Introduction and overview

Vertebrates have two eyes, ears and nostrils, in fact they are pretty much bilaterally symmetrical (although we have just one liver, for example). We can think of several reasons why this might be the case. Animals fight and get injured, or they get injured in other ways, so having two eyes or two ears (vets do a lot of business sewing cats' ears back up) provides some degree of redundancy. Having an eye on each side of the head makes it possible to see a large portion of the world, a more panoramic view. But in the sensory domain two channels enable the extraction of directional information. In the case of olfaction, the information, the differential arrival of odours at the two nostrils gives some indication of the direction of a source, but it remains imprecise. But for hearing and vision it is very precise indeed.

This chapter covers an important theoretical idea, the correspondence problem which underlies several aspects of sensory processing. When the information is collected by two spatially separate detectors, the signals coming in to each do not necessarily match perfectly. The differences can be used to infer something about the spatial properties of the signal. The two primary sonic differences are a variation in time of arrival and a difference in intensity arising because the signals have travelled slightly different routes.