The post-hoc montage of perception: deep layers of primary visual cortex encode postdictive percepts

One of the most puzzling aspects of perception is postdiction: later information can change how a previous stimulus is perceived [1,2]. This indicates that perception is not a livestream of the external world, but rather a post-hoc reconstruction. How does the brain revise perception? It has been proposed that recurrent processing in the sensory hierarchy may sustain earlier information long enough for later information to impact it [3]. We hypothesised that this allows feedback from higher-order multisensory regions to alter the representation of earlier visual stimuli in the primary visual cortex (V1) [4]. Here, we tested this hypothesis using two postdictive illusions [5], where the presence or absence of a tone determines whether an illusory flash is perceived, or an actually presented flash is perceptually suppressed, respectively. This allowed us to isolate neural signals reflecting illusory percepts from sensory inputs in retinotopic space. We collected 7T layer-specific fMRI data while participants (N=23) experienced the illusions. We found that postdictive illusions induced similar activity patterns as real percepts, specifically in the deep layers of V1, suggesting postdictive feedback. Informational connectivity analyses suggested that the source of this feedback was the Superior Temporal Cortex (STC). We conclude that the postdictive montage of perception is reflected in V1, implemented through a delaying feedback loop with higher-order sensory cortex. This extends the role V1 plays in perceptual inference: beyond receiving sensory inputs reflecting the present and predictions of the future, it is involved in the post-hoc revision of perception.