An attempt to push mental imagery over the reality threshold using non-invasive brain stimulation

Visual imagery and perception are associated with similar patterns of activation in the visual cortex. While an efficient re-use of neural resources, this overlap leads to the question of how imagery and perception are kept apart. One hypothesis is that signal strength determines perceptual reality judgements such that when activity in the visual cortex is strong enough to cross a ‘reality threshold’, that activity is inferred to reflect reality. Here, we aimed to test the causal role of sensory strength in perceptual reality monitoring by attempting to push mental imagery over the reality threshold using non-invasive brain stimulation. We used a combination of psychophysics, mental imagery and subthreshold Transcranial Magnetic Stimulation (TMS): a technique that is assumed to increase subthreshold neuronal activity. We applied subthreshold TMS to early visual cortex while participants simultaneously imagined and detected visual stimuli in noise. Both imagining the stimulus as well as brain stimulation to the early visual cortex (compared to vertex) increased reports of perceiving a real stimulus when none was presented. However, there was no interaction between brain stimulation and imagery, indicating that TMS failed to push imagined signals over the reality threshold. We discuss various explanations for our results and suggest how these methods could be used to investigate the causal mechanisms of perceptual reality monitoring in future research.