Phosphenes induced by transcranial magnetic stimulation (TMS) are sensations of light, whereas a missing region in the visual field induced by TMS is generally referred to as a scotoma. It is believed that phosphenes are caused by neural excitation, while scotomas are due to neural inhibition. In light of the recent literature it might, however, be surmised that both phenomena are the result of neural noise injected into the cortex by TMS and that the likelihood of perceiving the two kinds of percepts depends on the state of the cortex at the time of stimulation. In the present study, TMS was applied over the left occipital cortex under different background conditions (Experiments 1-2) and using different TMS intensities (Experiment 3). Behavioral responses indicate the visual system processes luminance in a standardized manner, as lighter percepts were reacted to faster than darker percepts; this effect, however, did not extend to percept size. Our results suggest that phenomenological characteristics of artificial visual percepts are in line with the proposed effects of TMS as the induction of random neural noise interfering with the neural dynamics (the state of the cortex) at the time of stimulation.

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Simple reaction times (RTs) are inversely related to the luminance of a visual region, with RT increasing as luminance decreases, and decreasing as luminance increases. A potential discrepancy in the link between luminance and RT, however, stems from the perception of luminance itself. Here, we tested whether RT is modulated by a measureable amount of light (luminance) or perceptual amount of light (brightness), as two test regions having the same luminance can be perceived as having different brightness. The current study investigates the effects of brightness using probes and artificial percepts, i.e., transcranial magnetic stimulation (TMS)-induced light and dark percepts. In Experiment 1, participants performed a RT task to light and dark probes presented over two backgrounds, one exhibiting a physical luminance and the other exhibiting perceptual brightness. Experiment 2 tested whether perceptual brightness could influence RT and frequency of artificial percepts. We found that while brightness contrast modulated RT to the dark probes, the frequency of artificial percepts was susceptible to both physical and perceived changes in luminance. These data suggest that some behavioral responses can be influenced by an illusion of brightness, wherein there is no actual change in luminance, as well as the perception of TMS-induced percepts.

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