Effects of Novelty, Habituation, Attention and Distraction on the Amplitude of the Various Components of the Somatic Evoked Responses

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Abstract
Amplitude changes of various somatic evoked responses (SER) components were studied during conditions of novelty, habituation, attention and distraction. Special care was taken to operationally define these conditions of higher nervous function. In addition, EEG, EKG and GSP were used to evaluate the concomitant level of ‘general alertness’ and reaction velocity and discrimination tasks for the evaluation of ‘selective attention’. Amplitude of early SER components (P2a and P3a) was not significantly affected by these experimental conditions, whereas amplitude of late SER components (P4a and P5a) depended upon the particular situation being superimposed on the subject. In some conditions, there was either direct or inverse correlation between the late SER components amplitude and general alertness level: both decreased when subjects shifted from novelty to habituation, both increased from habituation to attention but late SER components decreased while general alertness increased from attention to distraction. In some other conditions late SER components increased with selective attention under a similar level of general alertness: There was a direct correlation between late SER components amplitude and response velocity of different individuals under the same responding condition. Furthermore, in the same individual late SER components amplitude was larger for the attended stimuli delivered to one arm than to the unattended stimuli delivered to the other. These results suggest first, that early and late SER components are mediated through different mechanisms and second that the amplitude of late SER components depends more on the level of selective attention than on the level of general alertness.