Exploring the functional system of mental imagery by rTMS targeted at different cortical areas
Date
2013
Authors
Journal Title
Journal ISSN
Volume Title
Publisher
Tartu Ülikool
Abstract
The nature of visual mental imagery and its neural basis is complicated and not conclusively
understood. Neural basis of mental imagery seems to be quite well clarified in general (providing
more knowledge for its philosophical and phenomenal understanding), but partial similarities
with the visual perception actually make it complicated to differentiate brain mechanisms of
perception and imagery.
The specification of the role of visual cortex is essential and its function in relation to imagery
must be correctly understood. If the visual cortex plays the same role as in perception – imagery
may be reduced to one of its forms and differ only in degree, not in kind. If the visual cortex
functions differently and plays a different role, then imagery may be seen as a more independent
phenomenon, which only shares some similarities in degree, but differs in kind. Transcranial
magnetic stimulation (TMS) is a proper means to study the causal role of selected cortical areas
in mental functions and we capitalize on this possibility in the present thesis.
The experimental results reported in this study showed no statistically significant effect of
correct task performance and vividness ratings as dependent on rTMS conditions. In fact, this
situation may be due to both small sample size and/or relatively weak rTMS intensity. Although
many studies using 1-Hz rTMS stimulation have shown behavioral effects, this study failed to
find significant effects on performance in the cognitive task based on covert manipulation with
mental images. One possible reason for the absence of effects could be not too many rTMS
pulses. The use of 300 stimuli for each condition in this study had a reason that overall time of
one experimental day for each participant would be too long and make them tired, possibly
jeopardizing the imagery task performance. The relatively small intensity of 65% of phosphene
threshold was applied because with higher intensities, phosphenes would interfere with imagerytask
performance.
The main result of the present study showing a negative experimental effect is that this rTMS
design does not affect brain in a proper and/or strong enough manner, which means a necessity
of new experimental design with different stimulation regimens to be chosen for further
investigations of mental imagery aiming at exploring the relative roles of the selected cortical
areas in mental imagery.