Abstract
Issue: What are the temporal processes of visual target identification and response activation that control saccadic eye movements? And how do these processes interact? Methods: A detailed study was provided by saccade response times for visual search by two monkeys, Macacca mulatta and M. radiata. The identifiability of a singleton target and the discriminability of a Go/No-Go response cue were independently manipulated in a 2x2 factorial experiment. Both variables had large effects on the RTs and response probabilities in data collected from more than 19,000 trials for each monkey. Momentary response rates were measured by RT hazard rates at 3-4 ms intervals for each of the four conditions of singleton identifiability x response-cue discriminability. Results: Effects of the two variables were quantified at each time interval by ratios of hazard rates for the high vs low values of the variable. The influence of each variable was invariant with the value of the other variable. In each interval, response probabilities in the four conditions were accurately predicted (df = 1) as numerical products of two independent subprocesses. Conclusions: (1) At each moment, response rates in each condition were simultaneously influenced by two functionally independent subprocesses, singleton identification and response-cue discrimination. (2) Dynamics of perception and action were precisely described by their process rates (hazard rates) as functions of time. Evidently, neural processes for perception and action operated in parallel, influencing response probabilities continuously, simultaneously, and independently. RT hazard rates, rather than RTs per se, are likely behavioral correlates of the underlying neural activity.