Plasticity of the adult stereoscopic system : effects of TMS

Abstract

Brain plasticity in the visual system is restricted following a critical period in early childhood development. In recent decades, researchers have discovered that continuous theta burst stimulation (cTBS), despite its inhibitory effect on cortical excitability, induces visual plasticity and improves a wide range of visual capacities in adulthood, including stereopsis. However, the mechanisms underlying cTBS-induced visual plasticity remain controversial. This thesis is devoted to elucidating the neural mechanisms underlying cTBS-induced stereoscopic improvement by examining the behavioral and neural consequences following cTBS over various regions.

As cTBS over the occipital cortex has been demonstrated to improve stereoacuity in amblyopic vision, our first experiment used the healthy vision system as a model to further investigate whether this improvement is specific to certain stimulation sites or visual features. Specifically, we tested depth and luminance discrimination before and after cTBS over the occipital cortex (V1/V2) and lateral occipital complex (LOC), both of which have been shown to be involved in stereopsis. To test discriminative performance, we constructed disparity-defined random dot stereograms consisting of a central aperture and a surrounding annulus. In the fine depth task, participants were instructed to judge the small but clear depth position of the central dots relative to the surrounding reference. In the fine luminance task, participants were required to ignore the depth positions and judge whether the central dots are darker or lighter than the surround. Results of the fine depth task indicated that cTBS over LOC improved (decreased) disparity discrimination thresholds. Surprisingly, cTBS over the occipital cortex did not affect stereopsis in healthy adults, contrasting the findings previously reported in amblyopic patients, suggesting that cTBS differentially acts on the stereoscopic system in amblyopic versus healthy populations. Moreover, cTBS over the occipital cortex and LOC did not affect luminance discrimination thresholds in the fine luminance task, suggesting that the visual enhancements following LOC stimulation are specific to stereopsis.

In our second experiment, we further examined the neural responses during depth and luminance discrimination by means of functional magnetic resonance imaging (fMRI) before and after cTBS over LOC. Results of multivoxel pattern analysis indicated that, among all regions of interest, only LOC displayed differential response patterns in the fine depth task before vs after LOC stimulation. More importantly, we examined the brain-behavior correlations and found that only LOC responses appear to be functionally relevant to the changes in disparity discrimination thresholds. In turn, this suggests that LOC stimulation induces stereoscopic improvement by modulating multivariate response patterns in LOC.

Overall, our findings revealed that visually normal adults retain sufficient stereoscopic plasticity that can be induced by cTBS over LOC. As our fMRI data demonstrated that LOC stimulation did not affect unstimulated regions, we conjecture that cTBS improved depth discrimination by facilitating the neural processing of disparity information in LOC. Considering the inhibitory effect of cTBS, we speculate that the general inhibition following cTBS may enhance signal-to-noise ratio by suppressing internal noise in the nervous system and thereby contribute to better representation of disparity-defined stimuli during depth judgments.