Local contextual processing effects with increasing stimulus presentation rate.

We investigated the effects of stimulus presentation rates on local contextual processing. Local context was defined as the occurrence of a short predictive series of stimuli occurring before delivery of a target event. EEG was recorded in 12 subjects during two sessions: a slower (150 ms stimulus presentation, 1,000 ms interstimulus interval (ISI)) and a faster session (50 ms stimulus presentation, 800 ms ISI). Stimuli were presented to either the left or right visual field and consisted of 15% targets (downward facing triangle) and 85% of equal numbers of three types of standards (triangles facing left, upwards and right). Recording blocks consisted of targets preceded by randomized sequences of standards and by sequences including a predictive sequence signaling the occurrence of a subsequent target event. Subjects pressed a button in response to targets. Reaction times were faster for predicted compared to random targets in both sessions. Faster presentation rates of stimuli induced a greater P3b latency shift between predicted and random targets compared to slower presentation rates, such that P3b latency for predicted targets was shorter in the faster task compared to the slower task. These findings suggest that as stimulus presentation rates increase, predictive local context is utilized to a greater extent, increasing decision confidence in the detection of targets.