As pointed out in Section 2.3.2, there are numerous, independently variable sources of information about the spatial layout of the environment in the light sampled by the eye. In many cases, however, processing of stimulus information requires or is optimized by recruiting sources of auxiliary information from outside the visual system. These may be directly integrated with incoming visual information or used to change the weighting assigned to one or another source of optical stimulus information (Shams &amp; Kim 2010; Ernst 2012).