prestimulus

Prestimulus is a term used to describe the interval that occurs before a stimulus is presented in an experiment. In neuroscience and psychology, prestimulus refers to the neural, physiological, or cognitive state that exists prior to stimulus onset and that can influence subsequent perception, decision making, and behavior.

In practice, researchers define a pre-stimulus window, commonly a few hundred milliseconds before stimulus onset (for example, from −200 to 0 ms). Measurements are obtained with EEG, MEG, or fMRI. Analyses focus on prestimulus oscillatory power or phase, baseline neural activity, or patterns of network engagement, and how these states relate to the accuracy, speed, or visibility of the stimulus. Prestimulus activity is often contrasted with post-stimulus processing to understand how initial brain states predispose processing.

A substantial body of work links prestimulus brain states to performance. For example, lower alpha-band power before a visual stimulus is often associated with better detection, while the phase of ongoing oscillations can bias perceptual outcomes. Prestimulus activity can also reflect anticipatory attention, arousal, or motor preparation, which may confound interpretation if not properly controlled.

The concept is widely used but subject to caution: results can be influenced by task design, expectancy, or measurement noise. Researchers typically employ counterbalancing, jittered inter-stimulus intervals, and appropriate baselines to isolate genuine prestimulus effects and to distinguish them from post-stimulus processing or non-specific arousal.