The Possible Contribution of Neural Plasticity to ON, OFF and Steady-State Responses Elicited by Brief Trains of Repetitive Stimulation

Abstract

<p> The possible contribution of neural plasticity to ON, OFF and steady state responses elicited by brief, repetitive trains of stimulation was investigated in the intact human subject with the use of the electroencephalogram (EEG). Experiment One implemented trains of stimulation at three different repetition rates, 1.5Hz, 4Hz and 13Hz. The goal was to investigate the nature of the ON, OFF and steady state responses evoked at these repetition rates. The experiment was carried out in three modalities: visual (n=13), auditory (n=lO) and somatosensory (n=12). The main result was that the ON and OFF responses were enhanced at 13Hz compared to the lower repetition rates. Experiment Two sought to answer the question of whether enhancement depended on the repetition rate or the increased experience provided by the higher frequencies. The number of stimuli in the 13Hz trains was reduced to equal the 1.5Hz condition from Experiment One. Graded exposure was then provided to the 13Hz stimulation. This procedure was implemented in two groups of subjects: Replication One (n=12) used 13Hz stimulation and Replication Two (n=24) used 14Hz stimulation. A subset (n=IO) of the Replication Two subjects returned for a second session (Day 2) 24 hours after the first. An assessment of effects was made after minutes and hours. There were four main results. The OFF response was observed after nine 13Hz pulses and did not change over the course of the experiment. The ON response increased with exposure to the 13Hz trains. Steady state responses diminished and showed a phase shift over the experimental session. Results for Day 1 and Day 2 were not different. Within session changes, as a result of exposure to the stimulus, were seen. These effects were not long lasting. </p>