Neural activity profiles of the neocortex and superior colliculus after bimodal sensory stimulation

Current efforts at functional mapping of multisensory neurons are hampered by the need for both cellular-level resolution and the separate visualization of activity by different sensory cues. We have used a recently developed technique that exploits the differential time course of zif268 mRNA versus protein induction in neurons after sensory stimulation. Adult male rats were visually and acoustically deprived and then exposed to one of the following stimulation sequences: (i) no sensory stimulation; (ii) 2 h visual stimulation followed by 30 min auditory stimulation; (iii) 2 h auditory stimulation followed 30 min of visual stimulation; and (iv) 2 h compound visual and auditory stimulation. The neocortex and superior colliculus (SC) were then processed for fluorescent immunocytochemistry and in situ hybridization for staining of Zif268 protein and mRNA products. We have found that activity patterns in primary visual and auditory cortices were in accord with the sequence of the compound stimulus. We also show that SC superficial layers contained a pool of exclusively unimodal neurons, similar to that of visual cortex. Activity patterns of deep SC layers contained multimodal neurons with varying degrees of visual and auditory convergence. The deep SC layers also showed that auditory processing was largely carried out by a small, bimodal group of neurons whereas visual processing was coordinated by both a large unimodal and a small bimodal pool of neurons.