SPATIAL DETERMINANTS OF MULTISENSORY INTEGRATION IN CAT SUPERIOR COLLICULUS NEURONS

1. Although a representation of multisensory space is contained in the superior colliculus, little is known about the spatial requirements o f multisensory stimuli that influence the activity of neurons here. Cr itical to this problem is an assessment of the registry of the differe nt receptive fields within individual multisensory neurons. The presen t study was initiated to determine how closely the receptive fields of individual multisensory neurons are aligned, the physiological role o f that alignment, and the possible functional consequences of inducing receptive-field misalignment. 2. Individual multisensory neurons in t he superior colliculus of anesthetized, paralyzed cats were studied wi th the use of standard extracellular recording techniques. The recepti ve fields of multisensory neurons: were large, as reported previously, but exhibited a surprisingly high degree of spatial coincidence. The average proportion of receptive-field overlap was 86% for the populati on of visual-auditory neurons sampled. 3. Because of this high degree of intersensory receptive-field correspondence, combined-modality stim uli that were coincident in space tended to fall within the excitatory regions of the receptive fields involved. The result was a significan tly enhanced neuronal response in 88% of the multisensory neurons stud ied. If stimuli were spatially disparate, so that one fell outside its receptive field, either a decreased response occurred (56%), or no in tersensory effect was apparent (44%). 4. The normal alignment of the d ifferent receptive fields of a multisensory neuron could be disrupted by passively displacing the eyes, pinnae, or limbs/body. In no case wa s a shift in location or size observed in a neuron's other receptive f ield(s) to compensate for this displacement. The physiological result of receptive-field misalignment was predictable and based on the locat ion of the stimuli relative to the new positions of their respective r eceptive fields. Now, for example, one component of a spatially coinci dent pair of stimuli might fall outside its receptive field and inhibi t the other's effects. 5. These data underscore the dependence of mult isensory integrative responses on the relationship of the different st imuli to their corresponding receptive fields rather than to the spati al relationship of the stimuli to one another. Apparently, the alignme nt of different receptive fields for individual multisensory neurons e nsures that responses to combinations of stimuli derived from the same event are integrated to increase the salience of that event. Therefor e the maintenance of receptive-field alignment is critical for the app ropriate integration of converging sensory signals and, ultimately, el icitation of adaptive behaviors.