The Brain changing its Mind: bistable perception and voluntary control investigated with frontoparietal TMS

In bistable vision, the brain is unsure which of two percepts accurately represents the outside-world. As a result, the conscious percept switches back-and-forth continuously. Neuroimaging studies have shown that frontoparietal regions are activated during this process. The role of these regions constitutes one of the main outstanding questions in visual awareness research. Some have suggested that frontoparietal regions causally induce the perceptual switches. Alternatively, perceptual switches might induce frontoparietal activations, which would speak against a top-down causal influence from frontoparietal regions. During voluntary control, when people ‘will’ the percept to switch more often, top-down modulation should in any case occur. Subjects are able to voluntary increase the switch rate when watching, e.g., a bistable sphere-from-motion (SFM) stimulus. Are the same regions and/or mechanisms involved in voluntary control and passive bistable vision? Are frontoparietal regions causally relevant? In the current study we addressed these issues by directly interfering with frontal and parietal cortex activity, during passive bistable vision and during voluntary control, in ten participants. Offline rTMS lasting 5 minutes (1 Hz stimulation at 110% of individual motor threshold) was found to significantly reduce the amount of voluntary control over ambiguous SFM vision for 2 minutes after stimulation, as measured by perceptual switch rate. This was true for both parietal (p<0.05) and frontal (p<0.05) stimulation. Interestingly, no such TMS effects on switch rate were found when the same frontal and parietal cortices were stimulated in the same subjects as they passively viewed the bistable stimulus. This revealed frontoparietal cortices to be causally involved in top-down modulation exercised during voluntary control. Moreover, the exact same frontoparietal cortices did not seem to play a causal role during passive viewing. This speaks against a causal influence from the frequently reported frontoparietal activations during passive bistable vision.