Better ears with eyes open: effects of multisensory stimulation with nonconscious visual stimuli on auditory learning

Abstract

Audiovisual integration may improve unisensory perceptual performance and learning. Interestingly, this integration may occur even when one of the sensory modalities is not conscious to the subject, e.g., semantic auditory information may impact nonconscious visual perception. Studies have shown that the flow of nonconscious visual information is mostly restricted to early cortical processing, without reaching higher-order areas such as the parieto-frontal network. Thus, because multisensory cortical interactions may already occur in early stages of processing, we hypothesized that nonconscious visual stimulation might facilitate auditory pitch learning. In this study we used a pitch learning paradigm, in which individuals had to identify six pitches in a scale with constant intervals of 50 cents. Subjects were assigned to one of three training groups: the test group (Auditory + congruent unconscious visual, AV), and two control groups (Auditory only, A, and Auditory + incongruent unconscious visual, AVi). Auditory-only tests were done before and after training in all groups. Electroencephalography (EEG) was recorded throughout the experiment. Results show that the test group (AV, with congruent nonconscious visual stimuli) performed better during the training, and showed a greater improvement from pre-to post-test. Control groups did not differ from one another. Changes in the AV group were mainly due to performances in the first and last pitches of the scale. We also observed consistent EEG patterns associated with this performance improvement in the AV group, especially maintenance of higher theta-band power after training in central and temporal areas, and stronger theta-band synchrony between visual and auditory cortices. Therefore, we show that nonconscious multisensory interactions are powerful enough to boost auditory perceptual learning, and that increased functional connectivity between early visual and auditory cortices after training might play a role in this effect. Moreover, we provide a methodological contribution for future studies on auditory perceptual learning, particularly those applied to relative and absolute pitch training.