The information processing speed battery comprised two psychometric tests from the WAIS-IIIUK (digit symbol coding and symbol search) and two elementary cognitive tasks, RT and inspection time, which will be described briefly here; a full account of each test is given in Deary et al. (2007). Mean of correct trials on a simple RT (20 trials) and 4-choice RT (40 trials) task were used to assess speed of information processing. For simple RT, the participant is required to press a response key as fast as possible following the occurrence of a zero on the LCD screen. Four-choice RT requires the participant to press the corresponding response key when a target of 1, 2, 3 or 4 appears on the screen. The inspection time task is a two-alternative, forced choice, backward masking, visual discrimination task, requiring participants to make a discrimination, with no pressure on response time, regarding which of two parallel, vertical lines of markedly different lengths was longer. The stimuli and psychophysical procedure were the same as those used in Deary et al. (2004a), with a prescribed number of
