Figure 5.

How two LATER Units might describe the observed data.

It is assumed that a certain decision threshold must be reached to initiate a saccade. This threshold may be reached through two forms of ‘evidence’. As time passes following the amber onset, there is increasing expectation of the green light. This form of evidence is accrued slowly. Once the green light is lit, there is 100% evidence of the requirement for a saccade, so a faster decision process is initiated. Depending upon the amber duration and prior knowledge of the amber duration distribution, one process will win the race on any given trial. As these are biological systems, there is also noise (variability) in the rate of rise of each process. This results in a recinormal distribution of saccadic latencies even in the presence of identical trial conditions.