Visual cortex cTBS increases mixed percept duration while a-tDCS has no effect on binocular rivalry

Abstract

Neuromodulation of the primary visual cortex using anodal transcranial direct current stimulation (a-tDCS) can alter visual perception and enhance neuroplasticity. However, the mechanisms that underpin these effects are currently unknown. When applied to the motor cortex, a-tDCS reduces the concentration of the inhibitory neurotransmitter gamma aminobutyric acid (GABA), an effect that has been linked to increased neuroplasticity. The aim of this study was to assess whether a-tDCS also reduces GABA-mediated inhibition when applied to the human visual cortex. Changes in visual cortex inhibition were measured using the mixed percept duration in binocular rivalry. Binocular rivalry mixed percept duration has recently been advocated as a direct and sensitive measure of visual cortex inhibition whereby GABA agonists decrease mixed percept durations and agonists of the excitatory neurotransmitter acetylcholine (ACH) increase them. Our hypothesis was that visual cortex a-tDCS would increase mixed percept duration by reducing GABA-mediated inhibition and increasing cortical excitation. In addition, we measured the effect of continuous theta-burst transcranial magnetic stimulation (cTBS) of the visual cortex on binocular rivalry dynamics. When applied to the motor or visual cortex, cTBS increases GABA concentration and we therefore hypothesized that visual cortex cTBS would decrease the mixed percept duration. Binocular rivalry dynamics were recorded before and after active and sham a-tDCS (N = 15) or cTBS (N = 15). Contrary to our hypotheses, a-tDCS had no effect, whereas cTBS increased mixed percepts during rivalry. These results suggest that the neurochemical mechanisms of a-tDCS may differ between the motor and visual cortices.

Continuous theta burst TMS of area MT+ impairs attentive motion tracking

“…Participants then completed three cTBS sessions conducted on separate days. Session 1 involved active motor thresholding (AMT) to calibrate cTBS intensity (Abuleil et al, 2021) AMT was used for cTBS intensity calibration rather than phosphene threshold because not all people perceive phosphenes, AMT is objective and AMT is positively correlated with phosphene thresholds (Deblieck et al, 2008). Session 1 also included training to familiarize participants with the MOT paradigm.…”

Continuous Theta Burst Stimulation to the Secondary Visual Cortex at 80% Active Motor Threshold Does Not Impair Central Vision in Humans During a Simple Detection Task

“…In many cases, these results were also obtained based on extremely small samples (many had N < 10) (e.g. [17][18][19][20]), and therefore negative findings could simply reflect a failure to reject the null hypothesis due to the limited statistical power rather than a true absence of stimulation effect on vision. As a result of these issues, it is unclear whether cTBS can be safely applied to early visual areas without inducing transient blind spots in the central visual field.…”

“…From both the safety and scientific integrity perspectives, blind spots to the central visual field would be more problematic and thus warrant more attention for investigation. Additionally, findings from these studies have been inconsistent-reporting impairments [9,[12][13][14], improvements [16,17], and non-significant changes [17][18][19][20] to vision. This is likely due to variability in methods across studies, including differences in stimulation intensity, performance measurement, target localization method, and task demands.…”

Continuous Theta Burst Stimulation to the Secondary Visual Cortex at 80% Active Motor Threshold Does Not Impair Central Vision in Humans During a Simple Detection Task

“…From both the safety and scientific integrity perspectives, blind spots to the central visual field would be more problematic and thus warrant more attention for investigation. Additionally, findings from these studies have been inconsistent -reporting impairments (Rahnev et al, 2013;Cai et al, 2014;Chiou and Lambon Ralph, 2016;Chen et al, 2020), improvements (Waterston and Pack, 2010;Clavagnier et al, 2013), and non-significant changes (Waterston and Pack, 2010;Brückner and Kammer, 2014;Kaderali et al, 2015;Abuleil et al, 2021) to vision. This is likely due to variability in methods, including differences in stimulation intensity, performance measurement, target localization, and task demands.…”

“…This is likely due to variability in methods, including differences in stimulation intensity, performance measurement, target localization, and task demands. In many cases, these results were also based on extremely small samples (many had N < 10) (e.g., Waterston and Pack, 2010;Brückner and Kammer, 2014;Kaderali et al, 2015;Abuleil et al, 2021), and therefore negative findings could reflect a failure to reject the null hypothesis due to limited power rather than a true absence of stimulation effect on vision. As a result, it is unclear whether cTBS can be safely applied to early visual areas without inducing transient blind spots in central vision.…”