A critical technique for understanding how neuronal activity contributes to 
behavior is determining whether perturbing it changes behavior. The advent of 
optogenetic techniques allows the immediately reversible alteration of neuronal 
activity in contrast to chemical approaches lasting minutes to hours. 
Modification of behavior using optogenetics has had substantial success in 
rodents but has not been as successful in monkeys. Here, we show how optogenetic 
inactivation of superior colliculus neurons in awake monkeys leads to clear and 
repeatable behavioral deficits in the metrics of saccadic eye movements. We used 
our observations to evaluate principles governing the use of optogenetic 
techniques in the study of the neuronal bases of behavior in monkeys, 
particularly how experimental design must address relevant parameters, such as 
the application of light to subcortical structures, the spread of viral 
injections, and the extent of neuronal inactivation with light.