Merging of modalities in the optic tectum: infrared and visual integration in rattlesnakes.

The optic tectum of pit vipers (Crotalinae) contains a layer of infrared-sensitive neurons subjacent to the visual layer; these indirectly receive input from the facial pit organs. They respond transiently to the appearance or motion of warm objects within their 25 degrees to 70 degrees excitatory receptive fields (some have inhibitory regions) and presumably allow the snake to orient or strike toward prey. The infrared and visual spatiotopic tectal maps have similar but not identical axes; the infrared magnification is greater than that for vision. Bimodal neurons have receptive fields for each modality that reflect the disparity of the two maps. This finding suggests that (i) during development the infrared and visual fibers spread out independently to fill available tectal sites and (ii) bimodal neurons form local connections without regard to establishing spatial correspondence between the two modalities.