Transcranial magnetic stimulation (TMS)-induced Blindsight of Orientation is Degraded Conscious Vision

Meta-analysis of experimental factors influencing single-pulse TMS effects on the early visual cortex

Background

Single-pulse transcranial magnetic stimulation (spTMS) applied to the Early Visual Cortex (EVC) has demonstrated the ability to suppress the perception on visual targets, akin to the effect of visual masking. However, the reported spTMS suppression effects across various studies have displayed inconsistency.

Objective

We aim to test if the heterogeneity of the spTMS effects can be attributable to variations in experimental factors.

Methods

We conducted a meta-analysis using data collected from the PubMed and Web of Science databases spanning from 1995 to March 2024. The meta-analysis encompassed a total of 40 independent experiments drawn from 33 original articles.

Results

The findings unveiled an overall significant spTMS suppression effect on visual perception. Nevertheless, there existed substantial heterogeneity among the experiments. Univariate analysis elucidated that the spTMS effects could be significantly influenced by TMS intensity, visual angle of the stimulus, coil type, and TMS stimulators from different manufacturers. Reliable spTMS suppression effects were observed within the time windows of -80 to 0 ms and 50 to 150 ms. Multivariate linear regression analyses, which included SOA, TMS intensity, visual angle of the stimulus, and coil type, identified SOA as the key factor influencing the spTMS effects. Within the 50 to 150 ms time window, optimal SOAs were identified as 112 ms and 98 ms for objective and subjective performance, respectively. Collectively, multiple experimental factors accounted for 22.9% (r = 0.3353) and 39.9% (r = 0.3724) of the variance in objective and subjective performance, respectively. Comparing univariate and multivariate analyses, it was evident that experimental factors had different impacts on objective performance and subjective performance.

Conclusion

The present study provided quantitative recommendations for future experiments involving the spTMS effects on visual targets, offering guidance on how to configure experimental factors to achieve the optimal masking effect.

Neurologically Healthy Humans' Ability to Make Saccades Toward Unseen Targets

Some patients with a visual field loss due to a lesion in the primary visual cortex (V1) can shift their gaze to stimuli presented in their blind visual field. The extent to which a similar "blindsight" capacity is present in neurologically healthy individuals remains unknown. Using retinotopically navigated transcranial magnetic stimulation (TMS) of V1 (Experiment 1) and metacontrast masking (Experiment 2) to suppress conscious vision, we examined neurologically healthy humans' ability to make saccadic eye movements toward visual targets that they reported not seeing. In the TMS experiment, the participants were more likely to initiate a saccade when a stimulus was presented, and they reported not seeing it, than in trials which no stimulus was presented. However, this happened only in a very small proportion (∼8%) of unseen trials, suggesting that saccadic reactions were largely based on conscious perception. In both experiments, saccade landing location was influenced by unconscious information: When the participants denied seeing the target but made a saccade, the saccade was made toward the correct location (TMS: 68%, metacontrast: 63%) more often than predicted by chance. Signal detection theoretic measures suggested that in the TMS experiment, saccades toward unseen targets may have been based on weak conscious experiences. In both experiments, reduced visibility of the target stimulus was associated with slower and less precise gaze shifts. These results suggest that saccades made by neurologically healthy humans may be influenced by unconscious information, although the initiation of saccades is largely based on conscious vision.

Recognition and Processing of Visual Information after Neuronavigated Transcranial Magnetic Stimulation Session

Background: Transcranial magnetic stimulation (TMS) is a method of noninvasive and painless stimulation of the nervous system, which is based on Faraday’s law of electromagnetic induction. Over the past twenty years, the TMS technique has been deployed as a tool for the diagnosis and therapy of neurodegenerative diseases, as well as in the treatment of mental disorders (e.g., depression). Methods: We tested the inhibitory effects of repetitive TMS (rTMS) on reaction times to militarily relevant visual stimuli amidst distractors and on accompanying blood oxygenation level dependent (BOLD) signal functional magnetic resonance imaging (fMRI) in 20 healthy people. rTMS was applied over the visual cortices, V1, on both hemispheres with the inhibitory theta burst paradigm with the intensity of 70% of the active motor threshold fMRI in 20 healthy people. Results: Analysis of the reaction time to visual stimuli after using TMS to the V1 visual cortex revealed an increase in the number of incorrect recognitions, and the reaction time was from 843 to 910 ms. In the subgroup of participants (n = 15), after the stimulation, there were significant reductions of BOLD signal in blood flow within V1 cortices. Conclusions: The studies of reaction times after the rTMS revealed the inhibitory effect of rTMS on the reaction times and recognition performance of significant (military) objects in the visual field.

Promise and challenges for discovering transcranial magnetic stimulation induced "numbsense"-Commentary on Ro & Koenig (2021)

The notion that behavioral responses to stimuli can be mediated by separate unconscious and conscious sensory pathways remains popular, but also hotly debated. Recently, Ro and Koenig (2021) reported that when activity in somatosensory cortex was interfered with transcranial magnetic stimulation (TMS), participants could discriminate tactile stimuli they reported not consciously feeling. The study launches an interesting new area of research, helping to uncover mechanisms of unconscious perception that possibly generalize across different sensory modalities. However, we argue here that the study by Ro and Koenig also has several significant shortcomings, and it fails to provide evidence that pathways bypassing primary somatosensory cortex enable unconscious tactile discrimination. By referring to numerous studies investigating TMS-induced blindsight, we outline challenges in demonstrating unconscious sensory pathways using TMS. By facing to these challenges, research investigating TMS-induced numbsense has potential to stimulate progress in stubborn debates and reveal modality-general mechanisms of unconscious perception.