Responses to Loud Acoustic Stimuli: Short and Long Timescale Effects on Cortical and Sub-Cortical Preparation Reflect Conditional Probability

Loud acoustic stimuli (LAS) can trigger prepared motor responses at very short latencies: the StartReact effect. In this study, we tested the proposal that responses to LAS in the StartReact effect could be explained by stimulus intensity effects combined with movement-related preparation changes on nervous system excitability. Using a simple auditory reaction time task, we induced different levels of preparation by systematically manipulating the conditional probability and temporal location of the auditory stimulus (LAS or non-intense tone). We then examined how preparation-levels influenced motor responses to LAS and non-intense tones, as well as cortical and sub-cortical excitability — reflected by electroencephalographic (EEG) activity before the onset of the auditory stimulus, and the eye-blink startle reflex.On both tone and LAS trials, higher preparation conditions were accompanied with reductions in movement onset time, increased force, as well as enhanced cortical (in auditory and motor areas) and sub-cortical excitability. At the trial-level, we found that enhanced pre-stimulus EEG activity in sensory and motor areas was associated with earlier movement onset on tone trials, and shorter blink onset latencies were associated with earlier movement onset on LAS trials. The results show that movement preparation is associated with wide-spread changes in cortical and sub-cortical excitability, and that movement preparation (combined with stimulus-intensity effects) plays a critical role in shaping responses to both non-intense tones and LAS.