High visual demand following theta burst stimulation modulates the effect on visual cortex excitability

Abstract

Modulatory effects of repetitive transcranial magnetic stimulation (TMS) depend on the activity of the stimulated cortical area before, during, and even after application. In the present study, we investigated the effects of theta burst stimulation (TBS) on visual cortex excitability using phosphene threshold (PTs). In a between-group design either continuous or intermittent TBS was applied with 100% of individual PT intensity. We varied visual demand following stimulation in form of high demand (acuity task) or low demand (looking at the wall). No change of PTs was observed directly after TBS. We found increased PTs only if subjects had high visual demand following continuous TBS. With low visual demand following stimulation no change of PT was observed. Intermittent TBS had no effect on visual cortex excitability at all. Since other studies showed increased PTs following continuous TBS using subthreshold intensities, our results highlight the importance of stimulation intensity applying TBS to the visual cortex. Furthermore, the state of the neurons in the stimulated cortex area not only before but also following TBS has an important influence on the effects of stimulation, making it necessary to scrupulously control for activity during the whole experimental session in a study.

No Modulation of Visual Cortex Excitability by Transcranial Direct Current Stimulation

“…The design of the experiment was almost identical to that used in our former study [20] and is illustrated in Fig 1. Sitting in a comfortable chair during the whole session, subjects received biphasic magnetic pulses delivered with a Magpro X100 stimulator (MagVenture, Farum, Denmark) and a figure-of-eight coil MC-B70 placed over the occipital cortex.…”

“…Sitting in a comfortable chair during the whole session, subjects received biphasic magnetic pulses delivered with a Magpro X100 stimulator (MagVenture, Farum, Denmark) and a figure-of-eight coil MC-B70 placed over the occipital cortex. For details regarding phosphene perception criteria, familiarization procedure and PT measurements, see our former study [20]. For each subject, the coil position inducing the strongest phosphene perception was determined (“hot-spot”, cf.…”

“…For each subject, the coil position inducing the strongest phosphene perception was determined (“hot-spot”, cf. [20]) and kept constant using the frameless stereotactic positioning system BrainView (V2, Fraunhofer IPA, Stuttgart, Germany, cf. [26]).…”

“…For instance, “facilitatory” 5Hz rTMS produced an inhibitory effect [18], iTBS had no effect on PT [15] and anodal tDCS failed to produce any after-effect on contrast sensitivity [19]. Recently, it was suggested that at least TBS effects on visual cortex excitability depend on the state of the neurons even after TBS [20]. For tDCS, it was shown that neuronal activity before or during stimulation [21, 22] can modify the direction of modulatory effects.…”

“…Additionally, we aimed to examine whether the state of the neurons following tDCS, as varied by high or low visual demand after stimulation, has an influence on the expected modulatory effects. Thus, the design of the present study was exactly the same as published previously [20], but applying tDCS instead of TBS. Our hypotheses were: (a) anodal tDCS will decrease PTs and cathodal tDCS will increase PT [16, 17]; (b) if subjects are exposed to low visual demand following tDCS, the modulatory effect will last for at least 10minutes [16]; and (c) if subjects are exposed to high visual demand following tDCS, the modulatory effect will be modified, i.e., there will be a decrease in PT independent from tDCS polarity [23].…”