The human brain mechanisms of afterimages: From networks to cortical layers

Afterimages are common visual illusions that have long attracted scientific interest, yet their neural mechanisms remain little understood. We used high-spatial-resolution fMRI to investigate human whole brain and cortical layer activity in primary visual cortex (V1) linked with afterimages and perceptually matching animated images - stimuli designed to imitate the appearance of afterimages. Both afterimages and perceptually matched images engaged overlapping, widespread brain activity, particularly in visual sensory regions that follow the contralateral circuitry of the primary visual pathway. However, afterimages elicited weaker fMRI signals across many subcortical and cortical areas compared to images, except in salience network regions, where activity was enhanced for afterimages. Cortical layer-specific analyses in V1 revealed afterimages selectively engaged deep cortical layers, while images activated middle and superficial layers. In addition, we found that baseline eye measures and fMRI signals in arousal and visual networks differed depending on whether afterimages were perceived or not perceived. We argue that these results challenge the prevailing framing on the neurophysiological origins of afterimages as arising from either retinal or central neural processes. As with image perception, typical afterimages emerge by the interaction between retinal activity and central neural processing.