Factorial variation of saccade vigor with dual decision processes

Canonical stochastic models of decision-making treats decision and action as independent and sequential processes; however, studies involving limb movements consistently show that movement duration and kinematics are influenced by the quality of evidence. We examined the influence of evidence accumulation processes on eye-movement kinematics by testing whether saccade velocity varies with the quality of evidence in macaque monkeys performing a visual search Go/NoGo task. The elongation of a color singleton cued the Go/NoGo stimulus-response rule, while its location specified the saccade target. We factorially manipulated the efficiency of stimulus-response cue discrimination by varying the elongation of the singleton and the efficiency of singleton localization by varying the color similarity between the singleton and distractors. The effectiveness of the manipulations was revealed by the response times on correct trials that were separately modified by the singleton localizability and stimulus-response cue discriminability, by the incidence of localization and response selection errors with separately modified error response times. Saccade vigor (peak velocity normalized for amplitude) was higher on correct relative to error trials and was inversely proportional to response time. Saccade vigor was separately modified by singleton localizability and stimulus-response cue discriminability, controlling for variation of pupil diameter. Distinct patterns of error rates and saccade vigor across monkeys indicated individual differences in decision-making strategies. These findings demonstrate that the process selecting endpoints can influence both the timing and dynamics of saccadic eye movements. Incorporating saccade vigor can provide constraints on biologically plausible decision models and help address the persistent challenge of model mimicry.