The role of conscious and nonconscious processes in multimodal representation formation

Abstract

That subjective experience is a unified and multisensory whole suggests that multisensory integration may have a critical role to play in the generation of consciousness. This relationship has been formalised in several theories, the most influential being Global Workspace Theory (GWT; Baars, 1988). GWT posits a modular sensory processing system in which sensory inputs are processed in sequestered modality- or feature-specific networks; interaction between modules is only possible if their contents are globally broadcast throughout the brain via long-range connections (i.e., the global workspace). The GWT view that consciousness is necessary for multisensory processing is, however, difficult to reconcile with current multisensory research. Namely, multisensory processes are not homogenous, and operate on distinct types of input at early or late stages of stimulus processing to give rise to different behavioural effects. The present thesis thus aimed to clarify the distinction between types of multisensory process, and create a body of data that can inform the more underspecified aspects of GWT. In a series of four studies, I examined the requirement of consciousness for multisensory influence on perception and attention. Two approaches were taken in manipulating participants' awareness of stimuli: (1) stimulus information in both modalities was completely suppressed from consciousness (i.e., subliminal); (2) the entry of stimulus information of one modality into consciousness was delayed relative to the other. Both approaches yielded evidence of multisensory processes that were either unaffected or inhibited by these manipulations, reflecting nonconscious and conscious multisensory processes respectively. The combined findings indicated that nonconscious multisensory processes have relatively limited operation, e.g., an early-stage comparison process that detects physical correspondences between modality information streams, and that these do not alter the evaluation of content. Conversely, consciousness was required for more complex, late-stage multisensory processing that combines and re-evaluates stimulus information, or acts upon higher-order inputs (e.g., semantic information). Taken together, the present findings indicate that multisensory processing contributes to the generation of conscious experience through multiple mechanisms, and depict a richer and more nuanced picture of multisensory processing than initially envisioned in GWT.

Date of Award	2019
Original language	English