Entrainment and its perturbation in the auditory cortex of the rat: phase and frequency modulations

Entrainment and its perturbation in the auditory cortex of the rat: phase and frequency modulations Author: Anu Karvonen Supervisor: Markku Penttonen Master’s Thesis in psychology University of Jyväskylä September 2010 48 pages Objective of the present study was to build oscillatory entrainment in the auditory cortex of the rat and perturb it selectively. There is hardly any previous research on entrainment so the study was of exploratory nature and a unique experimental design was used. Local field potentials (LFPs) were measured from a total of fifteen urethane-anesthetized rats via an extracellular microelectrode placed over the dura. In the four main experiments 2,5 Hz entrainment was created with tone trains of nine stimuli, the entraining frequency of the tone being 4000 Hz and the perturbation frequency of the tone being 4500 Hz. Perturbation was always in the eight tone of the tone train, in which the phase and/or frequency of the tone was changed. Also jitter in the entraining frequency and single tone experiments were studied. It was noted that the rat can be either in a so called up & down or complex state during the measuring, from which the complex state is the preferred state for analyzing response differences. Phase perturbation results suggested that there is a specific influence when the phase is made longer rather than shorter. The longer phase response became more negative than the preceding entraining tone response approximately at 62-124 ms and rose to more positive levels at around 168-224 ms. Frequency perturbation, in turn, caused more positive levels than the preceding entraining tone response roughly at 86-138 ms suggesting a differing timeline of changes. When combining phase and frequency perturbation, both the longer and shorter phase perturbation presented more positive amplitudes than the preceding entraining tone response approximately between 41-160 ms. The longer phase response was again more negative than the shorter one, timeline of differences being 23-141 ms and from 252 ms onwards. It was found that the response to a frequency change remains even in the presence of a jitter in the entraining frequency, which suggests that it does not matter if entrainment is at a fixed interval or if it varies semirandomly. In addition, an important result was that at first responses to the two different tones do not differ from each other and do not change during the first single tone experiment. Differences were only observed after an entraining experiment, when the perturbation tone response became more positive than the entraining tone response. It is suggested from the results that entrainment can create memory traces in the nervous system, a probability being that the trace is actually stronger for the perturbation stimuli.