Visual cortical alpha rhythms : function and relation to other dynamic signatures in local networks

Hawkins, Karen

January 2016

Thesis or dissertation

© 2016 Karen Hawkins. All rights reserved. No part of this publication may be reproduced without the written permission of the copyright holder.

The alpha rhythm (8-12Hz) was the first EEG rhythm recorded by Hans Berger in 1929. Despite being the earliest rhythm discovered, alpha rhythms remain the most mysterious in terms of mechanism and function. In the visual system, post-stimulus alpha oscillations are observed upon closing of the eyes or removal of visual stimulus. Alpha rhythms have been implicated in functional inhibition and short term memory.

This thesis presents a rat in vitro model of the cortical alpha rhythm. This was achieved by mimicking the neuromodulatory changes that occur upon the removal of visual stimulus. Beta oscillations were induced by excitation of the visual cortex slice using the glutamate agonist kainate [800nM] to mimic sensory stimulation. This excitatory drive was then reduced using the AMPA and KA receptor antagonist NBQX [5µM], followed by the blocking of neuronal Ih current with DK-AH269 [10µM] to produce alpha frequency oscillations.

Alpha activity was seen throughout all cortical laminae, with alpha power predominating in layer IV of the V1. The rhythm was found to be critically
dependent upon NMDA receptor-mediated connections between neurons which required the need to be potentiated in the prior excitation phase leading to beta frequency oscillations. Alpha activity was also dependent upon gap junctional coupling and had neuromodulatory effects similar to the human profile of alpha.

Alpha oscillations were generated by pyramidal neurons found in layer IV of the V1 which elicited burst discharges. The alpha rhythm was not dominated by synaptic inhibition despite the functional inhibition role it is thought to play. Instead, the alpha rhythm appeared to dynamically uncouple activity in the primary thalamorecipient neurons (layer IV regular spiking cells) from down-stream activity in both supragranular and infragranular layers. In this manner, the alpha rhythm appears to be ideally constructed to prevent ascending visual information from both passing on to higher order visual areas, and also being influenced by top-down signal from these areas.

Hull York Medical School, The University of Hull and the University of York
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