Summary: SUPPLEMENTARY INFORMATION
Perceptual decision making in less than 30 milliseconds
Terrence R. Stanford, Swetha Shankar, Dino P. Massoglia, M. Gabriela Costello and Emilio Salinas
Note 1: what happens if the monkeys wait?
A potential concern with the compelled-response design is that, rather than responding immedi-
ately after the go signal, the monkeys could try to wait for the cue before initiating their responses.
Intuitively, such behavior might seem problematic because it could bias the reaction time (RT) val-
ues of the subjects in a non-uniform way. There are, however, three points to consider about this
issue: (1) the conditions of the task make it difficult for the subjects to wait beyond the go signal,
(2) there is no evidence in the data indicating that the subjects wait for the cue, and (3) unless such
waiting were extremely exaggerated, it would have no impact whatsoever on the measurement of
sensory processing speed.
First, to prevent the monkeys from waiting or predicting when the cue is revealed, gap inter-
vals were always delivered pseudo-randomly, and responses that took more than 600 ms were not
rewarded. It is unclear whether this time deadline was necessary or not, because trials with RTs
longer than 600 ms were always rare ( 1%). Still, these measures do not completely rule out the
possibility that, within the allotted time, the monkeys could gamble about how long to wait for
the cue and still execute the saccade in time.
If the monkeys waited, however, this would become apparent in two ways. First, their RTs
would likely change in proportion to the gap duration, because they would need to wait more at