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Abstract:
Temporo-nasal asymmetry in visual responses have been attributed to the anatomical asymmetry of fibres projecting to the Superior Colliculus (SC), even though this attribution is debated. The present study investigates temporo-nasal asymmetry in multisensory integration, and, by exploiting the absence of S-cone input to the SC, measures a behavioural response dependent strictly on the activity of the SC itself. We used a redundant signal paradigm for simple reaction times, with visual stimuli (red or purple) presented in either the temporal or the nasal hemifield. Participants responded quicker to concurrent audiovisual stimuli than to either an auditory or a visual stimulus alone, a phenomenon known as Redundant Target Effect (RTE). The nature of this effect was dependent on the colour of the visual stimuli, suggesting its modulation by collicular circuits. When spatially coincident audiovisual stimuli were visible to the SC (red stimuli), the RTE depended on a neural co-activation mechanism, suggesting an integration of multisensory information. When using stimuli invisible to the SC (purple stimuli), the RTE depended only on a simple statistical facilitation effect, in which the two sensory stimuli were processed by independent channels. Finally, we demonstrate that the multisensory integration effect was stronger for stimuli presented to the temporal than to the nasal hemifield.
