The influence of visual and auditory receptive field organization on multisensory integration in the superior colliculus

The spatial register of the different receptive fields of multisensory neur ons in the superior colliculus (SC) plays a significant role in determining the responses of these neurons to cross-modal stimulus combinations. Spati ally coincident visual-auditory stimuli fall within these overlapping recep tive fields and generally produce response enhancements that exceed the ind ividual modality-specific responses and can exceed their sum. Yet, in this context, it has not been clear how "spatial coincidence" is operationally d efined. Given the large size of SC receptive fields, visual and auditory st imuli could be within their respective receptive fields even when there are substantial spatial disparities between them. Indeed, previous observation s have raised the possibility that there may be a second level of determini sm in how SC neurons deal with the relative spatial locations of within-fie ld cross-modal stimuli; specifically, that multisensory response enhancemen ts become progressively weaker as the within-field visual and auditory stim uli become increasingly disparate. While the present experiments demonstrat ed that SC multisensory neurons have heterogeneous receptive fields, and th at the greatest number of impulses evoked were by stimuli that fell within the area of cross-modal receptive field overlap, they also indicate that th ere is no systematic relationship between cross-modal stimulus disparity an d the magnitude of multisensory response enhancement. Thus, two within-fiel d cross-modal stimuli produced the same proportionate change (i.e., multise nsory response enhancement) when they were widely disparate as they did whe n they overlapped one another in space. These observations indicate that cr oss-modal spatial coincidence can be defined operationally by the borders o f an SC neuron's receptive fields regardless of the size of those receptive fields and/or the absolute spatial disparity between within-field cross-mo dal stimuli.