Variations of the 2-line inspection time stimulus

Mackenzie and Bingham [(1985) Australian Journal of Psychology, 37, 257–268] used a variation of the 2-line Inspection Time (IT) task shifting the stimulus position between trials to prevent focussing of attention and obtained 71% thresholds which correlated -0.50 with WAIS Performance IQ. Nettelbeck [(1987) Speed of information-processing and intelligence. Norwood, NJ: Ablex] estimated the mean 97.5% threshold for Mackenzie and Bingham's subjects was 262 msec where other Inspection Time studies obtain estimated 97.5% thresholds of around 100 msec, and suggested the higher mean threshold estimate might be due to the changes in stimulus position. This experiment investigates whether the higher thresholds obtained by Mackenzie and his associates are due to the changes in stimuli used. Fifty non-retarded adults served as subjects for: M, stimuli which changed position as in Mackenzie and Bingham (1985); S, stimuli similar to M presented in fixed position; and N, stimuli more similar to the conventional 2-line stimuli in contrast and mask width. There were no significant differences between the 3 stimuli in their mean thresholds or in their correlations with progressive matrices (mean r = -0.61). The mean 79% threshold of 90 msec converts to an estimated 97.5% threshold of 216 msec. It follows that changing stimulus position (M vs S) or stimulus contrast or mask width (S vs N) are not responsible for the higher estimated 97.5% thresholds. The present study has obtained for the whole sample a correlation between mean IT and intelligence, -0.61, which is significantly higher than Hunt's [(1980) British Journal of Psychology, 71, 449–474] 0.3 barrier. Correcting possible skew in IT by log transform produces a correlation of -0.64, and there is no evidence that the correlation is substantially lower for subjects of above average IQ, after correcting for restriction of range r = -0.55.