The "amplifier" in Sokolov's Orienting Reflex (OR) mechanism: An EEG/ERP extension

Sokolov's theoretical mechanism of the Orienting Reflex (OR) includes two systems: a neuronal model/comparator system, and an amplifying system. The first of these relates the phasic OR to parameters of the eliciting stimulus; the second modulates output via the organism's arousal state, as reflected in the tonic OR. Research on elicitation and habituation of the OR has focussed largely on phasic aspects of the neuronal model/comparator system, with the amplifying system being largely ignored. Here we examine phasic and tonic OR profiles over trials from an auditory habituation/dishabituation study, using both electrodermal and central measures. Phasic OR measures were the skin conductance response (SCR) and the P300 ERP component, and corresponding tonic OR measures were the prestimulus skin conductance level (SCL) and prestimulus EEG alpha (8-13 Hz) amplitude. We predicted that the tonic measures of arousal at each stimulus presentation would modulate the phasic OR measures. The two phasic measures showed extensive rapid decrement over repeated standards, and recovery to a deviant, with a significant positive correlation over trials. Tonic OR measures also showed significant changes over trials, but with a significant negative correlation: decrement in SCL, increment in alpha. As predicted, the phasic OR measures were modulated by the prestimulus tonic OR levels, with large correlations between the phasic and tonic patterns over trials. Our evidence for the predicted tonic-as-amplifier aspect of Sokolovian OR theory opens new theoretical perspectives. Identification of both phasic and tonic peripheral and central measures of the OR should prompt an expansion of OR research.