Human brightness estimation often dissociates from objective viewing conditions. Rather than physical illumination, the subjective experience of brightness has been shown to correlate with temporal dynamics in the amplitude of cortical neural responses. This finding provides an important step towards understanding the basis of visual perception.
Human brightness estimation often pronouncedly dissociates from objective viewing conditions. Yet, the physiological substrate underlying subjective perception is still poorly understood. Rather than physical illumination, the subjective experience of brightness has been shown to correlate with temporal dynamics in the amplitude of cortical neural responses. Here, we aimed to experimentally manipulate visual flicker-evoked steady-state responses and related perception via concurrent modulation of cortical excitability by transcranial alternating current stimulation. Participants performed a brightness discrimination task of two visual flicker stimuli, one of which was targeted by same-frequency electrical stimulation at varying phase shifts. Transcranial electrical stimulation was applied with an occipital and a periorbital active control montage, based on finite-element method simulations of electric fields. Experimental results reveal that flicker brightness perception is modulated dependent on the phase shift between sensory and electrical stimulation, solely under stable flicker entrainment and exclusively under occipital electrical stimulation. The degree of induced brightness modulation was positively correlated with the strength of neuronal phase locking to the flicker, recorded prior to electrical stimulation. This finding was corroborated by a neural network model, demonstrating a comparable dependency between flicker-evoked phase synchronization and amplitude modulations of entrained neural rhythms by phase shifted visual and electric inputs. Our data suggest a causal role of the amplitude of neural activity in visual cortex for brightness perception in humans. This finding provides an important step towards understanding the basis of visual perception and further confirms electrical stimulation as a tool for advancing controlled modulations of neural excitability and related behavior.