Inspection time - The research issue
Nettelbeck and Lally (1976) were the first to use inspection time in research on intelligence. The hypothesis was that differences in speed of information processing might underlie IQ differences. This experiment aimed to take a novel index of the speed of perceptual encoding and examine whether ‘more intelligent’ people encode information faster than do ‘less intelligent’ people. Design and procedure Ten male participants aged 16–22 years with a mean IQ of 83 (range = 47–119) took part in the study. Three of the participants with the highest IQs were referred for the study because of problems associated with minor injuries or for assistance with behavioural problems. The seven with the lowest IQs were referred because of their inability to cope with open employment. The stimuli were drawn on cards and presented to the participant using a tachistoscope – a device that allows very fast presentation of visual material. (Nowadays, stimuli are presented on computer monitors.) The stimuli were two vertical lines o markedly different lengths joined at the top by a short horizontal bar. For one type of stimulus, the shorter of the lines was on the left, and for the other type it was on the right.
The participants simply had to decide whether the long line was on the left or the right of the stimulus. The task was made more difficult by covering up the stimulus with another card that showed two lines of equal length that were thicker and longer than the longest line in the test stimuli. In this way, the second card acted as a ‘backward masking’ stimulus that destroyed the information contained in the first test stimulus. The exposure duration of the test stimulus for each trial was manipulated by varying the time between the onset of the test stimulus and the onset of the masking stimulus (stimulus onset asynchrony, or SOA). On each trial, the participant had to press a key to indicate whether the short line on the test stimulus was on the left or on the right. Participants were given 105 trials of different exposure duration in each testing session. The first five trials were for practice only, and were excluded from subsequent analyses. The importance of accuracy rather than speed was emphasized. For each participant an inspection time was extrapolated from the data relating accuracy to SOA in order to estimate the shortest exposure required for 97.5 per cent accuracy in decision making.
Results and implications: Estimates of inspection time correlated −0.92 with performance IQ (PIQ) derived from administration of the Wechsler Adult Intelligence Scale (WAIS), but, interestingly, the relationship with verbal IQ (VIQ) derived from the WAIS was not statistically significant. So the study showed that a very simple measure of perceptual speed is related to IQ differences, although this association was specific to one form of IQ (i.e. performance rather than verbal IQ). Subsequent research has suggested that PIQ has a higher visual-spatial component than does VIQ, which might explain this finding. By today’s standards, this study is methodologically weak – there are too few participants, too many of them have low IQ for a study that hopes to extrapolate to normal variation in intelligence, and the psychophysical procedures are less than perfect. But the basic relationship with IQ observed here has held up, with meta-analyses suggesting that the correlation is about −0.5. So inspection time as a task has become a cornerstone of theories proposing that differences in general intelligence might be due to global differences in speed of information processing.
Nettelbeck and Lally (1976) were the first to use inspection time in research on intelligence. The hypothesis was that differences in speed of information processing might underlie IQ differences. This experiment aimed to take a novel index of the speed of perceptual encoding and examine whether ‘more intelligent’ people encode information faster than do ‘less intelligent’ people. Design and procedure Ten male participants aged 16–22 years with a mean IQ of 83 (range = 47–119) took part in the study. Three of the participants with the highest IQs were referred for the study because of problems associated with minor injuries or for assistance with behavioural problems. The seven with the lowest IQs were referred because of their inability to cope with open employment. The stimuli were drawn on cards and presented to the participant using a tachistoscope – a device that allows very fast presentation of visual material. (Nowadays, stimuli are presented on computer monitors.) The stimuli were two vertical lines o markedly different lengths joined at the top by a short horizontal bar. For one type of stimulus, the shorter of the lines was on the left, and for the other type it was on the right.
The participants simply had to decide whether the long line was on the left or the right of the stimulus. The task was made more difficult by covering up the stimulus with another card that showed two lines of equal length that were thicker and longer than the longest line in the test stimuli. In this way, the second card acted as a ‘backward masking’ stimulus that destroyed the information contained in the first test stimulus. The exposure duration of the test stimulus for each trial was manipulated by varying the time between the onset of the test stimulus and the onset of the masking stimulus (stimulus onset asynchrony, or SOA). On each trial, the participant had to press a key to indicate whether the short line on the test stimulus was on the left or on the right. Participants were given 105 trials of different exposure duration in each testing session. The first five trials were for practice only, and were excluded from subsequent analyses. The importance of accuracy rather than speed was emphasized. For each participant an inspection time was extrapolated from the data relating accuracy to SOA in order to estimate the shortest exposure required for 97.5 per cent accuracy in decision making.
Results and implications: Estimates of inspection time correlated −0.92 with performance IQ (PIQ) derived from administration of the Wechsler Adult Intelligence Scale (WAIS), but, interestingly, the relationship with verbal IQ (VIQ) derived from the WAIS was not statistically significant. So the study showed that a very simple measure of perceptual speed is related to IQ differences, although this association was specific to one form of IQ (i.e. performance rather than verbal IQ). Subsequent research has suggested that PIQ has a higher visual-spatial component than does VIQ, which might explain this finding. By today’s standards, this study is methodologically weak – there are too few participants, too many of them have low IQ for a study that hopes to extrapolate to normal variation in intelligence, and the psychophysical procedures are less than perfect. But the basic relationship with IQ observed here has held up, with meta-analyses suggesting that the correlation is about −0.5. So inspection time as a task has become a cornerstone of theories proposing that differences in general intelligence might be due to global differences in speed of information processing.
Nettelbeck, T., & Lally, M., 1976, ‘Inspection time and measured intelligence’, British Journal of Psychology, 67 (1), 17–22.
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