In Experiment 1, pigeons were presented with a sequence of light flashes an
d cued to peck a key for reward either after a fixed time or after a fixed
number of flashes. Curves that showed the rate of key pecking over time wit
hin trials indicated that peak rates of response were reached near the fixe
d time on timing-cued trials and near the fixed number of flashes on counti
ng-cued trials. In Experiment 2, the key cue was shifted from timing to cou
nting or from counting to timing midway through a trial. The peak times rea
ched reached after the cue change indicated that pigeons kept track of time
while cued to count but did not count while cued to time. These findings s
uggest a basic asymmetry in the dual-mode model of timing and counting.