If the brain only receives electrical impulses from the senses, what template does it use to construct reality?

As I understand it, the templates consist of the basic architecture and connectivity of the sensory cortices. These facilitate the conversion of raw sensory input into contextual maps of the perceptual world, but what these basic mappings mean must be learnt through experience. In the process, a lot of pruning and rewiring of neural connections occurs.

So, for example, the visual cortex processes the spike trains from the eyes into lines and shapes and colours, but how these correspond to the world, e.g. as objects of varying sizes and distances, must be learnt through experience.

It seems to me that the meaning of 'illusion' in this context is not so much 'it doesn't exist' as, 'it's not what it seems', in much the same way as the phantom limb pain an amputee may experience is illusory - it's a real pain experience, but is not what it seems to be (a pain in a limb). Similarly, the illusion of consciousness is that it is not what it seems; i.e. it feels like a consistent, unitary, continuous, agent self, when it really isn't.

In the case of colour, sometimes called a 'secondary quality', the 'illusion' is that objects don't themselves have colour, they reflect various wavelengths of electromagnetic radiation that we use to construct the experience of colour, but the wavelengths are only a guideline - we apply various transforms, such as colour constancy, to make our experience coherent.

I can only make sense of Dennett's denial of consciousness in these terms; I have similar difficulty with his compatibilist defence of moral responsibility....