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Amultidisciplinary collaborative study examining cognition in a large sample of twins is outlined. A common experimental protocol and design is used in The Netherlands, Australia and Japan to measure cognitive ability using traditional IQ measures (i.e., psychometric IQ), processing speed (e.g., reaction time [RT] and inspection time [IT]), and working memory (e.g., spatial span, delayed response [DR] performance). The main aim is to investigate the genetic covariation among these cognitive phenotypes in order to use the correlated biological markers in future linkage and association analyses to detect quantitativetrait loci (QTLs). We outline the study and methodology, and report results from our preliminary analyses that examines the heritability of processing speed and working memory indices, and their phenotypic correlation with IQ. Heritability of Full Scale IQ was 87% in the Netherlands, 83% in Australia, and 71% in Japan. Heritability estimates for processing speed and working memory indices ranged from 33–64%. Associations of IQ with RT and IT (−0.28 to −0.36) replicated previous findings with those of higher cognitive ability showing faster speed of processing. Similarly, significant correlations were indicated between IQ and the spatial span working memory task (storage [0.31], executive processing [0.37]) and the DR working memory task (0.25), with those of higher cognitive ability showing better memory performance. These analyses establish the heritability of the processing speed and working memory measures to be used in our collaborative twin study of cognition, and support the findings that individual differences in processing speed and working memory may underlie individual differences in psychometric IQ.
The n-back task was hypothesized to be a dual task,
permitting the imposition of parametrically increasing attentional
and working memory demands, while keeping constant the demands
of an embedded matching subtask. Visual targets were presented
for 200 ms every 2.2 s at pseudorandomly varying positions on
a computer screen. Participants were required to remember the
most recent 0, 1, 2, or 3 positions and responded with a choice
button push to whether the current target position matched the
position presented n items previously. P300 peak latency
was constant across n-back tasks, reflecting constant
perceptual and cognitive demands of the matching subtask. P300
peak amplitude decreased with increasing memory load, reflecting
reallocation of attention and processing capacity away from
the matching subtask to working memory activity. These data
support a dual-task nature of the n-back, which should
be considered when employing this paradigm.
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