The role of recurrent processing and NMDA receptor functioning in human conscious perception

Many theories of human consciousness propose that conscious perception requires the integration of information through recurrent processing—the dynamic interplay between higher- and lower-level brain regions. In parallel, animal studies have linked recurrent processing to NMDA receptors in neurons, yet this connection has remained largely unexplored in human consciousness research. This thesis bridges these fields by investigating the role of NMDA receptor-mediated recurrent processing in conscious perception. Using EEG decoding in human participants, we tracked the processing of different visual features—contrast, collinearity, and the Kanizsa illusion—as markers of feedforward, lateral, and recurrent processing, respectively. In Chapter 2, we refined the link between recurrent processing and consciousness, demonstrating that perceptual impairments of conscious access stem from early, local disruptions to recurrent processing, whereas attentional impairments emerge later and affect global processing. Chapters 3 and 4 examined the role of NMDA receptors by administering memantine, an NMDA receptor antagonist. Across two experiments with distinct participant groups and visual tasks, memantine selectively influenced recurrent processing while leaving feedforward and lateral processing intact. Notably, its effects were limited to fully conscious stimuli—those that were attended, task-relevant, and unmasked—where recurrent processing was strongest. Finally, Chapter 5 applied our paradigm to predictive processing, revealing that expectations modulate recurrent, but not feedforward, processing. Crucially, these effects occurred independently of attention. Together, these findings provide robust evidence that human conscious perception depends on recurrent processing, which, in turn, relies on NMDA receptor function.