The present study examined the effects of caffeine on the estimation o
f felt width of blocks employing haptic presentation. Following a betw
een-subject design, 160 male postgraduate students classified as high
or low impulsives received either placebo or one of four doses of caff
eine citrate (1, 2, 3 and 4 mg/kg body weight). A double-blind procedu
re was adopted for drug administration. Caffeine produced differential
effects on the performance of high and low impulsives, facilitated pe
rformance (decreased error in perceptual judgment) in high impulsives
but had no influence on the performance of low impulsives. The dose-re
sponse trends also followed different patterns in the two groups of su
bjects.