Exploring the functional system of mental imagery by rTMS targeted at different cortical areas
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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.