In the study of decision making, neuroscientists have largely focused on how the brain accumulates evidence during the formation of decisions. Less is known about how the accumulation of evidence is terminated. When decisions are communicated with an eye movement, neurons in the lateral intraparietal area (LIP) represent the accumulation of evidence. It has long been hypothesized that downstream areas terminate the decision process when LIP reaches a threshold level of activity. We recorded from neurons in the superior colliculus (SC), a primary downstream target of LIP, as a monkey performed a reaction-time, motion discrimination task. Simultaneously, we recorded from 80-250 LIP neurons using prototype, macaque neuropixels probes. Among these neurons, we identified subpopulations with response fields aligned to those of the SC neurons, allowing access on single trials to the putative LIP input to the SC population. Single-trial activity in LIP displayed drift-diffusion dynamics, as previously inferred. In contrast, single-trial dynamics in SC manifested as quiescence and bursts—one immediately before the saccade, sometimes preceded by smaller, non-saccadic bursts. Saccadic bursts were predicted by a distinct signal in LIP involving the derivative and magnitude of LIP responses. We hypothesized that the bursting dynamics in SC are the product of a threshold mechanism important for terminating the decision. Consistent with this, focal SC inactivation caused (i) an ipsiversive bias; (ii) longer contraversive reaction-times; and (iii) increased overall sensitivity. The combination of these effects is diagnostic of an increased decision threshold for contraversive choices. Simultaneous recordings in LIP showed a prolonged build-up of activity, further suggesting that SC inactivation caused an increase in the decision threshold. Together, the results shed light on the neural mechanism of a fundamental cognitive process—the transformation from deliberation to commitment.