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

Abstract

The presentation of Loud Acoustic Stimuli (LAS) during preparation can trigger motor actions at very short latencies in a phenomenon called the StartReact effect. It was initially proposed that a special, separate sub-cortical mechanism which by-passes slower cortical processes could be involved. We sought to examine the evidence for a separate mechanism against the alternative that responses to LAS can be explained by a combination of stimulus intensity effects and preparatory-states - as proposed by activation models of motor control. To investigate whether cortically mediated preparatory processes are involved in shaping reactions to LAS, we used an auditory reaction task where we manipulated preparation-level within each trial. We contrasted responses to non-intense tones and LAS and examined whether cortical activation, sub-cortical excitability (measured by pre-stimulus EEG and eye-blink startle reflexes, respectively) and the motor response were influenced by preparation-level. As predicted by the activation model, increases in preparation-level were marked by gradual reductions in RT coupled with increases in cortical activation and sub-cortical excitability - at both condition- and trial-levels. Changes in cortical activation influenced motor and auditory but not visual areas - highlighting the wide-spread yet selective nature of preparation. RTs were shorter to LAS than tones, but the overall pattern of preparation-level effects were the same for both stimuli. These results demonstrate that LAS responses are indeed shaped by cortically mediated preparatory processes. The concurrent changes observed in brain and behaviour with increasing preparation reinforces the notion that preparation is marked by evolving brain states which shape the motor response. Key PointsO_LIReactions to Loud Acoustic Stimuli can be explained by stimulus intensity and preparation state C_LIO_LIWe manipulated movement preparation by altering the temporal position of the imperative stimulus C_LIO_LIPreparation was marked by reductions in RT, and increased cortical and sub-cortical excitability C_LIO_LIPreparation had the same effect on reactions to Loud Acoustic Stimuli and non-intense tones C_LIO_LIThe results highlight the widespread, evolving, and strategic nature of movement preparation C_LI