Why can't we see sound or hear colour?

Different parts of our brain control our vision and hearing. The 'Occipital Lobe' is the back part of the brain that controls vision. The 'Temporal Lobe' is located on each side of the head above the ears.It controls hearing. In general these two separated brain regions function independently. In fact, apart from the fact that distinct regions in the brain are involved in processing vision and hearing, there are distinct sensory organs with specialised receptors corresponding to specific types of stimuli. Thus, the circuit that processes vision is distinct from the one that processes hearing. So, we don't usually 'see sound' or 'hear colour'.

However, there is a condition called synesthesia in which there is crosstalk between these regions and a person with this condition can actually 'see sound' and 'hear colour'. Synesthesia can connect other senses also. For instance a person can 'smell sound' or 'touch colour'. Synesthesis results from cross-wiring in the brain. In this condition neuronal connections are formed between distinct sensory areas and these cross connections are responsible for a person "seeing sound", or "hearing colour" or "smelling sound", for instance.