Proportion of cortex dedicated for vision and hearing I have read in some websites that the percentage of the cortex devoted to processing visual information is from 30-66% with some claiming even 90%. And compared to that, only about 3% is dedicated to hearing. 

Is this true? 

The only source I've found stating something similar is from Sells &amp; Fixott (1957) where 50% of the
neural tissue is devoted to vision and almost two-thirds of the
electrical activity of the brain is devoted to vision when the eyes are open.

I haven't found anything related to hearing though. Does anyone know of a paper where this is discussed?

References

Sells, S.B. &amp; Fixott, R. S. (1957). Evaluation of Research on Effects of Visual Training on Visual Functions. American Journal of Ophthalmology 44(2), 230—236DOI: 10.1016/0002-9394(57)90012-0
20-30% vision. See Sheth &amp; Young (2016) which cites Van Essen (2003).
27% vision, 8% auditory (Van Essen, 2003)
I have seen some earlier references that say 50%. I think this derives from two factors. First, it is 50% in Macaque (see above ref 1).  Second, my feeling (I haven't dug into this in enough detail to conclusively state it, so please don't quote me on this!) is that earlier studies on humans were more speculative and tended to follow that 50% as a best guess in absence of direct measurements. 
As some of the above answers point out, one needs to be careful in defining what part of the brain and what vision means.  If one were to include the subcortical structures (i.e., amygdala, superior colliculus, pulvina, LGN) the estimate of total brain (not just the cerebrum cortex) would be different. 



Sheth, B. R., &amp; Young, R. (2016). Two visual pathways in primates based on sampling of space: exploitation and exploration of visual information. Frontiers in integrative neuroscience, 10, 37.
Van Essen, D. C. (2003). Organization of visual areas in macaque and human cerebral cortex. The visual neurosciences, 1, 507-521.