Abstract
Subthreshold continuous theta burst stimulation of the visual cortex has been reported to cause inhibitory effects on phosphene threshold. In contrast, we observed no inhibition in a former study applying higher stimulation intensities. The main discrepancies between our experiments and the former studies were stimulation intensity and coil type. We aimed at investigating the role of these factors on the modulatory effects of continuous theta burst stimulation applied to the visual cortex. In a between-group-design, we used either a figure-of-eight-coil or a round coil, respectively. We measured phosphene thresholds prior and after continuous theta burst stimulation applied at 80% of individual phosphene threshold. With the figure-of-eight-coil, phosphene thresholds significantly decreased following stimulation. This is in line with the results of our former study but contrary to the increase observed in the other two studies. Using a round coil, no significant effect was observed. A correlation analysis revealed an inhibitory effect in subjects with higher phosphene thresholds only. Furthermore, the slope of the baseline phosphene threshold seems to predict the direction of modulation, independent from coil type. Thus, modulatory effects of continuous theta burst stimulation seem to depend on coil type and psychophysics parameters, probably due to different cortex volumes stimulated. Stochastic resonance phenomena might account for the differences observed.
Highlights
Phosphenes, defined as elementary visual percepts, can be evoked by pulses of transcranial magnetic stimulation (TMS) applied to the occipital pole (e.g., [1,2,3])
Whereas binocular light deprivation causes an increase in visual cortex excitability as indicated by decreased phosphene thresholds [4], the opposite was true for monocular light deprivation
We explored the aftereffects of continuous theta burst stimulation (cTBS) applied to the visual cortex on phosphene thresholds using either a round coil or a figure-of-eight-coil
Summary
Phosphenes, defined as elementary visual percepts, can be evoked by pulses of transcranial magnetic stimulation (TMS) applied to the occipital pole (e.g., [1,2,3]). Indicating the individual visual cortex excitability of the stimulated area, PT is very stable across multiple sessions showing high test-retest-reliability (e.g., [4,5,6,7,8,9]). PT was used as the dependent variable to investigate changes in visual cortex excitability. It was shown that the pattern of light deprivation has a substantial impact on visual cortex excitability. Whereas binocular light deprivation causes an increase in visual cortex excitability as indicated by decreased phosphene thresholds [4], the opposite was true for monocular light deprivation
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