The Determinants of Reaction Times: Influence of Stimulus Intensity

Abstract

The central nervous system has the ability to generate very rapid ‘temporally urgent’ sensory motor reactions in response to stimuli under certain conditions. For example, temporally urgent reactions have been shown to occur during control of protective responses, such as regaining balance after a perturbation or reacting rapidly to a startling auditory stimulus. Two different mechanisms have been proposed to the observation of rapid reaction times; 1) different (shorter) pathway for those reactions with very rapid reactions and/or 2) increased synaptic excitability to reduce the time required at each site of synaptic convergence leading to a reduction in total reaction time. The overarching hypothesis of this work is that the occurrence of rapid reactions is mediated by the facilitation of pathways through stimulus driven and/or central facilitation leading to significant reduction in reaction time. The current work is delimited to a focus on the determinants of reaction time in response to auditory stimuli. Two studies were conducted to determine the relationship between stimulus intensity and behavioural and neurological responses. Study one focused on influence of stimulus intensity on simple and choice reaction time performance. Choice reaction time is distinguished by need for higher level cortical processing for decision making. Stimulus induced changes in choice reaction time would be most likely accounted for by pathway facilitation. Results from study one showed an overall decrease in choice reaction time to an increase in stimulus intensity with no difference in errors suggesting an increase in stimulus intensity results in increase synaptic facilitation. Study two focused on electrophysiological events associated with auditory stimuli. It was proposed that evidence of more rapid electrophysiological events and increased amplitude would support a model of synaptic facilitation with increases in auditory stimulus intensity. Results from study two showed decreased peak latencies with high stimulus amplitudes as well as increased cortical activity prior to motor responses further suggesting reductions in reaction time to increases in stimulus intensity are a result of increased synaptic facilitation. Overall this thesis is focussed on developing a further understanding of stimulus intensity as a determinant of reaction time so that in the future one may better understand the factors that contribute to slowing in older adults and those with neurological impairment.