Object Perception

Up to this point, I've introduced the basics of the primary visual pathway of electrical information, which travels from the eyes into the cortex at the rear end of the occipital lobe. But knowledge of these mechanics doesn't do much to explain how objects are actually perceived. What's missing is the construction of recognition, memory, and judgement of the salience an object has for the individual. Only after these steps have occurred can an animal solve the problems that crop up while it's navigating through the external world.

This problem is the domain of the cortex, which is a vast, integrated processing system. It gathers the kind of elemental signals we've seen up to now, connects them to stored information, and ultimately gives them meaning, which is what perception truly means.

The primary visual pathway described in the previous section doesn't end in V1. It projects outward into other lobes of the brain. The most popular model for how it does so is the two streams hypothesis, which is very recent, dating from 1992. It postulates the existence of two pathways: the 'what' and the 'where'. The 'what' identifies, while the 'where' localizes.

This hypothesis has arisen because it seems that the brain identifies objects with one visual system, and unconsciously guides the body to respond with motor movements to those objects with another. The next few sections are concerned with the reasoning and evidence pointing to the existence of these two visual streams. It's interesting to note that this idea isn't localized to just the visual system - auditory perception might be composed of two separate pathways as well.