processing measure that is free from these problems and is therefore a promising candidate for a biomarker. This dissertation presents the first empirical investigation of this proposition. One hundred and fifty elderly participants were assessed on IT, traditionally used biomarkers (e.g. grip strength, visual acuity), a battery of cognitive tasks (e.g. fluid ability and crystallised ability) and measures of everyday functioning (e.g. activities of daily living). These individuals were assessed on three separate occasions over a period of 18-months. For the biomarkers, initial scores, 6-month change scores and 18-month change scores were generated and used to predict final scores and 18-month change scores on the functional outcomes (cognition and everyday functioning). Results revealed that slow IT at the start of the study was associated with dependence in activities of daily living and poorer fluid ability at the end of the study. There was also evidence that slow IT at the start was associated with decline in fluid reasoning over the subsequent 18-months. Moreover, consistent with the major aims of this study, decline in IT over time was associated with more cognitive problems in daily life and poor fluid ability at the end of the study. Given that initial and change scores for IT were independent, due to the methodology used to estimate them, the two measures explained unique variance in the functional outcome measures.These findings are extremely encouraging, particularly given the relatively short time frame for this study. IT has predictive validity for everyday functioning and cognitive aging over an 18-month period, and therefore, it is concluded that IT has promise as a valid biomarker for functional age. Recommendations for further research include investigating the link between IT and mortality, examining the association between IT and a broader range of functional age measures, the replication of these findings in a different sample, and means for improving the sensitivity and specificity of the current IT estimation procedure.